Pearls, Perils, and Pitfalls In the Use of the Electroencephalogram

Omkar N. Markand, MD, FRCPC


Semin Neurol. 2003;23(1) 

In This Article

EEG in Focal or Lateralized Cerebral Hemispheric Lesions

Since the advent of computerized tomography and MRI, the EEG has been utilized less for localizing focal cerebral lesions, including brain tumors. Nevertheless, the EEG is still extensively used to evaluate the epileptogenic potential of a focal cerebral process demonstrated on imaging studies. The EEG shows focal or lateralizing findings in localized lesions that involve a superficial assessable portion of a cerebral hemisphere.[61] There is slowing and decreased amplitude of the alpha rhythm on the side of the focal cerebral lesion. With extensive processes, the alpha rhythm disappears and is replaced by slower-frequency activity (theta/delta). Comparable changes can occur in the anterior beta activity and can be spontaneous or drug-induced. During NREM sleep, spindles may be less persistent and of lower amplitude as may vertex sharp transients. In massive or rapidly progressive hemispheric lesions such as a major hemispheric stroke or large glioblastoma, there may be severe depression of all EEG activities in that cerebral hemisphere.

Since Walters' observation[62] a focus (localized activity) of delta activity has become the sign "par excellence" of focal structural lesions. The delta activity is called polymorphic or arrhythmic (PDA) because it consists of waves of irregular shape that change in duration, shape, and amplitude (Fig. 15) and fall in the frequency range of 0.5 to 3.0 Hz. Focal PDA indicates a lesion that involves subcortical white matter. Greater variability in the waveform (irregularity), longer duration of waves (slower frequency), and greater persistence indicate a more severe and acute focal process. A fact less often appreciated is that in a large area of PDA the focal process is best localized to the area showing the lowest-amplitude or "flat" PDA, rather than the area showing high-amplitude PDA.[63] Destructive lesions most frequently associated with focal PDA include neoplasm, abscess, infarct, hematoma, and contusion. However, focal PDA can appear transiently after a complex migraine attack or focal epileptic seizure. Hence, in a patient with prominent focal PDA with a history of a recent epileptic seizure, a repeat recording in a few days is indicated to assess the persistence or transient occurrence of this focal abnormality. Rapid disappearance of focal PDA would suggest a postictal change but would also lend support to a focal epileptic process. Static lesions such as infantile hemiplegia or Sturge-Weber syndrome[64] are associated with marked attenuation and often total absence of rhythmic activities (alpha or beta activity) over the entire affected hemisphere (Fig. 16). In contrast to progressive hemispheric lesions, such as cerebral tumor, there is very little, if any, slow activity over the involved hemisphere in such lateralized static focal processes.

Figure 15.

EEG of a 43-year-old patient with right temporal glioma, showing polymorphic delta activity and low-amplitude spike discharges (*) over the right temporal region. (Reprinted from Daly and Markand,[61] with permission from Lippincott Williams & Wilkins.)

Figure 16.

EEG of a 16-year-old patient with Sturge-Weber syndrome of the right hemisphere, showing total absence of rhythmic activities over the entire affected hemisphere. (Reprinted from Daly and Markand,[61] with permission from Lippincott Williams & Wilkins.)

Certainly, attenuation, disorganization, and slowing of the background activity on the side of the focal cerebral lesion and presence of PDA are EEG hallmarks of a focal cerebral process. Less often, the amplitude of the background activity may be higher on the side of the focal cerebral lesion,[65] which may lead to an erroneous interpretation of the side of the lesion. Such increase in the amplitude of the background activity is encountered with cerebral infarcts that have "healed," with skull defect related to previous craniotomy or in patients with slowly progressive tumors (Fig. 17). Often the enhanced background activity (such as alpha rhythm) over the side of the focal cerebral process is slightly slower in frequency as well as less reactive to eye opening,[63] which should alert the interpreter to the abnormality. Breach rhythms[66] associated with skull defects are focal "mu-like" rhythms in Rolandic or temporal region with sporadic slow waves and spiky or sharp transients (Fig. 18). These rhythms are unrelated to epilepsy and do not indicate recurrence of a tumor. The "spiky" grapho-elements should not be overinterpreted as epileptogenic discharges. For proper assessment of EEG asymmetries, it is therefore essential to know if the patient has had a craniotomy or skull defect, which may enhance background activities on the side of the breach of the skull.

Figure 17.

EEG of a 47-year-old patient with a low-grade glioma of the left temporal lobe, showing slightly slow but higher amplitude alpha on the left side.

Figure 18.

EEG of a 47-year-old patient with history of previous left craniotomy, showing breach rhythm in the left temporocentral region.

Epileptiform activity, such as focal spikes, sharp waves, or spike wave discharges, also occur in localized hemispheric lesions usually of an indolent or static nature. With acute hemispheric lesions, epileptiform discharges are less common but when seen often have a periodic character. PLEDs consist of sharp waves, repeating more or less regularly at one per second over a relatively large area of the hemicranium during most of the EEG study (Fig. 19). This distinctive focal periodic pattern usually occurs in patients with acute hemispheric strokes, brain abscess, primary (usually glioblastoma) or metastatic neoplasms, and herpes simplex encephalitis.[44,67]


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