Ictal symptoms and signs reflect area(s) of the brain involved in the seizure. Events early in the seizure have greater localizing value than later ones as these latter may result from propagation. As certain symptoms, such as a rising abdominal sensation in a focal seizure, may result from involvement of any one of two or more anatomically distinct regions usual accompanying phenomena may help to distinguish possible ictal areas of origin. Therefore, a cluster of patient- and observer-reported phenomena will more accurately chart seizure origin and propagation than will a single symptom or sign. Knowledge of cortical, thalamic, and brain stem physiology will equip the physician with insightful questions of the patient and associates and will allow perceptive evaluations of video-telemetry seizure depictions. Not only will such scrutiny localize most focal seizures but also it will often distinguish primary generalized from secondarily generalized seizures.¹

Precipitating factors such as flashing lights or sleep deprivation need be sought. Enquiry about lifestyle may disclose potentially alternative diagnoses, such as excessive daytime sleep in a child thought to have absence or dyscognitive (temporal lobe) seizures. Epilepsy is likely more often overdiagnosed than underdiagnosed. Social and psychological consequences of incorrect diagnoses are difficult to reverse.²

As some phenomena will localize but not lateralize to a neural system, that is, visual or limbic, the neurological examination may help to determine the side of epileptogenesis. A decreased nasolabial fold or impaired fine finger movements are two of many physical signs of potential ictal lateralizing value. Moreover, normal aspects of the examination may eliminate certain regions, for example, full visual fields for calcarine occipital seizure onset. Therefore, thoughtful clinical evaluation will develop specific questions for electroencephalography (EEG), thus enhancing its value.

Appropriate questions for EEG derive from the clinical evaluation and include: (1) does the child have epilepsy; (2) what type is present—focal, generalized, or secondarily generalized; (3) what is its severity and thus its prognosis; and (4) are there any avoidable precipitating factors?

In addition to an understandable concern about overlooking a significant EEG phenomenon, three factors can lead to overinterpretation of EEGs, particularly pediatric EEGs.

The first are sharply contoured artefacts such as muscle, sudden movement, electrode malfunction, and bottle-sucking. Before assigning any label to a waveform, think artefact first.

Secondly, nonartefactual apiculate (sharply contoured) waveforms usually appear in normal background activity of childhood, especially while awake or drowsy. These potentials are either apiculate in themselves or combine with other, smoother waveforms to create an apiculate morphology. As rich mixtures of waveforms appear in the posterior head, central and temporal regions; nonspike apiculate waveforms occur particularly often at these sites.

The following criteria help to identify real spikes in this apparent minefield: (1) the discharge should be paroxysmal, that is, clearly distinguished from background activity; (2) an abrupt change in polarity should occur in the waveform to produce a sharp contour; (3) the duration of the phenomenon should not exceed 200 ms; (4) the spike should have an electrical field that would conform to physiological principles; and (5) spikes usually exhibit two or more phases. Sleep recordings resolve some of these dilemmas as some sharply contoured potentials recede.

Thirdly, several spike-containing or otherwise apiculate normal phenomena appear frequently in the EEG of an awake child: six per second spike-waves, rhythmic midtemporal discharges (“psychomotor variant”), apiculate alpha or mu rhythm, “posterior slow of youth,” and lambda waves. Those of non-REM sleep include: vertex sharp waves (“V-waves”), 14/6 per second positive spikes, and positive occipital sharp transients (POSTS).^2,3 For the more sustained of these normal features, two helpful principles can be applied: (1) anything maintaining an invariably regular frequency is likely to be normal, and (2) apiculate waves that could result from the superimposition of background components should be considered normal.²

Despite the aforementioned cautions, EEG spikes can confirm the presence of epilepsy, indicate whether it is focal or generalized, and help localize focal onsets. The following data will indicate the extent of EEG’s value in these domains. Eeg-Olofsson⁴ found spikes in 2% of 743 normal children. Cavazzuti et al⁵ and Okubo et al⁶ identified spikes in 3.5% and 4.5% of normal children, but most were spikes of the benign partial epilepsies of childhood (see further). In contrast, about 30% of children will demonstrate spikes after a first seizure,⁷ and such discharges appear on 50%–75% of initial EEGs of children with epilepsy.^8,9 The latter study⁹ found that spike sensitivity rose to 92% with three recordings. Several factors may increase the yield: younger age, sleep recording, generalized epilepsy, and recording the EEG after a seizure cluster.^8,10 However, focal spikes originating on inferior or mesial surfaces of the cerebral cortex may not appear on scalp recordings; thus no spikes does not equate with no epilepsy.

FOCAL SPIKES

Localization principles derived from adult studies apply equally to older children and adolescents.¹¹ However, focal epileptogenic lesions in children approximately 6-year old or less may be associated with multifocal or diffuse spikes, including hypsarrhythmia (HR).^12,13 The topographical lability of spikes in young children impairs their ability to localize epileptogenesis. However, this correlation improves if several recordings are performed; localization of most consistent spiking correlated with lobe of effective epilepsy surgery in 32 (67%) of 48 children.¹⁴ Reliability in this regard further increases if accompanied by regional nonepileptiform phenomena.

TEMPORAL SPIKES

Anterior temporal spikes of children share features of those occurring in adults. These discharges may therefore involve M1, F7, T3, and A1 electrode positions.¹⁵ Hyperventilation and non-REM sleep may be necessary to entice their appearance. Given the normally greater quantity of theta and delta activity in pediatric EEGs, identifying accompanying nonepileptiform abnormalities may more greatly challenge the EEGer but the effort should be undertaken. Compared to adults, EEGs of children with temporal lobe epilepsy (TLE) more commonly display multifocal spikes.¹⁶ However, in our experience,¹⁷ such multifocality almost always resides in the hemisphere ipsilateral to temporal seizure origin. Given a limbic seizure semiology and any MRI evidence of principally unilateral mesial temporal sclerosis (MTS), maximally ipsilateral spike activity may suffice to confidently identify the epileptogenic lobe in children.¹⁷

OCCIPITAL SPIKES

Eeg-Olofsson⁴ found occipital spikes in only 2% of the 743 normal children in his study. Spikes appear more commonly in the occipital area than anywhere else among children less than 4 years of age and this is the age group in which they most commonly appear.¹⁸ Of 31 children with occipital spikes in a Newfoundland population study, epilepsy was present in 29 (94%). Of these, 23 (74%) had benign, nonlesional epilepsy; lesion-based seizures occurred in 5 (16%). Others had less-defined conditions.¹⁹

Epileptogenic lesions may attenuate, distort, or slow the frequency of the abundant normal EEG features of the occipital lobe, such as alpha activity, “posterior slow of youth,” and photic driving. Such features provide more confirmation of spike localization for epileptogenesis than is found for any other lobe. However, studies of occipital spike localization of occipital seizure origin have reached conflicting conclusions. Williamson et al²⁰ found interictal occipital epileptiform potentials of little localizing value; however, our studies^21,22 found the majority of spikes ipsilateral to epileptogenesis in 79%–94% of patients. Rarely, occipital spikes may appear contralaterally if their dipole is orientated in such a way that a contralateral electrode will best record it—a property seen principally in discharges from mesial structures.

Occipital spikes appear almost always with the eyes closed and immediately attenuate with eye opening while the opposite occurs with lambda, the other sharply contoured occipital paroxysm of wakefulness. While presenting as isolated or clusters of single spikes during wakefulness, polyspikes (multiple spikes) may occur from the same region in sleep.

A host of conditions may be associated with occipital spikes. They may appear in EEGs of young children blind from ocular abnormalities.²³ Several syndromes encompass migraine and occipital and other seizure disorders.²⁴ Occipital spikes, epilepsy, and calcifications in the occipital area can be associated with celiac disease.²⁵ A progressive myoclonus epilepsy, such as Lafora body disease, may produce occipital spikes and light-sensitive seizures.²⁶

Associated EEG features largely influence the clinical significance of occipital spikes. An otherwise normal EEG suggests a benign partial epilepsy of childhood; the spike morphology resembles that of “Benign Rolandic Epilepsy” (see further). As indicated above, attenuated, or distorted alpha activity and focal excess delta may reflect a lesion-based epilepsy. Migraine-associated disorders may be associated with either a normal EEG or with focal abnormalities. Prominent epileptiform responses to photic stimulation raise the possibility of a progressive myoclonic epilepsy. The constellation of no alpha bilaterally and occipital spikes occurs in children with ocular abnormalities causing blindness from birth or infancy.

CENTRAL SPIKES

Several apiculate (sharply contoured) phenomena are recorded by central (C3,4) and adjacent electrodes during wakefulness and sleep. The apiculate nature of mu rhythm may challenge the EEG reader, particularly if a breach rhythm is present. V-waves, spindles, and beta, individually or superimposed, will create a normal apiculate appearance.

“Rolandic” spikes are stereotyped, distinct, abundant, high-voltage discharges whose location varies: central-parietal, occipital, frontal, and vertex in that approximate order. Thus, the label “Rolandic” refers as much to spike morphology as it does to location. A characteristic tangential dipole may be best displayed on an ear referential montage: electrode negativity of the major phase at C3–P3 with positivity at F3. This produces the prominent downward deflection of the F3–C3 derivation on bipolar recordings. Non-REM sleep augments their incidence. Such discharges occur most commonly at age 4–11 years and disappear between 15 and 18 years of age. The incidence of seizures in subjects with “Rolandic” spikes varies from 54% to 84%.^23,27,28 Thus, they may appear as incidental findings. However, their principal correlate is “Benign Rolandic Epilepsy” characterized by nocturnal generalized seizures and diurnal attacks usually implicating the lower Rolandic region.²⁹ “Rolandic” spikes are so prevalent in this disorder (a paradox, given the rarity of its seizures), that their lack on two awake and sleep recordings renders the diagnosis of “Benign Rolandic Epilepsy” untenable.

Lesion-based motor seizures are associated with a distinctly different EEG picture. Background awake and sleep rhythms, for example, mu and spindles, may be attenuated and often replaced by persistent delta or theta activity. Spikes lack the scalp-recorded dipole described above. However, some epileptogenic Rolandic lesions give little EEG evidence of their presence.

MULTIPLE INDEPENDENT SPIKE FOCI

This pattern, more common in children, can be defined as spike discharges that arise from three or more noncontiguous electrode positions with at least one focus in each hemisphere.³⁰ None of the 743 normal children in Eeg-Olofsson’s study⁴ had MISF. Seizures occur on over 90% of children with MISF of which 89% are generalized tonic–clonic.^30,31 Half the patients of these studies had more than one type of seizure.

PERIODIC PHENOMENA

Gross et al³² measured interparoxysmal intervals of “periodic” lateralized epileptiform discharges (PLEDs) and found their variability not to satisfy mathematical periodicity. “Repetitive” more accurately describes such complexes. Their presence indicates a physiologically acute process such as a recent seizure or viral encephalitis. Subacute lesions, such as regionally accentuated (Rasmussen’s) encephalitis or subacute sclerosing encephalitis (SSPE), will also produce repetitive complexes if they are progressive.^33,34

FOCAL SEIZURES

A focal seizure is represented by repetitive EEG activity from one region that is dissimilar to background rhythms and whose characteristics (sequential spikes, rhythmic waves, or both) evolve as progressive changes of morphology and/or frequency.³⁵ These criteria distinguish seizures from changes in state, that is, drowsiness or arousal, whose manifestations are more prominent and complex in children than in adults.^3,36

When applied to epileptiform potentials, “generalized” applies to those occurring bisynchronously and involving a substantial portion of each hemisphere.³⁷ This corresponds to “generalized” seizures that can be described as those beginning without a specific warning, whose motor manifestations are bilateral, and which end without focal postictal manifestations.

Prognosis for intellectual development and for seizure control correlates well with background features and with characteristics of the epileptiform potentials (Table 3–1). A discontinuous background and one with excessive quantity of delta activity in wakefulness each augur poorly for seizure control and intellectual development. A slower spike-wave repetition rate and the presence of fast rhythmic waves each reflect enhanced corticothalamic excitation, medical intractability, and ultimately limited cognitive ability.³⁸ Except for the epilepsies of progressive conditions, these unfavorable EEG aspects usually occur among patients with an early age of seizure onset. Therefore, the generalized EEG phenomena will be presented in this chapter in order of usual age at first appearance.

BURST SUPPRESSION

A burst-suppression EEG pattern persists during both wakefulness and sleep. The high-voltage (150–350 μV) bursts consist of 1–3 Hz waves with intermixed multifocal spikes.³⁹ These are separated by 2–10 seconds of diffuse attenuation. The epileptic spasms are associated with diffuse desynchronization upon which low-voltage, high-frequency activity is occasionally superimposed. Although the spasms may be accompanied by bursts, only attenuation occurs during a series of spasms.

The OS proceeds to West syndrome, and thence to the Lennox–Gastaut syndrome. OS mortality is high. Survivors become mentally and physically handicapped.⁴⁰

HYPSARRHYTHMIA (HR)

Described by Gibbs and Gibbs,⁴¹ HR consists of a chaotic mixture of high-voltage 1–3 Hz waves with intermingled multifocal spikes, as though the bursts of OS became continuous. In fact, this virtually continuous pattern in wakefulness and light sleep may become discontinuous in deep non-REM sleep, thus resembling the OS burst-suppression.

Several HR variants have been encountered.⁴² In “HR with episodic voltage attenuation” episodes of generalized or regional voltage attenuation occurs, thus resembling OS. “Asymmetrical HR” refers to a consistent amplitude asymmetry between the hemispheres, usually associated with asymmetrical structural abnormalities of the brain. HR may be maximally expressed over the more abnormal or the more normal hemisphere. In the latter instance, activity over the more abnormal hemisphere will be attenuated.

“HR with a consistent focus of epileptiform activity” consists of an active focus of interictal spikes or of seizures along with the HR pattern. Persistence of this focus over several recordings would raise the possibility of surgical resection to improve the seizure disorder. A rare variant is “HR with sparse epileptiform activity,” thus high-voltage 1–3 Hz waves with occasional multifocal spikes. Finally, “HR with increased interhemispheric synchronization” constitutes a transition to the Lennox–Gastaut syndrome.

HR is the most characteristic interictal pattern in patients with epileptic spasms, appearing in two-thirds of initial EEGs performed in such infants in one study,⁴³ but the proportion of infants with epileptic spasms whose EEGs contain HR varies from 7% to 75% in several studies.⁴⁴ Conversely, about two-thirds of patients with HR have epileptic spasms.⁴⁵ During spasms HR is replaced by diffuse attenuation, occasionally with superimposed low-voltage, high-frequency waves. If the etiology of HR and spasms is unclear, intravenous infusion of 50–100 mg of pyridoxine may be indicated; obliteration of HR would indicate a pyridoxine dependency or deficiency. HR is virtually confined to ages 3 months to 5 years, in parallel with the usual course of epileptic spasms. As HR resolves, the amplitude of its components declines and the spikes become less multifocal and more bilaterally synchronous as the transition to the slow spike-waves (SSWs) of the Lennox–Gastaut syndrome begins.⁴²