|dc.description.abstract||General anaesthesia is an induced state that enables a person to endure surgical procedures without pain or recollection. There is substantial individual variability in the response to anaesthesia and in order to avoid adverse effects caused by either under- or over-sedation, anaesthetic drug administration must be tailored to suit the individual patient. This requires a means to monitor the depth of general anaesthesia. The electroencephalogram (EEG) records the electrical activity of the brain and enables effects of anaesthetic drugs on brain functioning to be monitored. Quantitative EEG monitors, such as the Bispectral (BIS) index monitor, process the raw EEG and provide a numerical output that is often used to measure the depth of general anaesthesia during surgery. Due to a number of factors including clinical conditions and genetic variability in the EEG, the BIS value can at times be misleading.
To identify genetic variations associated with EEG phenotypes relevant to anaesthesia monitoring, an association analysis was performed for 34 single nucleotide polymorphisms (SNPs) in a sample of 125 surgical patients undergoing general, gynaecological or orthopaedic surgery. During surgery, patients were anaesthetised with the volatile anaesthetic agent desflurane, the depth of anaesthesia was measured using BIS monitoring and the raw was also EEG recorded. SNP genotyping was performed for 13 SNPs in five candidate genes; SGIP1, GABRA2, CACNA1G, HCN1 and HCN2, using the polymerase chain reaction and restriction fragment length polymorphism analysis. An additional 21 SNPs in 15 genes involved in various inflammatory and other immune-related pathways were genotyped by Sequenom MassARRAY at the Australian Genome Research Facility.
Six SNPs in five different genes were found to be associated with spindle amplitude (SGIP1, GABRA2, HCN1, IL1B and MYD88), and a further five SNPs were associated with either delta frequency (IL10), or end tidal desflurane concentration (ETDC) (CACNA1G, CRP, MYD88 and TGFB1). The strongest associations were identified for a single SNP located in the 3' UTR of MYD88 (rs6853). The rs6853 A/G genotype was associated with higher median spindle amplitude (p = 0.0040) and spindle amplitude relative to ETDC (p = 0.0006), and lower EDTC (p = 0.0095) than the A/A genotype. No rs6853 G/G homozygotes were identified in the study sample.
MYD88 acts as an adaptor protein in the interleukin-1 receptor and toll-like receptor signalling pathways. Within the brain, cytokines are thought to act as neuronal modulators and influence neurotransmitter signalling and ion channel activity. The association of MYD88 with spindle amplitude, in conjunction with IL1B, suggests that cytokines may influence the EEG during general anaesthesia. Thus cytokine mediated regulation of neuronal activity is speculated to underlie the reported associations. All reported effect sizes were small (0.77- to 1.55-fold) and associated genes had four distinct types of function; ion channels, neurotransmitter signalling, endocytosis, and cytokine signalling. This suggests that numerous genes in different pathways, each with a small, possibly additive effect, are involved in regulating the EEG during general anaesthesia.||