Monitoring at the Instituut voor Epilepsiebestrijding Meer en Bosch.

A requirement for intensive epilepsy monitoring arose from a demand for ictal EEG registration for diagnostic purposes and in the context of pharmacokinetic research. The monitoring facility developed gradually: first conventional EEG equipment was used on the wards, later miniaturized pre-amplifiers were introduced to allow the patient greater freedom, finally a comprehensive specialized monitoring facility was created with its own accommodation. For most clinical and research applications arising in the Institute intensive observation and video monitoring are necessary. The greater reliability, channel capacity and bandwidth of cable telemetry offer considerable advantages over alternative monitoring techniques as radio telemetry. For recording from patients in locations outside the monitoring unit but where restricted mobility is acceptable a combination of telephone telemetry with a local radio link is used. Synchronization of EEG and video images is achieved by various means, depending on the application: split-screen technique, superimposition of EEG on video images or annotating the paper EEG chart with digital time information derived from the video time-date generator. Buffering of EEG signals is also available by means of a computer disk or a shift register. During the development of the system evaluation studies have been performed showing a progressively increasing yield of clinically useful information. At the most recent assessment, 79% of the investigations answered the clinical question addressed, in 65% of instances the findings had consequences for management and in over 25% the decisions resulting from monitoring had long-term therapeutic benefits for the patient.

Temporal characteristics of seizures and epileptiform discharges.

The time relations of epileptic events have been studied in 3 sets of data: (I) counts of individual epileptiform discharges in twelve 48 h EEG recordings, (IIa) seizure calendars of 30 therapy-resistant outpatients participating in a drug trial, (IIb) seizure calendars of 10 mentally subnormal epileptic patients resident in a long-stay unit. The EEG data I were characterized most often by a Poisson distribution of intervals between discharges and the occurrence of marked periodicities, particularly at night. The periods of rhythmic nocturnal events ranged from 13 to 142 min and did not appear to correspond to the REM/non-REM cycle. In the seizure data IIa and b a Poisson distribution of intervals between events was found in half the patients. Periodicities occurred only in group IIa and did not correspond to weekly or monthly cycles. A stochastic process is considered to be the model which best fits these data.

The circadian distribution of interictal epileptiform EEG activity.

Eighteen continuous 48 h monitoring studies are reported from 17 patients with epilepsy. The numbers of epileptiform discharges over corresponding epochs of the 2 days were significantly positively correlated in 16 studies. However, this was explicable by masking due to the sleep/wake cycle and when waking and sleep were considered separately a minority of studies showed significant correlations. The difference in total 24 h production of discharges between the 2 days ranged from 1.3 to 30.3%, mean 15.1%. The maximum discharge rate in 75% of the studies occurred during sleep; during waking the distribution of discharges was random. Even in the waking state the 0.5 h discharge rate was extremely variable and in few patients could a single 30 min epoch be regarded as a reliable sample of the mean rate over the waking day. The intervals between events showed a Poisson distribution during 9 days and 5 nights, but there was no within-patient consistency between the first and second 24 h period. The occurrence of discharges was periodic significantly more often at night than during the day, but the periodicities did not clearly correspond to the REM cycle. Discharges increased overall during sleep in 14 studies, were unchanged in 2 and decreased in 4. The time of occurrence of maximal discharge rate during sleep was consistent from night I to night II only in patients exhibiting generalized regular spike-wave activity but random in the others. A negative correlation between antiepileptic drug levels and discharge rate was rarely observed.

Selective cognitive impairment during focal and generalized epileptiform EEG activity.

It is well established that generalized epileptiform EEG discharges unaccompanied by overt clinical change may nevertheless be associated with transitory cognitive impairment (TCI) detectable by appropriate psychological testing. However, the tests employed in most research studies of this phenomenon are of little use for routine clinical application. They are suitable for administration only over short periods of time and are therefore applicable only to patients with a high discharge rate, a serious limitation, as the discharges are generally depressed by the tests themselves. We have developed two short-term memory tasks, one using verbal, the other nonverbal material presented in the form of television games which patients are generally prepared to perform for up to an hour or longer. Forty-six patients with subclinical EEG discharges have been studied. They were screened by video monitoring before and during testing to exclude any with overt clinical changes during the discharges. Despite this rigorous selection, in 50 per cent TCI was demonstrable with a confidence level (within the individual patient) of 10 per cent. Discharges during stimulus presentation were most disruptive of performance and those confined to the period when the patient was responding were without demonstrable effect. A significant association was found between the laterality of focal or asymmetrical generalized discharges and impairment of one or other task, left-sided discharges being associated with errors in the verbal task and right-sided with impairment of the nonverbal test. Two case histories are cited illustrating patients who were clearly handicapped by TCI and whose functioning improved when the subclinical discharges were suppressed by medication. To determine how many patients suffer such disabilities or can be helped by appropriate medication, further prospective studies are required.