Intraneural ganglion cyst of the common peroneal nerve
Patient was referred to a neurologist by her primary care physician after a peroneal nerve palsy and pain in left lower leg which began two months prior. An EMG was congruent with peroneal nerve entrapment. The neurologist requested imaging of the left knee.
MRI of the left knee
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Long, homogeneous, multiloculated, thin-walled cyst along the course of the common peroneal nerve with a slender channel projecting anteriorly towards a horizontal tear in the midsection of the lateral meniscus. No apparent degenerative changes in the proximal tibiofibular joint. There is subtle peripheral enhancement after administration of intravenous gadolinium.
An intraneural ganglion cyst has been described in the literature as a rare cause of peroneal neuropathy whereby synovial fluid from a degenerative joint breaches the capsule and dissects the nerve sheath. In the published examples it arises from the proximal tibiofibular joint and extends along the perineural sheath of the articular branch of the peroneal nerve, reaching as high as the sciatic nerve.
We observed no communication with the proximal tibiofibular joint, but a complex route connecting the cyst with a lateral meniscal tear and associated meniscal cyst. We posit that this intraneural ganglion cyst arises from the meniscal tear rather than the proximal tibiofibular joint.
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