A 26-year-old man presents to his primary care physician with a 6-month history of increasing lower back pain. He first felt the pain while lifting boxes at work but thought that he had just strained a muscle. The pain appears to be worse in the mornings and after rest. Exercise and physical activity appear to temporarily make the pain better. He has taken acetaminophen and ibuprofen for the pain. He has no significant medical history and takes no medications. His temperature is 98.6°F (37.0°C), blood pressure is 115/70 mmHg, pulse is 70/min, and respirations are 12/min. On physical exam, he is found to have limited flexion and extension of his spine as well as tenderness to palpation over the insertion of his patellar tendons bilaterally. Results from laboratory tests are shown below:

Serum:
Erythrocyte sedimentation rate (ESR): 61 mm/hr
C-reactive protein (CRP): 36 mg/L (normal <10 mg/L)

Which of the following is the most accurate test for this patient's condition?
Magnetic resonance imaging
Bone scan
Ultrasound
Computed tomography
Magnetic resonance imaging
This young man presents with low back pain that is relieved by physical activity, decreased spinal mobility, enthesitis of his patellar tendons, and elevated ESR/CRP values most likely has ankylosing spondylitis (AS). The most accurate diagnostic test for early ankylosing spondylitis is MRI.

Multiple imaging tests can diagnose ankylosing spondylitis. Radiographs of the spine will show squaring of vertebrae with vertical or marginal syndesmophytes resulting in a "bamboo spine" appearance. The earliest radiographic sign is the erosion of the iliac side of the sacroiliac joint, but this finding is not sensitive in the early stages of the disease. MRI is sensitive in detecting sacroiliac inflammation, which makes this the best modality for early detection of ankylosing spondylitis in young patients. Initial therapy for ankylosing spondylitis includes lifestyle modifications (exercise, smoking cessation, physical therapy) and non-steroidal anti-inflammatory drugs (NSAIDs). Therapy for patients with inadequate response to NSAIDs includes tumor necrosis factor inhibitors (etanercept, infliximab, adalimumab) and interleukin 17 inhibitors (secukinumab, ixekizumab).

Jung et al. studied whether signs of inflammation on MRI are correlated with bone quality in patients with ankylosing spondylitis. The authors find that bone marrow edema is negatively correlated with the trabecular bone score. The authors recommend control of active bone inflammation to prevent osteoporosis in patients with ankylosing spondylitis.

Incorrect Answers:
Answer A: Bone scan will show inflammation in the sacroiliac joints, but this study lacks specificity for this disease compared with other inflammatory forms of arthritis.

Answer B: CT will show bony changes but not active inflammation, so it is most commonly used to diagnose cervical fractures in patients with ankylosing spondylitis. While it can support the diagnosis, it is less accurate than an MRI.

Answer D: Radiographs will show erosion of the iliac side of the sacroiliac joint, but this finding is not sensitive in the early stages of the disease when bony ankylosis has not yet occurred. Radiographs may be an appropriate initial test.

Answer E: Ultrasound is useful in evaluating muscle strains or tendon tears; however, it is not able to detect bony abnormalities and is therefore of limited use in ankylosing spondylitis.

Bullet Summary:
MRI is the most sensitive and accurate test for the early detection of ankylosing spondylitis.