What is the role of motor conduction studies in the workup of common peroneal nerve entrapment?

Updated: Oct 15, 2019
  • Author: Minoo Hadjari Hollis, MD; Chief Editor: Thomas M DeBerardino, MD  more...
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Answer

The most commonly performed test in determining peroneal conduction in the leg and across the fibular head is performed with the active electrode placed on the extensor digitorum brevis. The peroneal nerve usually is stimulated at the ankle, several centimeters below the fibular head and about 10 cm proximal to the fibular head, just medial to the biceps femoris tendon. This allows calculation of the NCV across the fibular head region, with comparison with the distal leg segment.

Comparison with the contralateral limb is often helpful. When significant extensor digitorum brevis atrophy is present (eg, with advanced age or with a polyneuropathy), the active electrode should be placed over the anterior tibial muscle. Generally, lower-extremity motor NCVs less than 40 m/sec are considered abnormal. Generally, proximal-segment NCVs should be greater than distal NCVs, given the greater axonal diameter in the proximal segment of the nerve.

If contralateral limb responses are normal, axonal loss can be estimated by expressing the compound muscle action potential (CMAP) on the affected side as a percentage of that on the unaffected side. This method is independent of the location of the active recording electrode and is valid in both circumstances. A 20-50% change (depending on the source) indicates a conduction block. The degree of conduction slowing and temporal dispersion may also be assessed to determine whether the lesion is mainly demyelinating or axonal.


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