AUTHOR AND EDITOR INFORMATION

Section 1 of 11  

• Authors and Editors
• Introduction
• Background
• Clinical Aspects of Acute Renal Colic
• Diagnosis
• Renal Colic Pain
• Imaging Studies
• Differential Diagnoses of Flank Pain
• Medical and Surgical Treatment
• Multimedia
• References

INTRODUCTION

Section 2 of 11  

• Authors and Editors
• Introduction
• Background
• Clinical Aspects of Acute Renal Colic
• Diagnosis
• Renal Colic Pain
• Imaging Studies
• Differential Diagnoses of Flank Pain
• Medical and Surgical Treatment
• Multimedia
• References

Acute renal colic is probably the most excruciatingly painful event a person can endure. Striking without warning, the pain is often described as being worse than childbirth, broken bones, gunshot wounds, burns, or surgery. Renal colic affects approximately 1.2 million people each year and accounts for approximately 1% of all hospital admissions. Most active emergency departments (EDs) treat an average of at least one patient with acute renal colic every day depending on the hospital's patient population. The ED physician is often the first to see and evaluate these patients. While proper diagnosis, prompt initial treatment, and appropriate consultations are clearly the primary responsibility of the emergency physician, substantial patient education, including preventive therapy program options, should also be started in the ED.

BACKGROUND

Section 3 of 11  

• Authors and Editors
• Introduction
• Background
• Clinical Aspects of Acute Renal Colic
• Diagnosis
• Renal Colic Pain
• Imaging Studies
• Differential Diagnoses of Flank Pain
• Medical and Surgical Treatment
• Multimedia
• References

The overall lifetime rate of kidney stones in the general population is approximately 12% for men and 4% for women. Having a family member with a history of stones doubles these rates. Approximately 30 million people are at risk in the United States. Peak incidence occurs in people aged 35-45 years, but the disease can affect anyone at any age. Initial stones in elderly people and in children are relatively uncommon; however, consider kidney stones whenever acute back or flank pain is encountered, regardless of patient age. When stones occur in persons in these uncommon age groups, a metabolic workup consisting of a 24-hour urine collection and appropriate serum laboratory testing is recommended.

The increasing incidence of kidney stone disease in the United States seems to be related to the socioeconomic status of the patient population. The lower the economic status, the lower the likelihood of renal stones. Other parts of the world with lower standards of living tend to have lower incidences of kidney stones but have higher rates of bladder calculi. Black people have a lower incidence of stones than white people, and people living in the South and Southwest have higher incidences of stones than people living in other parts of the United States.

Renal failure and stones

While nephrolithiasis is not a common cause of renal failure, certain problems, such as preexisting azotemia and solitary functional kidneys, clearly present a higher risk of additional renal damage. Other high-risk factors include diabetes and struvite staghorn calculi and various hereditary diseases such as primary hyperoxaluria, Dent disease, cystinuria, and polycystic kidney disease. Spinal cord injuries and similar functional or anatomical urological anomalies also predispose patients with kidney stones to an increased risk of renal failure. Recurrent obstruction, especially when associated with infection and tubular epithelial or renal interstitial cell damage from microcrystals, may activate the fibrogenic cascade, which is mainly responsible for the actual loss of functional renal parenchyma.

In emergent settings in which concern exists about possible renal failure, the focus of treatment should be on correcting dehydration, treating urinary infections, preventing scarring, identifying patients with a solitary functional kidney, and reducing risks of acute renal failure from contrast nephrotoxicity, particularly in patients with preexisting azotemia (creatinine >2 mg/dL), diabetes, dehydration, or multiple myeloma. Adequate intravenous hydration is essential to minimize the nephrotoxic effects of intravenous contrast agents. Choosing imaging studies that do not require intravenous contrast (eg, ultrasound, abdominal flat plate radiographs, noncontrast CT scans) is wise, especially in patients at increased risk for developing renal failure.

CLINICAL ASPECTS OF ACUTE RENAL COLIC

Section 4 of 11  

• Authors and Editors
• Introduction
• Background
• Clinical Aspects of Acute Renal Colic
• Diagnosis
• Renal Colic Pain
• Imaging Studies
• Differential Diagnoses of Flank Pain
• Medical and Surgical Treatment
• Multimedia
• References

Predicting spontaneous stone passage

In general, smaller stones are more likely to pass spontaneously, but stone passage also depends on the exact shape and location of the stone and the specific anatomy of the upper urinary tract in the particular individual. For example, the presence of a ureteropelvic junction (UPJ) obstruction or a ureteral stricture could make passing even very small stones difficult or impossible. If the stone is 4 mm or smaller, the stone is eventually passed 90% of the time. Stones 5-7 mm generally have a 50% chance of passing spontaneously. Calculi larger than 7 mm are unlikely to pass unassisted. However, nothing is more difficult in managing an acute kidney stone attack than predicting what will happen with the stone. Most experienced ED physicians and urologists have observed very large stones passing and some very small stones that do not move.

DIAGNOSIS

Section 5 of 11  

• Authors and Editors
• Introduction
• Background
• Clinical Aspects of Acute Renal Colic
• Diagnosis
• Renal Colic Pain
• Imaging Studies
• Differential Diagnoses of Flank Pain
• Medical and Surgical Treatment
• Multimedia
• References

The diagnosis is often made based on clinical symptoms alone, although confirmatory tests are usually performed. At this point, the goals and opinions of physicians in different specialties diverge. From the point of view of the ED physician, making the diagnosis of a renal or ureteral stone and excluding appendicitis or abdominal aortic aneurysm is sufficient. A urologist, who must ultimately make the decision about possible surgery, may require additional information. Before such a decision can be made, a urologist must know the exact size, shape, orientation, radiolucency, composition, and location of the stone and must know about overall kidney function, the presence of any infection, and other clinical information.

Clinical findings

The classic presentation for a patient with acute renal colic is the sudden onset of severe pain originating in the flank and radiating inferiorly and anteriorly. The pain is usually, but not always, associated with microscopic hematuria, nausea, and vomiting. The patient usually exhibits considerable costovertebral angle tenderness, but the rest of the examination findings are often unremarkable. Unlike patients with an acute abdomen, who usually try to lie absolutely still, patients with renal colic tend to move constantly, seeking a more comfortable position. (However, patients with pyonephrosis also tend to remain motionless.)

Approximately 85% of all patients with renal colic demonstrate at least microscopic hematuria, which means that 15% of all patients with kidney stones do not have hematuria. Lack of hematuria alone does not exclude the diagnosis of acute renal colic. Tachycardia and hypertension are relatively common in these cases, even in patients with no prior personal history of abnormal cardiac or blood pressure problems.

The presence of pyuria, fever, leukocytosis, or bacteriuria suggests the possibility of a urinary infection and the potential for an infected obstructed renal unit or pyonephrosis. Such a condition is potentially life threatening and should be treated as a surgical emergency.

In patients who are floridly septic or hemodynamically unstable, a percutaneous nephrostomy is a faster and safer way to establish drainage of an infected and obstructed kidney. Retrograde approaches to drainage require more sedation and anesthetic and may cause reflux of infected material from the ureter into the kidney and may exacerbate the infection. Retrograde endoscopy in these situations, if used at all, should be reserved for relatively mild cases in which patients are medically stable. Use appropriate urine cultures and antibiotics whenever a urinary infection is suspected in conjunction with hydronephrosis or renal colic.

RENAL COLIC PAIN

Section 6 of 11  

• Authors and Editors
• Introduction
• Background
• Clinical Aspects of Acute Renal Colic
• Diagnosis
• Renal Colic Pain
• Imaging Studies
• Differential Diagnoses of Flank Pain
• Medical and Surgical Treatment
• Multimedia
• References

The colicky-type pain known as renal colic usually begins in the upper lateral mid back over the costovertebral angle and occasionally subcostally. It radiates inferiorly and anteriorly toward the groin. The pain generated by renal colic is primarily caused by the dilation, stretching, and spasm caused by the acute ureteral obstruction. (When a severe but chronic obstruction develops, as in some types of cancer, it is usually painless.)

Colic is a misnomer because renal colic pain tends to remain constant, whereas intestinal or biliary colic is usually somewhat intermittent and often comes in waves. The pattern of the pain depends on the individual's pain threshold and perception and on the speed and degree of the changes in hydrostatic pressure within the proximal ureter and renal pelvis. Ureteral peristalsis, stone migration, and tilting or twisting of the stone with subsequent intermittent obstructions may cause exacerbation or renewal of the renal colic pain. The severity of the pain depends on the degree and site of the obstruction, not on the size of the stone. A patient can often point to the site of maximum tenderness, which is likely to be the site of the ureteral obstruction.

A stone moving down the ureter and causing only intermittent obstruction actually may be more painful than a stone that is motionless. A constant obstruction, even if high grade, allows for various autoregulatory mechanisms and reflexes, interstitial renal edema, and pyelolymphatic and pyelovenous backflow to help diminish the renal pelvic hydrostatic pressure, which gradually helps reduce the pain. The interstitial renal edema produced stretches the renal capsule, enlarges the kidney (ie, nephromegaly), and increases renal lymphatic drainage. (Increased capillary permeability facilitates this edema.) It may also reduce the radiographic density of the affected kidney's parenchyma when viewed on a noncontrast CT scan.

Distension of the renal pelvis initially stimulates ureteral hyperperistalsis, but this diminishes after 24 hours, as does renal blood flow. Peak hydrostatic renal pelvis pressure is attained within 2-5 hours after a complete obstruction. Within the first 90 minutes of a complete ureteral obstruction, afferent preglomerular arteriolar vasodilation occurs, which temporarily increases renal blood flow. Between 90 minutes and 5 hours after the obstruction, renal blood flow starts to decrease while intraureteral pressure continues to rise. By 5 hours after a complete obstruction, both renal blood flow and intraluminal ureteral pressure decrease on the affected side. Renal blood flow decreases to approximately 50% of normal baseline levels after 72 hours, to 30% after 1 week, to 20% after 2 weeks, and to 12% after 8 weeks. By this point, intraureteral pressures have returned to normal, but the proximal ureteral dilation remains and ureteral peristalsis is minimal.

Interstitial edema of the affected kidney actually enhances fluid reabsorption, which helps to increase the renal lymphatic drainage to establish a new, relatively stable, equilibrium. At the same time, renal blood flow increases in the contralateral kidney as renal function decreases in the obstructed unit.

In summary, by 24 hours after a complete ureteral obstruction, the renal pelvic hydrostatic pressure has dropped because of (1) a reduction in ureteral peristalsis; (2) decreased renal arterial vascular flow, which causes a corresponding drop in urine production on the affected side; and (3) interstitial renal edema, which leads to a marked increase in renal lymphatic drainage. Additionally, as the ureter proximal to the stone distends, some urine can sometimes flow around the obstruction, relieving the proximal hydrostatic pressure and establishing a stable, relatively painless equilibrium. These factors explain why severe renal colic pain typically lasts less than 24 hours in the absence of any infection or stone movement.

Experimental studies in animals have suggested that renal damage may begin within 24 hours of a complete obstruction and permanent kidney deterioration starts within 5-14 days. While some practitioners wait several months for a stone to pass in an asymptomatic patient, others argue that permanent damage is occurring as long as intervention is delayed. Based on personal experience and anecdotal cases, the author recommends waiting no longer than 4 weeks for a stone to pass spontaneously before considering intervention. Convincing asymptomatic patients of the need for surgical intervention may be difficult in the absence of a clear consensus in the urological community about the length of time to wait before surgical stone removal, fragmentation, or bypass.

If only a partial obstruction is present, the same changes occur, but to a lesser degree and over a longer period. Proximal ureteric and renal pelvic hydrostatic pressures tend to remain elevated longer, and ureteral peristalsis does not diminish as quickly. If the increased pressure is sufficient to establish a reasonable flow beyond the obstructing stone, glomerular filtration and renal blood flow approximates reference range baseline levels, although pain may be ongoing.

Phases of the acute renal colic attack

The actual pain attack tends to occur in somewhat predictable phases, with the pain reaching its peak in most patients within 2 hours of onset. The pain roughly follows the dermatomes of T-10 to S-4. The entire process typically lasts 3-18 hours. Renal colic has been described as having 3 clinical phases.

Acute, or onset, phase

The typical attack starts early in the morning or at night, waking the patient from sleep. When it begins during the day, patients most commonly describe the attack as starting slowly and insidiously. The pain is usually steady, increasingly severe, and continuous; some patients experience intermittent paroxysms of even more excruciating pain. The pain level may increase to maximum intensity in as little as 30 minutes after initial onset or more slowly, taking up to 6 hours or longer to peak. The typical patient reaches maximum pain 1-2 hours after the start of the renal colic attack.

Constant phase

Once the pain reaches maximum intensity, it tends to remain constant until it is either treated or allowed to diminish spontaneously. The period of sustained maximal pain is called the constant phase of the renal colic attack. This phase usually lasts 1-4 hours but can persist longer than 12 hours in some cases. Most patients arrive in the ED during this phase of the attack.

Abatement or relief phase

During this final phase, the pain diminishes fairly quickly, and patients finally feel relief. Relief can occur spontaneously at any time after the initial onset of the colic. Patients may fall asleep, especially if they have been administered strong analgesic medication. Upon awakening, the patient notices that the pain has disappeared. This final phase of the attack most commonly lasts 1.5-3 hours.

Pain in renal colic

Renal pain fibers are primarily preganglionic sympathetic nerves that reach spinal cord levels T-11 to L-2 through the dorsal nerve roots. Aortorenal, celiac, and inferior mesenteric ganglia are also involved. Spinal transmission of renal pain signals occurs primarily through the ascending spinothalamic tracts. In the lower ureter, pain signals are also distributed through the genitofemoral and ilioinguinal nerves. The nervi erigentes, which innervates the intramural ureter and bladder, is responsible for some of the bladder symptoms that often accompany an intramural ureteral calculus.

• Upper ureter and renal pelvis: Pain from upper ureteral stones tends to radiate to the flank and lumbar areas. On the right side, this can be confused with cholecystitis or cholelithiasis; on the left, the differential diagnoses include acute pancreatitis, peptic ulcer disease, and gastritis.

• Middle ureter: Midureteral calculi cause pain that radiates anteriorly and caudally. This midureteral pain in particular can easily mimic appendicitis on the right or acute diverticulitis on the left.

• Distal ureter: Distal ureteral stones cause pain that tends to radiate into the groin or testicle in the male or labia majora in the female because the pain is referred from the ilioinguinal or genitofemoral nerves. If a stone is lodged in the intramural ureter, symptoms may appear similar to cystitis or urethritis. These symptoms include suprapubic pain, urinary frequency, urgency, dysuria, stranguria, pain at the tip of the penis, and sometimes various bowel symptoms, such as diarrhea and tenesmus. These symptoms can be confused with pelvic inflammatory disease, ovarian cyst rupture, or torsion and menstrual pain in women.

Most of the pain receptors of the upper urinary tract responsible for the perception of renal colic are located submucosally in the renal pelvis, calices, renal capsule, and upper ureter. Acute distention seems to be more important in the development of the pain of acute renal colic than spasm, local irritation, or ureteral hyperperistalsis. Stimulation of the peripelvic renal capsule causes flank pain, while stimulation of the renal pelvis and calices causes typical renal colic. Mucosal irritation can be sensed in the renal pelvis to some degree by chemoreceptors, but this irritation is thought to play only a minor role in the perception of renal or ureteral colic.

In the ureter, an increase in proximal peristalsis through activation of intrinsic ureteral pacemakers may contribute to the perception of pain. Muscle spasm, increased proximal peristalsis, local inflammation, irritation, and edema at the site of obstruction may contribute to the development of pain through chemoreceptor activation and stretching of submucosal free nerve endings.

Nausea and vomiting are often associated with acute renal colic and occur in at least 50% of patients. Nausea is caused by the common innervation pathway of the renal pelvis, stomach, and intestines through the celiac axis and vagal nerve afferents. This is often compounded by the effects of narcotic analgesics, which often induce nausea and vomiting through a direct effect on GI motility and through an indirect effect on the chemoreceptor trigger zone in the medulla oblongata. Nonsteroidal anti-inflammatory drugs (NSAIDs) can often cause gastric irritation and GI upset.

The presence of a renal or ureteral calculus is not a guarantee that the patient does not have some other, unrelated medical problem causing the GI symptoms.

Acute onset of severe flank pain radiating to the groin, gross or microscopic hematuria, nausea, and vomiting not associated with an acute abdomen are symptoms that most likely indicate renal colic caused by an acute ureteral or renal pelvic obstruction from a calculus. Renal colic pain rarely, if ever, occurs without obstruction.

In some cases, a stone may pass before the definitive imaging procedure has been completed. In these cases, residual inflammation and edema still may cause some transient or diminishing obstruction and pain even without any stone being positively identified.

Nerve blocks

Nerve blocks have been used successfully in both the diagnosis and treatment of renal colic, although they are more helpful in chronic than in acute cases. Intercostal nerve blocks can be used to differentiate pain from chondritis, neuromas, and radiculitis from true renal pain. This is achieved by injecting an anesthetic agent, such as lidocaine, around the 11th or 12th intercostal nerve proximal to the site of the pain at a time when the patient is experiencing pain. If the injection causes abolition of the pain, a peripheral nerve or musculoskeletal etiology is suggested. Subsequent injections of various agents to produce neurolysis (eg, 10% phenol or 100% absolute alcohol) have been tried but often result in an intolerable denervation-related discomfort.

In the mid 1950s, a study was reported in which 51 patients with documented renal colic were successfully treated for pain using only a unilateral posterior splanchnic nerve block. Complete relief from pain, nausea, and vomiting was reported in all 51 patients treated.

Urinalysis

Microscopic examination of the urine is a critical part of the evaluation of a patient thought to have renal colic. Gross or microscopic hematuria is only present in approximately 85% of cases. The lack of microscopic hematuria does not eliminate renal colic as a potential diagnosis. In addition to a dipstick evaluation, always perform a microscopic urinalysis in these patients.

Pay attention to the presence or absence of leukocytes, crystals, and bacteria and to the urinary pH. In general, if the number of WBCs in the urine is greater than 10 cells per high-power field or greater than the number of RBCs, suspect a urinary tract infection (UTI). Determining urinary pH also helps because, (1) with a pH lower than 6.0, a uric acid stone should be considered, and (2) with a pH greater than 8.0, an infection with a urea-splitting organism such as Proteus, Pseudomonas, or Klebsiella is likely. Urinary crystals of calcium oxalate, uric acid, or cystine may occasionally be found upon urinalysis. When present, these crystals are very good clues to the underlying type and nature of any obstructing calculus.

While mild leukocytosis often accompanies a renal colic attack, a high index of suspicion for a possible infection should accompany any serum WBC count of 15,000/µL or higher in a patient presenting with an apparent acute kidney stone attack, even if afebrile.

IMAGING STUDIES

Section 7 of 11  

• Authors and Editors
• Introduction
• Background
• Clinical Aspects of Acute Renal Colic
• Diagnosis
• Renal Colic Pain
• Imaging Studies
• Differential Diagnoses of Flank Pain
• Medical and Surgical Treatment
• Multimedia
• References

Flat plate radiograph of the abdomen or plain abdominal film

The historical cornerstone of any evaluation of abdominal pain that could be a stone is a flat plate radiograph of the abdomen or kidneys, ureters, and bladder (KUB), which has been used for this purpose since 1896. All urinary calculi may not always be visible on the KUB radiograph because of their small size, stone radiolucency, or overlying gas, stool, or bone. The stones that are observed can be correlated with opacities found on other studies for identification and tracking progress.

Many calcifications observed on the KUB radiograph are phleboliths, vascular calcifications, calcified lymph nodes, appendicoliths, granulomas, various calcified masses, or even bowel contents. All can be confused with urinary tract calculi. Also, the insoluble radiopaque carrier for osmotically controlled-release oral system (OROS) pharmaceuticals can sometimes be mistaken for urinary calculi on KUB radiographs. (The OROS, developed by Alza Pharmaceuticals, is designed for once-daily dosing of various medications, including methylphenidate [Concerta], oxybutynin [Ditropan XL], doxazosin [Cardura XL], verapamil [Covera-HS], and prazosin [Minipress].)

The flat plate radiograph uses the same orientation and anatomical presentation that is observed on fluoroscopy images and retrograde pyelograms or during endoscopic ureteral surgery, such as ureteroscopy or intracorporeal lithotripsy. Even if a stone is not visible on a flat plate radiograph, the calculus could be a radiolucent uric acid stone that can be dissolved with alkalinizing medication. Such a stone is more likely if the urine pH indicates very acidic urine.

In practice, any patient with symptoms of acute renal colic who demonstrates a urine pH lower than 6.0 should be considered at risk for a possible uric acid stone and should have a KUB radiograph performed to help determine radiolucency.

The flat plate radiograph is inexpensive, quick, and usually helpful even if no specific stone is observed. It is extremely useful in following the progress of previously documented radiopaque calculi and checking the position of any indwelling double-J stents. The KUB radiograph can suggest the fluoroscopic appearance of a stone, which determines whether it can be targeted with extracorporeal shockwave lithotripsy (ESWL). The KUB radiograph is also quite accurate for helping determine the exact size and shape of a visible radiopaque stone, which can only be estimated using a CT scan alone because the x-ray beam of the CT scan does not always cross the stone at its widest point.

Differentiation between a phlebolith and an obstructing calcific stone becomes easier when the KUB radiograph demonstrates a lucent center, identifying the calcification as a phlebolith. This central lucency is not observed as often on CT scans. For these reasons, many urologists recommend the flat plate radiograph in addition to CT scan for any renal colic–type scenario.

A number of studies have suggested that the flat plate has a relatively low sensitivity and specificity for renal and ureteral calculi. Many patients have numerous pelvic calcifications that make pinpointing specific stones difficult. Any calcific density observed on a KUB radiograph that happens to overlie the course of the ureter is not guaranteed to be a stone. A number of emergency physicians argue that the flat plate radiograph adds little to the identification and treatment of a stone in the ED. Furthermore, obtaining a flat plate radiograph may cause delays, may unnecessarily increase the cost of the workup, and causes additional patient radiation exposure. Consequently, the flat plate radiograph is no longer uniformly required in the modern era, now that unenhanced CT scans are the criterion standard for diagnosis of acute renal colic.

A large clinical study from Johns Hopkins University in 2000 by Jackman and associates concluded "plain abdominal x-ray is more sensitive than scout CT for detecting radiopaque nephrolithiasis. Of the stones visible on plain abdominal x-ray, 51% were not seen on CT. To facilitate outpatient clinic follow-up of patients with calculi, plain abdominal x-rays should be performed." Many urologists, including this author, recommend that in addition to other studies (eg, noncontrast helical or spiral CT scans), a KUB radiograph be obtained in all patients with a clinical presentation of acute flank pain suggestive of renal colic. Knowing the exact size and shape of a stone, its position, fluoroscopic appearance, surgical orientation, and relative radiolucency is an advantage. Also, the progress of the stone can be easily monitored with a follow-up flat plate radiograph. These advantages far outweigh the few disadvantages associated with performing the examination.

A reasonable practical compromise has been suggested that would require the KUB only in cases in which the CT scan indicates ureteral or renal calculi. This would avoid the cost and radiation exposure in patients who do not actually have stones. Additionally, the digital CT scout radiograph can be used as a reasonable substitute for the KUB radiograph if cost factors, excessive delays, or logistical problems make obtaining a formal abdominal flat plate study difficult.

Renal ultrasound

Ultrasound is fast, easy, safe, and relatively inexpensive to perform. Ultrasound requires no intravenous contrast and can easily detect any significant hydronephrosis, although this must be differentiated from UPJ obstruction and an extrarenal pelvis. Ultrasound cannot be used to find small stones (ie, £5 mm) and does not help in the evaluation of kidney function. A large extrarenal pelvis or UPJ obstruction can easily be misread for hydronephrosis if ultrasound alone is used.

For some stones, ultrasound works quite well; however, in general, the test is less useful for diagnosing ureteral calculi than other modalities. Middleton et al reported perhaps the most successful use of ultrasound for renal colic in 1988 (ie, a stone detection rate of 91%). Most authors report detection rates of approximately 30%. The unusually high reported success rate achieved by Middleton et al can be explained in part by the fact that a radiologist specializing in ultrasound performed the studies, which typically required at least 15-20 minutes to complete. The success of diagnostic ultrasound is very dependent on operator skill and experience, which is probably demonstrated by the unique setting in which their study was performed.

Renal ultrasound works best in the setting of relatively large stones within the renal pelvis or kidney and sometimes at the UPJ. Whether the stones are radiolucent or opaque does not matter because an ultrasound picture is based strictly on density, not on calcium content. Ultrasound is a good way to monitor known stones after medical or surgical therapy if the stones are large enough to be detected by this modality and are in a suitable position. Ultrasound can also be used to check the abdomen for a possible abdominal aortic aneurysm or cholelithiasis, which can sometimes be mistaken for acute renal colic. It is also useful in differentiating filling defects observed on contrast studies because stones are much more echogenic than tumors, clots, or tissue. Ultrasound is the initial imaging modality of choice for patients with acute renal colic who are pregnant because it avoids all potentially hazardous ionizing radiation.

Ultrasound relies on indirect visualization clues to identify stones. Differentiating an extrarenal pelvis from an obstructed one is sometimes difficult when using ultrasound alone. Intermittent obstruction or mild hydronephrosis can be easily missed with ultrasound, and, with the few exceptions mentioned above, it generally does not provide much information about most other disease processes capable of causing acute flank pain.

Sometimes, a KUB abdominal flat plate radiograph is used in addition to the ultrasound to help identify and monitor suspected stones, especially if renal dilation is detected. As with the KUB radiograph alone, any density detected along the expected course of the ureter is not guaranteed to be an actual stone within the collecting system.

Combining a renal ultrasound with an abdominal flat plate radiograph has been proposed as a reasonable initial evaluation protocol when a CT scan cannot be performed or is unavailable. Ultrasound has reasonably good sensitivity for detection of other sources of abdominal pain, such as cholelithiasis and abdominal aortic aneurysms. When combined with the KUB radiograph, ultrasound can quickly and inexpensively provide substantial information about the urinary tract without the risk of contrast nephrotoxicity or hypersensitivity. Intravenous pyelograms (IVPs) can then be limited to those patients for whom additional information is required for a diagnosis or for whom the etiology of the pain remains unclear.

The intrarenal resistive index, as measured on Doppler studies, has been proposed as one way to diagnose acute renal obstruction using ultrasound. Elevation of the resistive index on the affected side, compared with the opposite healthy kidney, is evidence of obstruction. However, intrarenal resistive index does not identify partial or intermittent obstructions and is less helpful in the early phase of even complete ureteral blockage. It also does not provide any information about the radiolucency, size, shape, or position of any stone and cannot be used to differentiate between intrinsic and extrinsic urinary obstructions. Pyelosinus extravasation or fornix rupture, which occurs in up to 20% of patients with acute ureteral obstructions, leads to a loss of dilation and may be responsible for false-negative findings from studies. Other nonobstructive renal problems, such as renal failure, diabetic nephropathy, and renal compression, can affect the readings.

Under normal conditions, renal vascular resistance is relatively low and renal blood flow is excellent throughout the cardiac cycle, with a reasonable flow continuing even during diastole. During conditions associated with increased vascular resistance, such as an acute ureteric obstruction, the measured decrease in renal blood flow during diastole is proportionately greater than the decrease during systole.

The resistive index is calculated as peak systolic velocity minus end-diastolic velocity divided by the peak systolic velocity. An elevated resistive index of 0.7 or more is considered indicative of an acute ureteral obstruction. A change in the resistive index between the affected and contralateral (healthy) kidney of 0.04 or more also suggests a ureteral obstruction. (The affected kidney has the higher resistive index value.) This study may be particularly useful in pregnancy (when exposure to ionizing radiation must be minimized), severe contrast media allergy, and azotemia. For best results, measure the intrarenal resistive index during a pain attack but before any NSAIDs or other anti-inflammatory medications are administered.

Considering that up to perhaps 35% of patients with documented acute ureteral obstruction do not demonstrate any significant hydroureteronephrosis, the use of a noninvasive study such as Doppler ultrasound and intrarenal resistive index, which does not depend on visual ureteral or renal pelvic dilation, may eventually prove very useful. For now, additional studies on this technique are needed before intrarenal resistive index can be reliably used for diagnosing acute renal colic and ureteric obstruction.

Nuclear renal scans

A nuclear renal scan can be used to objectively measure differential renal function, especially in a dilated system for which the degree of obstruction is in question. This is also a reasonable study in pregnant patients, in whom radiation exposure must be limited.

Technetium Tc 99m (diethylene triamine pentaacetic acid) is the radioisotope used. It is eliminated from the body almost exclusively by glomerular filtration. The rate of clearance from the renal unit provides an excellent estimate of the glomerular filtration rate and the relative rate of drainage or clearing from each kidney. A drainage half-time that is 20 minutes or longer indicates obstruction, while a drainage half-time of 10 minutes or less is considered unobstructed. If the drainage half-time is 10-20 minutes, the result is indeterminate.

Plain renal tomography

Observing even a relatively large radiopaque stone located in the kidney or renal pelvis on a standard abdominal flat plate radiograph can be difficult or impossible if the patient has abundant gas or stool overlying the area. This can be overcome using noncontrast or plain renal tomography. Renal tomography requires moving the radiograph projector and film in such a way that a zone of photographic clarity is positioned at the stationary focus point of the radiograph beam. All other overlying material is eliminated on this type of radiograph. The focal point is adjusted along the anteroposterior axis a distance of 1 cm, and the radiograph procedure is repeated. Usually, a series of 4-6 films is needed to completely image both kidneys. Remember that if such a series of films is needed, it should be obtained before any intravenous contrast is administered; contrast obscures any stones present.

Plain renal tomography is most useful when checking for small stones that might be present in the kidney before starting a preventive therapy plan or when differentiating intrarenal calcifications that are likely to be stones from extrarenal opacities that are clearly not renal calculi.

Intravenous pyelogram

The IVP, or urogram, was the criterion standard for the detection and diagnosis of renal and ureteral calculi from 1927, when Swick first described it, until it was recently displaced by the unenhanced CT scan. The contrast agents used most since 1952 were iodinated benzoic acid derivatives until less allergenic nonionic agents were introduced in the 1980s. Various reactions have been attributed to intravenous contrast agents, including nausea, vomiting, urticaria, bronchospasm, cardiac arrhythmias, seizures, direct nephrotoxicity (especially in dehydrated or azotemic patients), and anaphylaxis.

The main advantage of the IVP is the clear outline of the entire urinary system that it provides, making visualization of even mild hydronephrosis relatively easy. This effect is enhanced by the osmotic diuretic effect of the contrast medium. When a patient has multiple pelvic calcifications, identifying the actual stone is simple with the IVP. IVP can also show nonopaque stones as filling defects. Disadvantages include the need for intravenous contrast material, which may provoke an allergic response or renal failure, and the need for multiple delayed films, which can take hours. The delayed films take time because the contrast material passes quite slowly into the blocked renal unit and ureter. Filming continues until the entire distal ureter is visible, which contributes to an increased radiation dose to the patient.

The dose of intravenous contrast is usually approximately 1 mL/kg. The bolus method of intravenous contrast administration is usually recommended for renal colic evaluations because it allows for a nephrogram-effect phase film. This normally occurs within the first minute after bolus contrast injection and cannot be obtained with the slow-drip infusion technique. Acute ureteral obstruction causes an intense persistent finding on nephrograms, caused by an increase in the intrarenal concentration of the contrast media. This may take several hours or more to fully visualize, which necessarily delays completion of the study.

Flat plate radiographs are obtained immediately before contrast administration and at 1, 5, 10, and 15 minutes afterwards or until visible contrast material fills both ureters. Prone films are sometimes obtained to enhance visualization of the ureters. When the bladder is full of contrast and the distal ureters contain sufficient contrast for visualization, the patient is asked to void; then a postvoid film is taken. Sometimes, oblique views are needed when bone or bowel contents overlie the area of interest.

Contrast-induced nephropathy is the third leading cause of hospital-acquired acute renal failure. A serum creatinine level of more than 2 mg/dL is a relative contraindication to the use of intravenous contrast agents. Patients with azotemia, multiple myeloma, pregnancy, or diabetes, especially if dehydrated, are particularly susceptible to acute contrast nephropathy which is defined as a 25% or greater increase in serum creatinine levels within 2-3 days of intravenous contrast exposure. Ischemia, direct intracellular high–contrast-concentration toxicity and free-radical injury are thought to be the causative mechanisms of contrast-induced nephropathy.

Low osmolarity or iso-osmolar contrast may help to reduce the risk of contrast-induced nephropathy. A renal vasodilator (fenoldopam mesylate [Corlopam]) has been used to minimize renal complications in higher-risk patients requiring intravenous contrast studies who would otherwise be at high risk for azotemia. Fenoldopam is a dopamine type 1A agonist that has been shown to increase renal plasma flow and to help prevent contrast nephropathy. Theophylline and N-acetylcysteine have also been used with some success, but the standard prophylactic therapy is intravenous saline at a rate of 1-3 mL/kg/h. Hemodialysis before and after intravenous contrast can also be used to minimize renal toxicity, but such a regimen is costly and too cumbersome for general use except in special high-risk situations.

A randomized study by Merten and colleagues from Charlotte, NC, published in JAMA in 2004 compared the standard intravenous saline hydration prophylaxis with a 154-mEq/L sodium bicarbonate solution. Identical volumes of each solution were used. They found a substantial benefit in the sodium bicarbonate–treated group compared with the sodium chloride–treated cohort. Patients treated with prophylactic sodium chloride hydration were 8 times more likely to develop nephropathy after contrast exposure than the patients treated with sodium bicarbonate. Such a treatment plan is practical, inexpensive, simple, safe, and effective, and the author now recommend sodium bicarbonate intravenous hydration as the prophylactic treatment of choice for prevention of contrast-induced nephropathy.

Serious allergic reactions to intravenous contrast occur in 0.1-0.2% of patients. Pretreatment with steroids, antihistamines, and H1 and H2 blockers can further reduce the likelihood of severe allergic reactions, especially if administered several hours prior to the study. Nonionic contrast media is more expensive but less likely to provoke an allergic response than the older ionic media, especially if the patient has a history of mild or moderate allergic reactions to contrast or injected dye.

A history of asthma or multiple drug allergies should also suggest the use of a nonionic agent. Many institutions currently use only the nonionic agent for all intravenous contrast studies, despite the added cost, because of the increased safety it provides. Glucophage (Metformin) should be discontinued at least 1 day before any intravenous contrast study, particularly in patients with proven or borderline azotemia, because of the risk of worsening renal function and the rare development of potentially life-threatening lactic acidosis. The medication can be resumed 48 hours after the contrast study if renal function has normalized.

Some urologists prefer the IVP to other imaging studies in the initial evaluation of possible ureteral stones because the size, shape, and position of any calculi are usually very clear and the orientation of the studies is very familiar because it is identical to the types of radiographs performed in endoscopic stone surgeries and shockwave lithotripsy. Another advantage is that IVPs demonstrate renal function and allow for verification that the opposite kidney is functioning normally. Typically, an IVP finding positive for a ureteral stone shows a delayed nephrogram effect and columnization. The ureter is peristaltic so the entire ureter is not usually visualized on a single film except when an obstruction is present, such as from a stone. Even without observing any specific stone, the presence of a nephrogram effect in one kidney with normal function of the opposite kidney is highly suggestive, but not diagnostic, of ureteral obstruction.

Extravasation of contrast around the collecting system may be a sign of a ruptured fornix, while pyelolymphatic backflow indicates that contrast has entered into the renal lymphatic drainage system. Both are considered signs of a more severe ureteric obstruction. However, no published study has indicated that the clinical course, treatment outcome, or residual renal damage is altered in any way in these patients. In fact, this information about the radiological assessment of the relative severity of the obstruction rarely affects clinical treatment decisions, except perhaps in persons with solitary kidneys.

Medullary sponge kidney

Generally, only an IVP provides sufficient detail to diagnose medullary sponge kidney (MSK), also called tubular or ductal ectasia or cystic dilation of the collecting ducts. While Beitzke first described the condition in 1908, the characteristic radiographic features were not recognized until 1939, by Lenarduzzi. MSK is generally a benign congenital condition that demonstrates dilation of the distal renal collecting tubules on IVP as the tubules fill with contrast. These normally invisible microscopic tubules show a whitish blush in the papilla in persons with MSK. In severe cases, stones, cysts, and diverticula can be present. The condition is bilateral in 70% of patients and is usually not discovered until the second or third decade of life, even though MSK is congenital.

The most accurate way to demonstrate MSK has been described as high-quality excretory urography (ie, IVP) with serial renal tomography starting just before the injection of the contrast media and continuing every 4 minutes for the next 20 minutes.

Most patients with MSK are asymptomatic; unless they have an IVP for an unrelated reason, the condition may never be diagnosed. The overall prevalence in the general population has been estimated at approximately 1 case per 5000-10,000 people, while the prevalence among persons receiving an IVP for any reason is approximately 1 case per 200 patients. No autopsy studies have been conducted to document the exact prevalence of the disease.

Of patients who are symptomatic, renal colic and calcium urinary stones are the most common problems. (UTIs and hematuria are the others.) Women are more likely to have MSK than men. The number of patients with MSK among all stone formers is not known; it ranges from 2.6-21% in various studies. A nephrolithiasis patient with MSK is estimated to pass 1.23 stones per year, compared with 0.66 stones per year in calcium stone formers without MSK. Hypercalcemia and hypercalciuria are quite common in patients with MSK, occurring in as many as 50% of these patients in some studies. Renal leak hypercalciuria is more common in MSK patients than in other hypercalciuric calcium stone formers. Hypocitraturia is also more common in MSK stone-forming patients, with a rate roughly double that of calcium stone formers without MSK.

Hypomagnesuria and distal renal tubular acidosis have been found more often in calcium stone–forming patients than in other calcium nephrolithiasis patients. UTIs also seem to be more prevalent in MSK patients, both with and without renal stone formation, compared with other patients (with a higher incidence in women, as expected). Renal failure is rare.

Some patients with MSK may report severe chronic renal pain without any evidence of infection, stones, or obstruction. The etiology of this pain is unclear. These patients may be treated best by physicians comfortable with the management of chronic pain disorders.

MSK is the most common anatomical problem found in calcium nephrolithiasis patients, affecting approximately 2% overall. Most stones in patients who have MSK are composed of calcium oxalate with or without calcium phosphate. Stones tend to be small and are usually passed spontaneously. Renal hemihypertrophy is more common in MSK patients. Up to 10% of all patients with hemihypertrophy of the kidney have MSK and 25% of all MSK patients demonstrate renal hemihypertrophy.

In most cases, MSK is not hereditary, although rare autosomal inherited forms have been described. The exact cause is unknown, but it could be caused by tubular obstruction due to calcium oxalate calculi from infantile hypercalciuria or collecting duct dilation from blockage by fetal uric acid stones, embryonal remnants, or other material.

Beckwith-Wiedemann syndrome (ie, high birth weight, macroglossia, omphalocele, adrenal cortical cysts, mental retardation, visceromegaly, renal enlargement, hemihypertrophy, and MSK) has a high tumor rate, particularly Wilms tumor, hepatoblastoma, and adrenal cancer. For this reason, when MSK is found in pediatric patients, routine screening for Wilms and other tumors is recommended, particularly when microscopic or gross hematuria is present. Hemihypertrophy also suggests the need for periodic screening for renal tumors or masses.

No specific treatment is available for MSK. Therapy is aimed toward avoiding complications of calcium nephrolithiasis and urinary infection. The author recommends early metabolic evaluations (ie, serum and 24-h urine determinations for preventive analysis) to identify correctable causes such as hypercalciuria and hypocitraturia. Appropriate hygienic advice and sometimes long-term, low-dose antibiotics (eg, nitrofurantoin, trimethoprim-sulfamethoxazole, cinoxacin) may be needed for UTI prophylaxis (see Preventing UTIs in Women).

CT scans

CT scans were first used in 1972 but have only become popular in the initial evaluation of acute renal colic since 1995. Part of the reason CT scans became popular was the need for faster scanners, such as the helical or spiral units that have been in widespread use since 1989. These improved scanners allowed studies to be completed much faster than earlier machines, usually during a single breath, which improves image quality by reducing respiratory motion and artifact.

Many institutions now have a renal colic study or protocol that consists of a noncontrast or unenhanced CT scan of the abdomen and pelvis with a helical or spiral machine. Very narrow cuts are taken through the kidneys and bladder areas, where symptomatic stones are most likely to be encountered. An abdominal flat plate or KUB radiograph is sometimes automatically included in a renal colic study, depending on the institution and the preferences of the medical staff. Technically, a relatively high pitch of more than 1.5 with thin collimation of 2-3 mm is generally considered a good compromise between imaging quality and radiation dosage. No rectal, oral, or intravenous contrast is used because contrast material obscures any calcium-containing stones; both the stone and the contrast material would appear radiopaque or white on the scans. Optimally, the patient's bladder is filled, which facilitates viewing the ureterovesical junction.

In equivocal cases in which an indeterminate calcification is found along the course of the ureter or an abrupt change in ureteral caliber is found without a conclusively identified stone, an overlapping retrospective series can be performed to better evaluate this specific area and eliminate any sampling error.

Phleboliths are often confused with calcific ureteral stones. On a KUB radiograph, the characteristic lucent center of a phlebolith is often visible, which is not present in a true calculus. Unfortunately, CT scans usually fail to reveal this central lucency or a bifid peak if a central lucency cannot be identified. Why this finding of a central transparency is so uncommon with CT scanning is unclear, but it may involve the orientation of the veins that form the phleboliths.

The "rim sign," originally reported by Smith in 1995, is described as a rim, ring, or halo of soft tissue visible on CT scans that completely surrounds ureteral stones. The effect is enhanced by the local inflammation a stone produces in the ureteral wall, with subsequent edema at the site of the calculus. The rim sign is generally missing or incomplete with phleboliths. While not absolutely definitive, the rim sign is strong evidence that the calcific density it surrounds is a stone and not a phlebolith. In several studies, more than 75% of all ureteral stones demonstrated a rim sign, while only 2-8% of phleboliths demonstrated it. The rim sign is more likely to be present in small or medium stones up to 5 mm in diameter. Larger stones, bigger 6 mm, tend to lose the rim sign, presumably from stretching and thinning of the ureteral wall around a relatively large calculus.

Another way to differentiate a phlebolith from a calculus is to find a comet's tail or comet sign, which is the noncalcified portion of a pelvic vein that is contiguous with the phlebolith. It appears as a small linear area of soft tissue that seems to pass obliquely through the CT scan section and attaches to the calcific density at one end. This is not observed in ureteral stones, although a ureter can mimic this sign to some degree. The comet sign is found in less than 20% of phleboliths, so its absence helps little, and its reliability is still unproved.

CT scans are readily available in most hospitals and can be performed and read in just a few minutes. Overall sensitivity from multiple studies is reported at 94-100%, with accuracy rated at 93-98%. CT scans do not require contrast, eliminating the risk of allergic reactions or renal failure. Stones, including relatively radiolucent uric acid calculi, cystine, matrix, and xanthine, show up as bright white spots, making identification easy. While they do not contain calcium, the stones are still much denser than the surrounding soft tissue. The only exception is stones made of various metabolites of medications, such as indinavir (Crixivan), which is a protease inhibitor used in the treatment of HIV disease; these stones are not be visible on CT scans. These patients require a contrast study, such as an IVP.

Spiral CT scans allow for anatomical 3-dimensional image reconstruction, which may be useful in the evaluation and pretreatment planning of large staghorn stones prior to percutaneous nephrostolithotomy. However, not all experts agree on the usefulness of this test for this purpose.

• Secondary CT signs of obstruction
  
  
  • Secondary signs of obstruction may be visible only on CT scans. In some cases, if a stone was passed shortly before the study, these signs may be the only evidence that the patient has or ever had a stone. These secondary signs include ureteral dilation with hydronephrosis, renal enlargement from interstitial edema (nephromegaly), and inflammatory changes, such as stranding or streaking in the perinephric fatty tissue.
  
  
  • In a 1996 study of 54 ureteral stone patients reported by Katz et al, hydronephrosis was present in 69%, proximal ureteral dilation was found in 67%, and perinephric stranding was detected in 65%. The other secondary signs had a similar frequency in adults and children. In the study, only 2 of the patients with ureteral calculi did not demonstrate any of the secondary signs of obstruction. The other secondary signs had a similar frequency in adults and children.
  
  
  • A similar 1996 study by Smith et al involving 220 patients found an even higher correlation between these secondary signs of obstruction and the presence of a ureteral calculus. In particular, the combination of collecting system dilation and perinephric stranding had a positive predictive value of 98%, while the absence of both of these secondary signs had a negative predictive value of 91%. However, perinephric stranding was found less often in children with ureteral calculi than in adults in a 2001 study by Smergel and associates; therefore, this secondary sign, at least in the pediatric population, may be less reliable.
  
  
  • An additional secondary sign of acute renal obstruction on noncontrast CT scans has been reported by investigators from Johns Hopkins University. This sign is defined as a reduction in renal parenchymal attenuation (radiologic density) on the nonenhanced CT scan of the acutely obstructed renal unit compared with the normal unobstructed contralateral kidney. The difference in density is at least 2 standard deviations. This sign was identified in 95% of patients with acute ureteral obstruction, which suggests it is a reliable indicator.
  
  
  • Rarely, in indeterminate cases in which the secondary signs are negative and a stone is strongly suspected clinically but not clearly visible on the unenhanced CT scan, intravenous contrast can be used to help visualize the ureter. Repeat scanning after contrast infusion allows for improved visualization of the ureters. This allows physicians to make direct comparisons with the earlier studies to help make the correct diagnosis. Flat abdominal radiograph films taken after the contrast provide information similar to an IVP, but delayed films or scans are likely to be needed.

Currently, CT scans can be used to estimate the relative stone density and composition to some extent, although the results certainly have not replaced the formal stone chemical composition analysis. However, this information may some day be used to help plan therapy. Higher-density stones may be more amenable to laser lithotripsy and ureteroscopy, while low-density readings are more likely to have good results with ESWL. Uric acid is the lowest-density stone material visible on unenhanced CT scans.

Researchers at the University of Wisconsin at Madison carefully studied the unenhanced CT scans of 129 patients with renal stones. They concluded that the peak Hounsfield attenuation level of a kidney stone, used either by itself or divided by the size of the calculus in millimeters, may be a useful indicator of the stone's chemical composition. An attenuation-to-size ratio of 80 or greater was found to be highly suggestive of calcium oxalate stone material, especially in larger calculi. Uric acid stones have relatively low peak attenuation levels, and their attenuation-to-size ratios were generally below 80. In this Wisconsin study, uric acid stones averaged a mean peak Hounsfield reading of 344 HU, while the mean for calcium oxalate calculi was 652 HU. Overall sensitivity was 94% and specificity was 84%, with a negative predictive value of 99%.

This information, combined with the KUB radiographic appearance and low urinary pH values, can be helpful in identifying uric acid calculi, which are amenable to alkalinization therapy. Calculating the peak attenuation level and attenuation-to-size ratio adds no financial cost, patient morbidity, or time delay. While this study and similar reports are interesting and suggestive, the precise clinical role of CT scans in predicting stone fragility and chemical composition remains unclear.

If the patient's true underlying pathology is something other than a kidney stone, the CT scan is more clinically useful than an IVP for examining other possible problems such as aneurysms, pancreatitis, appendicitis, ovarian problems, and various bowel disorders.

Disadvantages of a CT scan include increased cost and difficulties in identifying a stone if the patient exhibits limited hydronephrosis and multiple pelvic calcifications. A CT scan cannot provide precise information about relative renal function and usually cannot be used to diagnose MSK. CT scans can be used to estimate the overall size, width, and location of a stone, but they can only approximate the stone's shape. For example, the CT scan does not necessarily reveal the stone precisely at its widest point, which would provide an accurate measurement of the stone's maximum width. CT scan measurements of stone widths have a reported error rate as high as 12%. Further, a 2002 review by Narepalem et al found that CT scans are even less accurate and reliable in measuring ureteral stone length compared with a simple KUB radiograph. Stone location can be described in anatomical terms, but the scan lacks the surgical orientation that most urologists prefer.

Curved planar reformatting using CT software can be performed to convert the cross-sectional images of the CT scan into an image that approximates a traditional abdominal radiograph study. Because of the relatively high dose of ionizing radiation it uses, CT scans should not be performed on pregnant women.

Finally, the CT scan alone does not allow visual differentiation between a radiolucent and a radiopaque calculus. If a KUB or flat plate radiograph is performed at the same time as the CT scan, many of these objections and problems disappear. However, obtaining the extra films involves some additional delay, the patient is exposed to more ionizing radiation, and the total cost for the workup increases.

A digital CT scout radiograph has been suggested as a possible reasonable substitute for a formal KUB radiograph in some cases. Stones 3 mm and larger can be observed routinely on these studies. If the findings from a noncontrast CT scan are positive for a stone and the findings from the scout CT radiograph are negative, a separate KUB radiograph should be performed.

A computer program is available on the Internet (through UroComp) that is able to calculate the KUB stone appearance size from CT scan data. In cases in which a KUB radiograph is not available or KUB radiograph stone visualization is inadequate (as in uric acid calculi), this online program converts CT-derived stone size data into expected dimensions based on the KUB radiographic appearance. The clinical usefulness of such a tool is unclear; however, this new online stone size calculator could be the first of a series of online computer programs with true clinical utility in the future.

Certain medications (eg, protease inhibitors) can form urinary stones composed of drug metabolites that are invisible on CT scans. For this reason, if a patient is taking a protease inhibitor drug such as indinavir (Crixivan) and presents with a history suggestive of renal colic, the study of choice is an IVP.

In current clinical practice, the renal colic noncontrast CT scan with a modern helical or spiral machine is the standard of care in most EDs when a patient is thought to have renal colic or a patient presents with acute flank pain. Because of the limitations of CT scans, many urologists routinely request additional studies, such as KUB radiographs or IVPs, to help them make critical decisions about management, follow-up, and possible surgical interventions.

As noted earlier, the author recommends adding a KUB radiograph when a renal colic CT scan study is performed for acute flank pain, especially when the CT scan findings are positive for urolithiasis. The KUB radiograph not only provides more precise information about the size and shape of any stone, it also quickly reveals whether stones are nonopaque and radiolucent (eg, uric acid calculi), which indicates stones that are dissolvable with alkalinizing medication, or opaque and calcific, which indicates stones that are not dissolvable with medication alone.

Follow-up evaluations are easier because only a repeat KUB radiograph is needed for comparison. A KUB radiograph also helps the urologist determine if a stone will be visible on fluoroscopy images, which is essential for possible ESWL. This fluoroscopic visualization is needed for stone targeting and positioning. While the addition of an abdominal flat plate study (KUB radiograph) adds to the overall financial cost and requires additional time, the extra information the study provides is often quite valuable and ultimately beneficial to the patient.

Retrograde pyelograms

The most precise imaging method for determining the anatomy of the ureter and renal pelvis and for making a definitive diagnosis of any ureteral calculus is neither the IVP nor the renal colic CT scan. It is the retrograde pyelogram. In this study, the patient is taken to the operating room cystoscopy suite and an endoscopic examination is performed with the patient under anesthesia. After a cystoscope is placed in the bladder, a thin ureteral catheter can be inserted into the ureteral orifice on the affected side. A radiographic picture is taken at the appropriate time, while contrast material is injected through the ureteral catheter directly into the ureter. Any stone, even if radiolucent, and any ureteral kinks, strictures, or tortuousities that may not be visualized easily on other studies become clearly visible.

Urologists perform retrograde pyelograms when a precise diagnosis cannot be made by other means or when a need clearly exists for an endoscopic surgical procedure and the exact anatomical characteristics of the ureter must be clarified. They are also useful for estimating ureteral length prior to placement of an indwelling double-J stent. Retrograde pyelograms are rarely performed merely for diagnostic purposes because other less invasive studies are usually sufficient. They are considered essential when surgery is deemed necessary because of uncontrollable pain, severe urinary infection or urosepsis with a blocked kidney, a solitary obstructed kidney, a stone that is considered unlikely to pass spontaneously because of its large size (generally >7 mm), or the presence of possible anatomical abnormalities (eg, ureteral strictures).

Retrograde pyelograms can be performed safely both in patients highly allergic to intravenous contrast media and in patients with renal failure because the contrast medium never enters the bloodstream and therefore requires no renal filtration or excretion and causes no anaphylaxis.

Magnetic resonance imaging

MRI has virtually no role in the current evaluation of acute renal colic in the typical patient. Direct detection of most stones is not possible with MRI, and MRI should not be used for that purpose in most instances. MRIs are generally more expensive than other studies, such as CT scans, which reveal stones much better. However, MRI produces no dangerous radiation, the gadolinium contrast it uses has minimal nephrotoxicity, and the technology can readily show urinary obstruction even if the stones themselves are not easily visualized. These attributes make using MRI reasonable in selected cases in which other technologies are too toxic or potentially dangerous, such as in some children, in renal transplant recipients, in pregnant women, and in azotemic patients. MRI may also be appropriate in the evaluation of hydronephrosis when stones have been excluded based on CT scans and other means are too toxic or inadequate.

Gadolinium-enhanced 3-dimensional fast low-angle shot (known as 3D FLASH) magnetic resonance urographic imaging was found to be superior to the T2-weighted MRI technique, while combining the two produced the best overall results.

Use of MRI in pregnant patients is controversial. Long-term effects on the fetus are unknown, and MRI is not specifically indicated in pregnancy, although it is not specifically contraindicated either. Anecdotal reports suggest that MRI has no immediately detectable deleterious effects. When other imaging modalities cannot be used or are insufficient, magnetic resonance urographic imaging can be considered on a case-by-case basis when the benefits to the mother and fetus outweigh the potential risks.

IVP Versus CT Scan: Which Is Better?

*Many urologists find CT scans inadequate to help plan surgery, predict stone passage, or monitor patients.

^† This causes a delay, which may be significant in some institutions, and adds additional patient radiograph exposure and cost.

^‡These include significant allergic responses and renal failure.

Summary of IVP versus CT scan

The noncontrast or renal colic-type CT scan is good for the initial diagnosis of a stone, especially in unusual or atypical cases or when patients are unable to tolerate intravenous contrast because of allergy or azotemia. To be most effective and helpful, a CT scan must be coupled with a KUB radiograph. Without definite hydronephrosis, a CT scan may not be able to isolate a specific stone, although secondary signs, such as perinephric streaking and nephromegaly, may be present. The CT scan can be performed quickly in most institutions, even with an additional KUB radiograph, but it usually costs more than the IVP. In one series of 397 consecutive emergency urolithiasis patients from several university centers, the average fee for a CT scan was $1407, compared with $445 for an IVP.

CT scans are generally preferred by most ED physicians for the initial evaluation of patients with acute flank pain, except for HIV-positive patients who may be on protease inhibitors, who require an IVP, and pregnant women, who require ultrasound for their initial imaging modality.

The IVP is better for clearly outlining the entire urinary tract and determining relative renal function. This test clearly shows stones causing blockage, whether the stones are radiolucent or opaque. While an IVP can reliably help in the diagnosis of an MSK, the clinical importance of this diagnosis is limited. The IVP is sometimes preferred by urologists in certain situations because of its better orientation and superior value in predicting possible stone passage, although these advantages are mostly negated if a KUB radiograph routinely accompanies the CT scan.

DIFFERENTIAL DIAGNOSES OF FLANK PAIN

Section 8 of 11  

• Authors and Editors
• Introduction
• Background
• Clinical Aspects of Acute Renal Colic
• Diagnosis
• Renal Colic Pain
• Imaging Studies
• Differential Diagnoses of Flank Pain
• Medical and Surgical Treatment
• Multimedia
• References

Most common and significant causes of flank pain

Abdominal aortic aneurysms

Patients with abdominal aortic aneurysms may present with symptoms of flank pain that mimic renal colic. This occurs in approximately 10% of patients with abdominal aortic aneurysm who are initially thought to have a renal cause for their pain. The aneurysm is thought to be caused by injury or stretching of the nerve plexus around the celiac axis and other aortic arterial branches that also supply the nerve innervation for the kidneys. The aneurysm can lie immediately adjacent to the ureter and cause hematuria from local ureteral irritation or trauma. Lateral deviation of the ureters, especially on the left side, is a common finding. Abdominal aortic aneurysms are best diagnosed with ultrasound or a CT scan. This is one of the reasons why CT scanning is especially useful in cases of suspected renal colic in patients older than 50 years, who are at increased risk for an abdominal aortic aneurysm.

Pyelonephritis

Pyelonephritis is a common cause of flank pain, but its discomfort is usually described as a relatively mild dull ache rather than typical renal colic pain. Fever, chills, nausea, and vomiting are the most common symptoms, along with headache and a stiff neck. Flank and costovertebral angle tenderness are present and often are more severe than with renal colic. Patients with pyelonephritis often tend to lie very still in comparison to patients with renal colic, who move constantly in search of a comfortable position. Suprapubic pain with dysuria, frequency, and urgency often accompany the flank pain in persons with pyelonephritis, but distal ureteral calculi can produce similar symptoms. Fever, leukocytosis, and pyuria are typical findings in persons with pyelonephritis.

Extensive imaging studies may not be required in cases of obvious pyelonephritis, but, in some cases, a renal ultrasound may be helpful to identify hydronephrosis because pyelonephritis does not demonstrate any renal pelvic or ureteral dilation. Any collecting system dilation found suggests a possible obstruction with pyonephrosis. Perform appropriate studies immediately, and promptly institute surgical intervention for drainage. Uncomplicated pyelonephritis without obstruction can be treated with antibiotics alone.

Renal abscesses

Renal abscesses tend to cause more intense pain than pyelonephritis, possibly because the increased edema and inflammation produced further stretch the renal capsule. Sometimes, a flank mass may be palpable. Urinalysis often demonstrates a urinary infection, but, in some cases, the urinalysis findings may be normal. Fever and chills are common. Diaphragmatic irritation may be present, causing rales and decreased breath sounds on the affected side. People with diabetes are at increased risk.

Consider a possible renal abscess in any patient with pyelonephritis who does not improve with antibiotic therapy. Renal ultrasound and CT scans are the most useful imaging studies for making this diagnosis.

Renal abscesses can be divided generally into 2 types. The corticomedullary renal abscess is usually derived from an ascending urinary infection and is often associated with some type of urinary tract abnormality such as vesicoureteral reflux or obstructive uropathy. Escherichia coli is the most common organism involved, found in 75% of these lesions. The second type, the cortical renal abscess, develops from the hematogenous spread of bacteria, usually Staphylococcus aureus, from a primary focus point elsewhere in the body. Drainage of the abscess, either by open surgery or percutaneous methods, is the recommended treatment, along with culture-specific antibiotics whenever possible.

Blood clots

Blood clots sometimes cause an acute renal colic attack when they form a sudden ureteral obstruction. These blood clots can be from iatrogenic causes, such as percutaneous renal biopsies, or from underlying medical problems, such as blood dyscrasias, renal pelvis or parenchymal tumors, angiomyolipomas, hemophilia, sickle cell disease, or glomerulonephritis. Treatment with a ureteral stent or percutaneous nephrostomy is usually sufficient therapy for the immediate blockage. Direct infusion of a clot-dissolving medication such as streptokinase has been successful in selected cases.

Herpes zoster

Herpes zoster causes a moderately severe burning pain, usually in a band around the flank corresponding to a dermatome. The pain usually appears before characteristic skin changes appear. Eventually, the vesicles appear over the affected area, and the diagnosis can then be made easily. Although the pain distribution may be similar, the clinical presentation and location of the discomfort allow differentiation from renal colic. Suspect herpes zoster when the pain is described as burning and constant with a very limited and clearly demarcated distribution, even if no skin lesions are present. Treatment is symptomatic in nature, with analgesics and antiviral agents.

Muscle pain

Muscle pain from a strain or other injury of the back or flank can lead to a dull aching discomfort in the thoracolumbar area. The quadratus lumborum muscle may be involved. The discomfort is exacerbated by lifting and bending. The patient may reveal a history of heavy lifting or repetitive bending at work. The affected area is likely to be tender to palpation. Injection of a local anesthetic into the trigger points may afford significant relief and may help in the diagnosis. Treat such pains with analgesics, NSAIDs, and heat applications.

Papillary necrosis

Papillary necrosis can cause an acute ureteral obstruction from the sloughed papilla. Underlying pathologies include analgesic abuse, liver cirrhosis, and diabetes. A ring sign caused by the outline of a round or triangular filling defect surrounded by contrast in a renal calyx upon IVP characterizes this disorder. The actual sloughing of the renal papilla is caused by vascular ischemia, which leads to coagulative necrosis of the renal medullary pyramids. Women are affected more often than men. Underlying pathologies include analgesic abuse, liver cirrhosis, recurrent pyelonephritis, urinary tract obstruction, tuberculosis, sickle cell disease, systemic vasculitis, renal transplant rejection, and diabetes. Treatment of the underlying pathology and bypassing the obstruction with a nephrostomy or stent is the recommended therapy.

Pleuritis and pneumonia

Pleuritis can cause a sharp stabbing pain in the thoracolumbar region, which, in some cases, can simulate renal colic. The etiology of the pleuritis may be infectious causes (eg, tuberculosis, pneumonia) or noninfectious causes (eg, lupus, pulmonary emboli). The presence of respiratory symptoms or atypical pain involving the chest suggests these possible differential diagnoses and the need for chest radiography, which may help clarify the issue. Treatment is based on the underlying etiology.

Radiculitis

Radiculitis occurs when the lumbar or lower thoracic nerve roots are injured. An injury to the costovertebral junctions can also produce a radiculitis-type pain. When the pain affects the 10th, 11th, or 12th ribs, the discomfort mimics renal colic in its distribution but is usually described as sharp or stabbing. The pain is often acute and can radiate anteriorly and inferiorly, identical to renal colic pain. Movement exacerbates the pain. The fracture of a transverse process in the thoracic or lumbar regions can also produce this pain. Most patients are female. Predisposing factors include a history of rib fractures, arthritis, and a prior open renal surgery. The actual source of the pain is usually scarring or injury of the intercostal nerve. Radiculitis is a common cause of flank pain that is not of renal origin and should be suspected in patients with normal urinalysis findings and normal imaging findings from studies of the urinary system.

Rib pain

Rib pa