A 27-year-old man presents with worsening low back pain over the past 5 months. He reports that the pain is most severe in the morning and describes a “back stiffness” when getting out of bed. He reports moderate improvement with walking. The patient denies any systemic symptoms and reports an insignificant past medical history. On physical examination, the patient is afebrile with normal vital signs. On musculoskeletal examination, there is marked tenderness over the sacroiliac joints with limited range of motion on forward flexion of the lower back.
Of the following tests, which is the best test for the likely diagnosis in this patient?
A. MRI of the spinePlain film x-ray of the sacroiliac joints. The patient in this question is presenting with signs and symptoms consistent with ankylosing spondylitis (AS). AS is one of the seronegative spondyloarthropathies, which are joint diseases affecting the vertebral column that are characterized by a negative rheumatoid factor (RF) and ANA. AS is more common in men, typically occurs in the second and third decades of life, and is characterized by progressive low back pain and stiffness lasting at least 3 months in duration. Morning stiffness and improvement with exercise are also common with this condition. In order to confirm the diagnosis, anteroposterior (AP) x-ray of the sacroiliac (SI) joints is performed. Fusion of the SI joints (the presence of a “bamboo spine”) is diagnostic and a specific indicator for the diagnosis of AS.
(A) MRI of the spine would only be necessary if the x-ray of the lumbar spine returns negative and AS is clinically suggested. (B) Although patients with AS have an increased incidence of positivity for HLA-B27 (90% of patients), it is not a specific test because only a minority of patients demonstrating positivity for HLA-B27 actually has AS. (D) AS is a seronegative spondyloarthropathy, so by definition it is negative for RF levels.