Quantitative CMAP shape analysis differentiates between uniform and non-uniform motor nerve conduction slowing

Objective: Demyelination may cause a uniform reduction of the conduction velocity of all fibres of a peripheral nerve segment, or may non-uniformly affect only some nerve fibres while sparing others. In the latter case the routinely determined nerve conduction velocity (NCV) is normal while amplitude, duration, and shape of compound muscle action potentials (CMAP) vary with the distance between the stimulus and the muscle. This study was done to determine if and to what extent quantitative analysis of CMAP shapes has the potential to differentiate between uniform and non-uniform conduction slowing.

Methods: Data of patients with early inflammatory demyelinating polyneuropathy (IDP, non-uniform demyelination, n=20), hereditary neuropathy (CMT, uniform demyelination, n=8), motor neuron disease (MND, axon loss, n=20), and healthy controls (n=20) were analyzed. NCV, amplitude and duration of CMAP, minimal F-wave latency (FMIN), and F-wave chronodispersion (FDISP) in tibial nerves were measured. The high frequency attenuation (HFA) method was used to compare CMAP shapes.

Results: NCV and HFA best differentiated between IDP and CMT (Fig.1), as NCV was reduced only in patients with CMT (100%) and HFA was abnormal only in patients with early IDP (35%). CMAP duration was found increased in patients with early IDP (15%) and in patients with MND (10%). FDISP was increased in patients with early IDP (30%) and in patients with MND (20%). FMIN was increased in patients with early IDP (10%) and in patients with MND (10%), and in all patients with CMT in whom F-waves could be recorded.

Conclusion: The results stress that an increased HFA specifically indicates non-uniform slowing of nerve conduction while a reduced NCV indicates uniform slowing. With the help of HFA measurements, the diagnostic sensitivity of routine nerve conduction studies can be increased.