25.1 Patient Selection

The concept of VNS originated in the work of Zabara in 1992. He found that repetitive vagal stimulation stopped chemically induced seizures in dogs. Ensuing studies led, in 1997, to approval by the United States Federal Drug Agency for the treatment of medically refractory partial seizures in adults and adolescents over 12 years of age by VNS. There is Class I evidence for the safety and effectiveness of VNS therapy for epilepsy, but VNS is considered a palliative adjunct to pharmacotherapy with only rare instances of complete seizure remission. It is generally reserved for pharmacoresistant patients with partial or secondarily generalized epilepsy who are poor candidates for resection. These patients lack a localizable focus, have bilateral foci, or foci in eloquent cortex. VNS can significantly reduce medication needs. There have been several randomized controlled trials of high frequency VNS stimulation that showed statistically significant reductions in seizure frequency in 20–30% patients and at least a 50% decrease in seizure frequency in 31% of patients with either complex partial or secondarily generalized seizures. Because VNS is an extracranial procedure, it has a side effect profile that is safer than, for example, than corpus callostomy for disabling drop attack seizures. VNS can improve the quality of life by reducing anti epileptic drug burden and seizure frequency.

25.2 Preoperative Preparation

Pre-operative assessment for patients with medication refractory epilepsy includes inpatient video EEG monitoring, neuropsychological testing, and structural and functional neuroimaging including brain MRI and subtraction ictal SPECT co-registered to MRI. A multidisciplinary review of these results should eliminate candidacy for seizure focus resection.

Candidates must have an intact and functional vagus nerve. The nerve on the left side is the preferred site of treatment. Animal studies show that the right vagus nerve preferentially innervates the sinoatrial node of the heart. Stimulation of this side can produce bradycardia or even asystole; however, there are clinical reports of safe implantation and stimulation of the right vagus nerve. Pre-existing recurrent laryngeal nerve injury on the side opposite to the planned left side implant may preclude implantation. Patients who have had previous neck surgery or recurrent laryngeal nerve injury should have their vocal cord function evaluated preoperatively by video laryngoscopy.