Alkaptonuria is an autosomal recessive inborn error of metabolism caused by the accumulation of homogentisic acid due to a deficiency of an enzyme called HGO (homogentisate 1.2-dioxygenase).
It is characterized by:
- Urine that turns dark on standing
- Dark pigmentation of cartilage and connective tissue (ochronosis)
- In MRCP classically look for pictures with a dark patch on the sclera or pinna
- Arthritis
It is also associated with:
- Renal stones
- Aortic dilation
- Deterioration of cardiac valves
Spinal xrays show intervertebral calcification
Treatment:
- No definitive treatment at present.
- No clear benefit from low-protein diet or vitamin C supplements
- Possibly nitisinone in the future
Small print gems: the pigmentation is called ochronosis because when viewed microscopically it appears ochre, despite appearing grey-black to the naked eye. It is sometimes called Garrod's disease after the doctor who discovered it.
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