Alkaptonuria is a rare inherited disorder. It occurs when your
body can’t produce enough of an enzyme called homogentisic dioxygenase (HGD).
This enzyme is used to break down a toxic substance called homogentisic acid.
When you don’t produce enough HGD, homogentisic acid builds up in your body.
The buildup of homogentisic acid causes your bones and cartilage
to become discolored and brittle. This typically leads to osteoarthritis,
especially in your spine and large joints. People with alkaptonuria also have
urine that turns dark brown or black when it’s exposed to air.
Are the Symptoms of Alkaptonuria?
Dark stains on a baby’s diaper are one of the earliest signs of
alkaptonuria. There are few other symptoms during childhood.
Symptoms become more obvious as you age. Your urine may turn dark
brown or black when it’s exposed to air. By the time you reach your 20s or 30s,
you may notice signs of early-onset osteoarthritis. For example, you may notice
chronic stiffness or pain in your lower back or large joints.
Other symptoms of alkaptonuria include:
- dark spots in the sclera (white) of your eyes
- thickened and darkened cartilage in your ears
- blue speckled discoloration of your skin,
particularly around sweat glands
- dark-colored sweat or sweat stains
- black earwax
- kidney stones and prostate stones
- arthritis (especially hip and knee joints)
Alkaptonuria can also lead to heart problems. The buildup of
homogentisic acid causes your heart valves to harden. This can keep them from
closing properly, resulting in aortic and mitral valve disorders. In severe
cases, heart valve replacement may be necessary. The buildup also causes your
blood vessels to harden. This raises your risk of high blood pressure.
Alkaptonuria is caused by a mutation on your homogentisate
1,2-dioxygenase (HGD) gene. It’s an autosomally recessive condition. This means
that both of your parents must have the gene in order to pass the condition on
Alkaptonuria is a rare disease. According to the National
Organization of Rare Disorders (NORD), the exact number of cases is
unknown. It is estimated to occur in 1 of every 250,000 –1 million live births
in the United States. However, it’s more common in certain areas of Slovakia,
Germany, and the Dominican Republic.
Is Alkaptonuria Diagnosed?
Your doctor may suspect you have alkaptonuria if your urine turns
dark brown or black when it’s exposed to air. They may also test you for the
condition if you develop early onset osteoarthritis.
Your doctor can use a test called gas chromatography to look for traces
of homogentisic acid in your urine. They can also use DNA testing to check for
the mutated HGD gene.
Family history is very useful in making a diagnosis of
alkaptonuria. However, many people don’t know they carry the gene. Your parents
might be carriers without realizing it.
Is Alkaptonuria Treated?
There’s no specific treatment for alkaptonuria.
You may be put on a low-protein diet. Your doctor may also
recommend large doses of ascorbic acid, or vitamin C, to slow down the
accumulation of homogentisic acid in your cartilage. However, NORD warns that
long-term use of vitamin C has generally proven ineffective for treating this
Other treatments for alkaptonuria are focused on preventing and
relieving possible complications, such as:
- heart disease
- kidney stones
For example, your doctor may prescribe anti-inflammatory
medications or narcotics for joint pain. Physical and occupational therapy may help
you maintain flexibility and strength in your muscles and joints. You should also
avoid activities that put a lot of strain on your joints, such as heavy manual
labor and contact sports.
At some point in your life, you might need surgery. For example, NORD reports
that approximately half of people with alkaptonuria need a shoulder, knee, or
hip replacement, often by age 50 or 60. You may also require surgery to replace
your aortic or mitral heart valves, if they stop working properly. In some
cases, you may need surgery or other therapies to treat chronic kidney or
Researchers are currently studying the use of the drug nitisinone as a possible
treatment for alkaptonuria.
Is the Outlook for Alkaptonuria?
The life expectancy for people with alkaptonuria is fairly
normal. However, the disease puts you at much higher risk of certain disorders,
- arthritis in your spine, hips, shoulders, and
- tearing of your Achilles tendon
- hardening of your heart’s aortic and mitral
- hardening of your coronary arteries
- kidney and prostate stones
Some of these complications can be delayed with regular checkups.
Your doctor will want to monitor you regularly. Tests to monitor the progress
of your condition might include:
- spinal X-rays to check for disk degeneration and
calcification in your lumbar spine
- chest X-rays to monitor your aortic and mitral heart
- CT (computed tomography) scans to find signs of
coronary artery disease