Ochronosis is the bluish-black discoloration of certain tissues, such as the ear cartilage, the ocular (eye) tissue, and other body locations. It often occurs in individuals with alkaptonuria, a metabolic disorder; it can also occasionally occur from exposure to various substances. Alkaptonuria is caused by a mutation in the HGD gene, which results in the accumulation and deposition of homogentisic acid (HGA) in cartilage, causing the characteristic coloring of ochronosis. The affected tissue becomes weak and brittle with time, leading to chronic inflammation, degeneration, and osteoarthritis. Alkaptonuria is inherited in an autosomal recessive manner. When ochronosis is due to exposure to substances (called exogenous ochronosis), the condition is not inherited.
Last updated: 2/10/2011
What is alkaptonuria?
Alkaptonuria is an inherited condition that causes urine to turn black when exposed to air. The three major features of alkaptonuria are the presence of dark urine, ochronosis, a buildup of dark pigment in connective tissues such as cartilage and skin, and arthritis of the spine and larger joints. Ochronosis starts after age 30 and arthritis in early adulthood. Other features of this condition can include heart problems, kidney stones, and prostate stones. Alkaptonuria is caused by mutations in the HGD gene. It is inherited in an autosomal recessive fashion. There is still no cure for this disease. Treatment includes the management of joint pain, physical and occupational therapy, joint replacements and surgery when needed.
Last updated: 3/18/2016
How might alkaptonuria be treated?
There is no cure for alkaptonuria, but there is treatment for some individual signs and symptoms of the condition. Joint pain may be substantial in individuals with alkaptonuria, and close attention to pain control is usually necessary. Physical and occupational therapy can be important to promote muscle strength and flexibility. Knee, hip, and shoulder replacement surgeries may be options for managing significant arthritis. In general, however, the goal of joint replacement is pain relief rather than increased range of motion. Maintaining joint range of motion through moderate non-weight-bearing exercise such as swimming may have beneficial effects. Treatment of prostate stones and renal stones may include surgery.
Several therapies for alkaptonuria have been investigated. Treatment of alkaptonuria with nitisinone (also called NTBC) has been proposed; nitisinone is currently approved for the treatment of tyrosinemia type I, which is also a metabolic disorder. Further investigations to determine the benefits of nitisinone in slowing the progression of joint disease are in progress.Oral bisphosphonate therapy has been suggested to halt the progressive bone loss but studies have not confirmed the benefit. To see a list of completed clinical trials or trials currently enrolling individuals with alkaptonuria, click here.
No therapy has proven to prevent or correct the pigmentary changes of ochronosis.
Dietary restriction of phenylalanine and tyrosine has been proposed to reduce the production of HGA, but severe restriction of these amino acids is not practical in the long term and may be dangerous.
Last updated: 3/18/2016
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