Investig Magn Reson Imaging.  2019 Jun;23(2):142-147. 10.13104/imri.2019.23.2.142.

Accessory Belly of the Piriformis Muscle as a Cause of Piriformis Syndrome: a Case Report with Magnetic Resonance Imaging and Magnetic Resonance Neurography Imaging Findings

• Kim HJ
• Lee SY
• Park HJ
• Kim KW
• Lee YT

• KMID: 2452528
• DOI: http://doi.org/10.13104/imri.2019.23.2.142

Abstract

Piriformis syndrome caused by an accessory belly of the piriformis muscle is very rare. Only a few cases have been reported. Here, we report a case of piriformis syndrome resulting from an extremely rare type of accessory belly of the piriformis muscle originated at the proximal third portion of the main piriformis muscle and attached separately to the greater trochanter inferior to the insertion of the main piriformis muscle. A definitive diagnosis of piriformis syndrome was made based on magnetic resonance imaging and magnetic resonance neurography findings that were consistent with results of nerve conduction study and needle electromyography.

Keyword

Accessory muscle; Piriformis muscle; Piriformis syndrome; Magnetic resonance imaging; Magnetic resonance neurography

MeSH Terms

Diagnosis
Electromyography
Femur
Magnetic Resonance Imaging*
Needles
Neural Conduction
Piriformis Muscle Syndrome*

Figure

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  Fig. 1 Sagittal T2-weighted MR image (repetition time [TR] = 2750 ms, echo time [TE] = 104 ms) of the lumbar spine showing a mild degenerative bulging disc at L4-5 and L5-S1 levels.

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  Fig. 2 (a) Coronal T1-weighted MR image (TR = 691 ms, TE = 14 ms) revealing hypertrophy (asterisk) of the left piriformis muscle with an accessory belly (arrow) appearing at the inferior, proximal-third portion of the main piriformis muscle which coursed downward and laterally. (b) Axial T1-weighted MR image (TR = 709 ms, TE = 19 ms) showing the accessory belly (arrow) of the left piriformis muscle attaching to the greater trochanter, inferior to the insertion of the main piriformis muscle. (c) Sagittal T2-weighted MR image (TR = 3992 ms, TE = 100 ms) showing the accessory belly (arrowhead) at the inferior aspect of the left piriformis muscle along with thickening of the infrapiriformis portion of the left sciatic nerve (arrow). (d) Axial fat-suppressed T2-weighted MR image (TR = 4385 ms, TE = 70 ms) showing increased size and increased signal intensity of the left sciatic nerve (arrow). (e) Oblique coronal three-dimensional maximum intensity projection image obtained from fat-suppressed T2-weighted fast field echo (TR = 8 ms, TE = 4 ms, flap angle = 30°) clearly showing an abnormally thick left sciatic nerve (arrows) inferior to the hypertrophic left piriformis muscle (asterisk). (f) Coronal diffusion-weighted image with b value of 800 s/mm2 clearly demonstrating an abnormally thick left sciatic nerve (arrows) with diffusion restriction. (g, h) Axial T1-weighted (TR = 709 ms, TE = 19 ms) and axial fat-suppressed T2-weighted MR images (TR = 4385 ms, TE = 70 ms) showing no abnormal signal intensity suggesting denervation of left biceps femoris, semitendinosus, semimembranosus, and adductor magnus that are innervated by the left sciatic nerve.

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  Fig. 3 Drawings illustrating the six types of the Beaton and Anson's classification and our case regarding anatomical variation between the piriformis muscle and he sciatic nerve. Colored red and yellow indicate piriformis muscles and sciatic nerves (and its divisions), respectively. (a) Type A, undivided sciatic nerve exits the pelvis below the piriformis muscle. (b) Type B, sciatic nerve divides in the pelvis, common peroneal nerve pierces the piriformis muscle, and tibial nerve courses deep to the piriformis muscle. (c) Type C, sciatic nerve divides in the pelvis, common peroneal nerve courses over the piriformis muscle, and tibial nerve courses deep to the piriformis muscle. (d) Type D, undivided sciatic nerve exits the pelvis and pierces the piriformis muscle. (e) Type E, sciatic nerve divides in the pelvis, common peroneal nerve courses over the piriformis muscle, and tibial nerve pierces the piriformis muscle. (f) Type F, undivided sciatic nerve exits the pelvis and courses over the piriformis muscle. (g) Our case, accessory belly appears at the inferior aspect and proximal third portion of the main piriformis muscle, coursing laterally and inserting into the greater trochanter separately. Undivided sciatic nerve exits the pelvis below the main piriformis muscle and over the accessory belly.

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