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
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.  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,  traumatic myositis ossificans of the piriformis muscle  and pseudoaneurysm of the inferior gluteal artery.  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 , 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 , or anomalous attachment of piriformis muscle.  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. 
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.
Copyright 2011 - Neurology India
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