Misinterpretation of sural nerve conduction studies due to anatomical variation

• Sural sensory nerve action potential (SNAP) amplitudes may be falsely interpreted as decreased due to anatomical variation of the nerve.
• A sural nerve formation causing potential technical problems for nerve conduction studies with near-nerve needle technique or surface electrodes was found in 14.4% of 118 subjects with no evidence of polyneuropathy.
• If the sural SNAP amplitude is decreased in discordance to the clinical findings, a normal potential may be obtained by a more lateral or distal electrode placement compared to the traditional electrode placement 12–13 cm above the lateral malleolus.

ObjectiveAnatomical variation of the sural nerve has been documented in numerous cadaver studies. The sural nerve conduction parameters can potentially be influenced by the sural nerve type A formation formed by the union of the medial sural cutaneous nerve (MSCN) and the peroneal communicating branch (PCB) and the type C formation with the sural nerve formed solely by the PCB.MethodsIn 17 out of 240 prospectively examined subjects referred for polyneuropathy a suspicion of an anatomical variation of the sural nerve was raised due to decreased amplitude or substantial side-to-side variation (>50%) of the sensory nerve action potential (SNAP) in disproportion to the clinical findings. To verify the variation the sural nerve was examined further with surface electrodes and near-nerve technique, including extra lateral and distal needle placements.ResultsIn all 17 subjects an anatomical variation affecting the sural SNAP was confirmed as a normal sural SNAP could be obtained by changing the electrode placement. The most frequent variation, seen in 15 subjects, was a type A formation with union of the MSCN and the PCB distally at low calf, while a type C formation was seen in 2 subjects.ConclusionsIn case of a decreased sural SNAP amplitude or substantial side-to-side variation in disproportion to the neurologic evaluation, an anatomical variation instead of pathology could be suspected and a different electrode placement be considered.SignificanceNeurophysiologists should be aware of different types of formations of the sural nerve which may cause misinterpretations of nerve conduction studies, especially when needle electrodes are used.