This article discusses EEG patterns that usually are generalized and are not considered primarily ictal. Many of these patterns occur in encephalopathic states, which themselves can lower a patient's threshold for seizures. Some patterns can be considered epileptiform since they contain spikes, sharp transients, or rhythmic paroxysmal patterns. Remember that the term epileptiform is descriptive of an EEG's appearance only and does not necessarily imply that the pattern is epileptogenic. One must always consider how the patient appears clinically while the EEG is recording. Certainly, correlation with episodic behavior, reactivity, or changes in level of response may indicate that even a vague slow generalized pattern may imply seizures.[1]
Spindle coma is similar in appearance and implications but consists of monorhythmic 11- to 14-Hz activities, occurring paroxysmally on a delta background, without reactivity. Such patterns often are seen in anoxia, head trauma, and diffuse cerebral insults. Prognosis is usually that of the underlying etiology, although reactivity portends a better prognosis than unreactivity on these EEGs.[4] This pattern must be distinguished from normal alpha rhythm in the locked-in state and from slower segments of 10- to 18-Hz rhythms observed in various intoxications.
Myoclonic jerks often occur in association with the sharp waveforms, but the relationship is not constant. Late in the illness and during sleep, myoclonic jerks disappear, despite the persistence of the periodic EEG. The sharp waves typically react to external stimuli. Early in the disease, alerting the patient may elicit the periodic pattern; later, when the periodic pattern is readily apparent, rhythmic photic or other stimuli can drive the periodic frequency. Benzodiazepines or barbiturates can temporarily eliminate both myoclonic jerks and periodic patterns.
The normal EEG also varies depending on state. The EEG is used along with other measurements (EOG, EMG) to define sleep stages in polysomnography. Stage I sleep (equivalent to drowsiness in some systems) appears on the EEG as drop-out of the posterior basic rhythm. There can be an increase in theta frequencies. Santamaria and Chiappa cataloged a number of the variety of patterns associated with drowsiness. Stage II sleep is characterized by sleep spindles—transient runs of rhythmic activity in the 12–14t Hz range (sometimes referred to as the sigma band) that have a frontal-central maximum. Most of the activity in Stage II is in the 3–6e Hz range. Stage III and IV sleep are defined by the presence of delta frequencies and are often referred to collectively as slow-wave sleep. Stages I-IV comprise non-REM (or NREM ) sleep. The EEG in REM (rapid eye movement) sleep appears somewhat similar to the awake EEG.
