Piriformis syndrome is a controversial entity characterized by low back and buttock pain referred to the leg, which stems from the piriformis muscle or its effect on the sciatic nerve. The piriformis is a hip external rotator and abductor that sits in the greater sciatic foramen. Importantly, the sciatic nerve exits the pelvis inferior to the piriformis in 90% of cadavers, but sometimes exits superior to or through the piriformis, and may be divided into the tibial and fibular divisions either before or after it exits the pelvis.  Some studies purport that piriformis syndrome comprises up to 6%–8% of cases of low backpain/sciatica in the United States.  Piriformis syndrome can arise from trauma, hypertrophy, or spasm of the piriformis muscles or its effect on the sciatic nerve. There is no universally agreed upon diagnostic criteria for piriformis syndrome.  Hence, it is frequently a diagnosis of exclusion, and believed to be both over and underdiagnosed.  

The most common clinical findings—buttock pain and pain to palpation over the greater sciatic notch—were found in all cases of piriformis syndrome in one review.  Most patients have a history of minor trauma, including falls or a stretch during athletic activity.

Radiation into the leg is a common feature as well. Aggravating factors include sitting and rising from a seated position. Pain to external deep palpation of the sciatic notch is a near-universal feature, and some studies have noted a high sensitivity of reproduction of symptoms with internal palpation during a rectal or pelvic examination.  

The most accurate test is the active piriformis test (78% sensitive, 80% specific), during which the patient, in the side-lying position, abducts and externally rotates the hip against resistance, reproducing pain.

Imaging of the pelvis is not always helpful in confirming piriformis syndrome, as anatomic variants of the piriformis and its relationship with the sciatic nerve correlate poorly with the clinical syndrome.

MRI neurography may be an exception. The majority of patients with unexplained sciatica in one study had increased T2 signal in the sciatic nerve, usually at or just distal to the piriformis muscle, which correlates with a reproduction of symptoms when the hip is passively placed in the flexed, adducted, internally rotated (FAIR) position and with a good outcome with piriformis muscle release surgery in one large study.

EDX studies are often normal in piriformis syndrome, but abnormal spontaneous activity in sciatic-innervated muscles is very helpful in confirming sciatic nerve involvement when present. On the other hand, nerve conduction studies are largely unhelpful with the debatable exception of tibial H-reflexes. One study showed that when performed in the FAIR position compared to prone, using a threshold of 2 standard deviations above normal, a prolonged tibial H-reflex latency was 85% sensitive and 82% specific for piriformis syndrome according to clinical criteria, and it was 81% likely to predict a good response to conservative measures directed at piriformis syndrome.  This finding has not been replicated in other studies, and the rapid positional changes in nerve conduction studies required to explain the prolonged latency in the FAIR position lack a plausible physiological mechanism. Conservative treatments for piriformis syndrome include NSAIDs, muscle relaxers, anti-neuropathic pain medications, physical therapy, and injections, including botulinum toxin.