Short answer · Medically reviewed summary · Last updated: 2026-05-08
TL;DR: Alkaptonuria is a rare, inherited metabolic disorder where the body cannot properly break down two amino acids, tyrosine and phenylalanine, leading to the accumulation of a substance called homogentisic acid. This buildup causes urine to turn black upon exposure to air and eventually results in severe joint damage and ochronosis (bluish-black pigmentation of tissues). What causes Alkaptonuria? Alkaptonuria is caused by mutations in the HGD gene, which provides instructions for making an enzyme that breaks down homogentisic acid.
What is Alkaptonuria
What is Alkaptonuria? Plain-language, medically reviewed definition plus the lived reality told by patients.
TL;DR: Alkaptonuria is a rare, inherited metabolic disorder where the body cannot properly break down two amino acids, tyrosine and phenylalanine, leading to the accumulation of a substance called homogentisic acid. This buildup causes urine to turn black upon exposure to air and eventually results in severe joint damage and ochronosis (bluish-black pigmentation of tissues).
What causes Alkaptonuria?
Alkaptonuria is caused by mutations in the HGD gene, which provides instructions for making an enzyme that breaks down homogentisic acid. When this enzyme is inactive, the acid accumulates in the body and is excreted in urine. Over time, this acid deposits in connective tissues, such as cartilage and skin, causing them to darken and become brittle. This process, known as ochronosis, is the hallmark of Alkaptonuria.
How does Alkaptonuria affect the body?
While the condition is present from birth, symptoms often progress slowly throughout a patient’s life. The primary clinical manifestations of Alkaptonuria include:
- Urine discoloration: Urine that turns dark brown or black when exposed to air.
- Ochronosis: Bluish-black pigmentation visible in the ears, nose, or whites of the eyes.
- Arthropathy: Early-onset, severe arthritis, particularly in the spine and large joints like the hips and knees.
- Cardiac issues: Potential for aortic valve stenosis and stone formation in the kidneys or prostate.
How common is Alkaptonuria?
Alkaptonuria is a very rare condition with an estimated global prevalence of 1 in 250,000 to 1,000,000 individuals. It affects both males and females equally, as it is inherited in an autosomal recessive pattern. While it occurs worldwide, certain populations—such as those in Slovakia and the Dominican Republic—have shown higher clusters of cases due to genetic founder effects.
How is Alkaptonuria different from other arthritic conditions?
Unlike common osteoarthritis, which typically develops in later life due to wear and tear, the joint damage seen in Alkaptonuria is directly linked to the deposition of pigmented metabolic waste in the cartilage. This creates a distinct clinical profile that often requires specialized management, which is why 31 members of the DiseaseMaps.org community have connected to share their experiences managing this lifelong condition.
Next steps
- Consult a metabolic specialist or a rheumatologist familiar with rare metabolic bone diseases.
- Discuss dietary management, such as protein-restricted diets, with a clinical dietitian.
- Connect with the DiseaseMaps.org community to share experiences with others living with Alkaptonuria.
- Monitor joint health through regular imaging and cardiac function through echocardiograms.
Medical disclaimer: This content is for educational purposes only and does not constitute professional medical advice, diagnosis, or treatment. Always seek the advice of your physician or other qualified health provider with any questions regarding a medical condition.
References
- NIH Genetic and Rare Diseases Information Center (GARD): Alkaptonuria
- Orphanet: Alkaptonuria (ORPHA:67)
- Online Mendelian Inheritance in Man (OMIM): #203500 Alkaptonuria
- Alkaptonuria Society (AKU Society)
