The piriformis syndrome is a commonly diagnosed cause for sciatica. True sciatica can occur in the piriformis syndrome. The idea in piriformis syndrome is that the piriformis muscle is actually compressing or irritating the sciatic nerve. This was originally conceived when anatomists realized that the sciatic nerve is adjacent to, or even goes right through, the piriformis muscle. In cases of severe, persistent leg pain without an apparent cause shown by the lumbar MRI, imaging of the pelvis or leg is occasionally done. In rare cases, a mass is identified which truly compresses the sciatic nerve, but this is rare. The sciatic nerve actually goes right through the piriformis muscle in 20 to 33 percent of the time. The muscle serves as an external rotator of the thigh and as an abductor of the hip when the leg is flexed at the hip.
It is impossible to state the true incidence of the piriformis syndrome, because there is no “gold standard” of proving its diagnosis. An article from the Mayo Clinic suggests that it is responsible for 6 percent of sciatica. I think that the true percentage is far less than this. There is an effective way of disproving the diagnosis, moreover. Not infrequently, a patient that that gets this diagnosis ends up having a discectomy and subsequent resolution of the pain.
It is a natural diagnosis to consider because, with sciatica, the pain is often right where the sciatic nerve penetrates the piriformis muscle. We must remember, though, that the location of a pain is not always helpful in understanding its cause.