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Neurology India
Medknow Publications on behalf of the Neurological Society of India
ISSN: 0028-3886 EISSN: 1998-4022
Vol. 59, Num. 5, 2011, pp. 769-771

Neurology India, Vol. 59, No. 5, September-October, 2011, pp. 769-771

Letter to Editor

Accessory piriformis muscle: An easily identifiable cause of piriformis syndrome on magnetic resonance imaging

Anitha Sen1, S Rajesh2

1 Devi Scans, Kerala Institute of Medical Sciences, Trivandrum, Kerala, India
2 Kerala Institute of Medical Sciences, Trivandrum, Kerala, India
Correspondence Address: Anitha Sen, Devi Scans, Kerala Institute of Medical Sciences, Trivandrum, Kerala, India,

Date of Submission: 31-Jul-2011
Date of Decision: 31-Jul-2011
Date of Acceptance: 07-Aug-2011

Code Number: ni11232

PMID: 22019672

DOI: 10.4103/0028-3886.86562


A 28-year-old man with pain radiating to right leg was referred for sacral plexus magnetic resonance imaging (MRI) with special attention to imaging of the piriformis muscle as electromyographic findings were consistent with piriformis syndrome. MRI showed an abnormal sacral attachment of right piriformis muscle, with accessory muscle fibers crossing anterior to sacral foramen and right sacral nerve [Figure - 1]a, b, e. Bilateral piriformis muscles showed normal signal intensity. The right gluteal muscles and sciatic nerve showed increased signal intensity [Figure - 1]c-f on STIR sequences.

Piriformis syndrome is a rare cause of low backache and sciatica and is due to sciatic nerve entrapment at the greater sciatic notch. The causes include hypertrophy, inflammation, infection, reflex spasm or contracture of the piriformis muscle, anatomical variants, such as accessory piriformis muscle or tendon. [1] Predisposing factors include excessive exercise (athletes), cerebral palsy, leg length discrepancy (altered biomechanics leading to stretching and shortening of the piriformis muscle), narrowed sciatic foramen, and lumbar lordosis. Reported associations include dystonia musculorum deformans, [2] traumatic myositis ossificans of the piriformis muscle [3] and pseudoaneurysm of the inferior gluteal artery. [4] Incidentally, this patient had the habit of using right backpocket of trousers for keeping his wallet. Piriformis syndrome has been called back pocket sciatica or wallet sciatica since placing wallets in back pocket of trousers or jeans is said to predispose to this condition.

Although first described in 1928, piriformis syndrome is still frequently misdiagnosed or diagnosed late due to its rarity, nonspecific clinical features and absence of definite diagnostic tests. Clinical features include pain and paresthesias in unilateral gluteal and hip region with radiation to posterior thigh, tenderness over the belly of piriformis muscle in the gluteal region, isolated atrophy of the gluteus maximus, dysesthesia/hypoesthesia of the posterior aspect of the thigh, a sausage-shaped mass felt laterally on rectal examination, Freiberg sign, the sign of Pace and Nagle and positive Lasègue sign.

Nerve-conduction studies may demonstrate delayed F waves and H reflex. Computer tomography (CT) and MRI may show asymmetry [hypertrophy [5],[6] or atrophy] in the piriformis muscles. Bone scan may demonstrate abnormal uptake in the soft tissues of the pelvis. MRI may show an enlarged piriformis muscle [5],[7] or anomalous attachment of piriformis muscle. [1] If muscle injury or inflammation is present, increased signal may be seen within the piriformis muscle on T2 or STIR sequences. Sciatic nerve may be normal in size and signal characteristics if it is not compressed by the piriformis muscle while the patient is supine, if patient had been limiting physical activities that might precipitate his symptoms, or if the changes are chronic. MRI also helps in excluding other causes of low backache and sciatica, such as disc herniation, spinal canal stenosis, and lesions adjacent to piriformis muscle, such as pseudoaneurysm of the inferior gluteal artery. [4]

Treatment options include analgesics, corticosteroids, local injection of anesthetics and steroids, exercises and physical therapy, transrectal massage, and ultrasound. In refractory cases, neurolysis of the sciatic nerve and surgical release of the piriformis muscle are suggested.


1.Lee EY, Margherita AJ, Gierada DS, Narra VR.MRI of Piriformis Syndrome AJR Am J Roentgenol 2004;183:63-4.  Back to cited text no. 1    
2.Gandhavadi, B. Bilateral piriformis syndrome associated with dystonia musculorum deformans. Orthopedics 1990;13:350-1.  Back to cited text no. 2    
3.Beauchesne RP, Schutzer SF. Myositis Ossificans of the Piriformis muscle: An unusual cause of piriformis syndrome: A case report. J Bone Joint Surg Am 1997;79:906-10.  Back to cited text no. 3    
4.Papadopoulos SM, McGillicuddy JE, Albers JW. Unusual cause of piriformis muscle syndrome.' Arch Neurol 1990;47:1144-6.  Back to cited text no. 4    
5.Jankiewicz JJ, Hennrikus WL, Houkom JA. The appearance of the piriformis muscle syndrome in computed tomography and magnetic resonance imaging. A case report and review of the literature. Clin Orthop Relat Res 1991;262:205-9.  Back to cited text no. 5    
6.Palliyath S, Buday J. Sciatic nerve compression: Diagnostic value of electromyography and computerized tomography. Electromyogr Clin Neurophysiol 1989;29:9-11.  Back to cited text no. 6    
7.Rossi P, Cardinali P, Serrao M, Parisi L, Bianco F, De Bac S. Magnetic resonance imaging findings in piriformis syndrome: A case report. Arch Phys Med Rehabil 2001;82:519-21.  Back to cited text no. 7    

Copyright 2011 - Neurology India

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