Anomalous Dual Innervation of the Extensor Digitorum Brevis by the Tibial and Fibular Nerves: A Case Report
Sepideh Allahdadian1, Mansoureh Mamarabadi2
1Neurology, PennState Health Milton S. Hershey Medical Center, 2Penn State Milton S. Hershey Medical Center
Objective:
The main objective of this case presentation is to look for uncommon anatomical variation of the muscle's innervations in the distal leg and foot
Background:
The extensor digitorum brevis (EDB) is typically innervated by the deep branch of the fibular nerve. The most common one is the accessory deep fibular nerve which arises from the deep or superficial fibular nerve, running posterior to the lateral malleolus and contributes to the innervation of muscles and sensory areas typically supplied by the deep fibular nerve. We present a dual innervation pattern of the EDB.
Results:
Case presentation: A 41-year-old female presented with past medical history of left tibial fracture complaints of painless right foot drop since 2016. Physical examination revealed mild weakness in foot dorsiflexion, inversion, and toe extension, along with decreased sensation to pinprick up to ankle and lateral edge of foot. Nerve conduction study (NCS) of fibular nerve showed: (1) bilateral superficial sensory responses were normal. (2) The right common fibular nerve motor responses recording from EDB muscle were with normal limits except recording higher amplitude of compound motor action potential (CMAP) with proximal (fibular neck and popliteal fossa) stimulation compared to the distal (ankle, anterior to lateral malleolus). Stimulation at the posterior of the lateral malleolus recording from EDB did not produce CMAP. Surprisingly, stimulation of the right tibial nerve at the ankle (posterior to the medial malleolus) and popliteal fossa generated CMAPS higher than the one recorded by stimulating the fibular nerve.
Conclusions:
These NCS findings suggested an anomalous dual innervation of the EDB by both the tibial and fibular nerves, a rare anatomical variation that raised questions about the patient's nerve distribution in the lower extremity. The case highlights the importance of recognizing anatomical variations during nerve conduction studies to avoid misdiagnosis and optimize patient management.
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