What is the role of EEG in the workup of dementia and encephalopathy?

Updated: Oct 09, 2019
  • Author: Eli S Neiman, DO, FACN; Chief Editor: Selim R Benbadis, MD  more...
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Electroencephalography (EEG) is regarded as a fairly nonspecific measure of clinical states. A limited number of abnormalities that can be recognized in widely varied disease states exist.

On the other hand, an abnormal EEG is a sensitive measure of brain function. When the patient has clinically symptomatic encephalopathy or moderate dementia, the EEG is almost always abnormal.

Accordingly, the clinical utility of EEG must be appreciated in a different way from that of some other diagnostic procedures. In medicine, clear correlations and specific answers are desired; however, depending on the modality and the underlying principle of measurement, this goal cannot always be achieved.

In addition to EEG, magnetic resonance imaging (MRI) scan is very sensitive in showing various lesions in the brain. However, it is difficult to know exactly what these lesions represent on the basis of mere appearance. When interpreting MRI images, the clinician relies to a large extent on other relevant clinical information.

EEG measures electrical field variations, and a number of clinical conditions can disturb the normal electrical field of the brain. Simple state or electrolyte changes may alter the appearance and time variation of the brain-generated electrical fields; hence, a large number of conditions cause the EEG to appear abnormal. In EEG practice, the clinician has to rely to a large extent on the clinical history and the neurologic examination findings to make a clinically meaningful conclusion.

In most instances, the correct question may be whether the EEG is normal or abnormal. The next step is to decide how an abnormal EEG would help the clinical diagnosis; therefore, the EEG can be used to confirm clinical observation or suspicion or to determine the extent of the abnormality for prognostic purposes (ie, in attempting to predict outcome). Sometimes, EEGs serve as a "proof" to families that the patient's brain function is indeed so greatly disturbed that recovery is doubtful. In such cases, the EEG helps resolve anxiety and supports a more correct ethical decision.

Newer EEG techniques offer a number of conveniences and also enhance communication between the electroencephalographer and other clinical specialists. However, they may not make the record more specific but merely render it more easily understandable. Computer analysis, on the other hand, may offer features that, although present in the regular record, are difficult or time-consuming to extract and display.

EEG has a definite role in evaluating changes in mental states. It can confirm or rule out nonconvulsive status epilepticus (NCSE). EEG changes are usually proportional to the degree of metabolic, hepatic, or renal encephalopathy. EEG often is abnormal in subdural hematoma, normal pressure hydrocephalus (NPH), and Creutzfeldt-Jakob disease (CJD).

Computer analysis of the EEG may help reveal subtle changes in Alzheimer disease (AD). This application is promising and, it is to be hoped, will soon become clinically useful and available. In cases of clinical dementia, a normal EEG with preserved alpha might help establish the diagnosis of Pick disease, whereas a slowed and shifted alpha frequency is seen in AD and progressive supranuclear palsy (PSP). Low-voltage, flat EEG and the appropriate clinical presentation may raise the suspicion of Huntington disease.

EEG study should be requested if clear clinical indications are present or if the clinician has a reasonable presumption that it may give clinically relevant information. Such information would be expected to alter the clinical decision-making process. It would also help the referring source in diagnosis and treatment.

Another role is to help the patient or family to understand the ongoing disease process. EEG frequently is ordered to evaluate patients with different degrees of mental and behavioral changes and encephalopathy or coma. Usually, nonspecific abnormalities are present that do not give definite information about the cause of the underlying process but do provide information on its location and severity; therefore, unless epileptic seizures are a consideration, the EEG does not give direct unequivocal information on the cause of the patient's condition.

Nevertheless, EEG may differentiate between a generalized and a focal abnormality. This may guide the clinician to further appropriate imaging studies. On the other hand, if the abnormality is generalized, the EEG can be used to characterize and monitor the disease process. With coma, the EEG may help in predicting the neurologic prognosis. EEG is an important diagnostic tool in dementias in which specific morphologic lesions are not apparent on imaging studies.

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