Nonlinear EEG changes in postanoxic encephalopathy

Application of concepts from nonlinear dynamics to EEG has suggested the-ex istence of two different types of pathophysiology in diffuse encephalopathi es. The first is characterized by decreased cortical activation. The EEG is then linear stochastic. The second is characterized by neuronal hyperexcit ability and hypersynchronous oscillations. The corresponding EEC contains n onlinear: deterministic structure. We examined the hypothesis that anoxic e ncephalopathy belongs to the second category. EEGs were recorded in 20 healthy controls (mean age 63.4 year; SD 3.7; 12 f emales, 8 males) and 15 patients (mean age 65.6 year; SD 17.7; 5 females,:1 0 males):with postanoxic encephalopathy following an episode of cardiac arr est. EEG epochs (16 seconds; sample frequency 250 Hz; 16 bit A-to D) were e xamined for nonlinear structure with the algorithm of MNLCP (multi channel nonlinear cross prediction). This algorithm characterizes each epoch in ter ms of its predictability pred, amplitude asymmetry ama and time irreversibi lity tir. Significant amplitude asymmetry or time irreversibility indicate nonlinear dynamics. EEGs of patients were significantly better predictable and more amplitude a symmetric than those of controls. Stronger evidence for nonlinear structure corresponded with less reactivity of the EEG to painful stimuli and a wors e outcome. Evidence for nonlinear brain dynamics in postanoxic encephalopat hy was found. This lends further support to the concept of 'linear stochast ic' and 'nonlinear deterministic' encephalopathies characterized respective ly by decreased cortical :excitability and increased neuronal excitability and their associated EEG patterns. Nonlinear EEG analysis may provide infor mation which has pathophysiological:, relevance. Also, nonlinear dynamics i s associated with a worse prognosis in anoxic encephalopathy, which may be Clinically relevant.