[Ochronotic arthropathy in alkaptonuria. Radiological manifestations and physiopathological signs]

Alkaptonuria is a rare autosomal recessive metabolic disease, due to the lack of homogentisic acid oxidase. The following accumulation of homogentisic acid brings about a black discoloration of both the urine (alkaptonuria) and connective tissue (ochronosis). The ochronotic alterations into joint cartilages cause degenerative arthropathy and osteopenia. The radiological features of three unrelated cases of alkaptonuria are reported. Radiographic abnormalities of ochronotic arthropathy are found in both the spine and the extraspinal joints. In the spine, the progressive calcification and "vacuum" phenomenon of disc spaces are the most characteristic findings. Disc space narrowing is associated with calcification and marginal sclerosis of vertebral bodies and is accentuated by osteopenia. Osteophytes are usually absent or of small size; nevertheless progressive formation of marginal intervertebral bridges and obliteration of disc spaces at multiple levels ("pseudoblock vertebrae") may occur. In extraspinal sites, space narrowing, bone sclerosis and fragmentation may also be observed. Diagnosis of ochronotic arthropathy is often suggested by radiographs of the spine, and it is confirmed afterwards by clinical and laboratory findings. The characteristic radiological findings of ochronotic spondylitis, in the final stage, include narrowing of at least four lumbar disc spaces, associated with their calcification and "vacuum" phenomenon, "pseudoblock vertebrae", marginal sclerosis and osteopenia of vertebral bodies.