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Alkaptonuria

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.

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. The disease is most often discovered in 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

The build-up of homogentisic acid in the cartilage causes arthritis in many 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.

Alternative Names

AKU; Alcaptonuria; Homogentisic acid oxidase deficiency; Alcaptonuric ochronosis

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.

James WD, Berger TG, Elston DM. Mycobacterial diseases. In: James WD, Berger T, Elston DM, eds. Andrews' Diseases of the Skin. 12th ed. Philadelphia, PA: Elsevier; 2016:chap 16.

Kliegman RM, Stanton BF, St. Geme JW, Schor NF. Defects in metabolism of amino acids. In: Kliegman RM, Stanton BF, St. Geme JW, Schor NF, eds. Nelson Textbook of Pediatrics. 20th ed. Philadelphia, PA: Elsevier; 2016:chap 85.

Riley RS, McPherson RA. Basic examination of urine. In: McPherson RA, Pincus MR, eds. Henry's Clinical Diagnosis and Management by Laboratory Methods. 23rd ed. St Louis, MO: Elsevier; 2017:chap 28.

Review Date 5/1/2017

Updated by: Anna C. Edens Hurst, MD, MS, Assistant Professor in Medical Genetics, The University of Alabama at Birmingham, Birmingham, AL. Review provided by VeriMed Healthcare Network. Also reviewed by David Zieve, MD, MHA, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.