Supramaximal Stimulus Intensity as a Diagnostic Tool in Chronic Demyelinating Neuropathy

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.

<em>In Vivo</em> Electrophysiological Measurement of Compound Muscle Action Potential from the Forelimbs in Mouse Models of Motor Neuron Degeneration

“…Likewise, the required stimulus to reach supramaximal stimulus in these cases was similar for fore- and hindlimbs and varied between 5-12 mA. In CMT, the requirement for increased stimulus intensities has been recognized 17 and the same phenotype was seen in C61-PMP22 mice ( Figure 4C ). The phenomenon has been explained by increased electrical impedance from hypertrophic endoneurial changes 17 .…”

iMAX: A new tool for assessment of motor axon excitability. A multicenter prospective study

“…In CMT1a patients, motor axon hypoexcitability was particularly pronounced and systematically concerned the three studied parameters (minimal threshold, iUP, iMAX) at the three stimulation sites which was not the case for CIDP patients (Table 7). In a very similar approach (stimulus intensity value required to obtain a supramaximal CMAP), although retrospective, Parker et al (2016) observed in median and ulnar studies that mean supramaximal intensities were significantly higher in patients with CMT and CIDP than normal controls. Many studies have also been conducted using Qtrac software (threshold tracking techniques) to study motor nerve excitability in demyelinating neuropathies.…”

<em>In Vivo</em> Electrophysiological Measurement of Compound Muscle Action Potential from the Forelimbs in Mouse Models of Motor Neuron Degeneration

“…Likewise, the required stimulus to reach supramaximal stimulus in these cases was similar for fore-and hindlimbs and varied between 5-12 mA. In CMT, the requirement for increased stimulus intensities has been recognized 17 and the same phenotype was seen in C61-PMP22 mice (Figure 4C). The phenomenon has been explained by increased electrical impedance from hypertrophic endoneurial changes 17 .…”