Sensitivity of conventional motor nerve conduction examination in detecting patchy demyelination: a simulated model

Abstract

Objective: To evaluate, in 5 simulated motor nerves with patchy demyelination: (1) the sensitivity of the conventional motor conduction examination; (2) the conduction velocity of single axons (SA-CV).

Methods: Four damaged segments were simulated in each nerve. Myelin impairment was generated by varying two parameters: (1) percent reduction in conduction velocity, i.e. degree of damage (DEGREE); (2) percentage of affected axons, i.e. extent of damage (EXTENT). Myelin impairment was simulated in axons with different diameters. We evaluated: (1) conduction velocity; (2) temporal dispersion of the negative phase of compound motor action potential (CMAP); (3) amplitude decay of CMAP; (4) SA-CV of 20 randomly-chosen axons.

Results: When the damage involved both large and small axons, the conduction velocity was pathological only when severe myelin damage involved a large number of axons. Temporal dispersion and amplitude decay were more sensitive than conduction velocity in detecting the damage. In damage involving only large axons or only small axons, all parameters remained in the normal range. SA-CV evaluation was much more sensitive than the conventional studies, regardless of the diameter of the damaged axons.

Conclusions: Conventional studies are not sensitive in detecting minimal myelin damage. Decomposing the CMAPs and randomly studying 20 SA-CVs would increase the sensitivity of damage detection.

Significance: These results contribute to a better understanding of the relationship between axonal properties and neurophysiological findings in motor nerve demyelination.