Supramaximal Stimulus Intensity as a Diagnostic Tool in Chronic Demyelinating Neuropathy.

Vivien Parker,Julie Mills,Jodi Warman Chardon,Claire Goldsmith,Pierre R Bourque

doi:10.1155/2016/6796270

Abstract

Objective. The ability to correctly identify chronic demyelinating neuropathy can have important therapeutic and prognostic significance. The stimulus intensity value required to obtain a supramaximal compound muscle action potential amplitude is a commonly acquired data point that has not been formally assessed as a diagnostic tool in routine nerve conduction studies to identify chronic neuropathies. We postulated that this value was significantly elevated in chronic demyelinating neuropathy. Methods. We retrospectively reviewed electrophysiology laboratory records to compare the stimulus intensity values recorded during median and ulnar motor nerve conduction studies. The groups studied included normal controls (n = 42) and the following diagnostic categories: chronic inflammatory demyelinating neuropathy (CIDP) (n = 20), acquired inflammatory demyelinating neuropathy (AIDP) (n = 13), Charcot Marie Tooth (CMT) type 1 or 4C (n = 15), carpal tunnel syndrome (CTS) (n = 11), and amyotrophic lateral sclerosis (ALS) (n = 18). Results. Supramaximal intensities were significantly higher in patients with CMT (median nerve: 43.4 mA) and CIDP (median nerve: 38.9 mA), whereas values similar to normal controls (median nerve: 25.3 mA) were obtained in ALS, CTS, and AIDP. Conclusions. Supramaximal stimulus intensity may be used as an additional criterion to identify the pathophysiology of neuropathy. We postulate that endoneurial hypertrophic changes may increase electrical impedance and thus the threshold of excitation at nodes of Ranvier.

Highlights

The ability of an electrical stimulus to excite the nerve depends on several factors, including electrical impedance of intervening tissues and axonal excitability
We postulated that the supramaximal stimulus intensity, a value which is routinely acquired during conventional nerve conduction studies, may provide information about nerve excitability to facilitate diagnosis and prognosis for patients with chronic demyelinating neuropathies
In median and ulnar nerve studies, mean supramaximal intensities were significantly higher in patients with Charcot Marie Tooth (CMT) and chronic inflammatory demyelinating neuropathy (CIDP) than normal controls (p < 0.05)

Summary

Introduction

The ability of an electrical stimulus to excite the nerve depends on several factors, including electrical impedance of intervening tissues and axonal excitability. We observed that a substantially greater stimulus intensity was commonly required to achieve a supramaximal compound action potential amplitude in patients with chronic demyelinating neuropathy. This observation had not been validated in the literature and or published in commonly used monographs on nerve conduction study techniques. Studies have examined axonal excitability in demyelinating and axonal neuropathies using threshold electrotonus, threshold tracking, refractoriness, supernormality, and strengthduration behavior [1, 2]. These techniques are not typically performed during routine nerve conduction studies for clinical purposes. We studied the supramaximal stimulus intensities in normal subjects, patients with Charcot Marie Tooth (CMT), chronic

Methods

Results

Conclusion