Hyperuricemia

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Hyperuricaemia or hyperuricemia is an abnormally high level of uric acid in the blood. In the pH conditions of body fluid, uric acid exists largely as urate, the ion form.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name=Choi2005>Template:Cite journal</ref> Serum uric acid concentrations greater than 6 mg/dL for females, 7 mg/dL for males, and 5.5 mg/dL for youth (under 18 years old) are defined as hyperuricemia.<ref name=":0">Template:Cite journal</ref> The amount of urate in the body depends on the balance between the amount of purines eaten in food, the amount of urate synthesised within the body (e.g., through cell turnover), and the amount of urate that is excreted in urine or through the gastrointestinal tract.<ref name=Choi2005/> Hyperuricemia may be the result of increased production of uric acid, decreased excretion of uric acid, or both increased production and reduced excretion.<ref name=":0" />

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Signs and symptomsEdit

Unless high blood levels of uric acid are determined in a clinical laboratory, hyperuricemia may not cause noticeable symptoms in most people.<ref name=asc/> Development of gout Template:Ndash which is a painful, short-term disorder Template:Ndash is the most common consequence of hyperuricemia, which causes deposition of uric acid crystals usually in joints of the extremities, but may also induce formation of kidney stones, another painful disorder.<ref name="clev">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Gout symptoms are typically inflammation, swelling and redness of a joint, such as a toe or knee, accompanied by intense pain.<ref name="asc">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Not all people with hyperuricemia develop gout.<ref name=asc/>

CausesEdit

Many factors contribute to hyperuricemia, including genetics, insulin resistance, hypertension,<ref name=":0" /> hypothyroidism, chronic kidney disease, obesity, diet, iron overload, use of diuretics (e.g. thiazides, loop diuretics), and excessive consumption of alcoholic beverages.<ref>Template:Cite journal</ref> Of these, alcohol consumption is the most important.<ref name="Yamamoto+2005">Template:Cite journal</ref>

Causes of hyperuricemia can be classified into three functional types:<ref name="pmid18409507">Template:Cite journal</ref> increased production of uric acid, decreased excretion of uric acid, and mixed type. Causes of increased production include high levels of purine in the diet and increased purine metabolism. Causes of decreased excretion include kidney disease, certain drugs, and competition for excretion between uric acid and other molecules. Mixed causes include high levels of alcohol and/or fructose in the diet, and starvation.<ref name="pmid34502127">Template:Cite journal</ref>

Increased production of uric acidEdit

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A purine-rich diet is a common but minor cause of hyperuricemia. Diet alone generally is not sufficient to cause hyperuricemia (see Gout). Foods high in the purines adenine and hypoxanthine may aggravate symptoms of hyperuricemia.<ref>Template:Cite journal</ref>

Various studies have found higher uric acid levels to be positively associated with consumption of meat and seafood and inversely associated with dairy food consumption.<ref>Template:Cite journal</ref>

Myogenic hyperuricemia, as a result of the myokinase (adenylate kinase) reaction and the Purine Nucleotide Cycle running when ATP reservoirs in muscle cells are low (ADP>ATP), is a common pathophysiologic feature of glycogenoses such as GSD-III, GSD-V and GSD-VII, as they are metabolic myopathies which impair the ability of ATP (energy) production for the muscle cells to use.<ref name=":1">Mineo I, Kono N, Hara N, Shimizu T, Yamada Y, Kawachi M, Kiyokawa H, Wang YL, Tarui S. Myogenic hyperuricemia. A common pathophysiologic feature of glycogenosis types III, V, and VII. N Engl J Med. 1987 Jul 9;317(2):75-80. doi: 10.1056/NEJM198707093170203. PMID 3473284.</ref> In these metabolic myopathies, myogenic hyperuricemia is exercise-induced; inosine, hypoxanthine and uric acid increase in plasma after exercise and decrease over hours with rest.<ref name=":1" /> Excess AMP (adenosine monophosphate) is converted into uric acid. AMP → IMP → Inosine → Hypoxanthine → Xanthine → Uric Acid<ref name=":1" />

Hyperuricemia experienced as gout is a common complication of solid organ transplant.<ref name="pmid16392875">Template:Cite journal</ref> Apart from normal variation (with a genetic component), tumor lysis syndrome produces extreme levels of uric acid, mainly leading to kidney failure. The Lesch–Nyhan syndrome is also associated with extremely high levels of uric acid.<ref>Template:Citation</ref>

Decreased excretion of uric acidEdit

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The principal drugs that contribute to hyperuricemia by decreased excretion are the primary antiuricosurics. Other drugs and agents include diuretics, salicylates, pyrazinamide, ethambutol, nicotinic acid, ciclosporin, 2-ethylamino-1,3,4-thiadiazole, and cytotoxic agents.<ref name="pmid2070427">Template:Cite journal</ref>

The gene SLC2A9 encodes a protein that helps to transport uric acid in the kidney. Several single nucleotide polymorphisms of this gene are known to have a significant correlation with blood uric acid.<ref name="pmid18487473">Template:Cite journal</ref> Hyperuricemia cosegregating with osteogenesis imperfecta has been shown to be associated with a mutation in GPATCH8 using exome sequencing<ref>Template:Cite journal</ref>

A ketogenic diet impairs the ability of the kidney to excrete uric acid, due to competition for transport between uric acid and ketones.<ref name="pmid488876">Template:Cite journal</ref>

Elevated blood lead is significantly correlated with both impaired kidney function and hyperuricemia (although the causal relationship among these correlations is not known). In a study of over 2500 people resident in Taiwan, a blood lead level exceeding 7.5 microg/dL (a small elevation) had odds ratios of 1.92 (95% CI: 1.18-3.10) for renal dysfunction and 2.72 (95% CI: 1.64-4.52) for hyperuricemia.<ref name="pmid18514766">Template:Cite journal</ref><ref>Shadick NA, Kim R, Weiss S, Liang MH, Sparrow D, Hu H. (2000 ), Effect of low level lead exposure on hyperuricemia and gout among middle aged and elderly men: the normative aging study; J Rheumatol. 2000 Jul; 27(7):1708–12 (abstract).</ref>

Mixed typeEdit

Causes of hyperuricemia that are of mixed type have a dual action, both increasing production and decreasing excretion of uric acid.Template:Citation needed

Pseudohypoxia (disrupted NADH/NAD⁺ ratio), caused by diabetic hyperglycemia and excessive alcohol consumption, results in hyperuricemia. The lactic acidosis inhibits uric acid secretion by the kidney, while the energy shortage from inhibited oxidative phosphorylation leads to increased production of uric acid due to increased turnover of adenosine nucleotides by the myokinase reaction and purine nucleotide cycle.<ref>Template:Cite book</ref>

High intake of alcohol (ethanol), a significant cause of hyperuricemia, has a dual action that is compounded by multiple mechanisms. Ethanol increases production of uric acid by increasing production of lactic acid, hence lactic acidosis. Ethanol also increases the plasma concentrations of hypoxanthine and xanthine via the acceleration of adenine nucleotide degradation, and is a possible weak inhibitor of xanthine dehydrogenase. As a byproduct of its fermentation process, beer additionally contributes purines. Ethanol decreases excretion of uric acid by promoting dehydration and (rarely) clinical ketoacidosis.<ref name="Yamamoto+2005" />

High dietary intake of fructose contributes significantly to hyperuricemia.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref name="pmid18301272">Template:Cite journal</ref> In a large study in the United States, consumption of four or more sugar-sweetened soft drinks per day gave an odds ratio of 1.82 for hyperuricemia.<ref name="pmid18163396">Template:Cite journal</ref> Increased production of uric acid is the result of interference, by a product of fructose metabolism, in purine metabolism. This interference has a dual action, both increasing the conversion of ATP to inosine and hence uric acid and increasing the synthesis of purine.<ref name="pmid8213607">Template:Cite journal</ref> Fructose also inhibits the excretion of uric acid, apparently by competing with uric acid for access to the transport protein SLC2A9.<ref name="pmid18327257">Template:Cite journal</ref> The effect of fructose in reducing excretion of uric acid is increased in people with a hereditary (genetic) predisposition toward hyperuricemia and/or gout.<ref name="pmid8213607"/>

Starvation causes the body to metabolize its own (purine-rich) tissues for energy. Thus, like a high purine diet, starvation increases the amount of purine converted to uric acid. A very low calorie diet lacking in carbohydrates can induce extreme hyperuricemia; including some carbohydrate (and reducing the protein) reduces the level of hyperuricemia.<ref name="pmid7024153">Template:Cite journal</ref> Starvation also impairs the ability of the kidney to excrete uric acid, due to competition for transport between uric acid and ketones.<ref name="pmid7405247">Template:Cite journal</ref>

Gut MicrobiomeEdit

Radioisotope studies suggest about 1/3 of uric acid is removed in healthy people in their gut with this being roughly 2/3 in those with kidney disease.<ref name="l551">Template:Cite journal</ref> Uric acid metabolism is done in the human gut by ~1/5 of bacteria that come from 4 of 6 major phyla. Such metabolism is anaerobic involving uncharacterized ammonia lyase, peptidase, carbamoyl transferase, and oxidoreductase enzymes. The result is that uric acid is converted into xanthine or lactate and the short chain fatty acids such as acetate and butyrate.<ref name="v435">Template:Cite journal</ref> In mouse models, such bacteria compensate for the loss of uricase leading researchers to raise the possibility "that antibiotics targeting anaerobic bacteria, which would ablate gut bacteria, increase the risk for developing gout in humans".<ref name="v435" />

DiagnosisEdit

Hyperuricemia can be detected using blood and urine tests.Template:Citation needed

TreatmentEdit

Medications that aim to lower the uric acid concentrationEdit

Medications used to treat hyperuricemia are divided into two categories: xanthine oxidase inhibitors and uricosurics. For people who have recurring attacks of gout, one of these two categories of drugs is recommended.<ref name=":0" /> The evidence for people with asymptomatic hyperuricaemia to take these medications is not clear.<ref name=":0" />

Xanthine oxidase inhibitorsEdit

Xanthine oxidase inhibitors, including allopurinol, febuxostat and topiroxostat, decrease the production of uric acid, by interfering with xanthine oxidase.Template:Citation needed

UricosuricsEdit

Uricosuric agents (benzbromarone, benziodarone, probenecid, lesinurad, sulfinpyrazone, ethebencid, zoxazolamine, and ticrynafen) increase the excretion of uric acid, by reducing the reabsorption of uric acid once it has been filtered out of the blood by the kidneys.

Some of these medications are used as indicated, others are used off-label. In people receiving hemodialysis, sevelamer can significantly reduce serum uric acid,<ref>Template:Cite journal</ref><ref name="pmid19293539">Template:Cite journal</ref> apparently by adsorbing urate in the gut.<ref name="pmid19293539" /> In women, use of combined oral contraceptive pills is significantly associated with lower serum uric acid.<ref name="pmid2290309">Template:Cite journal</ref> Following Le Chatelier's principle, lowering the blood concentration of uric acid may permit any existing crystals of uric acid to gradually dissolve into the blood, whence the dissolved uric acid can be excreted. Maintaining a lower blood concentration of uric acid similarly should reduce the formation of new crystals. If the person has chronic gout or known tophi, then large quantities of uric acid crystals may have accumulated in joints and other tissues, and aggressive and/or long duration use of medications may be needed. Precipitation of uric acid crystals, and conversely their dissolution, is known to be dependent on the concentration of uric acid in solution, pH, sodium concentration, and temperature.Template:Medical citation needed

Non-medication treatments for hyperuricemia include a low purine diet (see Gout) and a variety of dietary supplements.Template:Medical citation needed Treatment with lithium salts has been used as lithium improves uric acid solubility.Template:Medical citation needed

pHEdit

Serum pH is neither safely nor easily altered. Therapies that alter pH principally alter the pH of urine, to discourage a possible complication of uricosuric therapy: formation of uric acid kidney stones due to increased uric acid in the urine (see nephrolithiasis). Medications that have a similar effect include acetazolamide.Template:Medical citation needed

TemperatureEdit

Low temperature is a reported trigger of acute gout.<ref>Template:Cite journal</ref> An example would be a day spent standing in cold water, followed by an attack of gout the next morning. This is believed to be due to temperature-dependent precipitation of uric acid crystals in tissues at below normal temperature. Thus, one aim of prevention is to keep the hands and feet warm, and soaking in hot water may be therapeutic.Template:Citation needed

PrognosisEdit

Increased levels predispose for gout and, if very high, kidney failure. Metabolic syndrome often presents with hyperuricemia.<ref>Template:Cite journal</ref> Prognosis is good with regular consumption of allopurinol or febuxostat. Template:Citation needed

ReferencesEdit

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External linksEdit

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