What is the recommended diagnostic test and treatment for sacroilitis?

MRI is the Gold Standard for Diagnosing Sacroiliitis

For suspected sacroiliitis, MRI of the sacroiliac joints is the recommended diagnostic test due to its superior ability to detect both active inflammation and structural changes before they appear on conventional radiographs. 1

Diagnostic Algorithm for Sacroiliitis

Initial Imaging

1. Conventional radiography (X-ray) of sacroiliac joints

   • First-line imaging modality for initial evaluation 1
   • Rated 9/9 for appropriateness in initial evaluation 1
   • Limitations: Low sensitivity (19%-72%) for early disease 1
   • Can detect chronic changes: erosions, sclerosis, and ankylosis

2. If radiographs are negative or equivocal but clinical suspicion remains high:

   • Proceed to MRI of sacroiliac joints (rated 8/9 for appropriateness) 1
   • MRI can detect inflammatory changes 3-7 years before they appear on radiographs 1

MRI Protocol for Sacroiliitis

• Required sequences:

  • T1-weighted sequences
  • Fat-suppressed fluid-sensitive sequences (T2-weighted fat-suppressed or STIR) 1
  • Intravenous gadolinium contrast is optional and generally not needed 1

• MRI findings to look for:

  • Active inflammatory lesions:

    • Bone marrow edema (most important finding)
    • Synovitis
    • Capsulitis
    • Enthesitis 2, 3

  • Structural (chronic) lesions:

    • Erosions
    • Subchondral sclerosis
    • Fat infiltration
    • Transarticular bone buds and bridges
    • Ankylosis 2

Alternative Imaging When MRI Cannot Be Performed

• CT of sacroiliac joints without contrast (rated 7/9 for appropriateness) 1
  • Better than X-ray for detecting subtle erosions and reparative changes
  • Useful for evaluating complex anatomy of SI joints
  • Limitation: Cannot detect active inflammation 1

Imaging Not Recommended for Routine Diagnosis

• Bone scintigraphy (including SPECT)
• FDG-PET/CT
• Ultrasound of sacroiliac joints 1

Treatment Approach for Sacroiliitis

Treatment depends on the underlying cause, with axial spondyloarthritis being the most common etiology:

1. Non-pharmacological interventions:

   • Physical therapy focusing on posture and spinal mobility
   • Regular exercise program

2. Pharmacological treatment:

   • First-line: NSAIDs at full anti-inflammatory doses
   • Second-line (if inadequate response to NSAIDs): Biological DMARDs
     • TNF-α inhibitors (adalimumab, etanercept, infliximab)
     • IL-17 inhibitors (secukinumab, ixekizumab)

3. Treatment monitoring:

   • MRI can be used to monitor disease activity and treatment response 1
   • STIR sequences are sufficient to detect inflammation; contrast medium not needed 1

Clinical Pearls and Pitfalls

• Key pitfall: Relying solely on radiographs can miss early sacroiliitis, as radiographic changes often lag behind clinical symptoms by 7+ years 1, 4

• Important caveat: Bone marrow edema on MRI, while sensitive for sacroiliitis, can also be seen in non-inflammatory conditions including:

  • Age-related degenerative changes
  • Postpartum patients
  • Chronic back pain from other causes
  • Athletes
  • Up to 30% of healthy controls 1

• Diagnostic confidence: When bone marrow edema is seen concomitant with synovial enhancement or erosion, the diagnosis of sacroiliitis is much more likely 3

• Monitoring frequency: If using radiographs to monitor structural changes, they should not be repeated more frequently than every two years 1