The electroencephalogram in altered states of consciousness.

The EEG can be very helpful in the evaluation of patients with altered states of consciousness. Diffuse slowing of background rhythms and the presence of triphasic waves suggests metabolic dysfunction, particularly hepatic. Generalized fast activity may be seen in patients with a drug intoxication. Abnormalities, such as PLEDs or focal continual PDA, support a diagnosis of supratentorial lesion, whereas a normal appearing EEG in a comatose patient suggests a brain-stem lesion. In addition, the EEG may reveal that the alteration in consciousness is due to continual epileptic activity without motor manifestations (nonconvulsive status) that had not been suspected. As indicated, certain patterns have prognostic implications. However, as these patterns are not specific for a single etiology, the EEG is of most help when the cause is known. For example, burst suppression or a diffuse alpha pattern coma can be seen in either hypoxia or a drug intoxication. With hypoxia, these patterns carry an extremely poor prognosis for useful recovery, whereas complete recovery is often seen in cases secondary to drug ingestion. As a more extreme example, a patient whose EEG shows ECS secondary to drug intoxication may recover but will not if the ECS is due to hypoxia or severe head trauma. If the etiology of the encephalopathy is unknown, then sequential records are needed for prognosis.