Gout
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-shaped crystals of the monosodium salt of uric acid. Pain typically comes on rapidly, reaching maximal intensity in less than 12 hours. In about half of cases, the joint at the base of the big toe is affected. It may also result in tophi, kidney stones, or kidney damage.
Gout is due to persistently elevated levels of uric acid in the blood. This occurs from a combination of diet, other health problems, and genetic factors. At high levels, uric acid crystallizes and the crystals deposit in joints, tendons, and surrounding tissues, resulting in an attack of gout. Gout occurs more commonly in those who regularly drink beer or sugar-sweetened beverages; eat foods that are high in purines such as liver, shellfish, or anchovies; or are overweight. Diagnosis of gout may be confirmed by the presence of crystals in the joint fluid or in a deposit outside the joint. Blood uric acid levels may be normal during an attack.
Treatment with nonsteroidal anti-inflammatory drugs, glucocorticoids, or colchicine improves symptoms. Once the acute attack subsides, levels of uric acid can be lowered via lifestyle changes and in those with frequent attacks, allopurinol or probenecid provides long-term prevention. Taking vitamin C and having a diet high in low-fat dairy products may be preventive.
Gout affects about 1–2% of adults in the developed world at some point in their lives. It has become more common in recent decades. This is believed to be due to increasing risk factors in the population, such as metabolic syndrome, longer life expectancy, and changes in diet. Older males are most commonly affected. Gout was historically known as "the disease of kings" or "rich man's disease". It has been recognized at least since the time of the ancient Egyptians.
Signs and symptoms
Gout can present in several ways, although the most common is a recurrent attack of acute inflammatory arthritis. The metatarsophalangeal joint at the base of the big toe is affected most often, accounting for half of cases. It can also involve midfoot structures, including the cuneiform bones. Other joints, such as the heels, knees, wrists, and fingers, may also be affected. Joint pain usually begins during the night and peaks within 24 hours of onset. This is mainly due to lower body temperature. Other symptoms may rarely occur along with the joint pain, including fatigue and high fever.Long-standing elevated uric acid levels may result in other symptoms, including hard, painless deposits of uric acid crystals called tophi. Extensive tophi may lead to chronic arthritis due to bone erosion. Elevated levels of uric acid may also lead to crystals precipitating in the kidneys, resulting in kidney stone formation and subsequent acute uric acid nephropathy.
Cause
The crystallization of uric acid, often related to relatively high levels in the blood, is the underlying cause of gout. This can occur because of diet, genetic predisposition, or underexcretion of urate, the salts of uric acid. Underexcretion of uric acid by the kidney is the primary cause of hyperuricemia in about 90% of cases, while overproduction is the cause in less than 10%. About 10% of people with hyperuricemia develop gout at some point in their lifetimes. The risk, however, varies depending on the degree of hyperuricemia. When levels are between 415 and 530 μmol/L, the risk is 0.5% per year, while in those with a level greater than 535 μmol/L, the risk is 4.5% per year.Lifestyle
Dietary causes account for about 12% of gout, and include a strong association with the consumption of alcohol, sugar-sweetened beverages, meat, and seafood. The dietary mechanisms and nutritional basis involved in gout provide evidence for strategies of prevention and improvement of gout, and dietary modifications based on effective regulatory mechanisms may be a promising strategy to reduce the high prevalence of gout. Among foods richest in purines yielding high amounts of uric acid are dried anchovies, shrimp, organ meat, dried mushrooms, seaweed, and beer yeast. Chicken and potatoes also appear related. Other triggers include physical trauma and surgery.Studies in the early 2000s found that other dietary factors are not relevant. Specifically, a diet with moderate purine-rich vegetables is not associated with gout. Neither is total dietary protein. Alcohol consumption is strongly associated with increased risk, with wine presenting somewhat less of a risk than beer or spirits. Eating skim milk powder enriched with glycomacropeptide and G600 milk fat extract may reduce pain but may result in diarrhea and nausea.
Physical fitness, healthy weight, low-fat dairy products, and to a lesser extent, coffee and taking vitamin C, appear to decrease the risk of gout; however, taking vitamin C supplements does not appear to have a significant effect in people who already have established gout. Peanuts, brown bread, and fruit also appear protective. This is believed to be partly due to their effect in reducing insulin resistance.
Other than dietary and lifestyle choices, the recurrence of gout attacks is also linked to the weather. High ambient temperature and low relative humidity may increase the risk of a gout attack.
Genetics
Gout is partly genetic, contributing to about 60% of variability in uric acid level. The SLC2A9, SLC22A12, and ABCG2 genes are commonly associated with gout and variations in them can approximately double the risk. Loss-of-function mutations in SLC2A9 and SLC22A12 causes low blood uric acid levels by reducing urate absorption and unopposed urate secretion. The rare genetic disorders familial juvenile hyperuricemic nephropathy, medullary cystic kidney disease, phosphoribosylpyrophosphate synthetase superactivity and hypoxanthine-guanine phosphoribosyltransferase deficiency as seen in Lesch–Nyhan syndrome, are complicated by gout.Medical conditions
Gout frequently occurs in combination with other medical problems. Metabolic syndrome, a combination of abdominal obesity, hypertension, insulin resistance, and abnormal lipid levels, occurs in nearly 75% of cases. Other conditions commonly complicated by gout include lead poisoning, kidney failure, hemolytic anemia, psoriasis, solid organ transplants, and myeloproliferative disorders such as polycythemia. A body mass index greater than or equal to 35 increases male risk of gout threefold. Chronic lead exposure and lead-contaminated alcohol are risk factors for gout due to the harmful effect of lead on kidney function.Medication
s have been associated with attacks of gout, but a low dose of hydrochlorothiazide does not seem to increase risk. Other medications that increase the risk include niacin, aspirin, ACE inhibitors, angiotensin receptor blockers, beta blockers, ritonavir, and pyrazinamide. The immunosuppressive drugs ciclosporin and tacrolimus are also associated with gout, the former more so when used in combination with hydrochlorothiazide.Pathophysiology
Gout is a disorder of purine metabolism, and occurs when its final metabolite, uric acid, crystallizes in the form of monosodium urate, precipitating and forming deposits in joints, on tendons, and in the surrounding tissues. Microscopic tophi may be walled off by a ring of proteins, which blocks interaction of the crystals with cells and therefore avoids inflammation. Naked crystals may break out of walled-off tophi due to minor physical damage to the joint, medical or surgical stress, or rapid changes in uric acid levels. When they break through the tophi, they trigger a local immune-mediated inflammatory reaction in macrophages, which is initiated by the NLRP3 inflammasome protein complex. Activation of the NLRP3 inflammasome recruits the enzyme caspase 1, which converts pro-interleukin 1β into active interleukin 1β, one of the key proteins in the inflammatory cascade. An evolutionary loss of urate oxidase, which breaks down uric acid, in humans and higher primates has made this condition common.The triggers for the precipitation of uric acid are not well understood. While it may crystallize at normal levels, it is more likely to do so as levels increase. Other triggers believed to be important in acute episodes of arthritis include cool temperatures, rapid changes in uric acid levels, acidosis, articular hydration and extracellular matrix proteins. The increased precipitation at low temperatures partly explains why the joints in the feet are most commonly affected. Rapid changes in uric acid may occur due to factors including trauma, surgery, chemotherapy and diuretics. The starting or increasing of urate-lowering medications can lead to an acute attack of gout with febuxostat of a particularly high risk. Calcium channel blockers and losartan are associated with a lower risk of gout compared to other medications for hypertension.