TIBIAL MOTOR CONDUCTION STUDY

Tibial motor conduction study is performed by placing active electrode at abductor hallucis brevis (AH) muscle, reference electrode at metatarsophalangeal joint of great toe and ground electrode b/w stimulator and active electrode.
Tibial nerve is stimulated behind medial malleolus and in popliteal fossa. Stimulation of nerve in popliteal fossa is difficult because nerve is lying deep in the fossa.

Usually, a distal latency in excess of 5.0 ms is taken as abnormal. Similarly, CMAP amplitude of less than 5 mv or conduction velocity of less than 40 m/s is taken as abnormal. However, these values may vary b/w various populations, machines etc, thus it is advisable to generate a normative data for each centre.




Reference:

1. Anatomy of Tibial Nerve


2. Routine Nerve Conduction Study


3. MNCS – Parameters


4. Recording Electrodes-For Motor Studies


5. Nerve Conduction Studies – Introduction


6. Recording Procedure-Nerve Stimulation

TIBIAL NERVE ANATOMY

The tibial nerve is the larger of the two major divisions of the sciatic nerve. It is derived from L5, S1 and S2 roots. It leaves the popliteal fossa between the heads of the gastrocnemius and supplies all muscles in the posterior compartment of the legs, i.e. gastrocnemius, soleus, plantaris, popliteus, flexor digitorum longus, flexor hallucis longus and tibialis posterior.

At the ankle the tibial nerve runs posterior to the medial malleolus under the flexor retinaculum (tarsal tunnel) to enter the foot. While coming out of (or within) tarsal tunnel, the nerve divides into four branches.

Two of these, the medial and lateral calcaneal nerves are purely sensory and supply sensation to the heel.

The other two branches, the medial and lateral plantar nerves innervate the intrinsic muscles of the foot and provide sensation to the medial and lateral sole respectively. Notably, medial plantar nerve supplies abductor hallucis brevis and lateral plantar nerve supplies abductor digiti quinti pedis.

Reference:

1. Richard S Snell, Clinical Anatomy: Lippincott Williams & Wilkins, 7th edition


2. Cimino WR. Tarsal tunnel syndrome: review of the literature. Foot Ankle 1990, 11:47.


3. Kimura J. Electrodiagnosis in disease of nerve and muscle: Principles and Practice, New York: Oxford V. Press, 3rd edition