Ochronosis is a rare hereditary disorder of amino acid metabolism. Patients lack the enzyme homogentisic acid oxidase in their liver, leading to the excretion of large amounts of unoxidized homogentisic acid in the urine and its deposition in various tissues and organs throughout the body. When homogentisic acid accumulates in the structural tissues of joints, it causes ochronotic arthritis.

bubble_chart Clinical Manifestations

Typically, this disease may have a familial history, with a difference in incidence between males and females, approximately 2:1. After birth, apart from the urine oxidizing and turning dark brown, staining the diaper black, patients exhibit no other symptoms. It is not until the age of 20 to 30, due to excessive accumulation of homogentisic acid, that a series of symptoms develop. The main symptoms include generalized skin, sclera, and corneal pigmentation appearing brownish-yellow, with the ears, nose, and cartilage potentially turning blue. The tympanic membrane edges become gray-black, and hearing often deteriorates. When uric acid deposits on the aortic and mitral valves, the valve membranes harden, leading to murmurs. Male patients often develop black prostatic stones. Bone and joint changes typically first affect the spine, followed by the knees, shoulders, and hips. The incidence of spondylitis is 10–15%, more common in males than females. Patients complain of low back pain, with examination revealing a stiff lumbar region, loss of lordosis, grade I kyphosis, and a posture resembling ankylosing spondylitis. The spine progressively stiffens, with disc degeneration, narrowing, calcification, marginal osteophyte formation, and involvement of intervertebral ligaments, eventually leading to bony ankylosis. The tissues of the limb joints also undergo degenerative changes due to pigment deposition, with loss of elasticity in the articular cartilage, fibrosis and thickening of the synovial membrane, subchondral bone erosion and cystic changes, along with bone sclerosis and osteophyte formation, which can result in joint ankylosis. The synovial membrane exhibits villous hyperplasia and pigmentation, and the joint fluid contains pigmented cartilage debris.

X-ray findings have distinct characteristics, sometimes appearing before clinical symptoms. X-rays reveal spinal osteoporosis, calcification and narrowing of intervertebral discs. Joint spaces in the knees, shoulders, and hips become narrowed, with subchondral bone sclerosis, cyst formation, calcification of the meniscus, osteochondral loose bodies, marginal osteophytes, and tendon calcification. Unlike rheumatoid arthritis, ochronosis typically does not involve the small joints of the hands and feet.

bubble_chart Treatment Measures

There is no specific effective treatment for this disease. The degenerative changes are similar to those in hypertrophic arthritis. Patients should engage in appropriate activities but avoid increasing joint load. If the patient experiences severe joint pain and significant impairment of motor function, joint arthroplasty may be considered. Theoretically, vitamin C should be avoided as it interferes with the oxidation and polymerization of homogentisic acid, potentially inducing ochronosis. Foods rich in vitamin C should be used with caution.