Alkaptonuria

Definition:

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.

Symptoms:

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

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.

Review Date: 10/26/2001
Reviewed By: David G. Brooks, M.D., Ph.D., Division of Medical Genetics, University of Pennsylvania Medical Center, Philadelphia, PA. Review provided by VeriMed Healthcare Network.

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