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.

Alternative Names

Alcaptonuria; Homogentisic acid oxidase deficiency; Ochronosis

Causes, incidence, and risk factors

Alkaptonuria is an autosomal recessive inherited disorder. It is a disorder of tyrosine (an amino acid) metabolism resulting from a defect in the enzyme homogentisic acid oxidase. 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.

Signs and tests

Urinalysis is positive for reducing substance. Further urine testing shows a positive ferric chloride test.

Treatment

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.

Expectations (prognosis)

The outcome is expected to be good.

Complications

Accumulation of homogentisic acid products in the cartilage causes arthritis in about 50% of older adults with alkaptonuria.

Calling your health care provider

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.

Prevention

Like most genetic diseases, alkaptonuria itself cannot be prevented, but some of the manifestations, such as arthritis, can be minimized by treatment.