Superiority of high frequency hypoxic ischemic EEG signals of fetal sheep for sharp wave detection using Wavelet-Type 2 Fuzzy classifiers

There is approximately a 6-8 hour window that exists from when a hypoxic-ischemic insult occurs, in utero, before significant irreversible brain injury occurs in new born infants. The focus of our work is to determine through the electroencephalogram (EEG) if such a hypoxic-ischemic insult has occurred such that neuro-protective treatment can be sought within this period. At present, there are no defined biomarkers in the EEG that are currently being used to help classify if a hypoxic ischemia insult has occurred. However, micro-scale transients in the form of spikes, sharps and slow waves exists that could provide precursory information whether a hypoxic-ischemic insult has occurred or not. In our previous studies we have successfully automatically identified spikes with high sensitivity and selectivity in the conventional 64Hz sampled EEG. This paper details the significant advantage that can be obtained in using high frequency 1024Hz sampled EEG for sharp wave detection over the typically employed 64Hz sampled EEG. This advantage is amplified when a combination of wavelet Type-2 Fuzzy Logic System (WT-Type-2-FLS) classifiers are used to identify the sharp wave transients. By applying WT-Type-2-FLS to the 1024Hz EEG record and to the same down-sampled 64Hz EEG record we demonstrate, how the sharp wave transients detection increases significantly for high resolution 1024Hz EEG over 64Hz EEG. The WT-Type-2-FLS algorithm performance was assessed over 3 standardised time periods within the first 8 hours, post occlusion of a fetal sheep, in utero. 1024Hz EEG results demonstrate the algorithm detected sharps with overall performance rates of 85%, 92%, and 87% in the Early/Mid and Late-latent phases of injury, respectively as compared to 25%, 55% and 31% in the 64Hz EEG. These results demonstrate the power of Wavelet Type-2 Fuzzy Logic System at detecting sharp waves in 1024Hz EEG and suggest that there should be a movement toward recording high frequency EEG for analysis of hypoxic ischemic micro-scale transients that does not occur at present.

I. Introduction

A major cause of brain injury in preterm infants is due to hypoxia-ischemia [1]. This is caused when the brain becomes starved of oxygen which can occur during a difficult delivery in child birth, resulting in Cerebral palsy and major handicap [2], [3]. It has been shown in the fetal sheep model of hypoxic-ischemia that the electroencephalogram (EEG) [2], [3] exhibits a 6–8 hours post insult period, known as the ‘latent phase’ after which epileptiform activity [2], [4] of high amplitude appears (shown in, Figure 1B). There are three distinct regions that are delimited in the latent phase period. These are known as the Early-latent, Mid-latent, and Late-latent phases which follow each other chronologically in the whole latent phase period. A very successful neuro-protective hypothermia treatment has been developed by world leaders in our team, Gunn et.al [5], that inhibits the epileptiform activity that leads Cerebral palsy and major handicap in newborns. However, such a treatment must be administered before the end of the late-latent phase to avoid brain-injury occurring.