Identical late motor responses in early Guillain-Barré syndrome: A-waves and repeater F-waves.

Electrodiagnostic (EDx) studies play a key role in the investigation of suspected Guillain-Barré syndrome (GBS), providing diagnostic and prognostic information. However, initial EDx findings may not fulfill the neurophysiological criteria for the disease. The aim of this study was to estimate the occurrence and characteristics of A-waves and repeaters F-waves (Freps), both late motor responses identical in latency and configuration, in early stages of GBS. We retrospectively analyzed the initial nerve conduction study (NCS) of 26 GBS patients performed within 10 days from symptom onset. The final subtype diagnosis was acute inflammatory demyelinating polyneuropathy (AIDP) in 16 patients (six met the criteria at the initial EDx study and 10 at follow-up) and acute motor axonal neuropathy (AMAN) in 10 patients (six initially). Identical late responses were commonly found in the majority of nerves (84%). A-waves were present in 59% and an increased frequency of Freps was calculated in 61% of the 105 studied nerves. A-waves morphology (single or complex) could not distinguish between AIDP and AMAN. Nerves with normal NCS had a significantly higher frequency of A-waves, either isolated or in combination with increased index total Freps, as compared to nerves with low compound muscle action potential (CMAP) amplitudes or conduction block. Our findings suggest that both late responses can be useful as early markers of conduction changes of various pathophysiology, being frequently present even prior to abnormalities of CMAP parameters.