Musculoskeletal ImagingAlkaptonuria
a rare hereditary metabolic disorder resulting from an inability to metabolize homogentisic acid due to absence of the enzyme homogentisic acid oxidase. Homogentisic acid in the urine is oxidized to a melanin-like product, which makes urine gradually turn dark.
Ochronosis refers to the bluish-black pigmentation of connective tissue that may ensue, and ochronotic arthropathy results from the pigmented deposits in the joints of the appendicular and axial skeleton.
Ochronotic arthropathy
The most characteristic manifestations of ochronotic arthropathy are widespread discal calcification, with loss of intervertebral disc height, and a distinctive arthropathy of axial and extra-axial joints (Table 1). Initial clinical manifestations may be seen in the hips, knees, and shoulders, with pain and limitation of motion. Joint effusions result from fragmentation of friable cartilage, with subsequent irritation of the synovial membrane. Stiffness and low back pain, obliteration of the normal lumbar curve, thoracic kyphosis, and restriction of motion are spinal manifestations of the disease. Elderly persons with alkaptonuria may be completely disabled.
Radiography reveals discal calcification, which is the most characteristic abnormality of the spine. The calcification consists of apatite crystals and occurs predominantly in the inner fibers of the anulus fibrosus. Narrowing of the intervertebral disc space and vacuum phenomenon are also characteristic manifestations of alkaptonuria (Fig.1). Involvement of extraspinal sites may lead to a radiographic appearance similar to that of degenerative joint disease. Certain features of ochronotic arthropathy usually permit its identification in these persons, however: involvement of unusual articular sites, unusual patterns of joint space loss, and severe abnormalities with extreme sclerosis, fragmentation and intra-articular cartilaginous and osseous bodies. At the symphysis pubis, joint space narrowing, calcification, bony eburnation and fragmentation may be seen. Joint space narrowing, sclerosis and osteophytosis can also be observed at the sacroiliac joints. The knee is the most common extraspinal site of abnormality, including articular space narrowing and subchondral sclerosis.
Alkaptonuria, Table 1. Diagnostic features of ochrontic arthropathy
| Spinal abnormalities | |
| Osteoporosis of vertebral bodies |
| Calcification and ossification of intervertebral discs |
| Disc space narrowing with vacuum phenomena |
| Small or absent osteophytes |
| Loss of lumbar lordosis |
| Extraspinal abnormalities | |
| Involvement of sacroiliac joints, symphysis pubis and large peripheral joints |
| Joint space narrowing |
| Bone sclerosis |
| Collapse and fragmentation with intra-articular osseous bodies |
| Small or absent osteophytes |
| Tendinous calcification, ossification and rupture |
| Unusual involvement of hands, wrists, feet, elbows and ankles |
DR/RB
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a, b. Frontal (a) and lateral (b) radiographs of the lumbar spine and pelvis demonstrate severe osteoporosis and widespread discal calcifications with disc space loss at multiple levels. Also note the sclerosis involving the sacroiliac joints bilaterally and ossification in the acetabular labra of the hips.
c. Frontal radiograph of the ear in the same patient demonstrates calcifications within the soft tissues.
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Alkaptonuria, Fig.1 (a) | | Alkaptonuria, Fig.1 (b) | | Alkaptonuria, Fig.1 (c) |