An EEG may also be of help in the diagnosis of coma that is due to ongoing seizures (“nonconvulsive status epilepticus”) or, when the pertinent history is not available and there was an unobserved convulsion. It may also point to an otherwise unexpected cause of coma, such as hepatic encephalopathy, intoxication with barbiturates or other sedative-hypnotic drugs, the effects of diffuse anoxia–ischemia, catatonia, or hysteria (in which the EEG is normal).
Alzheimer disease and other degenerative diseases that cause serious impairment of cerebrocortical function are accompanied by relatively slight degrees of diffuse slow-wave abnormality in the theta (4to 7-Hz) range; many recordings are normal in the early and midstages of illness. More rapidly progressive disorders—such as subacute sclerosing panencephalitis (SSPE), Creutzfeldt-Jakob disease, and to a lesser extent the cerebral lipidoses—often have, in addition, very characteristic and almost pathognomonic EEG changes consisting of periodic bursts of high-amplitude sharp waves, usually bisynchronous and symmetrical (Fig. 2-7K). In a negative sense, a normal EEG in a patient who is profoundly apathetic is a point in favor of the diagnosis of hysteria, catatonia, or schizophrenia.
Other Diseases of the Cerebrum
Many disorders of the brain cause little or no alteration in the EEG. Multiple sclerosis and other demyelinating diseases are examples, although as many as 50 percent of patients with advanced disease will have an abnormal record of nonspecific type (mild focal or diffuse slowing). Delirium tremens and Wernicke-Korsakoff disease, despite the dramatic nature of the clinical picture, cause little or no change in the EEG. Interestingly, the psychoses (bipolar disorders or schizophrenia), intoxication with hallucinogenic drugs such as lysergic acid diethylamide (LSD), and the majority of cases of mental retardation are associated either with no modification of the normal record or with only minor nonspecific abnormalities unless seizures are present.
Clinical Significance of Minor Electroencephalogram Abnormalities
The gross EEG abnormalities discussed above are by themselves clearly abnormal, and any formulation of the patient’s clinical state should attempt to account for them. Lesser degrees of abnormality form a continuum between the undoubtedly abnormal and the completely normal and are of correspondingly less significance. Findings such as 14and 6-per-second positive spikes or small sharp waves during sleep, scattered 5or 6-per- second slowing, minor voltage asymmetries, and persistence of “breakdown” for a few minutes after hyperventilation are interpreted as normal variants or borderline abnormalities. Whereas borderline deviations in an otherwise entirely normal person have no clinical significance, the same minimal EEG findings associated with particular clinical signs and symptoms become important. The significance of a normal or “negative” EEG in certain patients suspected of having a cerebral lesion was discussed above.
As a general clinical principle, the results of the EEG, like those of the EMG and electrocardiogram, are meaningful only in relation to the illnesses under consideration and to the clinical state of the patient at the time the recordings were made.