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0 Case Report

MRI OF PERONEAL NERVE ENTRAPMENT DUE TO A GANGLION CYST

JOHN LEON AND GARY MARANO

Department of Radiology, West Virginia University, School of Medicine, Morgantown, West Virginia 26506-6302

Ganglion cysts are relatively common entities, but impingement upon the peripheral nerves is rare. We describe a case in which peroneal nerve palsy was caused by a ganglion cyst demonstrated by MRI.

Keywords: Ganglion cyst, Peroneal nerve, MRI.

INTRODUCTION

A 24-year-old female developed numbness and pain on the lateral aspect of her left foot over a three- month period with weakness in the left foot. Physical examination revealed a lump on the lateral aspect of the left knee overlying the region of the head of the fibula and a left foot drop. Point tenderness was noted upon palpation of the lump over the head of the fibula.

Plain films of the knee revealed no evidence of bony abnormality. An EMG study revealed significant signs of acute denervation in the left anterior tibialis and left peroneus longus muscles. Mild signs of chronic denervation were present in the left posterior tibialis muscle. These findings were thought to be con- sistent with a left common peroneal nerve palsy.

Magnetic resonance imaging of the left knee showed an oval mass located lateral and slightly inferior to the head of the fibula. The mass exhibited a decreased signal intensity on T, weighted images and increased signal intensity on T, weighted images (Figs. IA and B).

At surgery, the mass was found to be a cystic struc- ture compressing the peroneal nerve at the level of the fibular neck (Fig. 2). The cyst was found to have a stalk which was dissected down to the tibiofibular joint.

Histologic studies of the mass revealed a fibrous- walled cyst with no specialized lining cells (Fig. 3). A final diagnosis of a fibrous-walled cyst, consistent with a ganglion cyst, was made.

DISCUSSION

Ganglion cysts are relatively common entities, but they rarely impinge upon the peroneal nerve. Hart- well, in 1901, was the first to describe peripheral nerve involvement by a ganglion cyst.’ In his patient, the median nerve was affected. In 1921, Salten reported a case of paralysis of the peroneal nerve caused by a ganglion cyste2 Since that time, many cases of gan- glion cysts involving the common peroneal nerve have been reported. 3-1’ To our knowledge, there has been no previous report of the MRI findings of a ganglion cyst impinging upon the peroneal nerve.

Ganglion cysts are idiopathic, multi-loculated cys- tic structures filled with viscous fluid which is very similar to synovial fluid. I2 Ganglion cysts arise either from joint capsules or from digital flexor tendon sheaths. The majority of these tumors occur around the wrist or in the hands and feet. The compression of peripheral nerves and underlying bone rarely occurs.

Conventional radiography has little to add to the clinical evaluation of a ganglion cyst. Ganglion cysts have been imaged on CT in the past. In fact a case of compression of the common peroneal nerve imaged by CT was reported in 1983.13 This report stated that CT was the current modality of choice for the detec- tion and precise localization of soft tissue tumors because of its ability to display the anatomy in an axial plane, and to distinguish between small differ- ences in the attenuation of various soft tissue compo- nents. We believe that MR is superior to CT in the evaluation of soft tissue tumors because of its superior

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B

Fig. 1. A, (sagittal) TR = 600, TE = 25. Well-circumscribed mass of decreased signal intensity (arrows) located lateral to the fibular head. The linear structure of increased signal passing through the mass is thought to represent the peroneal nerve. This correlates well with the surgical findings of a cystic structure compressing the peroneal nerve. B, (axial) TR = 1600, TE = 80. Well-circumscribed structure of increased signal intensity (arrow) seen lateral to the fibular head.

Fig. 2. Intraoperative photograph showing cystic mass which was found impinging upon the peroneal nerve.

MRI of peroneal nerve entrapment 0 J. LEON AND G. MARANO 309

Fig. 3. Photomicrograph shows a fibrous wall surrounded by adipose and loose fibrous connective tissue. Myxomatous changes were noted in some areas of the loose connective tissue. (Photograph courtesy of Dr. Sidney Schochet.)

inherent image contrast and its ability to provide review of thirteen collected cases. J. Bone Joint Sur.

direct sagittal and coronal images. 34B:391-400; 1952.

Weiss, Beltram, and Lubbers have recently re-

ported three cases of ganglion cysts around the wrist imaged by MR. I4 An SE sequence was used with a TR of 2000 msec and echo times (TEs) of 25, 50, 75, and 100 msec. The cysts had a characteristic low sig- nal on the first echo and appeared progressively brighter on later echoes. These findings correlated

with the findings in our case. The ganglion cyst in our case exhibited low signal intensity on the T, weighted

images and increased signal on T, weighted images (Figs. 1A and 1B).

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Clark, K. Ganglion of the lateral popliteal nerve. J. Bone Joint Surg. 43B:778-783; 1961. Ellis, V.H. Two case of ganglia in the sheath of the peroneal nerve. Br. J. Surg. 24:141-142; 1936-1937. Ferguson, L.K. Ganglion of the peroneal nerve. Ann. Surg. 106:313-316; 1937. Jenkins, S.A. Solitary tumors of peripheral nerve trunks. J. Bone Joint Surg. 43B:401-411; 1952. Parkes, A. Intraneural ganglion of the lateral popliteral nerve. J. Bone Joint Surg. 43B:784-790; 1961. Tupman, G.S. Axonotmesis of anterior tibia1 branch of lateral popliteal nerve due to ganglion of the nerve- sheath. Br. J. Surg. 45123-24; 1957. Wadstein, T. Two cases of ganglia in the sheath of the peroneal nerve. Acta Orthop. Stand. 2:22 1-23 1; 193 1. Allen, P.W. Pathology Annual, Part I. Volume 15, 1980. Appleton-Century-Crofts/New York. Firooznia, H.; Golimbu, C.; Rafii, M.; Chapnick, J. Computerized tomography in diagnosis of compression of the common peroneal nerve by ganglion cysts. Com- puterized Radiology, 343-345; 1983. Weiss, K.L.; Beltram, J.; Lubbers, L.M. High field MR surface coil imaging of the hand and wrist Part II. Pathologic correlations and clinical relevance. Radiol- ogy 160:147-152; 1986.

REFERENCES

Hartwell, AS. Cystic tumor of median nerve; opera- tion: Restoration of function. Boston Med. Surg. J. 144:528-X33; 1901. Sultan, C. Ganglion der Nervenscheide des Nervus Peroneus. Zentralbl. F. Chir. 48:963-96.5; 1921. Brooks, D.M. Nerve compression by simple ganglia. A