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Alkaptonuria

  • Definition
    • Alkaptonuria is a rare condition in which a person's urine turns a dark brownish-black color when exposed to air. Alkaptonuria is part of a group of conditions known as an inborn error of metabolism.

  • Alternative Names
    • AKU; Alcaptonuria; Homogentisic acid oxidase deficiency; Alcaptonuric ochronosis

  • Causes
    • A defect in the HGD gene causes alkaptonuria.

      The gene defect makes the body unable to properly break down certain amino acids (tyrosine and phenylalanine). As a result, a substance called homogentisic acid builds up in the skin and other body tissues. The acid leaves the body through the urine. The urine turns brownish-black when it mixes with air.

      Alkaptonuria is inherited, which means it is passed down through families. If both parents carry a nonworking copy of the gene related to this condition, each of their children has a 25% (1 in 4) chance of developing the disease.

  • Symptoms
    • Urine in an infant's diaper may darken and can turn almost black after several hours. However, many people with this condition may not know they have it until mid-adulthood (around age 40), when joint and other problems occur.

      Symptoms may include:

      • Arthritis (especially of the spine) that gets worse over time
      • Darkening of the ear
      • Dark spots on the white of the eye and cornea
  • Exams and Tests
    • A urine test is done to test for alkaptonuria. If ferric chloride is added to the urine, it will turn the urine black in people with this condition.

  • Treatment
    • Some people benefit from high-dose vitamin C. This has been shown to decrease the buildup of brown pigment in the cartilage and may slow the development of arthritis.

  • Outlook (Prognosis)
    • The outcome is expected to be good.

  • Possible Complications
    • People with this condition also can get arthritis in adulthood. The build-up of homogentisic acid in the cartilage causes arthritis in about half of older adults with alkaptonuria.

      • Homogentisic acid also can build up on the heart valves, especially the mitral valve. This can sometimes lead to the need for valve replacement.
      • Coronary artery disease may develop earlier in life in people with alkaptonuria.
      • Kidney stones and prostate stones may be more common in people with alkaptonuria.
  • When to Contact a Medical Professional
    • Call your health care provider if you notice that your own urine or your child's urine becomes dark brown or black when it is exposed to air.

  • Prevention
    • Genetic counseling is recommended for people with a family history of alkoptonuria who are considering having children.

      A blood test can be done to see if you carry the gene for alkaptonuria.

      Prenatal tests (amniocentesis or chorionic villus sampling) can be done to screen a developing baby for this condition if the genetic change has been identified.

  • References
    • Chakrapani A, Gissen P, McKiernan P. Disorders of tyrosine metabolism. In: Saudubray J-M, van den Berghe G, Walter JH, eds. Inborn Metabolic Diseases: Diagnosis and Treatment. 5th ed. New York, NY: Springer; 2012:chap 18.

      Rezvani I, Melvin JJ. Defects in Metabolism of Amino Acids. In: Kliegman RM, Behrman RE, Jenson HB, Stanton BF, eds. Nelson Textbook of Pediatrics. 19th ed. Philadelphia, PA: Elsevier Saunders; 2011:chap 79.