Clinical Uses of the EEG

The EEG is useful in the evaluation of patients with several types of neurological disorders, including seizures, encephalopathy, and focal cerebral abnormalities. It also provides an ancillary aid to the diagnosis of brain death and may suggest certain specific neurological diagnoses when characteristic EEG findings are recorded in patients with clinical disorders of uncertain nature.

EPILEPSY

The EEG is important in the evaluation of patients with known or suspected epilepsy. The interictal occurrence of epileptiform activity in a patient with an episodic behavioral disorder that may represent seizure activity increases markedly the probability of epilepsy. y In patients with epilepsy, the EEG findings help to classify the seizure disorder, often permitting a distinction between seizure types when this cannot be made clinically. For example, the EEG findings permit typical absence seizures to be distinguished from complex partial seizures and generalized seizures to be distinguished from partial seizures having rapid secondary generalization (

Fig 24-5 ). Such findings therefore suggest the appropriate anticonvulsant drug for treating individual patients and provide a guide to prognosis. A guide to prognosis is also provided by the background EEG

Figure 24-2 EEG showing a focal polymorphic slow-wave disturbance in a patient with a right frontal glioma.

Figure 24-3 Diffusely slowed EEG in an obtunded patient with a metabolic encephalopathy.

Figure 24-4 Electrocerebral silence in the EEG of a brain-dead patient following attempted resuscitation after cardiopulmonary arrest.

Figure 24-5 EEG in a woman with complex partial seizures, showing focal spike discharges in the right central region.

activity--a slowed background implies a poorer prognosis than otherwise. In addition, a normal background activity EEG permits certain epileptic syndromes with specific prognostic implications to be recognized. For example, benign rolandic epilepsy of childhood has a characteristic interictal EEG pattern (rolandic spikes) and a good prognosis, whereas anterior temporal spike discharges are typically associated with complex partial seizures and imply a poorer outcome.

When surgery is under consideration for the treatment of medically intractable seizures, the EEG has a major role in localizing seizure foci, provided that actual seizures can be recorded. This generally requires admission to a hospital, where the EEG can be recorded continually while patients are video-monitored.

The interictal EEG findings may suggest certain characteristic seizure disorders. Hypsarrhythmia refers to a pattern characterized by high-voltage, irregular slow activity with superimposed multifocal spike discharges. It is typically found in children with infantile spasms, although some children with infantile spasms do not have this EEG pattern and it is sometimes found in patients without infantile spasms. [io] Generalized spike-wave activity suggests primary generalized epilepsy when it occurs interictally at 3 Hz and is bilaterally symmetrical and bisynchronous ( .Fig 2,4.-6 ) and Lennox-Gastaut syndrome when it is at 1 to 2 Hz and irregular in frequency and morphology. Polyspike- wave activity occurs with myoclonic epilepsy, including juvenile myoclonic epilepsy of Janz.

The ictal EEG recorded during a tonic-clonic convulsion

Figure 24-6 Burst of generalized 3- to 4-Hz spike-wave activity occurring interictally in a patient with primary generalized epilepsy.

is always abnormal, but the findings may be obscured by muscle and movement artifact. A transient attenuation of ongoing electrocerebral activity or the development of low-voltage fast activity heralds the onset of the seizure (..Fig 24-7. ), followed by the appearance of rhythmic, repetitive activity at about 10 Hz that gradually becomes intermixed with increasing amounts of slow activity until the record is characterized by spike-wave activity as the tonic phase of the seizure is succeeded by the clonic phase. This is followed in turn by a transient attenuation of the background EEG and then by irregular slow activity before the EEG reverts to its preictal appearance. During tonic seizures the EEG shows generalized, repetitive spike activity or an attenuation of the ongoing background activity. During a typical absence attack, by contrast, the EEG is characterized by 3-Hz spike-wave activity; and during myoclonic seizures it is characterized by polyspike-wave activity at 4 to 5 Hz. The EEG may show no ictal accompaniments during simple partial seizures, and this should not be taken to imply that the seizures are nonorganic in origin. In other patients with such seizures and in those with complex partial seizures, the EEG shows rhythmic, organized, localized discharges (.Fig, 24-8 ) with or without spike activity that may become more generalized as the seizure proceeds and may be followed postictally by a transient flattening of the traces and then by polymorphic slow activity with a localized or generalized distribution.

During tonic-clonic (major motor or grand mal) status epilepticus the EEG shows repeated electrographic seizures. The EEG is not required for diagnostic purposes, however, except when patients have been paralyzed pharmacologically to facilitate management; when coma has been induced to control refractory status, the EEG has an important role in monitoring the level of anesthesia. In patients with nonconvulsive status epilepticus, the basis for the abnormal mental status can only be recognized by the EEG finding of continuous, generalized spike-wave activity (so-called spike-wave stupor) or repeated partial seizures. This underscores the importance of obtaining an EEG in any patient with a change in mental status of uncertain basis. FOCAL CEREBRAL PATHOLOGY

With the advent of new imaging techniques, the role of the EEG in screening patients for suspected focal intracranial pathological processes has declined, except in developing countries. In fact, however, the EEG detects disturbances of function whereas imaging procedures detect structural abnormalities, so that the two approaches are complementary, not alternatives. EEG abnormalities are sometimes generalized in patients with focal intracranial pathology, detracting from the value of the technique in localizing the lesion but emphasizing its importance in reflecting the extent of functional impairment. Thus, generalized slowing of the EEG commonly reflects a disturbance in the level of consciousness. Even when focal abnormalities are present, they generally provide no indication as to the nature of the underlying pathology. They typically consist of a localized slow-wave disturbance (see Fig 24-2. ) or an attenuation of background electrocerebral activity, sometimes accompanied by focal spikes or sharp waves.

Figure 24-7 EEG recorded at the onset of a generalized clonic-tonic-clonic seizure, showing the development of bisynchronous low-voltage generalized fast activity that then becomes intermixed with increasing amounts of slow activity as the seizure continues.

ABNORMAL STATES OF CONSCIOUSNESS

The EEG is an important means of investigating patients with a disturbance of consciousness. It may suggest the cause of the abnormal mental state, helps to determine the depth of coma, and provides a guide to prognosis. Moreover, the EEG is an important means of distinguishing between unconscious states and neurological disorders simulating unconsciousness, such as the locked-in syndrome, in which the EEG is normal or shows only minor nonspecific changes. In patients with a disturbance of consciousness due to diffuse cerebral dysfunction, the EEG typically showed generalized slowing (see Fig. 24-3 ) without focal or lateralizing features. In some metabolic encephalopathies, triphasic waves are conspicuous, but these are a nonspecific finding. When epileptiform discharges or electrographic seizures are found in the EEG, they suggest that a postictal state or status epilepticus is responsible for the obtundation.

In coma after head injury, the EEG is likely to be diffusely slowed but focal abnormalities (slowing, spike discharges, or an attenuation of activity) may be superimposed. These localized abnormalities may relate to hematoma or cerebral contusion, ischemia, or edema.

Alpha-pattern coma designates coma associated with alpha-frequency activity in the EEG. The alpha-frequency activity is usually widespread in distribution and nonreactive to external stimuli. It is found most commonly after a cardiac or cardiorespiratory arrest and also occurs after pontine strokes and occasionally with a drug overdose. Alpha-pattern coma was originally thought to imply a poor prognosis, but the outlook relates more to the cause of the coma than to the presence of alpha-frequency activity in the EEG. '11] , y

In some comatose patients the EEG shows a burst-suppression pattern, indicating a severe encephalopathic process such as an anoxic encephalopathy. The finding of electrocerebral silence indicates that irreversible brain damage and neocortical brain death have occurred. A similar finding may also relate to hypothermia or the presence of CNS depressant drugs, however, and these conditions must always be excluded before prognostic implications are ascribed to the eEg changes.

OTHER APPLICATIONS

In patients with encephalitis or meningoencephalitis, the EEG typically shows diffuse slowing without any specific features. Characteristic abnormalities are, however, found in certain infectious disorders of the brain. In herpes simplex encephalitis, periodic lateralized complexes may occur over one or both temporal regions, usually repeating once every 1 to 4 seconds (. Fig 24-9 ). '1.3' These discharges may take up to 2 weeks to develop, and their absence does not exclude the diagnosis. Their presence provides strong support for it, however, and their bilateral occurrence implies a poorer prognosis for survival than otherwise. In

Figure 24-8 Repetitive focal spike activity arising in the right anteromesial temporal region (right sphenoidal electrode at Sp2) at the onset of a complex partial seizure in a patient with epilepsy.

Figure 24-9 Repetitive complexes occurring in the right temporal region of a child with herpes simplex encephalitis.

Figure 24-10 EEG of a patient with Creutzfeldt-Jakob disease showing periodic complexes that often have a triphasic configuration.

subacute sclerosing panencephalitis, generalized periodic complexes are seen, often lasting for up to 3 seconds and with an interval 3 to 15 seconds between complexes. A similar pattern is sometimes observed in patients who have taken phencyclidine. y In Creutzfeldt-Jakob disease (iiiFig:iii,24-10 ), periodic complexes are typically triphasic in configuration and occur at a rate of approximately 1 Hz. Their occurrence in patients with a rapidly progressive dementia strongly supports the diagnosis.1^ , y

The EEG findings are of little relevance to the diagnosis of migraine. They are usually normal, but focal or generalized slow activity may be found, especially during or immediately after migrainous attacks. Epileptiform transients are also encountered in some patients.

Abnormalities may be found in demented patients and consist usually of a nonspecific loss of the alpha rhythm and the presence of excessive slow activity. Focal slowing may also occur, especially in Alzheimer's disease. A normal EEG in no way excludes a diagnosis of dementia, but a diffusely abnormal record supports a diagnosis of dementia as opposed to pseudodementia. The EEG findings may suggest the cause of the dementia, such as in Creutzfeldt-Jakob disease or subacute sclerosing panencephalitis, or when a focal structural lesion is present.

The EEG findings after head trauma reflect the severity of injury and level of consciousness. They do not improve the ability to predict which patients are likely to develop post-traumatic epilepsy.

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