| Definition:
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Alkaptonuria is a rare inherited disorder of metabolism characterized by urine which turns black when exposed to air. Another characteristic is the development of arthritis in adulthood.
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| Alternative Names:
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Alcaptonuria; Homogentisic acid oxidase deficiency; Ochronosis
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| Causes, incidence, and risk factors:
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Alkaptonuria is an autosomal recessive inherited disorder. In affected individuals, an amino acid known as tyrosine is not properly metabolized, due to a defect in an enzyme called homogentisic acid oxidase. Because of the defect, homogentisic acid is excreted in the urine and turns a brown color upon exposure to air. This is the result of a dark pigment with an ochre color (earthy red or yellow), which led to the name ochronosis. The bones and cartilage of the body can be brown colored.
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| Symptoms:
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Infant/child:
- Family history of alkaptonuria
- Urine in diaper may darken after several hours (can become almost black)
Adult:
- Progressive arthritis, especially of the spine
- Darkening of the ear (from black staining of the cartilage in the ear)
- Dark spots on the sclera (white of the eye) and cornea
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| Signs and tests:
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Urinalysis is positive for reducing substance. Further urine testing shows a positive ferric chloride test.
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| Treatment:
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Some patients benefit from high-dose vitamin C. This has been shown to decrease the build up of brown pigment in the cartilage, which may slow the rate of development of arthritis.
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| Expectations (prognosis):
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The outcome is expected to be good.
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| Complications:
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- Accumulation of homogentisic acid products in the cartilage causes arthritis in about 50% of older adults with alkaptonuria.
- Homogentisic acid products can accumulate on the heart valves, especially the mitral valve, sometimes leading to the need for valve replacement.
- Coronary artery disease may develop earlier in people with alkaptonuria.
- Kidney and prostate stones may be more common in people with alkaptonuria.
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| Calling your health care provider:
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Call your health care provider if you notice that your own urine or your child's urine becomes dark brown or black upon exposure to air.
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| Prevention:
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Like most genetic diseases, alkaptonuria itself cannot be prevented, but some of the manifestations, such as arthritis, can be minimized by treatment.
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