The prediction of both short and long term outcomes following severe brain injury using somatosensory evoked potentials

This thesis describes studies assessing the ability of somatosensory evoked potentials (SEPs) to predict outcome following severe brain injury by examining outcome and determining the predictive value of SEPs directly and in comparison to alternative tests in both patients and systematic reviews of the literature. Outcome was assessed using a functional and quality of life measure. It changed over time and was influenced by age, mechanism, timing and the type of outcome measure. When 5 year functional outcome was used, sensitivity and specificity for the initial SEPs were 63.2% and 93.3% with a positive predictive value of 92.3% for favourable outcome and 66.7%, 94.7% and 90.9% for unfavourable outcome prediction. SEPs predictive performance varied and was better in patients with 1 year outcomes, when outcome was measured with the quality of life tool and in patients suffering hypoxicischaemic encephalopathy. Importantly, only twelve false positives were identified in the systematic review of 55 studies from 903 patients with bilaterally absent SEPs. Eight of these false positives suffered focal lesions of the brain stem, large cerebral fluid collections or recent decompressive craniectomy which cause SEPs to be absent because of a mechanical disruption to the electrical signal. Comparisons between SEPs and other tests in the patient cohort and wider literature showed that SEPs were the best overall predictors of outcome but were outperformed by some clinical tests in specific areas. Specificity for unfavourable outcome prediction was better for ICP, CPP and the last pupillary response. In patients with any cause of brain injury, the combination of SEPs and Motor responses provided the best predictions for unfavourable outcome while for favourable outcome the best overall prediction and specificity were achieved with a combination of either SEPs or Motor responses and the best sensitivity with pupillary responses alone or a combination of either SEPs or Pupillary responses. The studies in this thesis provide a detailed evaluation of SEPs and showed that SEPs have a place in the prediction of outcome, alone or in combination with existing tests. Overall, they are superior to clinical tests and can be easily obtained at the bedside and in the presence of pharmacological paralysis and analgesia/sedation.