Abnormal EEG Activity

The pathophysiology of abnormal EEG activity is multifactorial and difficult to clarify. Increased excitability of the cerebral cortex probably accounts for cortical spike activity. More generalized spike-wave bursts probably depend on thalamocortical projections. The delta activity recorded under certain pathological circumstances probably reflects lesions involving predominantly the cerebral white matter. When subcortical nuclei are involved, either alone or in association with pathology affecting the cerebral cortex itself, paroxysmal slow activity is seen and may include spike discharges. '5!

Slow activity sometimes occurs with a localized distribution and varies in rhythm, rate, and amplitude with time. Such a focal polymorphic slow-wave disturbance, when present continuously, suggests an underlying structural lesion involving the cerebral white matter ( ...Fig.,.24-2 ) but provides no clue as to its nature. Transient polymorphic slow-wave activity is suggestive of either migraine or a postictal state after a partial (focal) seizure.

Intermittent rhythmical slow activity may occur with a frontal emphasis in adults or an occipital emphasis in children and is a nonspecific finding that has been described in a variety of settings, including metabolic disturbances, hydrocephalus, and diencephalic lesions.

A diffusely slowed record during wakefulness is nonspecifically abnormal but is seen most commonly in patients with diffuse encephalopathic processes such as a metabolic disturbance or encephalitic disorder (...Fig...2.4:3 ). It may also reflect long-standing diffuse cerebral dysfunction such as occurs in patients with a perinatal static encephalopathy.

Focal attenuation of EEG activity reflects destructive disease of the cerebral cortex, whereas more diffuse attenuation may result from severe encephalopathic processes and occurs also in certain degenerative disorders such as Huntington's disease. Electrocerebral silence or inactivity, in recordings made under conditions to enhance the presence of any underlying low-voltage EEG activity, is suggestive of neocortical brain death but may also occur in patients who are hypothermic or have taken an overdose of drugs that depress the central nervous system (CNS) (...Fig 24-4 ).'?' In some instances, the EEG consists of bursts of mixed-frequency activity occurring on a relatively quiescent background. This so-called burst-suppression pattern may occur with any severe encephalopathy, such as after severe cerebral anoxia or after head trauma, after overdose of CNS depressant drugs, and with anesthesia.

Epileptiform discharges consist of abnormal paroxysmal events containing sharp waves or spike discharges, at least in part. A spike discharge is a potential having a sharp contour and a duration of less than 80 msec, whereas a sharp wave has a duration of between 80 and 200 msec. They may occur without any clinical correlates or significance but are found interictally with a greater incidence than normal in patients with epilepsy.

Periodic lateralized epileptiform discharges (PLEDs) are lateralized epileptiform discharges that occur with a regular periodicity. They occur in association with acute hemispheric pathology such as an infarct or rapidly expanding tumor, y , '8 typically in obtunded patients with a focal neurological deficit and often recurrent seizures, and

Figure 24-1 A posteriorly predominant 9-Hz alpha rhythm is present when the eyes are closed and is attenuated by eye opening in the EEG of this normal subject. Electrode placements in this and succeeding figures are as follows: Fp, frontopolar; F, frontal; C, central; P, parietal; O, occipital; T, temporal; A, earlobe; Sp, sphenoid. Right-sided placements are indicated by even numbers, left-sided placements by odd numbers and midline placements by Z.

are commonly replaced after several weeks by a continuous polymorphic slow-wave disturbance.

Repetitive complexes consisting of slow waves, sharp activity, or both occur with a regular periodicity and generalized distribution in various disorders. Such periodic complexes are especially characteristic of certain infective disorders of the brain but may also occur after severe cerebral anoxia, head injury, or generalized seizures, with deep anesthesia, and in cerebral lipidosis.

Certain EEG patterns are worthy of mention because they are frequently regarded as abnormal when, in fact, they are simply normal variants of no pathological significance. Such patterns include 14- and 6-Hz positive spikes, small sharp spikes (sometimes referred to as benign epileptiform transients of sleep), wicket spikes, 6-Hz spike-wave activity, and rhythmical temporal bursts of sharpened theta activity, all of which occur most commonly during drowsiness or light sleep. y

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