EEG and Semiology in Generalized Epilepsies

Fig. 6.1

A generalized tonic-clonic seizure each EEG clip represents 20 s

Fig. 6.2

Representation of ictal discharge during generalized tonic–clonic seizure which lasted for 1 min and 45 s. Figure 6.1 shows generalized spike-and-wave discharges that gain rhythmicity as shown in Fig. 6.2

Fig. 6.3

Generalized myogenic artifact obscuring most cerebral activity

Fig. 6.4

Generalized clonic activity

Fig. 6.5

Generalized clonic activity, continued

Fig. 6.6

End of clonic activity followed by generalized voltage attenuation

Fig. 6.7

Representation of post-ictal state at 1 min showing generalized voltage attenuation and generalized slow activity

Fig. 6.8

Representation of post-ictal state at 5 min showing continued generalized slow activity

Interictally waking EEG is often normal and the yield of interictal epileptiform discharges is increased by hyperventilation and sleep. Photosensitivity (photic-stimulus-induced generalized epileptiform discharges) can be seen in up to 25 % of cases where there is also a family history of epilepsy.
Epileptiform discharges, when present, are generalized, high-voltage rhythmic spike-and-wave activity, anteriorly predominant and sometimes can only be seen anteriorly. In children, however, there can be posterior predominance.
Subtle asymmetry in the generalized discharges is commonly noted.
In NREM sleep, briefer, slower, and fragmented discharges can be seen.
Nonepileptiform activities such as generalized excessive low-voltage beta-activity or mild-generalized slow activity can be seen secondary to antiseizure medications. Chronic therapy with valproate and benzodiazepines can abolish epileptiform discharges. Frontal intermittent rhythmic delta activity (FIRDA) has also been reported.

Generalized spike-and-wave discharges can be seen in patients with no history of seizures, indicating a familial epilepsy trait. Similar discharges can also be seen in metabolic encephalopathies or drug-withdrawal states where they present as generalized tonic–clonic seizures.

Ictal EEG in Generalized Tonic–Clonic Seizures:

If generalized tonic–clonic seizures are preceded by absence (3 Hz spike-and-slow-wave) or myoclonic jerks (theta-range spikes or polyspikes and slow waves), then corresponding EEG ictal rhythms can be seen.

The tonic phase onset is predominated by muscle artifact obscuring the EEG. If neuromuscular blockers are used, the EEG shows higher amplitude and decreasing frequency discharge in the range of 9–10 Hz called the “epileptic recruiting rhythm.” Small side-to-side delay can be noted with computer analysis, but this delay is inconsistent. Slower mixed frequency discharges with increasing amplitude rhythmic spikes are seen bilaterally followed by repetitive complexes of high-amplitude spike-and-slow-wave activity in association with the tremor. Slowing of repetitions occurs with the start of violent jerks of the clonic phase, as cortical inhibition progresses. During the postictal state, an isoelectric EEG followed by diffuse slow activity can be seen, which corresponds to neuronal hyperpolarization. Focal abnormalities during postictal states are not expected and might suggest focal epilepsy with secondary generalization.

Absence Seizures [1, 6]

(i)

Typical Absence: This is the hallmark of the childhood absence epilepsy (CAE). Girls are affected more often than boys and present between 4 and 8 years of age with very frequent staring episodes (up to hundreds per day). Prognosis is excellent with treatment, and hence, a low threshold should be maintained to obtain EEG for diagnosis.

Semiology:

Staring episodes associated with cognitive impairment and change in facial expression lasting for approximately 10 s. Rhythmic lid or eye clonic–tonic activity, ocular retropulsive movements that might extend to involve head or trunk, and sometimes oral automatisms can be seen.
If the events last longer, there can be nonoral gestural motor automatisms.
Temporal semiological analysis in relation to the EEG discharge can show variability in the same patient.
Posture is usually maintained and falls are not often reported despite frequent episodes. The patients themselves are not aware of having these episodes. A child who has an event in the middle of a sentence will continue the sentence exactly where it was left off, while observers can appreciate the pause.
In the physician’s office, hyperventilation (in children, performed by blowing on a pinwheel) can elicit a seizure in approximately 90 % of patients with childhood absence epilepsy.

EEG correlation (Fig. 6.9): EEG record is ideally obtained between 8 and 10 am when the event frequency can be maximal.

Fig. 6.9

Absence seizure. This 10 s EEG clip shows ictal discharge—typical 3 Hz generalized high-voltage rhythmic spike-and-wave discharge train during which the patient demonstrated delayed responsiveness

“Interictal” and Ictal EEG—The interictal nature of these discharges is debatable. Detailed neuropsychological testing conducted in patients with the absence showed deficits even when discharges were shorter than 3 s. In 30 % of cases, a rhythmic slow posterior activity or occipital intermittent rhythmic delta activity (OIRDA) can be seen.
Generalized bilaterally synchronous high-voltage 2.5–3.5 Hz rhythmic spike-and-wave discharges with bifrontal predominance are classically seen during absence seizures. The initial negative deflection is of lower voltage compared to the after-wave, which is of very higher amplitude.
At the onset the discharge can be of higher frequency ~3.5 Hz and slows down over time. Occasionally, generalized polyspike-and-wave discharges, even bilateral independent focal frontal spikes, can occur in typical absence. These discharge trains often last 5–15 s but can last up to 30 s. Hyperventilation often precipitates longer trains of discharges.
During non-REM sleep, single or multiple spike-and-wave discharges are frequent.

(ii)

Juvenile Absence Epilepsy (JAE, Fig. 6.10): JAE has a typical age of onset between 9 and 13 years. Absence seizures typically occur in clusters upon awakening, and ocular retropulsive movements are less often seen. Seizures are less frequent than in CAE. EEG during absences shows generalized high-voltage spike-and-wave discharge, which may be slightly faster (3–4 Hz) than in CAE. About 80 % of patients also have generalized tonic–clonic and myoclonic seizures.

Fig. 6.10

Absence seizure in a patient with juvenile myoclonic epilepsy. Note the polyspike-and-wave discharge at the onset of ictal discharge (center of the EEG clip) that lasts ~3 s

(iii)

Atypical Absence: This is an important seizure type in patients with Lennox–Gastaut syndrome. Seizures occur lifelong and are more resistant to treatment, and photosensitivity is not a feature.

Semiology:

Onset and end of seizures is more gradual and eyelid myoclonus is not rhythmic. Forward head movement, perioral myoclonus, and drooling are notable. The seizure is usually less than 10 s in duration and the child can continue simple activities during an event.

EEG in atypical absence:

The ictal EEG shows 2–2.5 Hz slow spike-and-wave discharge, which can be irregular. Interictal EEG shows an abnormal background with diffuse slowing and multifocal interictal spikes.