What is the likely diagnosis for an adult patient with a history of physical activity, who experienced a slight pop in their back after lifting a lamp, followed by severe pain starting in the sacroiliac (SI) joint and becoming bilateral and cramping the next day?

Likely Diagnosis: Acute Sacroiliac Joint Dysfunction with Possible Muscle Spasm

This presentation is most consistent with acute sacroiliac (SI) joint dysfunction triggered by mechanical strain, with secondary bilateral paraspinal muscle spasm causing the cramping sensation. The delayed onset of severe pain (next morning) and progression from unilateral SI joint pain to bilateral cramping strongly suggests an initial SI joint injury followed by reactive muscle guarding 1, 2.

Clinical Reasoning

Why SI Joint Dysfunction is Most Likely

• The mechanism of injury fits perfectly: A "pop" sensation during lifting is a classic presentation for acute SI joint disruption, as the joint is vulnerable to rotational and shear forces during asymmetric loading 3, 1
• Delayed pain onset is characteristic: SI joint injuries commonly present with minimal immediate pain, followed by severe pain hours later (typically upon waking) as inflammation develops and muscles stiffen overnight 1, 2
• Pain location and pattern: Starting in the SI joint itself, then becoming bilateral and cramping, indicates the initial injury was to the SI joint with subsequent protective muscle spasm in the paraspinal and gluteal muscles 4, 2

The Bilateral Cramping Component

• Muscle spasm is a secondary phenomenon: When the SI joint is injured, the surrounding musculature (paraspinal muscles, gluteals, piriformis) reflexively contracts to splint and protect the injured joint, causing the bilateral cramping sensation 1
• This is not inflammatory back pain: The acute traumatic onset and lack of inflammatory features (no morning stiffness >30 minutes, no improvement with exercise, no night pain pattern) effectively rules out axial spondyloarthropathy 5, 6

Diagnostic Approach

Immediate Clinical Assessment

• Perform SI joint provocative maneuvers: At least 3 of 5 tests must be positive to suggest SI joint as pain source (FABER test, Gaenslen's test, thigh thrust, compression test, distraction test) - this has the highest diagnostic value 1, 2
• Assess for red flags: Rule out fracture, infection, or cauda equina syndrome through neurological examination and assessment of constitutional symptoms 5
• Pain pattern evaluation: SI joint pain typically localizes to the posterior superior iliac spine region, may radiate to buttock/posterior thigh, but does NOT follow dermatomal distribution 3, 2

Imaging Strategy

• Plain radiographs are appropriate initially: X-ray of pelvis/SI joints to rule out fracture, dislocation, or significant degenerative changes 6
• MRI is NOT indicated acutely unless red flags are present or symptoms persist beyond 6 weeks despite conservative treatment 5, 6
• Diagnostic SI joint injection can be considered if diagnosis remains uncertain after 4-6 weeks of conservative treatment, though this is controversial for acute presentations 7, 4

Treatment Algorithm

First-Line Conservative Management (Weeks 1-6)

• NSAIDs at therapeutic doses: Ibuprofen 600-800mg TID or naproxen 500mg BID for anti-inflammatory effect and pain control 4, 1
• Relative rest with early mobilization: Avoid aggravating activities (lifting, twisting) for 1-2 weeks, but maintain gentle movement to prevent stiffness 1
• Ice application: 15-20 minutes every 2-3 hours for first 48-72 hours to reduce inflammation 1
• Physical therapy referral: Pelvic stabilization exercises, focused stretching, and manual therapy should begin within first 1-2 weeks 4, 1

Second-Line Interventions (If No Improvement by 6 Weeks)

• Intra-articular corticosteroid injection: Can provide pain relief for >3 months in some patients when conservative measures fail 4, 1
• Consider pelvic belt: May provide symptomatic relief by stabilizing the SI joint, though evidence is limited primarily to postpartum patients 1

Third-Line Options (If Symptoms Persist Beyond 3 Months)

• Cooled radiofrequency ablation: Targets L5 dorsal ramus and S1-3 lateral branches, showing strongest evidence for chronic SI joint pain 4, 1
• SI joint fusion: Reserved for refractory cases after all conservative and interventional options have failed 8

Critical Pitfalls to Avoid

• Do not assume inflammatory arthropathy: The acute traumatic mechanism and lack of inflammatory features make axial spondyloarthropathy extremely unlikely in this presentation 7, 5
• Do not over-image: MRI is not indicated for acute mechanical SI joint dysfunction without red flags 5, 6
• Do not delay physical therapy: Early mobilization and stabilization exercises are crucial to prevent chronic dysfunction 4, 1
• Do not rely on imaging alone: SI joint dysfunction is a clinical diagnosis - imaging findings (even on MRI) do not correlate well with symptoms 7, 3