Technically, this is a myofascial pain syndrome. However, because of the strategic location of the muscle, pain sometimes is referred from the buttock to the foot.
The piriformis muscle connects the lateral sacrum to the greater trochanter of the femur. The muscle overlies the sciatic nerve.
Trauma to the piriformis (direct fall on the buttocks, twisting internal rotation injury of the hip, etc) can lead to a deep aching pain in the low buttocks, which can be transmitted down the sciatic nerve to the foot. Sitting worsens the pain as often does counterclockwise rotation of the right leg or clockwise rotation of the left leg. The pain is on one side. Often these patients walk with the foot turned out in order to avoid pain. There are some patients with anatomic variants of the relationship between the sciatic nerve and the muscle in which the muscle is spit by the nerve. Sacroiliac joint disease can activate the piriformis muscle, thereby secondarily producing pain from the buttock to the foot.
Diagnosis is made by performing a piriformis block using fluoroscopy and piriformis myography (above photo) to confirm needle placement. Once a local anesthetic block has confirmed the syndrome, treatment consists of stretching exercises, Botox injections into the muscle using electromyelographic guidance, intermittent local anesthetic injections, or piriformis excision surgery. The latter is reported to have a reasonable success rate, but is reserved for only the most severe refractory cases.