Excerpt from Therapeutic Injections for Pain Management

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This article focuses on the use of therapeutic injections to treat acute and chronic pain syndromes. Discussion of this topic begins with an overview of regional anesthesia, which includes the pharmacology of frequently administered medications and basic information regarding equipment and safety. The spectrum of injection procedures and their indications for specific pain disorders and pathoanatomic regions is addressed to include therapeutic options for the various tissues or structures characteristic of each area or syndrome.

The following anatomical divisions are somewhat arbitrary and overlap in some cases; however, this mode of presentation should prove relevant and accessible by using a format to address pain complaints by region and target tissues located in the spine, extremities, head and face, autonomic nervous system, and some viscera. A discussion of the clinical use of botulinum toxin is incorporated at the end of the article.

Neural blockade and similar injection procedures often are prescribed for therapeutic benefits; however, they also can be useful for diagnostic, prognostic, or prophylactic indications, or for a combination of these purposes.

Therapeutic blocks are appropriate for alleviating acute pain, especially in a self-limiting disorder (eg, postoperative, posttraumatic, or acute visceral pain syndromes). In general, they have been advocated to alleviate acute pain or an exacerbation of chronic pain and to provide direct and localized therapeutic action, especially in patients in whom pain is accompanied by swelling and inflammation. They help the patient (1) maintain an ambulatory or outpatient treatment status; (2) maintain participation in a physical therapy or rehabilitation program; (3) decrease the need for analgesics; and (4) in some cases, avoid or delay surgical intervention.

Sympathetic blocks in causalgia and reflex sympathetic dystrophy (ie, complex regional pain syndromes) permit more effective application of adjunctive treatment techniques including physical therapy and medication. In some cases, therapeutic injections help the physician gain patient cooperation, which may have been compromised not only by pain but also by fear, poor nutrition, and deconditioning.

Diagnostic blocks often help the treating physician determine the anatomic origin(s) of the patient's pain. These procedures also may facilitate differentiation of a local from a referred somatic pain source, a visceral from a somatic pain source, or a peripheral from a central etiology. Selective blocks can help determine which peripheral tissues are primary pain generators. In cases of presumed complex regional pain syndromes, neural blockade can be used to establish relative contributions of somatic and sympathetic nervous systems.

Prognostic blocks are intended to provide information regarding the efficacy of a planned neurolytic or neurosurgical ablative procedure or potential surgical outcomes. These blocks also may help the physician and patient decide whether to proceed with surgery or ablative procedures.

Prophylactic blocks are used to delay and reduce postoperative pain, to prevent complications caused by posttraumatic or visceral pain, to decrease the duration of hospitalization and convalescence, and to prevent development of certain chronic pain syndromes such as autonomic dystrophy and phantom limb pain.

For excellent patient education resources, visit eMedicine's Bone Health Center, Muscle Disorders Center, Public Health Center, and Procedures Center. Also, see eMedicine's patient education articles Chronic Pain, Pain After Surgery, BOTOX® Injections, and Pain After Surgery.

Guidelines for therapeutic application

Numerous technical and medical factors are pertinent to avoid potential pitfalls or complications when considering application of injections for the many indications outlined in the introduction. Historically, these procedures have been used empirically, often resulting in variable or temporary benefit, despite risk and potential complications. For these reasons, some of the basic clinical principles for utilization and safety are reviewed.

Physician criteria

A physician who intends to perform therapeutic injections should be qualified by education, training, and experience to diagnose and manage the specific disorder(s) to be treated, including the capacity to determine whether diagnostic evaluation has been complete and that verification of the disorder to be treated has been conclusive. Knowledge of the natural history and expected clinical course of these disorders should influence the physician's judgment as to what procedure should be performed, necessity of the procedure, and likelihood of success, and lead to true informed consent.

The treating physician should be aware of alternative or accessory therapies that can be applied before or following procedural intervention, and which may enhance the efficacy of treatment. Knowledge of the advantages, disadvantages, and limitations of each procedure, and the ability to manage complications, should be considered requisite. Knowledge of the anatomy and pharmacology of injected substances coupled with adequate experience and technical skill for performing prospective procedures are also requisite. The physician should be licensed with privileges to perform therapeutic procedures in the appropriate medical care settings.

Procedural methodology

Prior to performing or even scheduling injection procedures, the physician is obliged to assess the patient thoroughly, including the history of the present illness, past medical history, medications, and drug allergies, and the extent to which operant and psychological factors are salient with regard to the illness at hand. Of course, all such information should be documented thoroughly.

Prior to any medical treatment, especially neural blockade or therapeutic injection, the physician should inform the patient fully regarding technique, indications for the procedure, operative complications, typical time for convalescence, and cost.

The patient's pretreatment status should be documented carefully. A flow sheet and medical chart to record the procedure and document any complications or side effects from pretreatment to posttreatment is standard. On-call physician advice and care should also be available. Any and all adverse effects, whether related to the injection or not; objective observations such as changes in temperature, color, and/or edema affecting an extremity or other pertinent body region; and assessment of therapeutic efficacy should be documented.

Digital video or still photographic documentation of the physiological appearance of the involved extremity or anatomical region and, in some cases, the procedure, provide the physician with a visual record of the injection locale, including any preoperative cosmetic problems such as skin lesions, scars, or deformities. Postoperative photographic recording also can be obtained for comparison.

Further objective and meaningful information can be obtained using preoperative and postoperative visual analogue scales (VAS), pain and disability scales, quality-of-life measures, and injection-specific questionnaires. The purpose and medical necessity for therapeutic injection also should be well documented. Appropriate subspecialty consultation may be necessary in some cases to support the preoperative diagnosis and the medical necessity for application of specific procedures.

Furthermore, the use of adjunctive guidance such as electromyography (EMG), ultrasound, and radiologic studies is recommended in some cases. Injections are rarely the only suggested treatment; therefore, expectations regarding the extent to which the procedure may provide pain or symptom relief should be explained to the patient preoperatively. Most therapeutic injections are not curative; therefore, any assumption that a procedure is a panacea should be dismissed.

Overview: Technical application

The application of therapeutic injections and regional anesthesia requires knowledge of equipment that includes needles, syringes, and catheters. The Luer lock is a conical tip that allows easy exchange of needle to syringe and is named after the person who developed it. Today, most needles are disposable, with the bevel cut in 3 planes to minimize tissue laceration and discomfort. Needles that are used for deep injection during regional block should incorporate a security bead on their shafts so that the needle can be retrieved if the needle hub separates from the shaft.

Generally, disposable straight needles with a beveled or pencil-point shaped tip are used for spinal interventional procedures. Spinal and deep injections are best accomplished with a styletted needle, which has an outer cannula through which a smaller needle or catheter can be inserted. The inner stylet seals the cannula and prevents tissue from entering the cannula as the needle is advanced. The stylet should always remain entirely within the cannula when there is forward movement of the needle. Many advocate the use of a short needle bevel to reduce neural and vascular trauma.¹ Rounded needle tips have been advocated for puncturing the dura to access the subarachnoid space. In theory, rounded tips gently spread the dural fibers and may reduce the incidence of dural cuts that cause post spinal tap headaches. Caution must be exercised with long bevel needles because the soft metal tip is more likely to develop a hook or barb at the tip after striking a bony surface or with prolonged use during a procedure. Furthermore, needles with a smaller caliber (less than 20 gauge) or with a length greater than 3.5 inches are more difficult to steer through tissues of low resistance.

Knowing how to manage the bevel of the outer cannula and inner stylet are key to successful needle navigation. The hub of the needle usually has a notch that corresponds to the face of the bevel needle tip. After puncturing the skin, as the needle is advanced through the deeper soft tissues, the needle tip tends to veer slightly in the direction opposite to the hub notch; therefore, enter the skin as close to the target as feasible. The tendency for a needle to travel in a curved trajectory can be useful at times and can be enhanced by placing a small 5-10° bend in the tip. When traveling a significant distance with a bent needle tip, the needle must be continually rotated to prevent it from straying off course, which may cause significant tissue disruption. To counter this potential problem, a larger coaxial needle can be placed just proximal to the target, and then if a curved trajectory facilitates steering just beyond the needle tip, a bent needle can be inserted through the larger needle, which allows it to swerve or turn in the direction necessary to reach the anatomical objective.

The needle hub is held with the thumb on top pointing toward the notch. The index and middle fingers are place opposite the thumb at the junction of the hub and needle. The needle is pushed by the thumb and can be steered by turning the notch in a direction that is 180° opposite to the target. This maneuver places the sharp edge of the needle tip toward the direction that the needle is intended to travel. The stylet should always be contained entirely within the cannula while the needle is moving forward.

Before skin entry, the angle of the needle tip and its trajectory define its course. However, after the needle passes into deeper soft tissues, it cannot be steered by redirecting or pushing it sideways. Bowing of the needle is a technique, whereby, pressure is established both at the skin surface and at the proximal end of the needle. The needle bows toward the surface pressure. The hub is moved in a direction opposite to the notch, causing the needle to arc and the needle tip to travel in the same trajectory as the bow, opposite to the notch. Turning the hub changes the course of the needle, but always in a direction that is opposite to the bowed posture of the needle.

The needle should always be advanced slowly over short distances with frequent monitoring by fluoroscopy. The operating physician needs to be aware to move his hands out of the path of the x-ray beam when using intermittent fluoroscopy. The needle tip position can be determined by tissue feel (soft tissue vs bone), fluoroscopic visualization [lateral, oblique and AP planes] and using radiopaque contrast. Fluoroscopic localization requires an AP and lateral of the needle or one fluoroscopy view and contact with an identifiable bony landmark. Contacting bone during the procedure offers a unique opportunity to know needle tip position. Also, when the needle tip is resting on bone, it is unlikely to be in a dangerous venue, such as a blood vessel, neural tissues, or the intrathecal space. Injection of radiopaque dye can be used to further establish certainty of the needle location. Water-soluble contrasts are benign, even when injected into the intravascular or intrathecal space; however, the presence of contrast may obscure view of the needle's tip for continued placement. The physician operator must know the needle tip's location before injecting any active medication. If injected radiocontrast dye washes away rapidly from the needle tip during the injection be wary, because the contrast may be entering a blood vessel. The dye should stay at the injection site.²

Pharmacology: Local anesthetics

The application of any injectable substance can lead to allergic, idiosyncratic, or adverse side effects. Previous suspicious or unfavorable responses may be verified through prior hospital or office records. In some cases, a small amount of the substance in question may be injected subcutaneously to test the patient's reaction to exposure.

Safe and effective use of local or regional anesthesia requires thorough knowledge of the pharmacology of local anesthetics (LAs). Local infiltration for neural blockade can be accomplished by using dilute concentrations of LAs, as they rapidly penetrate the various tissues around targeted nerve endings. When large-diameter nerves are targeted, the quantity of drug reaching the central axonal core is reduced because of incomplete penetration of surrounding epineurium, perineurium, endoneurium, fat, blood vessels, and lymphatics, which can constitute as much as 40% of the peripheral nerve diameter.

Some of the injected substance is absorbed by local blood during its diffusion, which acts as another important mechanism for reducing the amount of drug that actually reaches the nerve axon. Higher concentration of LAs may cause local vasomotor paralysis, which increases local blood flow and enhances systemic absorption. Blood flow through injected tissues can be reduced by using an LA solution mixed with epinephrine, which decreases systemic absorption and improves penetration of the anesthetic to its target. Therefore, the vascularity of various tissues should be considered when deciding on the LA concentration and amount of injectate. Absorption into the bloodstream not only reduces potency of the injected material at the target site, but also increases systemic side effects. Low concentrations of LAs typically are used to block smaller, lightly myelinated and unmyelinated nerve fibers, such as C, A-delta, and B-preganglionic sympathetic fibers.

Several clinical characteristics should be considered when choosing an LA. The latency of onset of anesthetic action is an important clinical property; however, concentration, total dose, distance between the injection site and target, and relative penetrance of the compound also should be considered. Penetrance depends on target-tissue characteristics, including the thickness of superimposed, fibrous, and other intervening tissues. Tissue penetrance of specific LAs determines latency of onset and intensity of induced anesthesia. Duration of LA action depends on pharmacodynamic properties of the anesthetic, concentration, total dose, and vascularity of the region under scrutiny. LA toxicity relates to all of these factors and also to biotransformation.

All LAs have the same basic chemical structure with an aromatic and amino end joined by an intermediate chain. The amino esters use an ester link between the aromatic and intermediate chain. These drugs include cocaine, procaine, 2-chloroprocaine, and tetracaine. Cocaine was the first anesthetic used clinically and continues to be used for topical airway anesthesia because it is unique among LAs in also being a vasoconstrictor. The amino amides contain an amide link between the aromatic and intermediate chain. These medications include lidocaine, mepivacaine, prilocaine, oral pivacaine, bupivacaine, and etidocaine.

Lidocaine is a widely used LA because of its rapid onset, potency, and tissue penetration. Within this group bupivacaine is also a popular and frequently used LA for peripheral nerve block and epidural or spinal anesthesia. Commercially available concentrations of this drug range from 0.125-0.75%. Altering the concentration of bupivacaine can elicit a separate sensory or motor neural blockade, ie, lower concentrations primarily induce a sensory block, whereas higher concentrations cause motor blockade. Bupivacaine alters myocardial conduction more dramatically than lidocaine; therefore, the need for cardiorespiratory monitoring during the use of LAs should be emphasized.

Several agents are used to prolong or modify the action of LAs. As already discussed, epinephrine causes vasoconstriction, which reduces vascular and systemic absorption of the drug from the intended site of action, lowers the risk of systemic toxicity, and enhances LA efficacy on the target tissue. Epinephrine is the agent most often combined with LAs, which have a short to moderate duration of action. Epinephrine is contraindicated in some patients because of side effects or drug sensitivity or when a compromise of blood flow should be avoided (ie, when used in distal portions of the extremities, especially with coexisting peripheral vascular disease). Phenylephrine and norepinephrine (NE) also have been used as vasoconstrictors for spinal anesthesia; however, they do not appear to provide any significant advantage over the more commonly used epinephrine.

Alkalinizing agents are thought to facilitate the onset of action and prolong neural blockade when combined with LAs; however, recent double-blind studies in humans have failed to substantiate that this actually occurs. Nevertheless, addition of sodium bicarbonate to bupivacaine still is advocated to produce faster onset of epidural blockade with longer duration.

Corticosteroids

Injectable corticosteroids have been traditionally advocated to treat pain and inflammation associated with a myriad of musculoskeletal conditions, except when infection or skin breakdown is present at the target site, or in patients with poorly controlled diabetes.³ Several therapeutic actions have been proposed for their beneficial effects.⁴ They reduce inflammation by inhibiting the synthesis or release of a number of proinflammatory substances, including arachidonic acid and its metabolites (eg, prostaglandins, leukotrienes), some cytokines (eg, interleukins 1 and 6, tumor necrosis factor-α), and other acute phase reactants.⁵ Other proposed mechanisms of action include a direct membrane-stabilization effect, reversible inhibition of nociceptive C-fiber transmission, and modulation of nociceptive input within the dorsal horn substantia gelatinosa neurons.

Continuous large doses of a corticosteroid adversely affect collagen synthesis, and, therefore, connective tissue strength.^{6, 7} Frequency of injections and dosages must be monitored by the physician to prevent generalized or focal immune suppression such as infection or impaired soft tissue healing.³ Therefore, the amount of corticosteroids that can be applied over time to a specific tissue area can be detrimental, although the exact dose/time curve remains unknown.³ Concomitant use of medications that alter corticosteroid effects or clearance is usually not salient when injections are provided intermittently.

Physician preference among commonly used injectable corticosteroids is often arbitrary. Corticosteroid esters have long been preferred because of their relative safety and efficacy. The relative solubility of these solutions is considered a factor when determining the appropriate injectate.³ Highly soluble steroids such as betamethasone sodium phosphate-acetate are rapidly absorbed and pose a lower risk for connective tissue injury, such as tendon rupture, fat atrophy, and muscle wasting. Relatively insoluble steroid esters have a longer duration of action.³

Corticosteroids are among the most commonly used active substances for spinal intervention. Particulate steroids should not be placed into the cervical foramina, because foraminal arteries, specifically the radiculomedullary artery, can be occluded by the injection. Foraminal artery occlusion is also a consideration between spinal levels T10 to L4. Particulate steroids, when injected into a foraminal spinal artery, can cause paralysis, even death.⁸

Commonly experienced adverse reactions from corticosteroid injections include dizziness, nervousness, facial flushing, insomnia, and transient increased appetite.⁹ Flare-up of pain intensity at the injection site may occur, lasting for 24-48 hours in 10% of patients⁹ and presumed related to a local inflammatory response to corticosteroid crystals.³ The likelihood of a flare-up reaction is reduced by using a soluble, rapidly absorbed steroid. Rest and physical therapy are sometimes necessary in these cases. In addition, adverse reactions may occur in persons who have active peptic ulcer disease, ulcerative colitis, active infection, hypertension, congestive heart failure, renal disease, and psychiatric illness.⁹ Hyperglycemia in known diabetics warrants careful postprocedural monitoring. Other less serious side effects of corticosteroids include injection site hyperpigmentation, subcutaneous fat atrophy, peripheral edema, dyspepsia, and malaise. Systemic responses  frequently occur even in local injections of corticosteroids. Allergic reactions to systemic glucocorticoids in slow-release formulations have been reported to occur up to 1 week after injection.⁹

Spinal interventional procedures

Pain sensitive spinal structures within the 3 joint complex (composed of the disk and 2 posteriorly situated facet joints) include the nerve roots, dura, posterior longitudinal ligaments, outer annular fibers of the disk, facet joints, joint capsules, and cancellous bone. Intraspinal structures without proven pain innervation include the ligament flavum, inner annulus and nucleus pulposus. Spinal interventional techniques can isolate potential pain generators, and also provide therapeutic relief of pain and associated neurologic symptoms. Identification of a pain–producing structure can be inferred when the patient's characteristic pattern is provoked by radiocontrast agents or saline. Furthermore, diagnostic value can be derived from the patient's response to an injected local anesthetic, and sometimes the use of corticosteroids or neurolysis can provide durable therapeutic value. Safety and accuracy are enhanced when the physician performing these procedures is knowledgeable of spinal anatomy, experienced with the use of fluoroscopy, and skilled at steering needles within the soft tissues of the back.

The decision to perform a spinal interventional procedure should be based on sound medical evidence. Evidence-based medicine is a strategic approach to managing cost by managing care. It is the judicious use of the current best evidence for making decisions about the care of individual patients. Therefore, when clinical and research evidence support the benefit of a specific procedure for an individual patient problem, it should be advocated. If medical evidence suggests that no clear benefit is derived from a procedure for a specific indication, or if the procedure may harm the patient, either directly through adverse events or indirectly by wasting medical resources, then it should be avoided. When insufficient evidence exists to determine whether the procedure is beneficial, then the operating physician must practice the most conservative approach. Manchikanti and colleagues have defined guidelines that classify the strength of experimental evidence that supports decisions as to whether specific interventional pain procedures should be performed. This analysis includes the prevalence of specific spinal pain generators and the efficacy of performing specific procedures for therapeutic or diagnostic purposes.¹⁰

Although, fluoroscopy has revolutionized pain management by increasing the precision, safety, comfort, and outcomes of interventional techniques, the number of procedures and a variety of providers has increased. All physician interventionalists must be adequately trained and experienced to prevent adverse events from harming patients and coworkers. All somatic and spinal injection practices carry finite plausible risks that range from medication allergies or side effects, unwanted violation of body structures with neural or vascular content, to the ultimate possibility of death as a treatment outcome. Complications that are common or unique to each procedure will be discussed. However, the thesis by which this chapter exists is only its capacity to provide information. It does not provide the skill and knowledge that are necessary to perform the outlined interventional methods. University and other ABMS-accredited fellowship programs are now commonly offered. Pain societies and certification agencies such as the American Board of Anesthesia and the American Society of Interventional Pain Physicians provide learned guidelines, assistance through teaching and coursework, and board certification examinations for physician interventionalists. Expertise in performing the outlined procedures is a matter of forethought, not afterthought.

Fluoroscopy and radiation safety

Fluoroscopy has transformed interventional pain management, not only for more precise needle placement, but also for venturing into new treatment venues, especially within the spinal canal. Precise needle placement allows physicians to address multiple spinal pain generators with injections that include placement of radiographic contrast, local anesthetics, and corticosteroids into the epidural space, intra-articular facet joints, sacroiliac joints, and intervertebral discs. Symptomatic facet joints can be identified by median branch nerve blocks and then ameliorated with radio-frequency neurotomy or chemical neurolysis. New technologies have evolved, such as the use of spinal cord stimulators and a host of intradiskal procedures, including electrothermal coagulation, percutaneous mechanical disk decompression, laser disc decompression and radiofrequency intradiskal/annular neurolysis. Other new treatment methods include vertebroplasty and kyphoplasty for vertebral fractures. Fluoroscopy allows more precise localization of both stellate and lumbar paravertebral sympathetic blocks, visceral sympathetic blocks, celiac plexus and superior hypogastric plexus blocks, and neurolysis of the Impar ganglion.

Several studies have demonstrated the comparative accuracy of experienced injectors and anesthesiologists using fluoroscopy compared with previous blind injection techniques and have shown a superior success rate with imaged needle guidance.¹¹

Manchikanti et al advocate fluoroscopy as medically necessary for the performance of epidural corticosteroid injections.¹² Dye injection may reveal incorrect needle placement or inadequate penetration of the injectate to the level of pathology. Fluoroscopy eliminates the question of incorrect or suboptimal needle placement as compared with blind injections and can provide evidence of accurate needle positioning. Documentation of dye spread often mimics the probable flow of corticosteroids and other active medications, and therefore may correlate with the patient's response to treatment. Unintentional intravascular injection may occur during procedures despite negative aspiration through the needle. Vascular locations can be suspected when the contrast dye seems to wash away from the site of the needle tip after it is injected. Limited reasons for not using fluoroscopy include the avoidance of radiation, the cost of fluoroscopy, or allergy to contrast agents.

The fluoroscopy machine

The fluoroscopy machine is primarily composed of an x-ray tube, image intensifier, C-arm, and control panel. The electron flow, called tube current, is generated through an electrically heated negatively charged filament (cathode) and is expressed in milliamperes (mA). The x-ray tube fires a beam of electrons through a high voltage vacuum tube forming x-rays that are emitted through a small opening. X-rays are generated by engaging a high-voltage switch with the output expressed as the kilovolt peak (kVp). These x-rays pass into and through human tissue creating electrically charged ions. The image intensifier collects electromagnetic particles that pass through the patient and transforms them into a usable image that can be visualized on a television monitor. X-ray production ceases immediately when the switch is released. For this reason, radiation management in fluoroscopy is best accomplished by keeping the amount of beam-on time as short as possible.¹³

The C-arm facilitates optimal positioning of the fluoroscope for the physician to get the most favorable view, (eg, posterior-anterior, oblique, and lateral views of the patient). The control panel allows the technician to manually adjust the quality of the image or leave it to the automatic brightness control (ABC). The quality of the image contrast depends on the balance between the tube voltage and current. A higher kVp setting increases the penetrability of the x-ray beam, but reduces the contrast of the x-ray image, whereas the tube current increases both intensity and penetration. Balance of the tube current and tube voltage (kVp) creates the optimal contrast and image resolution. This is usually accomplished by the ABC system, whereby the computer automatically analyzes the image contrast and makes the appropriate adjustments to the kVp and mA to achieve the best balance between contrast and brightness of the image with the lowest dose-rate to the patient. Dose-rates are greater depending on the thickness or size of the patient. As patient size increases, image quality decreases, patient dose increases, and exposure rates to personnel increase. The control panel also allows for magnification and collimation of the image.¹⁴

Radiation concepts and safety

X-rays are a form of electromagnetic energy. When transferred through matter, x-rays ionize human tissue and produce electrically charged ions that can induce molecular changes, potentially leading to somatic and genetic damage. Radiologic nomenclature describes radiation quantities using terminology such as the absorbed dose, effective dose, equivalent dose, and Dose-area-product. A roentgen (R) measures exposure to ionizing radiation equivalent to the electrical charge per unit mass of air (1R=2.58x10-4 coulombs/kg of air). The concentration of energy that is deposited locally into a tissue is called the absorbed dose. This is measured in units of gray (Gyt) or milligray (mGyt). One gray of absorbed dose is equivalent to the energy deposition of 1 joule in 1 kg of tissue mass. Doses lower than 1 Gy generally do not cause notable acute effects other than slight cellular changes. The absorbed dose rate describes the rate of dose accumulation in mGyt/min. A typical skin entrance dose rate from fluoroscopy is about 30 mGyt/min. Effective dose is the quantity of radiation exposure affecting people who are not in a stationary or typically uniform space. It is the hypothetical dose received by the entire unprotected human body and poses the same health risk as the nonuniform dose received by an individual not wearing a protective apron. For the purposes of radiation protection, regulatory limits of whole-body exposures to personnel are given in terms of effective dose. This information is extracted from the data generated by film badges or other types of personal radiation monitors.

Therefore, the radiation-absorbed dose is the amount of energy deposited into human tissue from ionizing radiation sources and is measured in units of gray (Gy). Biologic effects of radiation are caused by the ionization of water molecules within the cells, producing light highly reactive free radicals that damage macromolecules of DNA. The acute effects occur at relatively high-dose levels, such as those given during radiotherapy treatments or from accidents. Chronic effects are more often the result of long-term low-dose exposure. The most common radiation-induced injuries affect the skin. Unlike a thermal burn, x-ray injuries develop slowly and may not become apparent until days or weeks later. Potential effects vary in severity from erythema to dermal necrosis and skin cancer. Additionally, the probability of induced cancer or leukemia is increased in the exposed individual. The latent period between radiation overexposure and cancer may be as short as 2 years.

To measure the effective dose (whole body dose) from occupational radiation exposure, the measure termed rad is converted to the unit of occupational exposure, which is designated as the radiation-equivalent-man (REM). The unit of dose equivalent to REM is measured by using the sievert (Sv); one REM is equivalent to 1 rad, and 100 REM is equivalent to 1 Sv. Radiation dose equivalents of 0.25 Sv (25 REM) may lead to measurable hematologic depression. Whole body total radiation doses exceeding 100 REM may lead to nausea, fatigue, radiation dermatitis, alopecia, testicular disturbance, and hematologic disorders. A maximal permissible dose (MPD) is the upper limit of the allowed radiation dose that an individual may receive without the risk of significant side effects. The annual whole body MPD limit for physicians is 50 mSv. The annual MPD for the lens of the eye is 150 mSv, and for the thyroid, gonads and extremities it is 500 mSv. The fluoroscopy's x-ray tube should be kept as far away from the patient as  possible. Federal regulations limit the maximum output for C-arm fluoroscopy to 10 R/min at 12 in (30 cm) from the image intensifier. Beam collimation reduces the area being irradiated, thereby reducing the amount of x-rays received by the patient. The use of live fluoroscopy should be minimized as much as possible. Furthermore, magnification should be limited since it increases the amount of radiation to human tissue. Image magnification by a factor of 2 increases the amount of radiation by 4 times.

Radiation exposure to ionizing radiation is unavoidable when performing fluoroscopic procedures. Only necessary personnel should be present in the fluoroscopy room. The primary source of radiation to the physician during such procedures is from scatter that is reflected back from the patient. Less prominent is the role of radiation leakage from the equipment. The cardinal principals of radiation protection are (1) maximize the distance from the radiation source, (2) use shielding materials, and (3) minimize exposure time. Radiation scatter can also be reduced by using the lowest tube current (mA) that is compatible with a good x-ray image. In conventional fluoroscopy, the x-ray tube is located beneath the table and the image intensifier is above the table. With a horizontal table, in this arrangement, most of the radiation scatter is in a downward direction and is absorbed into the floor or side panels of the table. In the opposite arrangement, it is often difficult to get adequate shielding to medical personnel. As mentioned, the beam-on time is the most important variable for controlling radiation exposure and should be kept to a minimum; most fluoroscopy machines are armed with a 5-minute alarm.

X-ray shielding can be fixed or mobile, including the commercially available protective apparel. Fixed shielding includes the thickness of walls, doors, and protective cubicles, which should have a lead equivalent of 1-3 mm. Mobile shielding is appropriate during fluoroscopy when a member of the staff needs to remain near the patient. Specific items of apparel that are used for personal shielding include lead aprons, gloves, thyroid shields, and glass spectacles.

Typically, physicians and assisting personnel are supplied with monitoring equipment in the form of a radiation or film badge that is packed with photographic film. These clips are typically light and slim for convenient placement on conventional clothing and apparel. Usually a "color badge" is worn outside the apron on the upper portion of the body, usually on the upper edge of the thyroid shield. This badge approximates radiation exposure to the lens of the eye. A second "behind the apron" badge is worn underneath lead apparel and clipped onto the waist of the physician. X-ray readings from this badge represent the actual dose to the gonads and major blood-forming organs. Also, a finger or ring badge can be worn with the film facing the underside part of the hand nearest the radiation source. Badges may also be placed on protective eyewear. These badges are usually processed monthly to monitor the type and amount of radiation exposure received by each clinical participant. Results are reported as monthly and 12-month accumulated dosages. Prompt exchange of badges on a monthly basis is required in most medical facilities.

Radiocontrast agents

Radiographic contrast agents aid in the localization of anatomical structures. Iodine atoms within these agents provide greater x-ray attenuation compared with human tissues, including bone. Osmolality describes a measure of the numbers of particles in a specific solution. The hyperosmolality of contrast agents relates directly to their toxicity. Second-generation radiocontrast agents have more physiologic properties, are labeled nonionic, and are more commonly used for spinal injections. The 2 most commonly used radiocontrast agents are iopamidol (Isovue-M) and iohexol (Omnipaque). Both are absorbed rapidly into the bloodstream from intrathecal, epidural, and paraspinal tissue injections. Plasma levels are measurable within an hour after injection. The mean half-life is 12 hours and 80-90% is excreted via the kidneys within 24 hours with minimal excretion via fecal route. Adverse reactions may be chemotoxic, osmolar-related, or allergic. Also, 90% of adverse effects occur within 15 minutes of exposure.

If an allergic reaction is suspected, patients should be observed for up to 60 minutes. The primary concern when using contrast media is unintentional intrathecal injection. For this reason, the above-mentioned water-soluble contrast media are recommended: iohexol (Omnipaque) or iopamidol (Isovue). Radiologic contrast media are not licensed for intrathecal use, but these 2 specific radiocontrast agents have not been reported to cause adhesive arachnoiditis and exhibit a low risk of seizures and neurotoxicity.

Patients at greater risk for an adverse reaction to radiocontrast media include those with a history of a previous adverse reaction, especially allergy. Any question regarding an allergic reaction can be avoided by giving oral prednisone 20-50 mg, ranitidine 50 mg, and diphenhydramine 25-50 mg orally 12-24 hours prior to exposure by injection. An additional 25 mg of diphenhydramine can be given by IV immediately before contrast injection. Further caution is required when administrating radiocontrast to patients with asthma; allergy/atopy; cardiac disease with decompensation, unstable arrhythmia, or recent MI; renal failure/nephropathy; feeble individuals with general debility (especially infants or the elderly); and patients with dehydration, metabolic disorders, or hematologic disorders. Adverse reactions vary from chemotoxic reactions (such as thyrotoxicosis or nephrotoxicity) hyperosmolar responses, or more typical allergic responses characterized by vasomotor responses, cutaneous reactions, bronchospasm, cardiovascular effects (hypotension), or anaphylactoid reactions.

Although fluoroscopy has revolutionized the precise and accurate practice of interventional pain management, radiation safety training is required for any physician who uses fluoroscopy in his practice. Furthermore, injectable radiocontrast media and active therapeutic agents require additional knowledge. Physician practice in this area of subspecialty requires additional training through recognized medical certification agencies or societies.¹⁵

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