Abnormal EEG: Epileptiformt

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Abnormal EEG: Epileptiform

William O. Tatum, IV

Interictal epileptiform discharges (IEDs) represent a distinct group of waveforms that are characteristically seen in people with epilepsy. Scalp detection of IEDs is based upon dipole localization and the surrounding field. The interictal electroencephalogram (EEG) plays a pivotal role in providing ancillary support for a clinical diagnosis of epilepsy. Abnormal focal IEDs on EEG represent a heightened predisposition for the expression of focal seizures. Epileptiform discharges appear in different morphologies. Both spikes and sharp waves are referred to as IEDs and are defined by their duration. Mid-temporal IEDs occur in patients with temporal lobe epilepsy. Benign childhood epilepsy with centrotemporal spikes is a common childhood genetic localization related epilepsy syndrome. Frontal spikes are often found in patients with frontal lobe epilepsy, although they may be absent in up to one-third of patients. Central IEDs can occur with symptomatic focal epilepsies at any age.

clinical diagnosis, epilepsy, frontal spikes, interictal electroencephalogram, interictal epileptiform discharges, scalp detection

Electroencephalography, Epilepsy

Interictal epileptiform discharges (IEDs) represent a distinct group of waveforms that are characteristically seen in people with epilepsy. Variations of normal background rhythms, a variety of artifacts, and variants of uncertain significance may mimic abnormal IEDs and lead to overinterpretation of the EEG and untoward patient consequences (Chapter 1). IEDs have reliably been associated with epilepsy at rates sufficient to be clinically relevant. Although prominent intra-patient and inter-patient variability in frequency and morphology of IEDs may occur, those patients with prominent IEDs on the EEG are not necessarily the patients with more severe epilepsy. Scalp detection of IEDs is based upon dipole localization and the surrounding field. The resultant scalp detection of the source may appear different on the scalp EEG than the actual site of seizure genesis. In most cases, an IED reflects a radial dipole that is oriented to be detected on the scalp. However tangential dipoles may commonly occur in certain epilepsy syndromes (i.e., self-limited epilepsy with centrotemporal spikes [SLECTS]) or from developmentally or surgically altered cortex may produce unusual dipoles that challenge the EEG reader. Horizontal dipoles are better detected by magnetoencephalography which is often complimentary to source localization. Rarely, normal individuals may possess IEDs on EEG without the phenotypic expression of seizures. The photoparoxysmal (PPR) response, generalized spike-and-wave (GSW), and centrotemporal IEDs are the most frequent asymptomatic IEDs encountered in the absence of clinical seizures. When they are encountered, they typically reflect the genotype of an inherited trait that is represented on the EEG without the phenotypic expression of clinical seizures. Additionally, focal IEDs have a variable association with clinical epilepsy and depend upon the location of their appearance. For example, central, parietal, and occipital spikes, in general, may be less likely to be associated with clinical epilepsy than IEDs associated with the frontal and temporal location in the absence of a structural lesion. IEDs have seen in migraine, certain drugs such as lithium or clozapine, the autism spectrum disorder, cerebral palsy, and blindness, among other conditions.

The interictal EEG plays a pivotal role in providing ancillary support for a clinical diagnosis of epilepsy. The presence of abnormal IEDs occurs in <1% to 2% of normal individuals. When IEDs are present in the interictal EEG, they can help to classify the type of seizure, the type of epilepsy, or the epilepsy syndrome. EEG support of the clinical diagnosis is provided by identifying the mechanism that would apply to the patient’s particular clinical semiology or semiologies (i.e., epilepsy with generalized tonic–clonic seizures). Classification of the epilepsies is based upon differentiating patients with seizures that have a focal onset from those that are generalized in onset. When classification of epilepsy syndromes is able to be determined the type and distribution of IEDs noted on EEG is often a cornerstone to diagnosis. Focal IEDs may be either focal, regional, lateralized, or secondarily generalized discharges (aka secondary bilateral synchrony [SBS] on EEG) and are characterized by their field of involvement. Their presence may help provide information that is useful in localizing the epileptogenic zone for the purposes of surgical treatment. Frontal, anterior temporal, and midline IEDs have the highest correlation with seizures. Furthermore, there is treatment information that can be clinically relevant in following the response to therapy (i.e., as in the case of the medical management of absence seizures). In addition, IEDs may provide prognostic information when considering a trial of antiepileptic 100drug taper and signify a lower likelihood of success when persistent IEDs are evident on the EEG prior to tapering. In the absence of IEDs, epilepsy is not excluded because of the deep cortex, fissures, gyri, and sulcal neuroanatomy that may not readily be represented at the scalp during routine recording. The EEG, while ideally suited for evaluating patients with epilepsy, is also not specific for etiology when demonstrating IEDs. The scalp EEG may demonstrate similar interictal and ictal discharges, appearing in the same or different regions of the brain. While various morphologies of IEDs exists, their distribution usually implies one type of epilepsy as opposed to more than one type (i.e., focal and generalized epilepsy).

101FOCAL EPILEPTIFORM DISCHARGES

Abnormal focal IEDs on EEG represent a heightened predisposition for the expression of focal seizures. The temporal lobe is the most epileptogenic lobe of the brain. The location also reflects the anticipated semiology expected from a seizure arising from that region in patients with focal epilepsies.

**FIGURE 5.1.** Intracranial (top 16 channels) and simultaneous scalp EEG recording (bottom 6 channels) obtained during a presurgical evaluation for drug-resistant epilepsy. Sensitivities in the top channels are 75 µv versus 7 µv/mm at the scalp. Note the absence of IEDs in the scalp EEG compared to the intracranial EEG where they occur at 1/sec (arrows).

It is often said that the presence of a normal interictal EEG does not exclude a clinical diagnosis of epilepsy. The rationale for this reflects limited cortical sampling by surface-based scalp EEG and an incomplete representation of the entire brain. Many deep-seated cortical gyri are unable to be “seen” unless intracranial electrodes are placed directly into or on top of the underlying cortex (Figure 5.1). Because scalp potentials are volume-conducted potentials through cerebrospinal fluid and meninges, skull, and subcutaneous tissue of the scalp, “buried” or low-amplitude potentials may be underrepresented at the level of the scalp EEG. Therefore, difficulty with source detection at the level of the scalp may arise because the generators of the IEDs are deep-seated (i.e., mesial frontal), small regions of involved cortex, exhibit rapid cortical propagation, or are obscured by movement or myogenic artifact.

**FIGURE 5.2.** Different morphologies that occur with interictal epileptiform discharges (IEDs) include sharp waves (seen during seconds 1 and 2), spikes and sharp waves (in second 3), polyspike-and-slow waves (in second 4), and spike-and-wave discharges (in the last second of the figure). The above example was recorded during an ambulatory EEG in a patient with localization-related epilepsy.

Epileptiform discharges appear in different morphologies. Both spikes and sharp waves are referred to as IEDs (transients) and are defined by their duration. They are clearly distinguished from the background rhythm but have no difference with respect to epileptogenicity. A spike has a duration of 20 to 70 msec while a sharp wave lasts 70 to 200 msec and appears more “blunted” can occur with or without an after-going slow wave. Polyspikes (or multispikes) are another common type of IED. Combinations of IEDs often occur in the same patient at different times (see Figure 5.2). Both spikes and sharp waves are generated at the top of the cortical gyrus and have a polarity that is most often negative at the surface of the human scalp EEG.

**FIGURE 5.3.** EEG demonstrating a couplet of left anterior temporal spike-and-slow waves. Note the very brief duration of approximately 20 msec.

Focal IEDs include epileptiform “spikes” that imply that a focal mechanism exists in a patient with a clinical diagnosis of epilepsy or seizures. The polarity of an abnormal epileptiform discharge designated as a spike is very frequently negative at the surface of the scalp EEG with a duration of 20 to 70 msec (Figure 5.3). Those discharges of <20 msec are suspicious for noncerebral potentials such as artifact that is generated by muscles. There may or may not be an after-going slow wave discharge. The location usually determines the potential for epileptogenicity with temporal locations usually carrying the highest association with clinical seizure expression. Furthermore, the seizure semiology can be inferred with anterior temporal IEDs carrying a greater risk for the expression of focal seizures of temporal lobe origin.

**FIGURE 5.4.** Left temporal sharp waves in the EEG of a 43-year-old man after a left temporal lobectomy who was being evaluated for reoperation. Note the positive phase reversal at T3.

Positive spikes are rarely encountered in the routine clinical EEG (Figure 5.4). IEDs (spikes and sharp waves) are almost always surface negative, generating the typical negative phase reversal in a bipolar montage. The situation commonly encountered in clinical practice when positive polarity of the IEDs are seen occurs in patients who have had surgery and have an altered cortical anatomy (i.e., cortical dysplasia). In neonatal EEG, positive IEDs reflect periventricular injury and are not uncommon, although with development, unless congenital brain malformations or prior brain surgery is evident, positive sharp waves are rarely encountered.

**FIGURE 5.5.** EEG showing a left anterior temporal sharp wave during second 5 (arrow) in a patient with drug-resistant temporal lobe epilepsy due to hippocampal sclerosis. Note the parameters of the sharp wave in the box.

The degree of epileptogenicity varies with location but the temporal region is the most common epileptogenic region in the brain. Anterior temporal spikes or sharp waves with a duration of 70 to 200 msec (Figure 5.5) have a clinical association with focal seizures of temporal lobe origin more than 90% of the time. These discharges have maximal electronegativity at the F7/F8 derivations using the 10 to 20 system of electrode placement. However, the amplitude of these IEDs is usually greatest in the “true temporal” (at T1 and T2), ear, or sphenoidal electrodes when additional electrodes are utilized. In one-third of patients, the discharges are seen bilaterally, are activated by sleep, and localize best in wakefulness or rapid eye movement (REM) sleep when present.