MRI demonstration of arachnoiditis in cauda equina syndrome of ankylosing spondylitis.

The cauda equina syndrome is a rare but well-recognised complication of longstanding ankylosing spondylitis, usually presenting when the joint disease is quiescent. The clinical and radiological findings in a patient with only a 3-year history, in whom the onset of joint and neurological symptoms was apparently simultaneous, are presented. MRI revealed characteristic expansion of the lumbar spinal canal with scalloping of the pedicles, laminae and spinous processes, related to numerous posterior dural diverticula. The quantity and extent of such diverticula are unusual. We demonstrated adherence of individual nerve roots to the arachnoid surface of these diverticula and to each other. In a second patient, with a much longer history of both ankylosing spondylitis and cauda equina syndrome, MRI again showed florid, multilocular dural ectasia, marked irregularity and thickening of nerves, and adherence to the dural diverticula. These cases provide evidence for the role of arachnoiditis in the pathogenesis of the cauda equina syndrome of ankylosing spondylitis.