Test your medicine knowledge with the MKSAP challenge, in partnership with the American College of Physicians.

A 20-year-old man is evaluated for a 6-month history of low back pain accompanied by prolonged morning stiffness. His symptoms improve over the course of the day, but he is now unable to play recreational soccer. Rest, physical therapy, and acupuncture have not improved his symptoms. Use of ibuprofen or diclofenac provides only partial relief. He has no other pertinent medical history and takes no additional medications.

On physical examination, vital signs are normal. There is loss of normal lumbar lordosis, and flexion of the lumbar spine is decreased. The low back and pelvis are tender to palpation. Pain increases when the patient crosses his legs. Reflexes and muscle strength are intact.

Radiographs of the lumbar spine and sacroiliac joints are normal.

Which of the following studies is most likely to establish the diagnosis in this patient?

A: Bone scan
B: CT of the sacroiliac joints
C: MRI of the lumbar spine
D: MRI of the sacroiliac joints

MKSAP Answer and Critique

The correct answer is D: MRI of the sacroiliac joints. This item is available to MKSAP 16 subscribers as item 41 in the Rheumatology section.

MKSAP 16 released Part A on July 31. More information is available online.

This patient most likely has ankylosing spondylitis, and MRI of the sacroiliac joints is most likely to establish a diagnosis. Radiographic evidence of sacroiliitis is required for definitive diagnosis and is the most consistent finding associated with this condition. Onset of ankylosing spondylitis usually occurs in the teenage years or 20s and manifests as persistent pain and morning stiffness involving the low back that is alleviated with activity. This condition also may be associated with tenderness of the pelvis.

Typically, the earliest radiographic changes in affected patients involve the sacroiliac joints, but these changes may not be visible during the first few years from onset; therefore, this patient’s normal radiographs of the sacroiliac joints do not exclude sacroiliitis. MRI findings of the sacroiliac joints can include bone marrow edema, synovitis, and erosions. Bone marrow edema is the earliest finding and can precede the development of erosions. MRI, especially with gadolinium enhancement, is considered a sensitive method for detecting early erosive inflammatory changes in the sacroiliac joints and spine and can assess sites of active disease and response to effective therapy.

Bone scan can demonstrate increased uptake of the sacroiliac joints in patients with ankylosing spondylitis but is less sensitive and specific than MRI.

CT is the most sensitive modality available to demonstrate bone changes such as erosions; however, it cannot detect early changes such as bone marrow edema that precede erosive change in patients with ankylosing spondylitis.

In the diagnosis of early ankylosing spondylitis, sacroiliac joint MRI is more sensitive than lumbar spine MRI. Although changes to the lumbar spine can be detected on MRI, they are usually preceded by changes in the sacroiliac joints. Therefore, if imaging of the lumbar spine is negative, subsequent imaging of the sacroiliac joints would still be necessary to exclude ankylosing spondylitis.

Key Point

• MRI is considered the most sensitive method for detecting early erosive inflammatory changes in the sacroiliac joints when radiographs are normal.

Learn more about ACP’s MKSAP 16.

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