What is Ochronosis or alkaptonuria ? Causes and treatment

Ochronosis or alkaptonuria, is an uncommon autosomal recessive condition that occurs secondary to complete deficiency of homogentisic acid oxidase. As a result, the metabolism of phenylalanine and tyrosine results in excessive retention of homogentisic acid, a pigmented polymer that is subsequently deposited in cartilage, skin, and the sclera. 

In articular cartilage, it is bound to collagen in the deeper layers, resulting in a more brittle articular surface that is easily fibrillated and eroded. Diseased cartilage is prone to fragment and develop osteochondral loose bodies. In addition, homogentisic acid has a propensity for deposition in the nucleus pulposus, resulting in a brittle and stiffened disc space . 

Onset of symptoms occurs typically in the fourth decade of life.In ochronotic spondylosis patient may present with  acute back pain. Progression of the spondylosis results in a stiffened lumbar spine and loss of normal lordosis. Peripheral arthropathy typically involves the large joints and is characterized by prominent osteochondral loose body formation, chronic effusions, and secondary chondrocalcinosis 

Radiographs demonstrate multiple disc vacuum signs with eventual ossification of the discs and intervertebral ligaments. In contrast to ankylosing spondylitis, the sacroiliac and apophyseal joints are spared. 

The peripheral joints are characterized by loose bodies and osteoarthritic changes, with involvement of the shoulder, hip, and knee, and sparing of the smaller joints. 

The diagnosis is suggested by a history of the patient’s passing black urine and is confirmed upon observation of freshly passed urine that turns black upon exposure to air. 

Synovial fluid is characteristically clear and yellow because it does not darken with alkalinization. Symptomatic treatment is similar to that for other osteoarthritic conditions. Arthroscopic removal of osteochondral loose bodies is indicated for mechanical symptoms of locking or instability.

 Total joint arthroplasty is the treatment for end-stage arthritis.