Diffuse Idiopathic Skeletal Hyperostosis

Diffuse idiopathic skeletal hyperostosis (DISH), also known as ankylosing hyperostosis or Forestier disease, is not an arthropathy. The articular cartilage, adjacent bone margins, and synovium are not affected. DISH appears to be a bone-forming diathesis in which ossification occurs at skeletal sites subjected to stress, primarily at tendinous and ligamentous attachments. It is a common disorder, occurring in 12 percent of the elderly population. Its radiographic manifestations have been mistaken for ankylosing spondylitis, other spondyloarthropathies, and osteoarthritis. Although it may coexist with an arthropathy, it should not be mistaken for a manifestation of that arthropathy. Knowledge of the radiographic criteria allows the correct diagnosis to be made. The radiographic findings are the following:

1.

Normal mineralization
2.

Flowing ossification of at least four contiguous vertebral bodies
3.

Preservation of disc spaces
4.

Ossification of multiple tendinous and ligamentous sites in the appendicular skeleton
5.

Absence of joint abnormality
6.

Sporadic distribution
7.

Distribution primarily in the spine

The radiographic manifestations of DISH divide into those associated with the spine and those that are extraspinal. Extraspinal changes without spinal involvement are possible but extremely unusual.

Spinal manifestations

Ossification of the ligaments and soft tissues that surround the vertebral bodies occurs in DISH. This must be observed around four or more contiguous vertebral bodies in order to make the diagnosis of DISH. The thickness of the ossification can range from 1 to 20 mm. Bone excrescences of various shapes may be observed. The ossification may be so extensive as to render the spine as immobile as one with ankylosing spondylitis. Such a spine can fracture and develop a pseudoarthrosis similar to that which can occur in ankylosing spondylitis. Fracture may occur with relatively minor trauma, and these injuries can be easily overlooked ( Fig. 15-1 ).

The Thoracic Spine

The thoracic spine is the most common site of involvement. Flowing ossification is observed here in 97 to 100 percent of patients with DISH. It is usually seen anteriorly and on the right side in the lower thoracic spine, from T7 to T11 ( Fig. 15-2 ). The thickness of the ossification can range from 1 to 20 mm. It may be smooth or bumpy in contour, depending upon the configuration of the bone excrescences at the disc levels. The radiolucent disc may appear to protrude into the flowing ossification, creating an L-, T-, or Y-shaped defect at the disc level ( Fig. 15-3 ).

If the flowing ossification is thin and smooth, it may be mistaken for the ossification seen in ankylosing spondylitis ( Fig. 15-4 ). Usually at some point a lucent line separates the flowing ossification from the adjacent vertebral body and thus distinguishes DISH from ankylosing spondylitis ( Fig. 15-5 ).

The Cervical Spine

The cervical spine is involved in 78 percent of patients with DISH. The abnormalities seen are most common in the lower cervical region. The flowing ossification anteriorly can vary from 1 to 12 mm in thickness. It may be very smooth in contour and appear to be an extension of the anterior border of the vertebral body ( Fig. 15-6 ), or it may be very bumpy, with the bumps occurring at the disc levels ( Fig. 15-7 ). The ossification may impinge upon the esophagus, causing dysphagia ( Fig. 15-8 ). The disc heights are preserved, and the apophyseal joints are uninvolved. In some patients the posterior longitudinal ligament may be ossified, creating spinal stenosis.