The piriformis muscle is a deep pyramidal shaped flat muscle in the buttock region which originates over the sacrum and inserts over the greater trochanter of the femur. This muscle lies deep to the gluteus maximus and helps to externally rotate (or rotate away from the center of the body) the hip. The sciatic nerve can course through or in proximity to the muscle and if there are spasms of the muscle this can irritate the sciatic nerve. This is actually a rare syndrome that presents with numbness and tingling in the buttock area which can radiate into the leg and foot.
Piriformis syndrome is diagnosed after imaging tests, electrophysiologic studies (EMG studies), and physical exam do not show the lumbar spine as the causative factor of sciatica. Causes of piriformis syndrome can include trauma, adhesions, enlargement of the muscle, chronic inflammation of the muscle, sitting for prolonged periods, sacroiliac joint inflammation and the way the muscle was formed at birth (in up to 20% of the population the muscle is split and the sciatic nerve courses through dividing the muscle.) In some cases, a lumbar radiculopathy can cause piriformis syndrome as the muscle is irritated.
Diagnosis is made by ruling out other causes of sciatica. Physical exam can be helpful and, in some cases, advanced imaging may be helpful. Treatment consists of physical therapy to stretch the muscle and reduce spasm, modalities such as ultrasound, electrical stimulation and manual techniques may be helpful. If these measures fail to help, an image guided (ultrasound or fluoroscopy) local anesthetic and corticosteroid injection into the muscle belly can be helpful. For persistent piriformis syndrome, Botox may be helpful or surgery may be an option.