Piriformis Syndrome on MRI & X-ray: Imaging Findings

Piriformis syndrome is a clinical diagnosis of exclusion, requiring that lumbar spine pathology be ruled out as the source of sciatic nerve irritation. MRI of the lumbar spine excludes disc herniation, foraminal stenosis, and intrapelvic mass compressing the sacral plexus. If lumbar MRI is normal, MRI of the pelvis with dedicated coronal and axial sequences through the greater sciatic notch can identify piriformis muscle hypertrophy, asymmetry, anomalous fiber bundles splitting around the sciatic nerve, myositis, hematoma, or a bifid piriformis — structural variants that predispose to nerve entrapment. Electrodiagnostic studies (EMG and nerve conduction velocity) can corroborate sciatic nerve dysfunction at the piriformis level, although normal studies do not exclude the diagnosis. Diagnostic injection of local anesthetic and corticosteroid into the piriformis muscle under CT or ultrasound guidance provides both diagnostic confirmation and therapeutic benefit.

Beaton and Anson classified the relationship between the piriformis and sciatic nerve into six types. Type A (approximately 84% of individuals) has the undivided sciatic nerve exiting below an undivided piriformis — the standard anatomy. In type B, the peroneal division of the sciatic nerve passes through the piriformis while the tibial division exits below it; this affects roughly 12% of individuals and is the most common variant predisposing to piriformis syndrome. Rarer types involve the entire nerve passing through the muscle or the peroneal division above it. In these variants, contraction, spasm, or hypertrophy of the piriformis directly compresses the intramuscular sciatic nerve component. MRI with high-resolution sequences through the sciatic notch can occasionally delineate these relationships and explain why certain patients have atypical distributions of sensory loss or refractory symptoms.

When physical therapy targeting piriformis stretching, hip rotator strengthening, and lumbopelvic stabilization fails to resolve symptoms after 6 to 8 weeks, image-guided injection of the piriformis muscle is the next intervention. Ultrasound or CT guidance is used to inject a combination of local anesthetic and corticosteroid directly into the muscle belly, with reported short-term response rates of 60–80%. Botulinum toxin type A injection is an alternative for patients with frequent recurrence, providing longer-lasting neuromuscular blockade and muscle relaxation over 3 to 4 months. For truly refractory cases with confirmed anatomical nerve entrapment — particularly in patients with a type B or rarer sciatic nerve variant demonstrated on imaging — surgical decompression via open or endoscopic piriformis release, releasing the fibromuscular constriction around the nerve, is a viable option with favorable outcomes in carefully selected patients.