The Anklyosed Spine: Fractures in DISH and Anklyosing Spondylitis

The Anklyosed Spine: Fractures in DISH and Anklyosing Spondylitis Lee F. Rogers, MD ACCR, Oct 26, 2012 Dallas, Texas Diffuse idiopathic skeletal hyperostosis (DISH) and ankylosing spondylitis (AS) are the most common diseases associated with spinal ankylosis. While they share the characteristic of ankylosis they are, in fact two separate diseases with distinct clinical, pathologic, and radiologic features. DISH: Diffuse idiopathic skeletal hyperostosis is a disease of older persons characterized by extensive ossification of the paraspinal ligaments anteriorly and laterally, bridging the intervening disc spaces. Bony bridging may be continuous or discontinuous. The anterior cortex of the vertebral body can be seen within the ossification. These findings are much more pronounced in the thoracic and lower cervical spine than in the lumbar area. Minor expressions of this disorder are commonly encountered in the mid-dorsal spine on lateral views of the chest. DISH characteristically spares the SI joints. The SI joints remain patent and clearly visible on radiographs or CT even in the presence of extensive disease in the thoracolumbar and cervical spine. DISH is also characterized by enthesopathy; ossification of ligaments or tendon insertions, forming so-called entheses. These appear as whiskering of the iliac crest, ischial tuberosites and greater trochanters. The radiographic appearance bears a superficial resemblance to that of AS. In DISH the spinal ossification is very irregular and unlike the thin, vertical syndesmophytes see in AS. The relative absence of changes in the lumbosacral spine, the patency of the SI joints, and absence of ankylosis of the facet and costovertebral joints should allow differentiation of DISH from AS. Ankylosing spondylitis: AS is typically a disease of young adult men, with a male-female ratio of about 15:1. The earliest clinical manifestation is usually persistent low back pain of insidious onset. The disease involves the SI joints and then the spine, progressively from the lumbar to the cervical spine. All patients have SI joint involvement. The absence of SI joint disease rules out this diagnosis. SI joint disease is bilateral and symmetrical eventually leading to sclerosis and 1 complete obliteration of the joint. Early changes are best seen by MRI. The initial radiographic manifestation of spinal involvement is squaring of the lower lumbar vertebral bodies as seen on the lateral view; the superior and inferior corners of the vertebrae are almost square, without the normal concavity. Later, syndesmophytes form from ossification of the outer layers of the annulus fibrosis of the intervertebral disk. When extensive, they result in the typical “bamboo” spine appearance so characteristic of this disease. The apophyseal, facet, and costovertebral joints also ankylose. As the joints fuse, the spine and pelvis become osteoporotic and susceptible to fractures, particularly through or in juxtaposition to the disc space as well as the posterior elements. Dislocations and spinal cord injuries are frequent. In 30% of patients the proximal peripheral joints, the hip, shoulders, and knees are involved. Uveitis, aortic valvular disease with aortic insufficiency, pulmonary fibrosis of the upper lobes (commonly mistaken for tuberculosis), and other cardiac and renal diseases are associated with ankylosing spondylitis. The rheumatoid factor is negative but histocompatibility antigen HLA-B27 is frequent. Differential diagnosis: Extensive, advanced cases of DISH in the elderly are frequently misinterpreted and diagnosed as ankylosing spondylitis. While this confusion is understandable, the differentiation of DISH from ankylosing spondylitis can be readily established by findings clearly depicted on radiographs and computed tomography. In well-developed cases of ankylosing spondylitis, fusion of the sacroiliac joints will be obvious as seen on radiographs and CTs of the abdomen and pelvis; whereas the SI joints will be patent in DISH. Look for prior radiographs or CTs of the abdomen and pelvis to make this determination. The facet joints and costovertebral joints are fused in ankylosing spondylitis, while they remain unfused and patent in DISH. This is easily seen on a CT obtained to evaluate a potential or known spinal fracture in an anklyosed spine. Fractures of the anklyosed spine: Individuals afflicted with anklyosed spines as a consequence of DISH (diffuse idiopathic skeletal hyperostosis) or ankylosing spondylitis are highly susceptible to spinal fractures and attendant neurologic injury. Spinal fractures with neurologic deficits commonly arise from simple, low impact trauma; falls from a standing height as in trip, stumble, and fall; falls in the bathroom, or falls when standing on a stool or climbing a step ladder. The majority of these injuries are readily apparent on radiographs and CT 2 due to obvious dislocations of the vertebrae. However, a significant sample sustains minimally displaced or virtually undisplaced fractures within or immediately adjacent to the intervertebral disc that may be difficult to recognize or are completely overlooked if one is not fully informed about the potential for and the need to look specifically for these subtle fractures. Fractures of the spine may result from even the most trivial trauma; the patient may have no memory of the event. Any patient with an anklyosed spine who develops back pain or whose pattern of pain has worsened or changed should be suspected of an underlying spinal fracture until proven otherwise. A radiographic examination of the spine should be obtained and a CT examination considered even if the radiographs are unrevealing. 3 .

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