Neurophysiological investigations

Published on 09/04/2015 by admin

Last modified 22/04/2025

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Neurophysiological investigations

The electroencephalogram

The electroencephalogram (EEG) records the electrical activity of the brain. Its major use is in the diagnosis and characterization of epilepsy. An EEG is usually recorded between seizures (interictal) but sometimes during seizures (ictal). The diagnosis of epilepsy is clinical and a normal interictal EEG does not exclude a diagnosis of epilepsy. The EEG is also useful in the diagnosis of encephalitis, coma and dementia, especially if rapidly progressive. It is complementary to imaging techniques such as CT and MRI and should not be used to look for structural pathology.

The patient rests back and 20 electrodes are attached over the scalp with glue (Fig. 1). These are connected to a multichannel recorder, which generates a paper tracing or a computer record. This is often synchronized with a video recording of the patient. A skilled technician monitors the recording throughout, to detect and eliminate artefacts. The EEG is recorded with the patient’s eyes open and closed and several methods may be used to enhance sensitivity of the technique, routinely including forced hyperventilation for 3 min and stroboscopic photic stimulation at 1–50 Hz. Another technique to increase sensitivity is to deprive the patient of sleep before the EEG, then allow the patient to fall asleep during the recording.

Fig. 1 EEG leads on a patient.

Spikes and sharp waves may be focal (affecting only part of the brain) or generalized (simultaneously affecting all parts of the brain) (Figs 2 and 3). Focal spikes suggest epilepsy due to a focal disturbance and imply a focal structural cause; neuroimaging should be considered. Generalized spikes are seen as part of the generalized epilepsies, which usually start in childhood or adolescence (pp. 74–75). The EEG shows normal background rhythms with frequent paroxysmal spike discharges. Photic stimulation triggers the discharges in about 5% of patients with primary generalized epilepsy (PGE) but rarely in other forms of epilepsy. Other generalized patterns may be seen in children with an abnormal background EEG who have a poorer prognosis and are associated with mental retardation or structural pathology such as tuberose sclerosis.

Fig. 2 Spike and wave.

Fig. 3 Focal EEG disturbances.

Sensitivity and specificity of paroxysmal interictal EEG discharges

The EEG has a relatively low sensitivity and a higher specificity for the diagnosis of epilepsy. The results depend heavily on the population being tested. The false-positive rate of epileptiform interictal EEG abnormalities (epileptiform EEG changes in individuals who definitely do not have epilepsy) is 0.5–2% of randomly selected individuals and 5–10% of first-degree relatives of patients with epilepsy. The first EEG may show epileptiform changes in 50% of patients with definite epilepsy. A sleep-deprived EEG increases the diagnostic yield to 75% and repeating the EEG up to 80%. As many as 15% of patients with epilepsy consistently have a normal interictal EEG.

Ictal EEG

None of the interictal EEG changes in themselves makes the diagnosis of epilepsy, but diagnostic changes can be seen if the patient has a seizure during the recording. This can usually be achieved in absence epilepsy, where the typical 3 Hz spike and wave discharges can almost invariably be triggered in untreated patients by vigorous hyperventilation. Other seizures are seen more rarely.

The electrographic appearance of a focal seizure is characterized by an evolving seizure discharge. Normal rhythms are replaced by rhythmic activity which may be spiky, slow or fast but increases in prominence during the ictus and then subsides, to be replaced by slow activity, for minutes to hours until the EEG returns to its usual interictal appearance. Nearly all seizures can be detected on ictal scalp EEG.

With the increase in surgical treatment for epilepsy, it has become more important to define the region of seizure generation with EEG. Prolonged EEG monitoring using portable EEG equipment with video monitoring of the patient is commonly used to detect seizure discharges in presurgical evaluation.

Important non-epileptic EEG patterns

Some non-epileptic EEG patterns are listed in Table 1.

Table 1 EEG abnormalities in conditions other than epilepsy

Herpes simplex encephalitis	Very regular sharp wave (e.g. 1 Hz) activity over one or both temporal regions with underlying slow activity called periodic lateralized epileptiform discharges (PLEDs)
Metabolic encephalopathy	Generalized slow activity. Triphasic waves typically seen in hepatic encephalopathy
Creutzfeldt–Jakob disease (CJD)	Widespread rhythmic triphasic waves at 0.5–2 Hz, often associated with near-simultaneous myoclonic jerks (may be absent in new variant CJD)

Evoked potential studies

Evoked potential studies are a method of testing the integrity of sensory pathways from end-organ to cerebral cortex. They are useful in identifying subclinical sites of nervous system involvement, providing evidence of multiple lesions in CNS demyelination. The optic nerve is a particularly common site of subclinical involvement in multiple sclerosis (MS) and the visual-evoked potential (VEP) is generally the most useful test (Fig. 4). VEPs give a relatively poor indication of the site of disease along the sensory pathway and they have become less important in the diagnosis of MS since the advent of MRI scanning. Indications for evoked potentials are summarized in Table 2.

Fig. 4 Visual evoked potential. (a) Normal; (b) delay in the left eye indicating optic nerve demyelination.

Table 2 Indications for evoked potentials

Investigation	Possible indications
Visual-evoked potentials	Monocular visual loss, optic nerve disease, multiple sclerosis
Auditory-evoked potentials	Unilateral hearing loss, multiple sclerosis
Somatosensory-evoked potentials	Spinal cord lesions, radiculopathy, multiple sclerosis