Prolongation of the tibialis anterior CMAP duration in chronic inflammatory demyelinating polyneuropathy.

Objective: To assess the duration of the distal compound muscle action potential (dCMAP) recording from the tibialis anterior (TA) as a supportive electrodiagnostic feature in chronic inflammatory demyelinating polyneuropathy (CIDP).

Methods: We performed peroneal motor conduction studies with recording from the TA in 35 CIDP patients, 30 normal controls, and 21 disease controls. The normal cut-off for the TA dCMAP duration was determined to be 14 ms.

Results: Prolonged TA dCMAP durations were detected in 34% of CIDP patients (12/35) and in 33% (2/6) of patients in whom only one demyelinating lesion was identified by conventional motor conduction studies. Prolonged TA dCMAP durations were present in 28% (5/18) of patients with normal duration dCMAPs recorded from the abductor hallucis (AH) and in 42% (5/12) of patients with normal duration dCMAPs recorded from the extensor digitorum brevis (EDB). In patients with AH or EDB dCMAP amplitudes <1 mV, TA dCMAP durations were prolonged in 28% and 23% of patients, respectively.

Conclusions: Determination of TA dCMAP duration appears to be useful for detecting demyelination in CIDP, especially when there is significant coexisting axon loss.

Significance: In patients with potential CIDP and limited electrodiagnostic abnormalities by routine studies, the finding of additional demyelinating findings, such as increased TA dCMAP duration, could allow for improved diagnostic sensitivity.