Piriformis syndrome

Piriformis syndrome is a rare entrapment neuropathy resulting in radicular pain radiating into the buttock and hamstrings. The entity is controversial as are the putative causes.

Clinical presentation

Sciatic pain reproduced on passive internal rotation/adduction of a flexed hip is considered suggestive of the syndrome. Typically, no neurological deficit will be elicited on examination as well as a negative straight leg raise test ⁷. 

Pathology

The general, although by no means unanimous consensus is that piriformis syndrome is caused when hypertrophy, inflammation, injury or anatomical variation of the piriformis muscle results in compression of the sciatic nerve as they both exit the pelvis through the greater sciatic notch.

Normally, the sciatic nerve typically passes immediately anterior to the piriformis muscle. This relationship is variable, however, as the nerve occasionally passes through the muscle, or splits early, with part of it passing through the muscle. 

Additionally, an accessory piriformis muscle that arises from the more medial part of the sacrum can be implicated.

Aetiology

• early sciatic nerve division and passage through the muscle belly or above it
• narrowed sciatic foramen
  • accessory piriformis muscle
  • muscle hypertrophy
  • lumbar lordosis
  • ganglion or cyst 
  • any mass lesion 
• spasm
  • athletes
  • cerebral palsy
• bursitis and inflammation

Radiographic features

MRI

MRI is the only modality that can adequately visualise the region. The radiographic appearance depends on the cause, and often no abnormality is noted. If muscle injury or inflammation is present then increased signal within the piriformis muscle may be seen on T2 MRI. An accessory piriformis muscle may be identified.

Treatment

• physiotherapy with stretching (especially if a spasm is thought to be the cause)
• administration of non-steroidal anti-inflammatory agents
• injection of local anaesthetic and steroid
• injection of botulinum toxin to atrophy the piriformis muscle
• surgical release of the piriformis muscle
• neurolysis

History and etymology

Entrapment of the sciatic nerve by the piriformis muscle was first described By W Yeoman in 1928 ¹.

Differential diagnosis

Clinically it should be distinguished from hamstring syndrome.