Hyperuricemia

In the body, uric acid is the final product of purine metabolism. Synthesis of uric acid occurs mainly in the liver, and to a small degree in the small intestine. Normally, two thirds of uric acid excretion occurs through the kidneys and one third through the intestines. ^[4]  Uric acid in the blood is saturated at 6.4-6.8 mg/dL at ambient conditions, with the upper limit of solubility placed at 7 mg/dL.

In the kidney, urate is freely filtered at the glomerulus, reabsorbed, secreted, and then again reabsorbed in the proximal tubule. The cloning of certain urate transporters will facilitate the understanding of specific mechanisms by which urate is handled in the kidney and small intestines.

A urate/anion exchanger (URAT1) has been identified in the brush-border membrane of the kidneys and is inhibited by an angiotensin II receptor blocker, losartan. ^[5] A human organic anion transporter (hOAT1) has been found to be inhibited by both uricosuric drugs and antiuricosuric drugs, ^[6] while another urate transporter (UAT) has been found to facilitate urate efflux out of the cells. ^[7] These transporters may account for the reabsorption, secretion, and reabsorption pattern of renal handling of urate.

Intestinal excretion of urate is handled by uric acid transporters in intestinal epithelial cells, which transport uric acid from the blood to the intestinal lumen. Multiple transporters participate in this process, but mainly ABCG2 and SLC2A9. Intestinal excretion also involves the catabolism of uric acid by intestinal flora. ^[4]

Urate excretion does appear to correlate with the serum urate concentration because a small increase in the serum concentration results in a marked increase in urate excretion.

Hyperuricemia may occur because of decreased excretion (underexcretors), increased production (overproducers), or a combination of those two mechanisms.

Underexcretion accounts for most causes of hyperuricemia. Urate handling by the kidneys involves filtration at the glomerulus, reabsorption, secretion, and, finally, postsecretory reabsorption. Consequently, altered uric acid excretion can result from decreased glomerular filtration, decreased tubular secretion, or enhanced tubular reabsorption.

While decreased urate filtration may not cause primary hyperuricemia, it can contribute to the hyperuricemia of kidney insufficiency. Decreased tubular secretion of urate occurs in patients with acidosis (eg, diabetic ketoacidosis, ethanol or salicylate intoxication, starvation ketosis). The organic acids that accumulate in these conditions compete with urate for tubular secretion. Finally, enhanced reabsorption of uric acid distal to the site of secretion is the mechanism thought to be responsible for the hyperuricemia observed with diuretic therapy and diabetes insipidus.

Overproduction accounts for only a minority of patients presenting with hyperuricemia. The causes for hyperuricemia in overproducers may be either exogenous (diet rich in purines) or endogenous (increased purine nucleotide breakdown). A small percentage of overproducers have enzymatic defects that account for their hyperuricemia. These include a complete deficiency of hypoxanthine guanine phosphoribosyltransferase (HGPRT) as in Lesch-Nyhan syndrome, partial deficiency of HGPRT (Kelley-Seegmiller syndrome), and increased production of 5-phospho-alpha-d-ribosyl pyrophosphate (PRPP) activity. Accelerated purine degradation can result from rapid cell proliferation and turnover (blast crisis of leukemias) or from cell death (rhabdomyolysis, cytotoxic therapy). Glycogenoses types III, IV, and VII can result in hyperuricemia from excessive degradation of skeletal muscle adenosine triphosphate (ATP).

Combined mechanisms (underexcretion and overproduction) can also cause hyperuricemia. The most common cause under this group is alcohol consumption, ^[8] which results in accelerated hepatic breakdown of ATP and the generation of organic acids that compete with urate for tubular secretion. Enzymatic defects such as glycogenoses type I and aldolase-B deficiency are other causes of hyperuricemia that result from a combination of overproduction and underexcretion.

Urate crystals can engage an intracellular pattern recognition receptor, the macromolecular NALP3 (cryopyrin) inflammasome complex. ^{[9, 10]} NALP3 inflammasome may result in interleukin 1 (IL-1) beta production, which, in turn, incites an inflammatory response. Inhibition of this pathway has been targeted as a treatment for hyperuricemia-induced crystal arthritis, with reports documenting the efficacy of the IL-1 inhibitors canakinumab and rilonacept for preventing gout flares during the initiation of allopurinol therapy. ^[11]

Zinc and magnesium are important nutrients with anti-inflammatory properties. Studies have linked low dietary levels to hyperuricemia in men. A study by Xie et al in 2697 men and 2471 women indicated that dietary zinc intake was inversely associated with hyperuricemia in middle-aged and older men, but not in women. ^[12] Wang et al reported that in 5168 subjects, dietary magnesium intake was inversely associated with hyperuricemia, independent of some major confounding factors, but only in men. ^[13] Studies of approximately 25,000 adults in the United States, using data from the National Health and Nutrition Examination Survey (NHANES), found that in both men and women, lower magnesium and zinc intake was associated with increased hyperuricemia risk. ^{[14, 15]}

Hyperuricemia is generally divided into the following three pathophysiologic categories:

• Uric acid underexcretion
• Uric acid overproduction
• Combined causes

Underexcretion

Causes of uric acid underexcretion include the following:

• Idiopathic

• Familial juvenile gouty nephropathy: This is a rare autosomal dominant condition characterized by progressive renal insufficiency. Affected persons have a low fractional excretion of urate (typically 4%). Kidney biopsy findings indicate glomerulosclerosis and tubulointerstitial disease but no uric acid deposition.

• Kidney insufficiency: Kidney failure is one of the more common causes of hyperuricemia. In chronic kidney disease, the uric acid level does not generally become elevated until the creatinine clearance falls below 20 mL/min, unless other contributing factors exist. This is due to a decrease in urate clearance as retained organic acids compete for secretion in the proximal tubule. In certain kidney disorders, such as medullary cystic disease and chronic lead nephropathy, hyperuricemia is commonly observed even with minimal kidney insufficiency.

• Metabolic syndrome: This syndrome is characterized by hypertension, obesity, insulin resistance, dyslipidemia, and hyperuricemia, ^[16]  and is associated with a decreased fractional excretion of urate by the kidneys.

• Drugs: Causative drugs include diuretics, low-dose salicylates, cyclosporine, pyrazinamide, ethambutol, levodopa, and nicotinic acid.

• Hypertension

• Acidosis: Types that cause hyperuricemia include lactic acidosis, diabetic ketoacidosis, alcoholic ketoacidosis, and starvation ketoacidosis.

• Preeclampsia and eclampsia: The elevated uric acid associated with these conditions is a key clue to the diagnosis because in healthy pregnancies, uric acid levels are lower than normal.

• Hypothyroidism

• Hyperparathyroidism

• Sarcoidosis

• Lead intoxication (chronic): History may reveal occupational exposure (eg, lead smelting, battery and paint manufacture) or consumption of moonshine (ie, illegally distilled corn whiskey) because some, but not all, moonshine was produced in lead-containing stills).

• Trisomy 21

Overproduction

Uric acid overproduction may be idiopathic. Known causes include the following:

• Hypoxanthine guanine phosphoribosyltransferase (HGPRT) deficiency (Lesch-Nyhan syndrome): This is an inherited X-linked disorder. HGRPT catalyzes the conversion of hypoxanthine to inosinic acid, in which PRPP serves as the phosphate donor. The deficiency of HGPRT results in accumulation of 5-phospho-alpha-d-ribosyl pyrophosphate (PRPP), which accelerates purine biosynthesis with a resultant increase in uric acid production. In addition to gout and uric acid nephrolithiasis, these patients develop a neurologic disorder that is characterized by choreoathetosis; spasticity; growth; intellectual disability; and, occasionally, self-mutilation.

• Partial deficiency of HGPRT (Kelley-Seegmiller syndrome): This is also an X-linked disorder. Patients typically develop gouty arthritis in the second or third decade of life, have a high incidence of uric acid nephrolithiasis, and may have mild neurologic deficits.

• Increased activity of PRPP synthetase: This is a rare X-linked disorder in which patients make mutated PRPP synthetase enzymes with increased activity. These patients develop gout when aged 15-30 years and have a high incidence of uric acid kidney stones.

• Diet: A diet rich in high-purine meats, organ foods, and legumes can result in an overproduction of uric acid.

• Increased nucleic acid turnover: This may be observed in persons with hemolytic anemia and hematologic malignancies such as lymphoma, myeloma, or leukemia.

• Tumor lysis syndrome: This may produce the most serious complications of hyperuricemia.

• Glycogenoses III, V, and VII

• Exposure to persistent organic pollutants (eg, organochlorine pesticides) ^[17]

Combined causes

Combined causes include the following:

• Alcohol ^[8] : Ethanol increases the production of uric acid by causing increased turnover of adenine nucleotides. It also decreases uric acid excretion by the kidneys, which is partially due to the production of lactic acid.

• Fructose-sweetened soft drinks: Fructose raises serum uric acid levels by accentuating degradation of purine nucleotides and increasing purine synthesis, and epidemiologic studies have documented a link between sugar-sweetened soft drink intake and serum uric acid levels in several populations. ^{[18, 19, 20, 21]}  More recently, Lecoultre et al found that fructose-induced hyperuricemia is associated with a decreased uric acid excretion by the kidneys. ^[22]

• Exercise: Exercise may result in enhanced tissue breakdown and decreased kidney excretion due to mild volume depletion.

• Deficiency of aldolase B (fructose-1-phosphate aldolase): This is a fairly common inherited disorder, often resulting in gout.

• Glucose-6-phosphatase deficiency (glycogenosis type I, von Gierke disease): This is an autosomal recessive disorder characterized by the development of symptomatic hypoglycemia and hepatomegaly within the first 12 months of life. Additional findings include short stature, delayed adolescence, enlarged kidneys, hepatic adenoma, hyperuricemia, hyperlipidemia, and increased serum lactate levels.

According to the National Health and Nutrition Examination Survey (2007-2016), the prevalence rate of hyperuricemia in the general population of the United States is estimated at 20%. The prevalence of gout is 5.2% in men and 2.7% in women, with the rates for both remaining stable over the decade. ^[23]

Worldwide, the prevalence of hyperuricemia has increased substantially in recent decades. The progressive increase in serum levels of uric acid levels may be linked to the rising prevalence of overweight and obesity, as well as the increase in consumption of sugar-sweetened beverages, foods rich in purines, and alcohol. ^{[24, 25]}

A Japanese study that used an administrative claims database to ascertain 10-year trends in the prevalence of hyperuricemia concluded that the prevalence of hyperuricemia in the overall study population increased during the 10-year follow-up. When stratified by age, the prevalence increased among groups older than 65 years in both sexes. In those younger than 65 years, men had a prevalence 4 times higher than that in women, but in those older than 65 years, the gender gap narrowed to 1:3 (female-to-male ratio) with gout and/or hyperuricemia.

A high prevalence of hyperuricemia exists in indigenous races of the Pacific, which appears to be associated with a low fractional excretion of uric acid. ^[26] In the United States, Blacks develop hyperuricemia more commonly than whites.

Hyperuricemia, and particularly gouty arthritis, are far more common in men than in women. Only 5% of patients with gout are female, but uric acid levels increase in women after menopause. ^[27]

The normal serum uric acid level is lower in children than in adults. The upper limit of the reference range for children is 5 mg/dL (0.30 mmol/L). The upper limit of the reference range for men is 7 mg/dL (0.42 mmol/L) and for women is 6 mg/dL (0.36 mmol/L). ^[28] The tendency to develop hyperuricemia increases with age.

Hyperuricemia has a higher prevalence (25-40%) in individuals with hypertension and has been associated with increased morbidity in these patients. ^[29]  In a study of 837 elderly patients with hypertension followed up over 3.5 years, Lin et al found that increases in uric acid levels were independently associated with decline in renal function. ^[30]  Ding et al reported that serum uric acid concentration and prevalence of hyperuricemia were positively associated with osteoarthritis of the knee in a cohort of Chinese women. ^[31]

The cause for these associations is unknown, but hyperuricemia is probably a marker for comorbid risk factors rather than a causative factor, per se. Results of a cross-sectional study by Yang et al suggested that levels of high-sensitivity C-reactive protein (a nonspecific marker for inflammation) are positively associated with the prevalence of hyperuricemia. ^[32]

Although observational studies on hyperuricemia and stroke have yielded conflicting results, a meta-analysis by Li et al concluded that hyperuricemia may modestly increase the risk of stroke incidence and mortality. ^[33]  The authors reviewed 15 studies that together included 22,571 cases of stroke and 1,042,358 participants. The risk ratio (RR) for the incidence of stroke in patients with hyperuricemia was 1.22 (95% CI, 1.02-1.46) and the RR for mortality was 1.33 (95% CI, 1.24-1.43). The pooled estimate of multivariate RRs of both stroke incidence and mortality were higher in women than in men (1.25 vs 1.08 and 1.41 vs  1.26, respectively). ^[33]

Possible complications of hyperuricemia include the following:

• Gout
• Acute uric acid nephropathy
• Uric acid nephrolithiasis
• Chronic kidney disease ^[34]