Alkaptonuria is a rare genetic disorder wherein the body fails to produce adequate homogentisate dioxygenase enzyme. This enzyme helps break down homogentisic acid, a toxic substance. Inadequate homogentisate dioxygenase leads to accumulated homogentisic acid in the body, causing cartilage and bones to become brittle and discolored. It results in osteoarthritis, typically in your large joints and spine. People with alkaptonuria also produce urine that turns black or dark brown when exposed to air.

Causes
In alkaptonuria the body fails to break down amino acids like phenylalanine and tyrosine. Hence, homogentisic acid accumulates in the skin and other body tissues. Alkaptonuria is a genetic disease, meaning it passes down in families. When both parents possess a nonfunctional copy of the gene associated with this condition, their offsprings face a 25 percent (1 in 4) likelihood of developing the disease.

Symptoms
Alkaptonuria can lead to several symptoms that can affect different body areas. Some prominent signs you may experience with the condition are:

Urine changes
In childhood and infancy, one of the noticeable alkaptonuria signs is a change in urine color. Individuals suffering from alkaptonuria have urine that turns black on exposure to air or dark urine. You can notice this change in diapers.

Tissue discoloration
Some people do not know they have alkaptonuria until adulthood. As individuals age beyond 30 years, the progression of ochronosis becomes evident through the discoloration of tissues in certain areas, such as the:

The skin on the hands
White areas in the eyes
Outer ear cartilage

Bone and joint issues
People with alkaptonuria may have decreased joint mobility, stiffness, and shoulder, knee, and hip pain. Lowered mineral content in the bones aggravates your susceptibility to fractures.

Breathing difficulty
When the muscles and the bones around the lungs become stiff, it becomes challenging for your chest to expand efficiently. It may result in difficulty breathing and shortness of breath.

Heart conditions
Homogentisic acid deposits around the heart valves result in:

Heart failure
Coronary artery disease
Irregular heart rate
Heart disease

In the later stages of life, people with this condition are susceptible to developing aortic valve stenosis. This condition develops with the aorta valve narrowing. It can cease or lower the blood flow from the heart to the rest of the body.

Prostate, bladder, and kidney stones
Homogentisic acid deposits can result in the formation of bladder, prostate, and kidney stones. Studies suggest that fifty percent of people with the disorder have a kidney stone history by age 64.

Treatment
Unfortunately, there exists no definite treatment for alkaptonuria. Hence, most treatment measures help with symptom management. While many therapies have been tried, most have proven unhelpful or harmful in the long run. Some alkaptonuria treatments focus on managing associated complications, such as kidney stones, arthritis, and heart disease.

Occupational and physical therapy can also help maintain strength and flexibility in joints and muscles. Further, it is vital to avoid activities that strain the joints, such as contact sports and heavy manual labor. Over time, one may need surgery too. Adults may experience a homogentisic acid build-up in the cartilage that causes arthritis and may demand a hip, knee, or shoulder replacement. Individuals may also need surgery to replace the mitral or aortic heart valves or require therapies or surgeries to cure prostate or chronic kidney stones.