Piriformis syndrome is a pain and numbness in the buttock that can radiate down the back of the leg into the foot. It involves the sciatic nerve. The sciatic nerve runs just beneath the piriformis muscle. When the piriformis muscle is irritated or damaged, it may spasm or swell and put pressure on the sciatic nerve making it hard to sit comfortably. The pain may get worse during activity. The main symptom of piriformis syndrome is sciatica.