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One of the most common and well studied innervation anomalies in the upper limbs is the Martin-Gruber anastomosis.1 2In the lower limbs, the anomaly is uncommon except for the accessory deep peroneal nerve.1 2 Recently, an exclusive innervation of the extensor digitorum brevis by the tibial nerve, “all tibial foot” has been reported.3-5 We experienced a similar patient with “all tibial foot”, who, in addition, showed sensory anomaly.
A 23 year old man with encephalitis had nerve conduction studies (NCSs) to exclude coexistent peripheral neuropathy. The studies were normal except for the anomalous innervation in the bilateral lower limbs. Peroneal nerve stimulation at the ankle, fibular head, and popliteal fossa elicited only a negligible compound muscle action potential (CMAP) over the extensor digitorum brevis. The accessory deep peroneal nerve was not demonstrated by stimulation behind the lateral malleolus.1 2 A normal CMAP from the extensor digitorum brevis was elicited by stimulating the tibial nerve at the ankle and popliteal fossa (figure A). Although CMAP of the anterior tibial muscle was normally elicited by stimulating the common peroneal nerve at the fibular head, a small CMAP was recorded also by the stimulation of the tibial nerve at the popliteal fossa (figure B).
Sensory studies of the sural, superficial peroneal and medial plantar nerves were normal. Stimulation of the deep peroneal nerve at the ankle gave rise to a normal antidromic sensory nerve action potential (SNAP) in the skin between the first and second toes, where an obvious SNAP was recorded even after the stimulation of the tibial nerve behind the medial malleolus (figure C).
Our patient had “all tibial foot” for the motor innervation, the anomalous dual innervation of the anterior tibial muscle, and the sensory coinnervation of the skin between the first and second toes by the tibial and deep peroneal nerves. Findings were similar on both sides. We speculate that, in our patient, the deep peroneal nerve becomes almost pure sensory after branching motor fibres for the anterior tibial muscle, and that the extensor digitorum brevis is innervated by the tibial nerve. Further, the tibial nerve had a motor branch for the anterior tibial muscle and a sensory branch to supply the area typically innervated by the deep peroneal nerve. Although rare, we should keep in mind this anomaly in the practice of nerve conduction studies.