Accessory Belly Of The Piriformis Muscle As A Cause Of .

pISSN 2384-1095eISSN 2384-1109iMRI 23.2.142Accessory Belly of the Piriformis Muscleas a Cause of Piriformis Syndrome: aCase Report with Magnetic ResonanceImaging and Magnetic ResonanceNeurography Imaging FindingsHae-Jung Kim1, So-Yeon Lee1, Hee-Jin Park1, Kun-Woo Kim2, Young-Tak Lee21Case ReportReceived: February 18, 2019Revised: April 8, 2019Accepted: May 9, 2019Correspondence to:So-Yeon Lee, M.D., Ph.D.Department of Radiology,Kangbuk Samsung Hospital, 29,Saemunan-ro, Jongno-gu, Seoul03181, Korea.Tel. 82-2-2001-1031Fax. 82-2-2001-1030E-mail: capella27@gmail.comDepartment of Radiology, Kangbuk Samsung Hospital, Sungkyunkwan University School ofMedicine, Seoul, Korea2Department of Physical & Rehabilitation Medicine, Kangbuk Samsung Hospital, SungkyunkwanUniversity School of Medicine, Seoul, KoreaPiriformis 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 syndromeresulting from an extremely rare type of accessory belly of the piriformis muscleoriginated at the proximal third portion of the main piriformis muscle and attachedseparately to the greater trochanter inferior to the insertion of the main piriformismuscle. A definitive diagnosis of piriformis syndrome was made based on magneticresonance imaging and magnetic resonance neurography findings that wereconsistent with results of nerve conduction study and needle electromyography.Keywords: Accessory muscle; Piriformis muscle; Piriformis syndrome;Magnetic resonance imaging; Magnetic resonance neurographyINTRODUCTIONThis is an Open Access article distributedunder the terms of the Creative CommonsAttribution Non-Commercial 4.0/) which permits unrestrictednon-commercial use, distribution, andreproduction in any medium, providedthe original work is properly cited.Copyright 2019 Korean Societyof Magnetic Resonance inMedicine (KSMRM)142Piriformis syndrome is a rare entrapment neuropathy in which the sciatic nerveis compromised by an abnormal piriformis muscle (1). It is usually caused by anabnormal condition of the piriformis muscle such as hypertrophy, spasm, contracture,inflammation, or anatomic variation (1, 2). An accessory piriformis muscle is one suchanatomic variation. Although possible anatomical relationships between sciatic nerveand piriformis muscle were classified by Beaton and Anson in 1937 into six types (3),only a few cases have been reported regarding the accessory piriformis muscle. Most ofprevious studies describing the accessory piriformis muscle (1, 4-6) reported one of thesix types described by Beaton and Anson (3). Ravindranath et al. (7) and Natsis et al.(8) have reported a few cases not classified by Beaton and Anson (3). Here, we report avery rare case of an accessory piriformis muscle not described in the Beaton and Ansonclassification as a cause of piriformis syndrome. Its magnetic resonance imaging (MRI)and magnetic resonance neurography (MRN) imaging findings are also described.www.i-mri.org

https://doi.org/10.13104/imri.2019.23.2.142CASE REPORTA 24-year-old female was referred to our institutioncomplaining of a nineteenth-month history of allodyniaat the lateral side of her calf and the dorsum of her footon the left side. In order to exclude disc abnormalities, alumbar spine MRI was initially performed at an outsidelocal hospital. A mild degenerative bulging disc at L4-5and L5-S1 levels was noted on sagittal T2-weighted imageof the lumbar spine MRI (Fig. 1). Nerve conduction study(NCS) and needle electromyography (EMG) test wereperformed to confirm the presence of peripheral nerveinjury. Sensory NCS revealed no sensory nerve actionpotential response in left superficial peroneal, left deepperoneal, or left sural nerves. In motor NCS, the left tibialnerve showed low compound motor action potentialswhile the left peroneal nerve showed no response. EMGrevealed abnormal spontaneous activities in bicepsfemoris, tibialis anterior, and peroneus longus muscles.Electrophysiologic assessment demonstrated abnormalitiesin muscles innervated by the left sciatic nerve. She wasdiagnosed with left sciatic neuropathy. No abnormalitieswere found on EMG performed at different levels (L4, L5)of paraspinal muscles. The patient underwent pelvic boneMRI with MRN for further evaluation. Coronal T1-weightedMR images showed hypertrophy of the left piriformismuscle with an accessory muscle belly lying at the inferiorand originating at the proximal third portion of the mainpiriformis muscle (Fig. 2a). The accessory piriformis musclewas directed downward and laterally. It inserted separatelyinto the greater trochanter, inferior to the insertion of mainpiriformis muscle on axial T1-weighted MR image (Fig. 2b).Increased size and increased signal intensity of the infrapiriformis portion of the left sciatic nerve were noted onsagittal T2-weighted (Fig. 2c) and axial fat-suppressed T2weighted MR images (Fig. 2d). On MRN, oblique coronalthree-dimensional maximum intensity projection imageobtained from fat-suppressed T2-weighted fast field echoshowed abnormal thickening of the infra-piriformis portionof left sciatic nerve with concomitant hypertrophy of theleft piriformis muscle (Fig. 2e). Coronal diffusion-weightedimage with b value of 800 s/mm2 showed thickening ofthe left sciatic nerve with restricted diffusion (Fig. 2f).Axial T1-weighted (Fig. 2g) and axial fat-suppressed T2weighted MR images (Fig. 2h) showed no abnormal signalintensity, suggesting denervation of muscles innervated bythe left sciatic nerve. MRI and MRN imaging findings wereconsistent with piriformis syndrome. After the diagnosis, shewww.i-mri.orgwas treated conservatively with analgesics. Her symptomslightly improved. Surgical treatment was considered forrefractory case. However, the patient was lost to follow-up.Thus, no further evaluation was done.DISCUSSIONAlthough piriformis muscle as an etiologic factor forsciatic pain and lower back pain was first describedby Yeoman (9) in 1928, correct diagnosis of piriformissyndrome is often delayed and frequently misdiagnoseddue to its rarity. In 1937, Beaton and Anson (3) classifiedpossible anatomical relationships between sciatic nerve andFig. 1. Sagittal T2-weighted MR image (repetition time [TR] 2750 ms, echo time [TE] 104 ms) of the lumbar spineshowing a mild degenerative bulging disc at L4-5 and L5S1 levels.143

Accessory Piriformis Muscle Causing Piriformis Syndrome Hae-Jung Kim, et al.piriformis muscle into six types (Fig. 3a-f). However, onlya few cases of anatomic variation of the piriformis musclehave been reported as a cause of piriformis syndrome,especially regarding the accessory piriformis muscle. Leeet al. (1) and Sen and Rajesh (4) have reported accessorymuscle fibers of piriformis muscles extending mediallyover the sacral foramen and crossing over the sacralnerves. These corresponded to a type D variation (Fig. 3d).Yadav et al. (5) and Polesello et al. (6) have reported twodistinct bellies of piriformis muscles that are fused to forma common tendon and inserted to the greater trochanter.aAs a nerve branch of the sciatic nerve passed betweenthe accessory belly and the main piriformis muscle ineach case, these cases were type B variations (Fig. 3b).However, there are also a few case reports of the accessorypiriformis muscle not described in the Beaton and Ansonclassification. Ravindranath et al. (7) have reported threecadaver cases of accessory piriformis muscle originatedfrom the sacrotuberous ligament or the fascia overlying thegluteus medius. Accessory slips of these cases merged withthe main tendinous part of piriformis muscles. However, thesciatic nerve was deeply related to the accessory slip andbdc144Fig. 2. (a) Coronal T1-weighted MR image (TR 691 ms, TE 14 ms) revealinghypertrophy (asterisk) of the left piriformis muscle with an accessory belly(arrow) appearing at the inferior, proximal-third portion of the main piriformismuscle which coursed downward and laterally. (b) Axial T1-weighted MRimage (TR 709 ms, TE 19 ms) showing the accessory belly (arrow) of theleft piriformis muscle attaching to the greater trochanter, inferior to theinsertion of the main piriformis muscle. (c) Sagittal T2-weighted MR image(TR 3992 ms, TE 100 ms) showing the accessory belly (arrowhead) at theinferior aspect of the left piriformis muscle along with thickening of the infrapiriformis portion of the left sciatic nerve (arrow). (d) Axial fat-suppressed T2weighted MR image (TR 4385 ms, TE 70 ms) showing increased size andincreased signal intensity of the left sciatic nerve (arrow).www.i-mri.org

https://doi.org/10.13104/imri.2019.23.2.142efghthe main piriformis muscle. Natsis et al. (8) have reportedan extremely rare cadaver case of anatomical variation,showing that the piriformis muscle has three muscle bellies.Unlike other cases described previously, our patient had anaccessory belly of the piriformis muscle which originatedat the inferior aspect and the proximal third portion of themain piriformis muscle, coursed deep to the sciatic nerve,and attached separately into the greater trochanter (Fig.3g). In addition, coexisting hypertrophy of the piriformismuscle was noted as an additional cause of piriformissyndrome. This is an extremely rare anatomic variation.To the best of our knowledge, such case has not beenpreviously reported.Although the diagnosis of piriformis syndrome ischallenging due to its nonspecific clinical symptoms andthe absence of definitive diagnostic tests, MRI can be usedto make a confirmative diagnosis of suspected piriformissyndrome by detecting abnormal anatomy of the affectednerve and evaluating the size and anatomic variation of thewww.i-mri.orgFig. 2. (e) Oblique coronal threedimensional maximum intensityprojection image obtained fromfat-suppressed T2-weighted fastfield echo (TR 8 ms, TE 4 ms,flap angle 30º) clearly showingan abnormally thick left sciaticnerve (arrows) inferior to thehypertrophic left piriformis muscle(asterisk). (f) Coronal diffusionweighted image with b value of800 s/mm2 clearly demonstratingan abnormally thick left sciaticnerve (arrows) with diffusionrestriction. (g, h) Axial T1weighted (TR 709 ms, TE 19ms) and axial fat-suppressedT2-weighted MR images (TR 4385 ms, TE 70 ms) showingno abnormal signal intensitysuggesting denervation of leftbiceps femoris, semitendinosus,semimembranosus, and adductormagnus that are innervated bythe left sciatic nerve.piriformis muscle (1, 2). In our patient, an accessory bellyand hypertrophy of the left piriformis muscle were noted onthe patient’s MRI. In addition, increased size and increasedsignal intensity of the infra-piriformis portion of left sciaticnerve were seen on T2-weighted MRI images and MRN.These were sufficient for an imaging diagnosis of piriformissyndrome, consistent with results of the NCS and EMG.However, the present case has not yet been surgicallyconfirmed as the initial treatment for piriformis syndromeis conservative. Surgical release of the sciatic nerve andsectioning of the accessory belly of the piriformis musclemay be considered in refractory cases (2).In conclusion, an accessory belly of the piriformis muscleis a rare but important etiology of piriformis syndrome.Radiologists in particular must be aware of this etiologybecause early recognition using MRI may prevent delayeddiagnosis of piriformis syndrome.145