Besides a detailed and comprehensive history, an EEG assessment is the most important diagnostic method in the diagnosis of epilepsy. Obtaining an EEG recording in patients with ID is difficult and time consuming in many cases. This is true especially in timid and restless persons. In those cases it is often helpful to customize the patient slowly to the situation. Providing a familiar situation by allowing the patient to hear her or his favorite music or to bring a familiar object would have a calming influence.
It is advisable to keep the time the EEG assistant takes to apply the electrodes to the head as brief as possible. Among the many different EEG electrode application techniques, electrocaps are especially suited for patients with ID and/or behavioral problems. They are made of an elastic fabric with electrodes firmly integrated into the caps. Electrocaps are available in different seizes and provide a significantly shorter application time than other systems. Their main drawback is that their fixed electrodes can be placed only roughly adjusted to the individual head size and shape according to the international 10-20 positioning system. Many patients with ID will not cooperate sufficiently so as to obtain effects of hyperventilation on the EEG. Using a wind wheel is a practicable tool in those cases.
When an EEG is performed to evaluate patients with suspected seizures the first recording should be performed for at least 20 minutes and should, when possible, involve hyperventilation and intermittent photic stimulation.46 There continues to be debate on the degree of usefulness of the EEG in the diagnosis of epilepsy in persons with ID47 ; however, clinicians in their day-to-day practice still prefer to undertake an EEG assessment.
As it is very unlikely to record a patient's habitual seizure during routine EEG, interictal epileptiform discharges (IEDs) are the most significant EEG changes when epilepsy is in question. Interictal epileptiform discharges are rare in individuals, especially adult persons, without epilepsy. In patients without epilepsy (in the community) they are found in 3.3% with no provoked or unprovoked seizures during follow-up.47 The occurrence of IEDs in selected populations of healthy adults is reported to be much lower.48
A first EEG recorded from patients with epilepsy shows IEDs in only 20-50%.47,48 The yield of EEG can be substantially increased by repeating recordings and activation techniques like hyperventilation, intermittent photic stimulation, and sleep deprivation.49 Hyperventilation is more effective in generalized epilepsies, where it provokes 3/s spike wave activity; hyperventilation seems also able to enhance slow spike and wave activity in symptomatic generalized epilepsies, which occur frequently in persons with ID.50 Intermittent photic stimulation can elicit different EEG responses, of which the photoparoxysmal response (generalized paroxysms with spike-/polyspike and slow waves) is specifically associated with epilepsy. It is most probably a genetic trait and is particularly seen in young patients with generalized epilepsies.49 Sleep EEG increases the sensitivity of EEG by a substantial amount of 30-70% (including a sampling effect, due to the additional sleep EEG with a longer recording time), sometimes even up to 90% when it is recorded after sleep deprivation.49,51
Among EEG changes in patients with ID and epilepsy there are various typical patterns characterizing (1) a specific epileptic syndrome, (2) a peculiar disorder of the cortical development, or (3) genetic syndromes associated with ID and epilepsy. Hypsarrhythmia is the specific EEG correlate of infantile spasms (West syndrome).
Slow spike and wave activity is usual 2n the Lennox Gastaut syndrome but is of little specificity, whereas sleep-related frontal spike bursts (runs of rapid spikes) with and without tonic seizures are much more distinctive.52,53 Ohtahara's syndrome shows characteristic EEG features with suppression-burst pattern, where bursts last 2-6s and comprise high-voltage slow waves intermingled with spikes.2 4 Dravet's syndrome, myoclonic astatic epilepsy, or the syndrome of myoclonic absences do not show any characteristic interictal EEG pattern; EEG photosensitivity, however, is very frequently seen in children with Dravet's disease.7,55,56
Several distinct structural lesions of the brain that cause epilepsy (and ID) show characteristic changes of the interictal EEG. In Sturge-Weber syndrome the most consistent interictal EEG finding is a depression of amplitude over the affected hemisphere or parts of the hemisphere. Epileptiform activity appears frequently over the contralateral hemisphere or, in children, as a bilateral synchronous paroxysmal activity.57 Interictal EEG patterns in focal cortical dysplasia consist of continuous or quasicontinuous rhythmic spiking or repetitive spiking in many patients.58,59
In hemimegalencephaly, background activity is often asymmetric with pathological alpha activation (higher amplitudes over the hemimegalencephalic hemisphere) and loss of physiological activity (e.g., sleep spindles) on the side of the pathology as well as suppression burst activity and/or hemihypsarrhythmia.20,61 There are rather uniform EEG findings in bilateral subcortical band heterotopia (double cortex syndrome): more or less slow background activity and paroxysmal high-voltage bilateral synchronous slow activity with intermingled spikes and sharp waves as well as sporadic focal epileptiform activity.62 The EEG in Type I lissen-cephaly frequently shows an absence of the normal distribution of local activities; diffuse, rhythmic fast alpha and beta activity; high-voltage focal, multifocal, or bisynchronous spike wave activity.63
There are several genetic syndromes associated with ID and epilepsy in which frequently appearing and typical EEG patterns are described. In Angelman syndrome (AS) characteristic EEG patterns do not differentiate between patients with and without seizures. The most frequent finding, independent of the patient's age, is high-amplitude 2-3 Hz rhythmic activity over the frontal regions with interspersed epileptiform elements. In childhood high-amplitude rhythmic 4-6 Hz activity over the occipital regions with associated spikes, facilitated by eye closure, is a frequent pattern in AS.64
In Wolf-Hirschhorn syndrome paroxysms of bisynchronous 2-3Hz waves of high amplitude associated with sharp waves and spikes of lower amplitude seem to be a characteristic EEG pattern, enhanced during drowsiness and sleep.65 66 In Kabuki syndrome, a clinical entity in which a consistent genetic abnormality has not been defined yet, characteristic EEG findings with temporo-occipital spikes (during sleep) are described.27 In many other syndromes associated with ID and epilepsy no characteristic EEG patterns could be found.
Long-term EEG or video-EEG monitoring can add substantial information and serve as the gold standard in relation to many clinical questions:
1. To differentiate between epileptic seizures and nonepileptic paroxysmal events.
2. To reveal sleep-related seizures.
3. To obtain more information on the semiology of seizures.
4. To obtain information on the electroclinical correlation of seizures with respect to epilepsy surgery.
5. To relate (subtle) seizures to possible cognitive impairment.
All aspects mentioned above can be applied to patients with ID. In individuals with severe ID, especially in Rett's syndrome, paroxysmal (repetitive, hyperactive) behavior is frequently mistaken for epileptic events. In many cases a correct diagnosis is only achieved by video-EEG monitoring, where, on the other hand, subtle seizures not previously recognized by the caregivers can be uncovered.68 Even in cases with multifocal cerebral lesions and multifocal interictal epileptiform discharges, TSC ictal recordings together with magnetic resonance imaging (MRI) can identify the focus of seizure activity. Epilepsy surgery is beneficial in many patients with TSC who become seizure free after resection of the epileptogenic tuber.69
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