To measure the duration of chronic electrocorticography (ECoG) needed to attain stable estimates of the seizure laterality ratio in patients with drug-resistant bilateral temporal lobe epilepsy (BTLE).

Temporal lobe epilepsy (TLE) is the most common drug-resistant focal epilepsy amenable to surgical resection. The benefits of surgery for improving seizure control and neuropsychological function in TLE are greatest when a majority of seizures originate from one side. Unilateral temporal resection predicated on the seizure laterality ratio determined by RNS chronic electrocorticography (ECoG) can be highly effective. However,  the minimum duration of chronic ECoG before proceeding to resection remains undefined.

We studied 13 patients with drug-resistant BTLE implanted for at least one year with a responsive neurostimulation device (RNS^® System) that provides chronic ambulatory electrocorticography (ECoG). Bootstrap analysis and non-linear regression were applied to model the relationship between chronic ECoG duration and the probability of capturing at least one seizure. Laterality of electrographic seizures in chronic ECoG was compared with the seizure laterality ratio from phase 1 scalp video-EEG (vEEG) monitoring. The Kaplan-Meier estimator was used to evaluate time to laterality ratio convergence.

Seizure laterality ratios from phase 1 scalp vEEG monitoring correlated poorly with those from RNS chronic ECoG (r=0.31, p-value=0.30).  A 50% probability of recording at least one electrographic seizure required 9.1d of chronic ECoG, and 90% probability required 44.3d of chronic ECoG. A median recording duration of 5 months was needed before confidence intervals reliably contained the long-term seizure laterality ratio. The median recording duration before the seizure laterality ratio converged to a stationary value was 236.8d (7.9 months).

RNS chronic electrocorticography overcomes temporal sampling limitations intrinsic to inpatient phase 1 vEEG evaluations. For individuals who are candidates for unilateral resection based on seizure laterality, optimized recording duration may help avert morbidity associated with delay to definitive treatment.