EEG | The Massachusetts General Hospital Handbook of Neurology

Authors: Flaherty, Alice W.; Rost, Natalia S.

Title: Massachusetts General Hospital Handbook of Neurology, The, 2nd Edition

> Table of Contents > Adult Neurology > EEG (Electroencephalography)

EEG (Electroencephalography)

A. Indications

Confirming, localizing, or classifying seizures but a normal EEG does not rule out the possibility of seizures. EEGs aid dx of CJD and encephalopathy.

B. Montages

1. Bipolar: Good for localization; bad because distort widely distributed potentials, e.g., vertex waves. Location of spike = at phase reversal.
2. Referential: Good for sleep studies. More EKG artifact. Location of spike = at max. amplitude.

C. Studies

1. Routine
2. Sleep-deprived: More sensitive for epileptiform activity, which is seen especially in sleep-wake transitions.
3. Continuous: Indications include ACD coma in status epilepticus, nonconvulsive status, capturing seizures to determine the hemisphere in which the seizure began (for epilepsy surgery), and distinguishing electrographic seizures from nonepileptic spells (see p. 104).

D. Normal rhythms

1. Alpha: 8-12 Hz, posterior. Resting rhythm of thalamocortical network. Disappears when eyes open or when drowsy.
2. Beta: Fast, 20-40 Hz, anterior. Cortical activity during alertness.
3. Theta: 4-8 Hz. Normal during drowsiness or sleep; abnormal if alert.
4. Delta: <4 Hz. Normal during drowsiness or sleep; abnormal if alert.
5. Gamma: 40-80 Hz. Not analyzed in clinical EEG, but important for binding and coordinating the activity of different brain regions.

P.40

6. Sleep: Generalized slowing, vertex waves, K complexes, sleep spindles (14 Hz), drowsy hypersynchrony.
7. Normal effects of stimulation:
- a. Hyperventilation: Can cause generalized slowing in normals.
- b. Photic stimulation: Occipital driving; often more with eyes closed, can be normal.

E. Abnormal rhythms

1. Slowing: Theta or delta while alert. Nonspecific if generalized; may help localize lesion if focal.
2. Seizure: High-voltage, chaotic or rhythmic, focal or generalized.
3. Spikes and sharp waves: Interictal evidence for epilepsy.
4. FIRDA: Frontal intermittent rhythmic delta activity. Nonspecific encephalopathy.
5. Diffuse periodic sharps: DDx = CJD, lithium toxicity, baclofen toxicity, metabolic encephalopathy, subacute sclerosing panencephalitis (has complexes with longer periodicity).
6. PLEDS: Periodic lateralized epileptiform discharges. Do not try to eradicate them; just aim for normal ACD levels. May have different meaning in different contexts.
7. Triphasic waves: Usually from metabolic abnormality, although they look ictal.