Nerve excitability changes in chronic inflammatory demyelinating polyneuropathy: A new clinical diagnostic biomarker

The diagnosis of chronic inflammatory demyelinating polyneuropathy (CIDP) mainly relies on clinical presentation and traditional nerve conduction studies. However, diagnosing CIDP with an atypical presentation remains a challenge. Availability of an additional diagnostic utility, such as the nerve excitability test (NET), can improve clinicians' ability to diagnose CIDP. In this article, we present a review of published papers on the changes in nerve excitability parameters in CIDP. Among the nerve excitability parameters, a baseline increase of the threshold current in a stimulus-response curve, decreased strength-duration time constant, and "fanning-out" pattern of the threshold electrotonus are consistently noted. The recovery cycle might show increased superexcitability and the current-voltage relationship might show inward rectification, but these changes are less consistently noted. These parameters are compatible with membrane hyperpolarization in CIDP. On longitudinal follow-up, normalization of nerve excitability parameters is noted after intravenous immunoglobulin treatment. We also report a case of acute-onset focal CIDP with a longitudinal nerve excitability study, where nerve excitability changes consistent with previous studies have enabled early diagnosis. NET may be a useful tool for clinical neurophysiologists for early diagnosis and follow-up of CIDP.