Alkaptonuria

Written by Janet Barwell and Elizabeth Boskey, PhD | Published on July 16, 2012
Medically Reviewed by George Krucik, MD

What Is Alkaptonuria?

Alkaptonuria is an inherited disorder. People with alkaptonuria can not properly metabolize phenylalanine and tyrosine. These two amino acids are normally used to produce hormones, pigments, neurotransmitters, L-dopa, epinephrine, and norepinephrine in the body.

People with alkaptonuria have urine that turns dark brown when it is exposed to air. This is because their livers make too much of a chemical called homogentisic acid. This pigment-like polymer accumulates in the body over time.

What Causes Alkaptonuria?

Alkaptonuria is caused by a mutation of the homogentisate 1,2-dioxygenase (HGD) gene. It is an autosomally recessive condition. That means that both of your parents must have the gene in order to pass the condition to you.

Alkaptonuria is a rare disease. It affects only one out of every 100,000-200,000 people worldwide. However, it is much more common in certain areas of Slovakia and the Dominican Republic. This may be because isolated populations of carriers live in those areas.

What Are the Symptoms of Alkaptonuria?

Dark stains in a baby’s diaper are one of the earliest signs of alkaptonuria. There are few other symptoms during childhood. However, the pigment continues to accumulate in the body over time.

Symptoms become more obvious in middle age. By the time you hit your thirties or forties, signs include:

  • brown or blue discoloration of the sclera (white) of the eyes
  • stained cartilage that may be visible in your ear
  • darkening of the skin, particularly around sweat glands
  • chronic low back pain
  • arthritis in the hips and knees
  • kidney stones and prostate stones

Alkaptonuria can also lead to heart problems. Homogentisic acid can cause calcification of your heart valves. This may keep them from closing properly, resulting in aortic and mitral valve disorders. In severe cases, heart valve replacement may be necessary. Calcification in the arteries may also put people at risk of coronary artery disease.

How Is Alkaptonuria Diagnosed?

This disease is usually first suspected after parents notice a child’s urine turning dark when exposed to air. However there are also specific tests for the disease:

Gas chromatography can detect traces of homogentisic acid in the urine.

DNA testing can look for the mutated HGD gene.

Family history is very useful in making a diagnosis of alkaptonuria. However, many people do not know they carry the gene.

How Is Alkaptonuria Treated?

There is no specific treatment for alkaptonuria. Large doses of ascorbic acid (vitamin C) may slow down the accumulation of pigment in the cartilage. This could slow the development of arthritis.

You may be put on a low-protein diet to reduce amino acid levels. Phenylalanine should be eliminated from your diet. Some foods that contain phenylalanine include:

  • meats
  • milk products
  • soy
  • nuts and seeds
  • many types of artificial sweeteners

Reducing phenylalanine in the diet has been shown to reduce levels of homogentisic acid in children. Evidence is less clear in adults. Doctors still don’t know if a low-protein diet in childhood can improve adult health. .

Non-dietary treatments for alkaptonuria are focused on preventing possible complications. These include arthritis, kidney stones and heart disease.

What Is the Prognosis for Alkaptonuria?

The life expectancy for people with alkaptonuria is fairly normal. However, the disease puts you at much higher risk for certain disorders including:

  • arthritis in the spine, hips, shoulders, and knees
  • calcification of the heart’s mitral valve
  • tearing of the Achilles tendon
  • calcification of the coronary arteries
  • kidney and prostate stones

Some of these complications can be delayed with regular checkups. Your doctor will want to monitor you regularly for potential effects of the disease. Tests to monitor disease progress might include:

  • spinal X-rays to watch for disk degeneration and calcification in your lumbar spine
  • chest X-rays to monitor the aortic and mitral valves
  • CT (computed tomography) scans to find signs of coronary artery disease
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