Muscle disease as a cause of kyphotic deformity in ankylosing spondylitis.

The effects of ankylosing spondylitis on skeletal muscle were investigated in nine consecutive patients referred for correction of severe spinal deformity. Enzymatic studies (creatinine phosphokinase, aldolose), electromyography, and paraspinal muscle biopsy were performed. The enzyme studies and electromyography yielded only variable results, but muscle biopsy uniformly demonstrated evidence of severe skeletal muscle disease. Small, scattered, sharp angular fibers were present in all specimens along with atrophy of Type I and Type II muscle fibers. Core or targetoid fibers were present in all but one patient. These findings suggest that muscle disease may be present in all ankylosing spondylitis patients with spinal flexion deformity.