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The boy had back pain for more than a year. Neither physical therapy nor anti-inflammatory drugs had helped. He had no significant history of injury and no family history of lower back pain. He had no skin rashes, colitis, or other complaints. He previously had an MRI of his lumbar spine, which appeared normal, and a normal CT scan of his pelvis,

When he presented to a neurosurgeon, he had lower back pain and significant limitation of movement in his lumbar spine. An MRI of the sacroiliac revealed significant inflammation on both sides of the joint (see images), compatible with sacroiliitis, a finding highly suggestive of ankylosing spondylitis (AS). He was referred to Dr. Norman B. Gaylis, a rheumatologist in Miami.

Patients withAS tend to present with sacroiliitis, especially when they are human leukocyte antigen B27-positive. “Many patients with back pain have their diagnosis missed,” he said, because either physicians don't think of AS or the x-ray is negative. If an MRI is done, it is typically on the lumbar spine.

The differential diagnosis for lower back pain includes disk damage, trauma, or muscle strain, but sometimes patients are dismissed as imagining the pain.

Historically, diagnosis of AS has depended on the radiographic finding of sacroiliitis. “The findings on MRI [for AS], similar to those for rheumatoid arthritis, occur far earlier than you will see on x-ray,” said Dr. Gaylis. MRI findings of AS—synovitis and intense enhancing bone marrow edema/osteitis, as in this case—can precede x-ray evidence by 3 years (J. Rheumatol. 1999;26:1953–8).

MRIs show synovitis and intense enhancing bone marrow edema/osteitis.