SEPs and BAEPs are extensively and increasingly used in intensive care units to assess the extent of injury and predict the outcome for comatose patients. This makes it essential to be able to take relatively noise-free measurements and document the response peaks by making at least two successive replications of the measurements.
Both the general outcome and the interpretation of the responses are influenced by the cause of the coma. There are four main clinical groups of coma conditions: anoxic ischaemic brain damage following cardiac arrest or respiratory arrest, traumatic brain damage, cerebrovascular disease or space-occupying lesions in the central nervous system and metabolic or toxic coma. In most cases, the cause of the coma will be known after hospital-based investigations during the first 24 hours. In contrast to electroencephalography (EEG), EP studies do not provide essential aetiological information, but they may often provide good prognostic information. With metabolic/toxic coma, normal SEP findings will strengthen clinical suspicion of a reversible condition. Patients with traumatic head injuries, cerebral haemorrhage or infarction will more often have complications such as oedema, herniation or hydrocephalus, and repeated EP testing is often indicated. Anoxia primarily has a diffuse effect on grey matter, and the outcome depends on the duration of the anoxia.
Somatosensory evoked potentials are very valuable for early prediction of the outcome of post-anoxic brain damage. Bilateral absence of response from the sensory cortex (N20) is the best predictor of failure to regain consciousness, i.e. death, a persistent vegetative state or minimal consciousness, with a specificity of 100 % (12, 13) (Fig. 1). The same applies if anoxia patients receive hypothermia therapy, at least down to 33°C (14). On the other hand, the presence of a N20 response peak from the sensory cortex (on one or both sides) can not be interpreted as a definite favourable predictor, because about half of these patients will not regain consciousness.
In contrast to EEG, somatosensory evoked potentials are not substantially affected by general anaesthesia or sedatives, and N20 is measurable even in cases of sedation to isoelectric EEG. According to the most recently published international recommendations (15), somatosensory evoked potentials can be conducted 24 hours or earlier after cardiac arrest, but we often recommend, particularly with children (16), monitoring the course with repeated tests over several days.
SEPs are also a useful predictor with children, but in contrast to the situation with adults, a good prognosis has been reported for a few children even in the bilateral absence of N20 (17). Few articles about SEP in children have been published (17) and the findings must therefore be interpreted more cautiously than for adults.
Normal BAEP findings have no predictive value for anoxic brain injuries, because cortical neurons are more sensitive to hypoxic-ischaemic damage than neurons in the brainstem. Preserved brainstem activity, on the other hand, has far greater predictive value for traumatic brain damage. Normal BAEPs will be a good sign in this situation, and given normal SEPs in addition, the predictive value for waking up is 90 % and the probability of a favourable outcome 75 – 80 % (13). Pathological BAEP tests (particularly the absence of wave V) indicate pontine dysfunction and are associated with death or a persistent vegetative state in over 90 % (15). Bilateral absence of somatosensory evoked potentials from the sensory cortex is not associated with death or a vegetative state in all brain damaged patients, but is a good predictor for this group as well (18).
EPs can have similar predictive value for patients with coma due to cerebrovascular causes (Fig. 2). In the event of substantial supratentorial bleeding or infarction, the absence of SEPs and BAEPs predicts a poor outcome, while normal findings are equally uncertain predictors as in cases of anoxic damage (19). Repeated EP testing beyond the first week elicits little supplementary information in this group (20).