Etiology:
Rare entrapment neuropathy, causing lower back pain and sciatica, due to sciatic nerve entrapment at greater sciatic notch
Usually due piriformis hypertrophy; other causes include inflammation, anatomic variations, trauma, pseudoaneurysm of inferior gluteal artery, excessive exercise, inflammation and spasm of piriformis muscle, infection
Clinical features:
Frequently delay in the diagnosis or misdiagnosed
Usually diagnosis of exclusion
6% of lower back pain and sciatica may be caused by piriformis syndrome
Pain and paresthesias in the unilateral gluteal region radiating to the hip and posterior thigh in a sciatic radicular distribution
Symptoms can be reproduced by digital pressure over the belly of the piriformis muscle in the gluteal region and on the lateral pelvic wall of the affected side
Freiberg sign - pain with forced internal rotation of extended thigh
Sign of Pace and Nagle - pain with resistance to abduction and external rotation of thigh
Imaging:
MR is investigation of choic although CT may be useful
Hypertrophy of pyriformis the most common finding
Bone scan may show abnormal uptake in the soft tissue
Treatment:
Treat the cause, usually surgically
Local injection is possible
References:
Lee EY et al. MRI of pyriformis syndrome. AJR 2004; 183:63-64
Beauchesne RP et al.Myositis Ossificans of the Piriformis Muscle: An Unusual Cause of Piriformis Syndrome: A Case Report. The Journal of Bone and Joint Surgery 79:906-10 (1997)
