The piriformis is a muscle in the back of the thigh by the buttocks. The muscle helps externally rotate the hip. Piriformis syndrome refers to the piriformis muscle irritating the sciatic nerve. The sciatic nerve typically passes over this muscle superficially, however, in some people the nerve may split into 2 divisions with one division passing directly through the muscle.
There are two types of piriformis syndrome—primary and secondary. Primary piriformis syndrome has an anatomic cause, such as a split piriformis muscle, split sciatic nerve, or an anomalous sciatic nerve path. Secondary piriformis syndrome occurs as a result of a precipitating cause, including macrotrauma, microtrauma, ischemic mass effect, and local ischemia. Among patients with piriformis syndrome, fewer than 15% of cases have primary causes.
The most common presenting symptom of patients with piriformis syndrome is increasing pain after sitting for longer than 15 to 20 minutes. Many patients complain of pain over the piriformis muscle (ie, in the buttocks), especially over the muscle’s attachments at the sacrum and medial greater trochanter. Symptoms, which may be of sudden or gradual onset, are usually associated with spasm of the piriformis muscle or compression of the sciatic nerve. Patients may complain of difficulty walking and of pain with internal rotation of the ipsilateral leg, such as occurs during cross-legged sitting or ambulation.
Procedural options are targeted to where the nerve is compressed. “Hydrodissection” can be used to free up the nerve from adhesions or minor muscular entrapment. With this procedure, saline is injected around the nerve under ultrasound.