Signs and symptoms[edit]
Alkaptonuria is often asymptomatic, but the sclera of the eyes may be pigmented (often only at a later age),[1] and the skin may be darkened in sun-exposed areas and around sweat glands; sweat may be coloured brown. Urine may turn brown or even inky black if collected and left exposed to open air, especially when left standing for a period of time. Kidney stones and stone formation in the prostate (in men) are common and may occur in more than a quarter of cases.[1]
The main symptoms of alkaptonuria are due to the accumulation of homogentisic acid in tissues. In the joints this leads to cartilage damage, specifically in the spine, leading to low back pain at a young age in most cases. Cartilage damage may also occur in the hip and shoulder. Joint replacement surgery (hip and shoulder) is often necessary at a relatively young age.[1]
Intervertebral discs Calcification
Valvular heart disease, mainly calcification and regurgitation of the aortic and mitral valves, may occur, and in severe and progressive cases valve replacement may be necessary. Coronary artery disease may be accelerated in alkaptonuria.[1]
A distinctive characteristic of alkaptonuria is that ear wax exposed to air turns red or black (depending on diet) after several hours because of the accumulation of homogentisic acid.[4]