Alkaptonuria Definition, Reason, Diagnosis, Symptom, Treatment


Alkaptonuria is an inherited health condition and is quite rare. Alkaptonuria can occur when the body does not produce an enzyme called Homogentisic Dioxygenase (HGD) in sufficient quantities.

This enzyme is used to break down a toxic substance called homogentisic acid. When the body does not produce enough HGD, homogentisic acid can build up in the body.

The buildup of homogentisic acids can cause bones and cartilage to change color and become brittle. This condition can result, especially in the spine and large joints. Individuals with alkaptonuria may also experience a change in the color of their urine to dark brown or black when exposed to air. According to Magnet Brains Alkaptonuria – Principles of Inheritance & Variation


Alkaptonuria is caused by a mutation in the homogentisate 1,2-dioxygenase (HGD) gene and is an autosomal recessive condition. This means that both parents must have the gene in order to pass the condition on to their child.

Alkaptonuria is a rare disease. According to the National Organization of Rare Disorders (NORD) in the United States, the exact number of cases is unknown, but it is thought to occur in 1 in about 250,000–1,000,000 live births in the country.


The presence of dark-colored stains on a baby’s diaper can be one of the earliest signs of alkaptonuria. In addition, there are also some other signs and symptoms that can occur in childhood.

Signs and symptoms may become more pronounced with age. Urine may change color to dark brown or black when exposed to air.

By the time they reach the age of 20 to 30, an individual with this condition may notice signs and symptoms of early-onset arthritis. For example, the person may experience chronic stiffness or pain in the lower back or large joints.

Other signs and symptoms of alkaptonuria may include:

  • Black spots on the sclera (the white part of the eye)
  • Thickening and darkening of the ear cartilage
  • There is a picture of bluish spots on the skin, especially around the sweat glands
  • Sweat or sweat stains
  • Black earwax
  • Kidney stones and prostate stones
  • Arthritis (especially in the hip and knee joints

Alkaptonuria can also cause heart health problems. The buildup of homogentisic acid can cause the heart valves to harden.

This condition can inhibit the closure of the heart valves completely, which in turn causes abnormalities in the aortic and mitral valves. In severe cases, heart valve replacement may be required. This buildup also causes hardening of the arteries, which can increase the risk of developing high blood pressure.


The diagnosis of alkaptonuria can be determined from a detailed medical interview, direct physical examination, and certain investigations if deemed necessary.

Doctors can suspect alkaptonuria if the color of the urine turns dark brown or black when exposed to air. In addition, alkaptonuria can also be examined if a person has early-onset osteoarthritis.

The doctor may perform an examination known as gas chromatography to check for the presence of small amounts of homogentisic acid in the urine. A DNA test can also be performed to evaluate the presence of a mutated HGD gene.

Family history can also be very helpful when making the diagnosis of alkaptonuria. However, most people do not know if they are carriers of the gene or not.


There is no specific treatment for alkaptonuria. People with this condition may be advised to eat a low-protein diet.

Doctors may also recommend taking high doses of ascorbic acid, or vitamin C, to slow the accumulation of homogentisic acid in cartilage. However, long-term use of vitamin C has generally not been shown to be effective in treating this condition.

In addition, other treatments for alkaptonuria are aimed at preventing and treating complications that may occur, such as:

For example, your doctor may prescribe anti-inflammatory or narcotic medications for joint pain. Physical and occupational therapy can help to maintain flexibility and strength in muscles and joints.

In addition, people with this condition are also advised to avoid activities that can burden the joints, such as lifting heavy objects and contact sports.

At some stage, people with alkaptonuria may need surgery. One available study reports that half of people with alkaptonuria may require shoulder, knee, or hip joint replacement surgery, often between the ages of 50 and 60.

In addition, people with this condition may also need an aortic or mitral valve replacement, if they are no longer functioning. In some cases, surgery or other therapy may also be needed to treat chronic kidney or prostate stones.


Since alkaptonuria is a genetic disease that is present from birth, there is no proven effective way to completely prevent this condition from occurring.

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