Among ancillary neurophysiologic tests, electroencephalography (EEG) and somatosensory-evoked potential (SSEP) have been extensively studied in assessing prognosis after brain injury of various etiologies. EEG is particularly useful early in the evaluation of patients by helping to differentiate anoxic coma from psychiatric conditions and from nonconvulsive status epilep-ticus. Beyond this, the degree of cortical damage roughly correlates with various features of the EEG (23). Several EEG patterns are associated with poor prognosis, including alpha-coma pattern, burst-suppression pattern, and an isoelectric recording. Alpha-coma pattern can be seen with brain stem ischemia after CA and is associated with a mortality rate of about 90% (23). In a systematic review of the literature, Zandbergen found that 5 of 6 studies reported 100% specificity for poor outcome with either an isoelectric or burst-suppression pattern. Using pooled data, the false positive rate for the presence of either pattern was estimated at 0.2% to 5.9%.
Among electrophysiologic tests, median nerve SSEPs have emerged as potentially the most useful. Bilateral absence of cortical SSEP to median nerve stimulation occurs with severe cortical brain injury and is associated with very poor prognosis. Robinson et al. reported an extensive review of the literature of SSEP in predicting prognosis of awaking from coma and extracted data from 41 articles for pooled analysis (24). Among 1136 adults with coma after CA (extracted from 18 articles), 336 had bilateral absence of median nerve SSEP, and none of these patients awoke from coma. The typical follow-up time of these studies was 6 to 12 months after CA. The 95% confidence interval for chance of awakening with bilaterally absent SSEP was 0% to 1%. In contrast, patients with present SSEP of some sort had a 41% chance of awakening, and the chances were better if the SSEP was normal (Fig. 2).
The above analysis was restricted to papers with a sufficient number of patients and data that could be extracted and analyzed. In their literature review, however, Robinson noted two articles reporting a total of 3 patients with absent SSEP who did awaken from postanoxic coma (24). In all cases, however, the SSEP was obtained < 24 hr from CA, and in one of these patients, the SSEP was present when repeated on day 3. Hence, median nerve SSEP may not be useful if obtained within the first 24 hr but should be obtained on day 2 or 3 after CA. Overall, absent median nerve SSEP identified only about half of patients who did not awaken (median sensitivity among studies reviewed, 0.45) but who had a very high specificity (with 95% confidence interval of 0-1% chance of awakening).
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