Elsevier

Clinical Neurophysiology

Volume 125, Issue 10, October 2014, Pages 2115-2121
Clinical Neurophysiology

Misinterpretation of sural nerve conduction studies due to anatomical variation

https://doi.org/10.1016/j.clinph.2014.01.030Get rights and content

Highlights

  • 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.

Abstract

Objective

Anatomical 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.

Methods

In 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.

Results

In 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.

Conclusions

In 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.

Significance

Neurophysiologists 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.

Introduction

Nerve conduction studies are important diagnostic tools to evaluate the integrity and function of the peripheral nervous system. The sural nerve is one of the most commonly examined nerves by nerve conduction studies, mainly for the diagnosis of polyneuropathy, but it is also useful in the evaluation of focal nerve injury of the lumbosacral plexus and the sciatic and tibial nerves.

The sural nerve is traditionally described by three different formation types, designated A, B, and C (Huelke, 1957). Type A, the most common type, is formed by the union between the medial sural cutaneous nerve (MSCN), which is a branch of the tibial nerve, and the peroneal communicating branch (PCB) of the common peroneal nerve, while type B is the direct continuation of the MSCN with the PCB absent, and type C is formed by the PCB only (Fig. 1). The union in type A may take place anywhere between the popliteal fossa and the lateral malleolus. Numerous cadaver studies have been conducted worldwide documenting the anatomical variations of the sural nerve (Eid and Hegazy, 2011, Huelke, 1957, Madhavi et al., 2005, Mahakkanukrauh and Chomsung, 2002, Pyun and Kwon, 2008, Shankar et al., 2010). In one of these, sural nerve conduction studies from healthy adults were done in addition to the cadaver studies showing highly variable sural nerve formation (Pyun and Kwon, 2008). Recently, an ultrasound study of anatomic variants of the sural nerve has shown similar variations as the cadaver studies (Zhu et al., 2011). These studies have mainly focused on surgical implications such as reconstruction of peripheral nerves, since the sural nerve is commonly used for nerve biopsies as well as a convenient source for nerve grafting.

From a neurophysiological point of view, the type A with a very distal union between MSCN and PCB as well as type C can cause problems, while type A with proximal union and type B show similar nerve conduction studies, probably without giving rise to any technical problems. However, despite the fact that the variability in the formation of the sural nerve may affect the parameters of sural nerve conduction studies, neurophysiologists have in general not paid much attention to the anatomical variation. In this study, we present 17 subjects with anatomical variation of the sural nerve examined electrophysiologically with both surface electrodes and near-nerve needle technique.

Section snippets

Material

We examined prospectively 240 consecutive subjects referred on suspicion of polyneuropathy. In all subjects bilateral motor conduction studies in the peroneal and tibial nerves and sensory conduction studies in the sural nerve were performed. The median and ulnar nerves were examined on one side in subjects with either electrophysiological changes in the lower extremities or symptoms in the upper extremities. Surface electrode recording was used for motor conduction studies, while sensory

Methods

The sural nerve was further examined in the 17 subjects with a suspected anatomical variation of the sural nerve in order to exclude pathology of the nerve and confirm the variation and identify its type. These supplementing examinations comprised antidromic surface recordings (Falck et al., 1994) and orthodromic near-nerve needle recordings with both the one-threshold (Behse and Buchthal, 1971, Buchthal and Rosenfalck, 1966, Rosenfalck and Rosenfalck, 1975, Trojaborg, 1992) and the two-

Type A formation

A type A formation with distal union of MSCN and PCB was found in 15 subjects. With orthodromic near-nerve two-threshold recording at mid calf at the lateral edge of the Achilles tendon, the SNAP amplitude was decreased bilaterally in 2 subjects and unilaterally in 13 subjects; right-sided in 6 and left-sided in 7. Of the subjects with unilateral variation, 9 had decreased SNAP amplitude at one side, while in 4 subjects the sural SNAP amplitudes were within normal ranges but showed an asymmetry

Discussion

In this study an anatomical variation instead of pathology of the sural nerve was confirmed in 17 out of 118 subjects (14.4%) with no neurophysiological evidence of polyneuropathy. A type A formation with union of MSCN and PCB at the low calf distal to the electrode placement 12.5 cm above the lateral malleolus was found in 15 subjects, while a type C formation of the sural nerve was found in 2. The variation was bilateral in both subjects with type C formation and in 2 subjects with type A

References (25)

  • B. Johnsen et al.

    Electrodiagnosis of polyneuropathy

    Neurophysiol Clin

    (2000)
  • F. Behse et al.

    Normal sensory conduction in the nerves of the leg in man

    J Neurol Neurosurg Psychiatry

    (1971)
  • F. Buchthal et al.

    Evoked action potentials and conduction velocity in human sensory nerves

    Brain Res

    (1966)
  • J.H. Coert et al.

    Clinical implications of the surgical anatomy of the sural nerve

    Plast Reconstr Surg

    (1994)
  • E.M. Eid et al.

    Anatomical variations of the human sural nerve and its role in clinical and surgical procedures

    Clin Anat

    (2011)
  • B. Falck et al.

    Sensory nerve conduction studies with surface electrodes

    Methods Clin Neurophysiol

    (1994)
  • D.F. Huelke

    The origin of the peroneal communicating nerve in adult man

    Anat Rec

    (1958)
  • D.F. Huelke

    A study of the formation of the sural nerve in adult man

    Am J Phys Anthropol

    (1957)
  • C.H. Kim et al.

    The relative contributions of the medial sural and peroneal communicating nerves to the sural nerve

    Yonsei Med J

    (2006)
  • C. Krarup

    Compound sensory action potential in normal and pathological human nerves

    Muscle Nerve

    (2004)
  • S. Kumar et al.

    Variability in the extent of sensory deficit after sural nerve biopsy

    Neurol India

    (2004)
  • D. Lacomis et al.

    Approach to vasculitic neuropathies

    J Clin Neuromuscul Dis

    (2007)
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