Musculoskeletal manifestations of alkaptonuria: A case report and literature review.

Alkaptonuria (AKU) is a rare autosomal recessive disorder that results from the deficient activity of homogentisate 1,2-dioxygenase and leads to increased levels of homogentisic acid (HGA) and its oxidized product benzoquinone acetic acid (BQA). Both HGA and BQA form polymerized deposits that lead to a bluish-black discoloration of the cartilage as well as degeneration, inflammation, and calcification of the tendons, ligaments, intervertebral discs, and large joints and increased bone resorption. A brittle and fragmented cartilage forms and leads to aberrant loading of the subchondral bone. These fragments then adhere to the synovial membrane and cause fibrosis or chondromatosis, leading to ochronotic arthropathy. Ochronotic tendinopathy most commonly affects the patellar or Achilles tendon and can lead to enthesopathy or spontaneous tendon ruptures. Ochronotic pigments deposited in the bone impair the bone mineralization process and lead to osteopenia or osteoporosis. Here, we report a case of a patient with several musculoskeletal manifestations of AKU and reviewed the literature to summarize the pathophysiology, clinical characteristics, and radiologic findings of the rheumatic features of AKU. Though medical treatment options are limited, early identification of AKU can facilitate prompt surgical intervention.