Pirifomis Muscle Syndrome

Piriformis muscle syndrome (also piriformis syndrome) is a controversial entity, which, in the absence of reproducible and reliable diagnostic criteria, remains a diagnosis of exclusion.

It is thought to be due to compression of the sciatic nerve through hypertrophy, spasm, contracture, or inflammation and scarring of the piriformis muscle. While the initiating insult is usually unknown, trauma (with resultant muscle injury, hematoma, and scarring) or acute or chronic piriformis muscle stretching associated with gait disturbances can serve as the initial insult leading to the common endpoint of piriformis muscle syndrome.

Imaging can be helpful by delineating the relationship between the sciatic nerve and the piriformis muscle. The diagnosis of piriformis muscle syndrome can be inferred from alterations in the appearance of the sciatic nerve, with sciatic nerve edema being associated with symptoms of piriformis muscle syndrome in ~90% of patients.

Muscle asymmetry, on the other hand, appears to be less useful. Muscle anomalies and significant variations in size of the piriformis muscle in asymptomatic individuals limit the role of imaging in assessing piriformis muscle abnormalities. One study found that muscle asymmetry by itself had a specificity of ~65% and sensitivity of ~45% in identification of patients with piriformis muscle syndrome who responded well to piriformis surgery (that is, they had muscle-based piriformis muscle syndrome).

Use of both piriformis muscle asymmetry (hypertrophy or atrophy) and sciatic nerve edema can lead to ~90% specificity and ~65% sensitivity in predicting response to piriformis surgery.

References

Petchprapa CN, Rosenberg ZS, Sconfienza LM, Cavalcanti CF, Vieira RL, Zember JS. MR imaging of entrapment neuropathies of the lower extremity. Part 1. The pelvis and hip. Radiographics. 2010 Jul-Aug;30(4):983-1000.