Abstract

Diffuse idiopathic skeletal hyperostosis (DISH) and ankylosing spondylitis (AS) share the involvement of the axial skeleton and the peripheral entheses. Both diseases produce bone proliferations in the later phases of their course. Although the aspect of these bone proliferations is dissimilar, confusion of radiologic differential diagnosis between the two diseases exists mostly as a consequence of a lack of awareness of their characteristic clinical and radiographic features. The radiologic spinal findings are so different that it is possible to recognize the changes due to each disease even in patients in whom both diseases occur.

On radiographs DISH is characterized by “flowing mantles” of ossifications occurring in the anterior longitudinal ligament and to a lesser extent in the paravertebral connective tissue and the peripheral part of the annulus fibrosus. Frequently, on lateral view radiolucency is noted between the new bone and the vertebral body. DISH may also involve the sacroiliac joints. The ossification of the joint capsule on the anterior surface of the joint can resemble the fusion of the sacroiliac joints on anteroposterior pelvis x-rays that may mistakenly be interpreted as post-inflammatory ankylosis of the joint (grade 4 sacroiliitis). Computed tomography (CT) can be helpful in these cases by demonstrating the normal aspect of the joint space and bony margins together with the presence of the anterior capsular ossification.

On x-rays AS is characterized by the bone damage caused by sacroiliitis and by inflammation at the discovertebral junction at the attachment of the annulus fibrosus. The consequent close subchondral osteitis is radiologically characterized by a destructive vertebral lesion and sclerosis confined to the anterior corners of the vertebral bodies (“Romanus lesion”). The healing of these lesions and the adjacent periosteal reaction results in “squaring” of the vertebral bodies on lateral view of the spine. Simultaneously, the healing process proceeds in the periphery of the annulus fibrosus resulting in the formation of syndesmophytes that are vertical bony bridges joining adjacent vertebral bodies anteriorly and laterally and ultimately forming a “bamboo-spine”.

The confusion between DISH and AS may also extend to the clinical field because patients with DISH can occasionally have severe limitation of spinal mobility, along with postural abnormalities that resemble long-standing advanced AS.