A pilot study using retrospective data to describe the raggedness of the compound muscle action potential(CMAP) in Guillain Barre Syndrome(GBS) using Poincare plots M Lai(1),I Schofield(1),J Hunton(4),Jaiser,S(1,2),D Wu(3),S Baker(2),C Fisher(5),J Miller(6),M Baker(1,2). (1)Clinical Neurophysiology RVI Newcastle,(2)ION,Newcastle University,(3)Bournmouth University,(4)Durham University,(5)James Cook Hospital,Middlesborough Standard methods of quantification of the compound muscle action potential to peripheral nerve stimulation are latency, amplitude, conduction velocity and duration. Although raggedness of the CMAP is a common observation in GBS, there is no metric that describes or quantifies this phenomenon. The Poincare plot (PP) is an established metric of the heart rate variability, and is a plot of the intervals of the RR intervals against the subsequent RR interval–ie RR(n) plotted against RR(n+1).In this study the PP has been applied to the sample points of the digital signal of the rising slope of the CMAP of the abductor pollicis brevis(APB) to median nerve stimulation in patients deemed to have “normal” median nerves, median nerves in patients with a clinical diagnosis of GBS, and median nerves with electrophysiological evidence of entrapment at the wrist. The geometry of the PP in patients with normal median nerves comprise of a simple ellipsoid like loop. In patients with GBS the geometry of the PP of the CMAP is strikingly different especially in those with clear raggedness. Whilst the loop can still be seen, it increases in complexity becoming embedded with many smaller loops. Explored methods of quantification of the PP complexity will be presented. One method is to measure the length of the PP and express this as a ratio to its spread–ie a function of the maximum and minimum value of the PP. This metric demonstrated differences between “normal” median nerves and those of GBS and between median nerve pathology due to entrapment at the wrist, and GBS.