Piriformis syndrome resulting from a rare anatomic variation.

Study Design: Case report.
Objectives: We report a rare case of piriformis syndrome accompanying anatomic variation in the piriformis muscle and sciatic nerve.
Summary of Background Data: Beaton classified anatomic variation in the piriformis muscle and sciatic nerve into 6 types based on cadaver studies. There has been no report in the English literature of surgical treatment for a case of piriformis syndrome accompanying Beaton type d anatomic variation.
Methods: A patient with sciatica showing no nerve root compression in lumbar MRI underwent pelvic MRI and perineurography of the sciatic nerve followed by CT. The findings in these images suggested piriformis syndrome accompanying anatomic variation of the piriformis muscle and sciatic nerve. Surgical treatment was performed.
Results: Surgical exploration of the piriformis muscle revealed Beaton type d anatomic variation. Both anterior and posterior lobes of the piriformis muscle were resected. The pain in the leg had completely disappeared after surgery.
Conclusions: This is a very rare case of surgically treated piriformis syndrome resulting from type d anatomic variation in Beaton's classification. Pelvic MRI and perineurography of the sciatic nerve were useful for diagnosis in this case.