Chapter 9: Focal Epilepsy

Interictal epileptiform discharges (IEDs) associated with epilepsy

• ▪ With clinical correlation, the high sensitivity and the specificity of IEDs for seizure disorders support the use of IEDs as the electrophysiological signature of an epileptogenic brain.

• ▪ IEDs represent the macroscopic field created by the summation of potentials from pathologically synchronized bursting neurons.

Common types of IEDs

• ▪ Spikes, polyspikes, sharp waves, and spike-and-slow-wave complexes, which can be either focal or generalized.

• ▪ The main types of generalized IED patterns are:

  • ▸ 3-Hz spike and slow wave

  • ▸ Sharp and slow wave

  • ▸ Atypical repetitive spike and slow wave

  • ▸ Multiple spike and slow wave

  • ▸ Paroxysmal fast activity (PFA)

Definition of IEDs and features

• ▪ The epileptiform sharp waves and spikes are:

  • ▸ Transient waveforms that may repeat, and arise abruptly out of the EEG background activity.

  • ▸ The waveforms are asymmetric with more than one phase (usually two or three). In contrast, nonepileptiform, sharply contoured transients such as wicket waves are often approximately symmetric.

• ▪ The epileptiform spikes and sharp waves are often followed by a smoothly contoured slow wave (spike-and-slow-wave complexes), which disrupts the ongoing EEG background rhythm.

• ▪ The sharp wave or spike is produced by an abrupt change in voltage polarity that occurs over several milliseconds. The duration of an epileptiform sharp wave is between 70 and 200 msec, and the duration of spikes is less than 70 msec, although the distinction is of unclear clinical importance.

• ▪ The epileptiform discharge should have a physiologic field and not be confined to a single electrode except in newborn.

• ▪ Besides interictal epileptiform spikes and sharp waves, intermittent rhythmic delta activity over the temporal region (TIRDA) has similar specificity for temporal lobe epilepsy.

• ▪ There are also localized, periodic patterns of IEDs that are associated with seizures. These periodic patterns of IEDs can be lateralized to one hemisphere, as seen in periodic lateralized epileptiform discharges (PLEDs), or can be bilateral or multifocal (BiPLEDs).

Types of interictal EEG patterns

• ▪Spikes, spike-wave discharges, polyspikes, or sharp waves (interictal epileptiform discharges [IEDs]).

• ▪ Broad sharp or polyphasic sharp waves (duration >200 msec).

• ▪ Periodic lateralized epileptiform activity (PLEDs) or bilateral independent periodic lateralized epileptiform activity (BiPLEDs) in an acute/subacute cerebral insult.

• ▪ Paroxysmal fast activity (PFA)—most commonly in Lennox-Gastaut syndrome.

• ▪ Hypsarrhythmia in infantile spasm (hemihypsarrhythmia in focal, symptomatic infantile spasm).

• ▪ Temporal intermittent rhythmic delta activity (TIRDA) in mesial temporal epilepsy.

• ▪ Occipital intermittent rhythmic delta activity (OIRDA) in generalized epilepsy, especially absence epilepsy.

• ▪ Frontal intermittent rhythmic delta activity (FIRDA) rarely represents IED.

• ▪ Continuous, near-continuous, or long trains of localized spikes or rhythmic sharp waves (intrinsic epileptogenicity) in structural abnormalities, especially focal cortical dysplasia (FCD).

• ▪ Regional polyspikes (especially in extratemporal region) are highly associated with FCD (80%).

• ▪ Focal low-voltage fast activity or electrodecrement in ...