Asymptomatic hyperuricemia | |
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Uric acid | |
Specialty | Endocrinology ![]() |
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. [1] [2] 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. [3] 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. [2] Hyperuricemia may be the result of increased production of uric acid, decreased excretion of uric acid, or both increased production and reduced excretion. [3]
Unless high blood levels of uric acid are determined in a clinical laboratory, hyperuricemia may not cause noticeable symptoms in most people. [4] Development of gout – which is a painful, short-term disorder – 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. [5] Gout symptoms are typically inflammation, swelling and redness of a joint, such as a toe or knee, accompanied by intense pain. [4] Not all people with hyperuricemia develop gout. [4]
Many factors contribute to hyperuricemia, including genetics, insulin resistance, hypertension, [3] hypothyroidism, chronic kidney disease, obesity, diet, iron overload, use of diuretics (e.g. thiazides, loop diuretics), and excessive consumption of alcoholic beverages. [6] Of these, alcohol consumption is the most important. [7]
Causes of hyperuricemia can be classified into three functional types: [8] 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. [9]
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. [10]
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. [11]
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. [12] 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. [12] Excess AMP (adenosine monophosphate) is converted into uric acid. AMP → IMP → Inosine → Hypoxanthine → Xanthine → Uric Acid [12]
Hyperuricemia experienced as gout is a common complication of solid organ transplant. [13] 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. [14]
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. [15]
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. [16] Hyperuricemia cosegregating with osteogenesis imperfecta has been shown to be associated with a mutation in GPATCH8 using exome sequencing [17]
A ketogenic diet impairs the ability of the kidney to excrete uric acid, due to competition for transport between uric acid and ketones. [18]
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. [19] [20]
Causes of hyperuricemia that are of mixed type have a dual action, both increasing production and decreasing excretion of uric acid.[ 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. [21]
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. [7]
High dietary intake of fructose contributes significantly to hyperuricemia. [22] [23] [24] 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. [25] 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. [26] Fructose also inhibits the excretion of uric acid, apparently by competing with uric acid for access to the transport protein SLC2A9. [27] The effect of fructose in reducing excretion of uric acid is increased in people with a hereditary (genetic) predisposition toward hyperuricemia and/or gout. [26]
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. [28] Starvation also impairs the ability of the kidney to excrete uric acid, due to competition for transport between uric acid and ketones. [29]
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. [30] 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. [31] 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". [31]
Hyperuricemia can be detected using blood and urine tests.[ citation needed ]
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. [3] The evidence for people with asymptomatic hyperuricaemia to take these medications is not clear. [3]
Xanthine oxidase inhibitors, including allopurinol, febuxostat and topiroxostat, decrease the production of uric acid, by interfering with xanthine oxidase.[ citation needed ]
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, [32] [33] apparently by adsorbing urate in the gut. [33] In women, use of combined oral contraceptive pills is significantly associated with lower serum uric acid. [34] 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.[ medical citation needed ]
Non-medication treatments for hyperuricemia include a low purine diet (see Gout) and a variety of dietary supplements.[ medical citation needed ] Treatment with lithium salts has been used as lithium improves uric acid solubility.[ medical citation needed ]
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.[ medical citation needed ]
Low temperature is a reported trigger of acute gout. [35] 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.[ citation needed ]
Increased levels predispose for gout and, if very high, kidney failure. Metabolic syndrome often presents with hyperuricemia. [36] Prognosis is good with regular consumption of allopurinol or febuxostat. [ citation needed ]
Uric acid is a heterocyclic compound of carbon, nitrogen, oxygen, and hydrogen with the formula C5H4N4O3. It forms ions and salts known as urates and acid urates, such as ammonium acid urate. Uric acid is a product of the metabolic breakdown of purine nucleotides, and it is a normal component of urine. High blood concentrations of uric acid can lead to gout and are associated with other medical conditions, including diabetes and the formation of ammonium acid urate kidney stones.
Gout is a form of inflammatory arthritis characterized by recurrent attacks of pain in a red, tender, hot, and swollen joint, caused by the deposition of needle-like crystals of uric acid known as monosodium urate crystals. Pain typically comes on rapidly, reaching maximal intensity in less than 12 hours. The joint at the base of the big toe is affected (Podagra) in about half of cases. It may also result in tophi, kidney stones, or kidney damage.
Xanthine oxidase is a form of xanthine oxidoreductase, a type of enzyme that generates reactive oxygen species. These enzymes catalyze the oxidation of hypoxanthine to xanthine and can further catalyze the oxidation of xanthine to uric acid. These enzymes play an important role in the catabolism of purines in some species, including humans.
Allopurinol is a medication used to decrease high blood uric acid levels. It is specifically used to prevent gout, prevent specific types of kidney stones and for the high uric acid levels that can occur with chemotherapy. It is taken orally or intravenously.
Phosphoglucomutase is an enzyme that transfers a phosphate group on an α-D-glucose monomer from the 1 to the 6 position in the forward direction or the 6 to the 1 position in the reverse direction.
Tumor lysis syndrome (TLS) is a group of metabolic abnormalities that can occur as a complication from the treatment of cancer, where large amounts of tumor cells are killed off (lysed) from the treatment, releasing their contents into the bloodstream. This occurs most commonly after the treatment of lymphomas and leukemias and in particular when treating non-Hodgkin lymphoma, acute myeloid leukemia, and acute lymphoblastic leukemia. This is a potentially fatal complication and people at an increased risk for TLS should be closely monitored while receiving chemotherapy and should receive preventive measures and treatments as necessary. TLS can also occur on its own although this is less common.
Lesch–Nyhan syndrome (LNS) is a rare inherited disorder caused by a deficiency of the enzyme hypoxanthine-guanine phosphoribosyltransferase (HGPRT). This deficiency occurs due to mutations in the HPRT1 gene located on the X chromosome. LNS affects about 1 in 380,000 live births. The disorder was first recognized and clinically characterized by American medical student Michael Lesch and his mentor, pediatrician William Nyhan, at Johns Hopkins.
Glycogen storage disease type I is an inherited disease that prevents the liver from properly breaking down stored glycogen, which is necessary to maintain adequate blood sugar levels. GSD I is divided into two main types, GSD Ia and GSD Ib, which differ in cause, presentation, and treatment. There are also possibly rarer subtypes, the translocases for inorganic phosphate or glucose ; however, a recent study suggests that the biochemical assays used to differentiate GSD Ic and GSD Id from GSD Ib are not reliable, and are therefore GSD Ib.
Protein toxicity is the effect of the buildup of protein metabolic waste compounds, like urea, uric acid, ammonia, and creatinine. Protein toxicity has many causes, including urea cycle disorders, genetic mutations, excessive protein intake, and insufficient kidney function, such as chronic kidney disease and acute kidney injury. Symptoms of protein toxicity include unexplained vomiting and loss of appetite. Untreated protein toxicity can lead to serious complications such as seizures, encephalopathy, further kidney damage, and even death.
Uricosuric medications (drugs) are substances that increase the excretion of uric acid in the urine, thus reducing the concentration of uric acid in blood plasma. In general, this effect is achieved by action on the proximal tubule of the kidney. Drugs that reduce blood uric acid are not all uricosurics; blood uric acid can be reduced by other mechanisms.
Hypouricemia or hypouricaemia is a level of uric acid in blood serum that is below normal. In humans, the normal range of this blood component has a lower threshold set variously in the range of 2 mg/dL to 4 mg/dL, while the upper threshold is 530 μmol/L (6 mg/dL) for women and 619 μmol/L (7 mg/dL) for men. Hypouricemia usually is benign and sometimes is a sign of a medical condition.
Nucleic acid metabolism is a collective term that refers to the variety of chemical reactions by which nucleic acids are either synthesized or degraded. Nucleic acids are polymers made up of a variety of monomers called nucleotides. Nucleotide synthesis is an anabolic mechanism generally involving the chemical reaction of phosphate, pentose sugar, and a nitrogenous base. Degradation of nucleic acids is a catabolic reaction and the resulting parts of the nucleotides or nucleobases can be salvaged to recreate new nucleotides. Both synthesis and degradation reactions require multiple enzymes to facilitate the event. Defects or deficiencies in these enzymes can lead to a variety of diseases.
Ribose 5-phosphate (R5P) is both a product and an intermediate of the pentose phosphate pathway. The last step of the oxidative reactions in the pentose phosphate pathway is the production of ribulose 5-phosphate. Depending on the body's state, ribulose 5-phosphate can reversibly isomerize to ribose 5-phosphate. Ribulose 5-phosphate can alternatively undergo a series of isomerizations as well as transaldolations and transketolations that result in the production of other pentose phosphates as well as fructose 6-phosphate and glyceraldehyde 3-phosphate.
Purine metabolism refers to the metabolic pathways to synthesize and break down purines that are present in many organisms.
Hyperuricosuria is a medical term referring to the presence of excessive amounts of uric acid in the urine. For men this is at a rate greater than 800 mg/day, and for women, 750 mg/day. Notable direct causes of hyperuricosuria are dissolution of uric acid crystals in the kidneys or urinary bladder, and hyperuricemia. Notable indirect causes include uricosuric drugs, rapid breakdown of bodily tissues containing large quantities of DNA and RNA, and a diet high in purine.
Solute carrier family 22, member 12, also known as SLC22A12 and URAT1, is a protein which in humans is encoded by the SLC22A12 gene.
A xanthine oxidase inhibitor is any substance that inhibits the activity of xanthine oxidase, an enzyme involved in purine metabolism. In humans, inhibition of xanthine oxidase reduces the production of uric acid, and several medications that inhibit xanthine oxidase are indicated for treatment of hyperuricemia and related medical conditions including gout. Xanthine oxidase inhibitors are being investigated for management of reperfusion injury.
The Purine Nucleotide Cycle is a metabolic pathway in protein metabolism requiring the amino acids aspartate and glutamate. The cycle is used to regulate the levels of adenine nucleotides, in which ammonia and fumarate are generated. AMP converts into IMP and the byproduct ammonia. IMP converts to S-AMP (adenylosuccinate), which then converts to AMP and the byproduct fumarate. The fumarate goes on to produce ATP (energy) via oxidative phosphorylation as it enters the Krebs cycle and then the electron transport chain. Lowenstein first described this pathway and outlined its importance in processes including amino acid catabolism and regulation of flux through glycolysis and the Krebs cycle.
Lesinurad is a urate transporter inhibitor for treating high blood uric acid levels associated with gout. It is recommended only as an adjuvant with either allopurinol or febuxostat when these medications are not sufficient.
Gout suppressants are agents which control and prevent gout attacks after the first episode. They can be generally classified into two groups by their purpose: drugs used for induction therapy and that for maintenance therapy.