Electrophysiological studies and electrodiagnostic criteria

Nerve conduction studies were performed according to standardized techniques ^[22]. In median, ulnar, peroneal and tibial motor nerves the following parameters were measured: distal motor latency (DML), amplitude of the negative peak of compound muscle action potential (CMAP), duration of CMAP, conduction velocity (CV), and minimal F wave latency. CMAP duration was measured manually at a sensitivity of 500 μV/division from the onset of the first negative phase to return to baseline of the last negative deflection ^{[2], [14]}. Proximal/distal (p/d) amplitude and duration ratios of CMAPs from different stimulation sites were also assessed.

Electrophysiological findings were normalized as percentages of upper and lower limits of normal (ULN, LLN) according to the reference values of each participating laboratory. The ULNs for distal CMAP (dCMAP) duration were determined using the control duration values reported for 2 Hz low frequency filter +2SD ^[14]. Distal CMAP duration was considered to be in the demyelinating range when this attained >120% of the ULN i.e.: 9.5 ms for the median, 10.2 ms for the ulnar, 9.5 ms for the peroneal, and 8.9 ms for the tibial nerve ^[22].

Sensory studies were performed antidromically in median, ulnar and sural nerves and amplitude of sensory nerve action potential (SNAP) was measured from baseline to negative peak.

The criteria set used here for electrodiagnosis of GBS subtypes was originally devised on the basis of two serial electrophysiological studies but also showed highest diagnostic accuracy in the first study compared with two other criteria sets, in a cohort with a balanced number of AIDP and axonal GBS patients. This approach has already been employed in the electrodiagnosis of GBS associated with Zika virus infection (table, supplementary material) ^{[22], [23]}. This criteria set is mainly characterized by the following features: (1) cut-off values for demyelination are quite stringent (DML > 130% ULN, motor CV <70% LLN); (2) duration of dCMAP is evaluated because it has been shown that increased duration is a sensitive and specific indicator of acquired demyelination ^{[2], [16]}; (3) because of the recognition that conduction failure (even completely reversible) can be an expression of axonal pathology, p/d CMAP amplitude ratio <0.7 is considered only for axonal GBS subtypes, whereas an increased (>130%) p/d CMAP duration is considered indicative of demyelination ^{[22], [24]}; (4) isolated F-wave absence, defined as unrecordable or markedly reduced persistence (<20%) of F wave with otherwise normal conductions, is introduced as a parameter suggestive of axonal pathology ^{[12], [22]}; (5) sural sparing, defined as abnormal ulnar and normal sural SNAP amplitude, is also taken into account ^[3].