Outcome measurement after vagal nerve stimulation therapy: Proposal of a new classification

Purpose: Vagal nerve stimulation (VNS) is an adjunctive palliative therapy for refractory epilepsy. Effects of treatment are varied and some, such as the use of an external magnet for seizure termination, are unique to VNS. No accepted standard exists for outcome measurement after VNS treatment. We present a novel classification for outcome, which includes assessment of both seizure frequency and severity in VNS-treated patients. Methods: We devised a classification system modeled on the Engel classification for surgically treated patients, but tailored for use in VNS therapy, which incorporates five classes of outcome. We retrospectively reviewed VNS-treated patients in our centre, and used the data to illustrate our system and compare it with the Engel model. Results: With this system, 48 patients (mean age, 30 years) were followed up over a median of 18 months. Seventy-eight percent had partial epilepsy. Sixteen and a half percent experienced class I outcome (> 80% seizure-frequency reduction). Twenty percent had class II improvement (50-79% seizure-frequency reduction). One-third had no improvement (class V). The remaining patients comprised class III (seizure-frequency reduction < 50%) or class IV (magnet benefit alone) outcomes. Class I-III outcomes were further subdivided according to effects on ictal or postictal severity. Conclusions: We propose a new classification, which can be used for all epilepsies and which reflects outcome measures beyond seizure-frequency reduction alone. Use of this system would allow greater comparison between future studies of VNS therapy.