Purpose: Limited capability exists to predict when vagus nerve stimulation (VNS) battery deterioration becomes significant. Initial models last 2-5 years. We evaluated the first 18 patients with pharmacoresistent epilepsy after reimplantation to examine the clinical course observed during VNS end of service (EOS).
Methods: Of 72 patients with VNS, 18 patients had generator replacement. EOS was estimated based on duration of use and stimulus parameters in accordance with manufacturer guidelines. Eight males and ten females had pharmacoresistent epilepsy for a mean of 17.9 years. Thirteen with localization-related epilepsy (LRE) and 5 nonverbal patients with symptomatic generalized epilepsy (SGE) failed a mean of 11.1 antiepileptic drugs (AEDs) over 21.5 years. Seven had intracranial evaluations and five failed epilepsy surgery. Reimplantation was performed after a mean of 34.4 months. Symptoms at end of service (EOS) were addressed by postoperative survey submitted at initial reprogramming within 2 weeks of reimplantation. Stimulus parameters were compared before and after surgery.
Results: Nine of thirteen (69.2%) verbal patients and 11 of 18 (61.1%) total patients had signs or symptoms prior to replacement, suggesting clinical EOS, and 4 of 18 (22.2%) failed interrogation denoting battery failure without symptoms; however, this did not reach significance (chi2=0.359,p=0.54). Increased seizures were the most frequent sign in 8 of 18 (44.4%), with intensification in 7 of 18 (38.9%). Irregular stimulation was detected in 5 of 18 (27.7%), with less intense stimulation in 4 of 18 (22.2%). Painful stimulation and behavioral worsening each occurred in 2 of 18 (11.1%). A subjective improvement in function after reimplantation was noted in 12 of 13 (92.3%) verbal patients, with greater intensity and consistency. Maximally tolerated reimplant current averaged -0.56 mA less. All but one (94.4%) felt surgery should be performed before clinical EOS occurred.
Conclusions: We conclude that clinical signs and symptoms may arise during VNS EOS and following replacement. Seizure increase or a change in seizure pattern was most frequently observed. The tolerated reimplant current was less than the preoperative output current in most cases. Battery replacement before EOS appears desirable from a patient perspective.