Disclosures: Hassen Berri, FAAPMR: No financial relationships or conflicts of interest

Case Diagnosis: Intraneural perineurioma (IP) of the common peroneal nerve (CPN)

Case Description:

Patient: 14 y/o healthy female with spontaneous left foot and ankle weakness beginning insidiously. • Month 2: Initial pediatrician visit. Patient referred her to neurology for painless weakness of ankle eversion and dorsiflexion without sensory deficits. • Month 5: Neurologist completes EMG: 1+ fibrillations in tibialis anterior, no information on recruitment, low amplitude peroneal motor response, and lower superficial peroneal sensory amplitude. Read as sensorimotor axonal peroneal mononeuropathy “likely below the knee.” • Month 6: No return of function. Referred to Orthopedist. MRI-knee ordered, read as unremarkable. • Month 7: Peroneal decompression performed by orthopedist at the level of fibular head. • Month 11: No improvement; pediatrician recommends tertiary care PM&R referral. • Month 12: PM&R consultation and MSK Ultrasound with loss of fascicular architecture of CPN just distal to sciatic bifurcation. Repeat EMG shows severe, incomplete sensorimotor CPN mononeuropathy, localizing proximal branch to anterior tibialis with sparing of the short head of biceps femoris • Month 13: Repeat MRI: 7cm segmental abnormality of the CPN correlating to ultrasound • Month 15: Neurosurgery for CPN exploration, neurolysis, biopsy, and decompression

Setting: Tertiary care medical centerAssessment/

Results: Pathology of CPN fascicles consistent with IP

Discussion: IPs were first identified in 1968 and are extremely rare. IPs occur most commonly in adolescence or young adulthood. Nerves most frequently involved include: sciatic, median, ulnar, and nerves of the brachial plexus. Less than 20 IPs isolated to the CPN have been reported in medical literature to date.

Conclusion: This case brings awareness to a rare etiology for a commonly seen issue. It emphasizes good localization on initial EMG, and the value of MSK ultrasound as a diagnostic tool. Patient is currently recovering without any worsening neurologic deficits. Further surgery may be considered pending clinical course.

Level of Evidence: Level V

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PM&R Meeting Abstracts - https://pmrjabstracts.org/abstract/adolescent-foot-and-ankle-weakness-not-your-typical-foot-drop-a-case-report/