Updated in NEJM.
…patients whose pupillary reactions were absent at day 3 after cardiac arrest had poor outcomes (false positive rate, 0%; 95% confidence interval [CI], 0 to 3).
...the absence of a corneal reflex at 72 hours was associated with no false positives for a poor outcome (95% CI, 0 to 14 in one study18 and 0 to 41 in the other).
If the patient received a therapeutic hypothermia protocol I assume time zero would be at completion of the protocol rather than the time of the arrest but the review is not clear on that point. It does mention therapeutic hypothermia as a potential confounder in assessing for these neurologic signs over time.
The motor response to noxious stimuli also provides useful prognostic information. Several prospective studies, including one multicenter study involving more than 400 patients with cardiac arrest, showed that a motor response to noxious stimuli that was no better than extensor posturing (i.e., a decerebrate response or no response) at 72 hours was associated with no false positives for a poor outcome …
The author cautions that if the patient received therapeutic hypothermia this sign may not be reliable until day 6 or beyond.
In a prospective study involving 407 patients, myoclonic status epilepticus at 24 hours after arrest was associated with no false positives...
The caution here is that this syndrome must be differentiated from other seizure like syndromes, which means you'll want to get an EEG and some help from a neurologist.