EEG Patterns in Stupor and Coma

Slow-Wave Voltage

Low-voltage slow waves intermixed with the patient’s baseline background activity may be the first EEG sign of encephalopathic change (see Figure 12-1 ). An increase in the amount or amplitude of slow-wave activity suggests an increase in the severity of the encephalopathy. With deepening coma, slow-wave amplitude may continue to increase, and very high-voltage slow-wave patterns may be seen. Rather than intermixing with the background activity, the high-voltage slow-wave activity becomes the background. As cerebral function is increasingly affected, however, slow-wave amplitude can only increase to a certain point. With yet more severe cortical dysfunction, cortical rhythms begin to decrease in amplitude. With the most severe neurological processes cortical function becomes depressed and the brain becomes less able to maintain slow-wave voltages, resulting in diminished background activity and voltage. Thus, very low-voltage patterns in coma ( voltage depression ) are considered more ominous than high-voltage slow-wave patterns. The EEG patterns associated with the most severe degrees of cortical dysfunction show marked suppression of voltages or even electrocerebral inactivity.

Given this described sequence of initially increasing, then decreasing slow-wave amplitude with increasingly severe encephalopathy, a linear relationship between slow-wave amplitude and severity of encephalopathy cannot be assumed. When amplitudes are seen to decrease, this could represent either a trend toward normalization or signal a trend toward voltage depression and increasing dysfunction. In such cases, other EEG features (discussed later) such as frequency and reactivity of the background may help clarify the meaning of the change (see Figures 12-2 and 12-3 ).

The evolution of slow-wave activity during the improvement phase of a neurologic process may be less tightly linked to the patient’s neurologic status. The clearing of slow-wave activity often lags behind the patient’s clinical improvement. In a patient who is recovering from a dramatic encephalopathy, EEG slow-wave activity may still be present even as the patient wakes up, sits up, and begins talking. The persistence of slow-wave activity in the face of an improving neurologic picture is not necessarily a poor neurologic sign as long as there is a trend toward EEG improvement. Likewise, the slow-wave activity that follows a seizure ( postictal slowing ) may persist well past the point that patients report feeling back to their preseizure baseline. Slow-wave activity may persist after a seizure for hours, commonly a few days, but occasionally for as long as 3 to 4 weeks depending on the type of seizure, the duration of the seizure, and the general neurologic health of the individual.

Slow-Wave Frequency

The relationship of slow-wave frequency to coma severity is more straightforward than it is for slow-wave amplitude. In general, decreasing slow-wave frequencies suggest increasing severity of encephalopathy. A decrease in slow-wave amplitude can be associated with either improvement or deterioration in neurologic status as described earlier. Counting wave frequency is a useful tool for distinguishing between the two possibilities. If background frequency is increasing, this is a good sign; slower slow waves suggest deterioration. A similar approach is taken when comparing two hemispheres with slow-wave activity—one with higher voltages than the other. Higher voltage slowing may mark the more affected hemisphere, but it may be that the opposite hemisphere manifests lower voltages because it is the more abnormal side. In such cases, comparing the frequencies generated by each side may clarify which is the relatively “healthier” hemisphere, identified by its higher frequency.

Reactivity

EEG reactivity is an additional useful feature in assessing the depth coma. The EEG is monitored for change when the patient is stimulated. The stimulus may be as simple as calling the patient’s name or could include purposeful noxious tactile stimulation. Intensive care unit procedures such as endotracheal tube suctioning or venipunctures also provide an opportunity to observe EEG reactivity. An unreactive EEG is one that shows no change in response to stimulation. Reactive EEGs show a change with stimulation, such as an increase in amplitude and rhythmicity in low-voltage tracings or a relative flattening of the background in higher voltage tracings.

Presence of Normal Sleep Elements

The presence of identifiable sleep elements in the EEG in coma is felt to be associated with a relatively better neurologic prognosis. The presence of sleep features implies that there is enough cerebral structure intact to generate these elements. Sleep spindles are the most commonly identified sleep feature in this setting. In rare cases, the higher centers that generate sleep elements are intact, but there has been a severe injury at lower levels of the central nervous system, resulting in a poor outcome despite the persistence of sleep elements.