Histological appearance of a chronically stimulated vagus nerve in a pediatric patient

Vagus Nerve Stimulation Electrode Impedance Over Time in Children With Lennox-Gastaut Syndrome

OBJECTIVE

This manuscript describes the behavior of impedance of vagus nerve stimulation (VNS) electrode over time in a cohort of children with Lennox-Gastaut syndrome.

MATERIALS AND METHODS

Nineteen consecutive pediatric patients with Lennox-Gastaut syndrome submitted to VNS were studied. All patients had at least four years of follow-up. Serial impedance measurements were carried out during every out-patient visit. A baseline value was obtained one month after surgery, before generator activation and yearly values were recorded for the next four years. Outcome regarding seizures was obtained through analysis of standardized seizure diaries filled out by the patient, relatives, or caregivers.

RESULTS

There were 12 boys. Age ranged from four to 14 years (mean = 7.2). Mean impedance value was 2635 Ω at baseline, 2576 Ω after one year, 2418 Ω after two years, 2340 Ω after three years, and 2241 Ω after four years. There was a mean impedance decrease of 17% after four years. This decrease was statistically significant compared with baseline by the second year of follow-up: p = 0.342 after one year, p = 0.007 after two years, p = 0.001 after three years, and p = 0.001 after four years. There was no significant relationship between impedance values and seizure outcome at any time point.

CONCLUSIONS

VNS electrode impedance significantly decreased during long-term follow-up in children with Lennox-Gastaut syndrome. To our knowledge, this is the first report on such findings regarding VNS in the literature. These findings suggest that the electrode/nerve interface is stable during long-term follow-up of VNS therapy and that this preserved anatomical relationship might be related to our ability to safely stimulate and review/explant the system whenever needed.

Vagus nerve stimulation in children with intractable epilepsy: a randomized controlled trial

AIM

The aim of this study was to evaluate the effects of vagus nerve stimulation (VNS) in children with intractable epilepsy on seizure frequency and severity and in terms of tolerability and safety.

METHOD

In this study, the first randomized active controlled trial of its kind in children, 41 children (23 males; 18 females; mean age at implantation 11y 2mo, SD 4y 2mo, range 3y 10mo-17y 8mo) were included. Thirty-five participants had localization-related epilepsy (25 symptomatic; 10 cryptogenic), while six participants had generalized epilepsy (four symptomatic; two idiopathic). During a baseline period of 12 weeks, seizure frequency and severity were recorded using seizure diaries and the adapted Chalfont Seizure Severity Scale (NHS3), after which the participants entered a blinded active controlled phase of 20 weeks. During this phase, half of the participants received high-output VNS (maximally 1.75mA) and the other half received low-output stimulation (0.25mA). Finally, all participants received high-output stimulation for 19 weeks. For both phases, seizure frequency and severity were assessed as during the baseline period. Overall satisfaction and adverse events were assessed by semi-structured interviews.

RESULTS

At the end of the randomized controlled blinded phase, seizure frequency reduction of 50% or more occurred in 16% of the high-output stimulation group and in 21% of the low-output stimulation group (p=1.00). There was no significant difference in the decrease in seizure severity between participants in the stimulation groups. Overall, VNS reduced seizure frequency by 50% or more in 26% of participants at the end of the add-on phase The overall seizure severity also improved (p<0.001).

INTERPRETATION

VNS is a safe and well-tolerated adjunctive treatment of epilepsy in children. Our results suggest that the effect of VNS on seizure frequency in children is limited. However, the possible reduction in seizure severity and improvement in well-being makes this treatment worth considering in individual children with intractable epilepsy.

Brain stimulation for epilepsy

Brain stimulation has been receiving increasing attention as an alternative therapy for epilepsy that cannot be treated by either antiepileptic medication or surgical resection of the epileptogenic focus. The stimulation methods include transcranial magnetic stimulation (TMS) or electrical stimulation by implanted devices of the vagus nerve (VNS), deep brain structures (DBS) (thalamic or hippocampal), cerebellar or cortical areas. TMS is the simplest and least invasive approach. However, the most common epileptogenic areas (mesial temporal structures) probably lie too deep beneath the surface of the skull for effective TMS. The efficacy of VNS in reducing the frequency or severity of seizures is quite variable and depends on many factors which are currently investigated. VNS is well-tolerated and approved in many countries. DBS is much more invasive than either TMS or VNS. Currently, a number of targets for DBS are investigated including caudate, centromedian or anterior thalamic nuclei, and subthalamic nucleus. Direct stimulation of the epileptic cortical focus is another approach to the neuromodulation in epilepsy. Finally, another line of research investigates the usefulness of implantable seizure detection devices. The current chapter presents the most important evidence on the above methods. Furthermore, other important issues are reviewed such as the selection criteria of patients for brain stimulation and the potential role of brain stimulation in the treatment of depression in epileptic patients.

Dual mode of signalling of the axotomy reaction: retrograde electric stimulation or block of retrograde transport differently mimic the reaction of motoneurons to nerve transection in the rat brainstem

Axotomy of a peripheral nerve causes a complex central response of neuronal perikarya, astroglia and microglia. The signal initiating this axotomy reaction is currently explained either by deprivation of target-derived trophic factors after interruption of transport (trophic hypothesis) or by electrophysiological disturbances of the axotomized neurons (electric hypothesis). In 108 adult Wistar rats we have compared the time course and intensity of the axotomy reaction in the hypoglossal nucleus after (1) resection of the nerve (permanent axotomy), (2) one-time electric stimulation (intact nerve, brief transient electric disturbance), and (3) colchicine block of transport (intact nerve, prolonged transient loss of trophic factors). Nerve resection activated microglia at 2-35 days post-operation (dpo), elevated GFAP in astrocytes at 3-35 dpo and increased CGRP in motoneurons at 2-15 dpo. Fluorogold prelabeling revealed neurophagocytosis and 25% neuron loss at 25 dpo. Colchicine block similarly activated microglia at 5-35 dpo, elevated GFAP at 7-35 dpo and upregulated CGRP at 7-25 dpo. Neurophagocytosis and 15% motoneuron loss were evident at 25 dpo. Electric stimulation (15 min, 4 Hz, 0.1 msec impulse, 2 mAmp) of the intact nerve activated microglia at 1-10 dpo, elevated astroglial GFAP-expression at 7-35 dpo, and upregulated CGRP at 1-10 dpo, but no neuron death and neurophagocytosis were detected. Hence electric stimulation elicited a faster, shorter-lasting response, but transport block as well as axotomy a slower, longer-lasting response. This suggests a dual mode of signaling: Onset and early phase of the axotomy reaction are triggered by electric disturbances, late phase and neuron death by deprivation of trophic factors.

Brain stimulation for epilepsy

Neural stimulation is a promising new technology for the treatment of medically-intractable seizures. Vagus-nerve stimulation (VNS) is licensed in several countries as an adjunctive therapy. VNS is as effective as antiepileptic drug therapy, and serious complications are rare. Transcranial magnetic stimulation is simple, non-invasive, and widely used in neurophysiology. Therapeutic results in a few studies are equivocal at best. Deep brain stimulation, although experimental, has been applied to the cerebellum, caudate nucleus, centromedian thalamus, anterior thalamus, subthalamus, hippocampus, and neocortical seizure foci. Preliminary results are encouraging, but not conclusive. Electrode implantation in the brain for indications other than seizures has been associated with a 5% risk for intracranial haemorrhage and 5% for infection. A controlled study of anterior thalamic stimulation in patients with intractable partial and secondarily generalised seizures has been started. Future investigations are likely to study extrathalamic sites of stimulation, and effects of stimulation contingent upon detection of or prediction of EEG patterns of epileptiform activity.

Complications of chronic vagus nerve stimulation for epilepsy in children

OBJECT

The aim of this study was to define better the incidence of surgical complications and untoward side effects of chronic vagus nerve stimulation (VNS) in a population of children with medically refractory epilepsy.

METHODS

The authors retrospectively reviewed the cases of 74 consecutive patients (41 male and 33 female) 18 years of age or younger (mean age 8.8 years, range 11 months-18 years) who had undergone implantation of a vagal stimulator between 1998 and 2001 with a minimum follow up of 1 year (mean 2.2 years). Of the 74 patients treated, seven (9.4%) had a complication ultimately resulting in removal of the stimulator. The rate of deep infections necessitating device removal was 3.5% (three of 74 patients who had undergone 85 implantation and/or revision procedures). An additional three superficial infections occurred in patients in whom the stimulators were not removed: one was treated with superficial operative debridement and antibiotic agents and the other two with oral antibiotics only. Another four stimulators (5.4%) were removed because of the absence of clinical benefit and device intolerance. Two devices were revised because of lead fracture (2.7%). Among the cohort, 11 battery changes have been performed thus far, although none less than 33 months after initial implantation. Several patients experienced stimulation-induced symptoms (hoarseness, cough, drooling, outbursts of laughter, shoulder abduction, dysphagia, or urinary retention) that did not require device removal. Ipsilateral vocal cord paralysis was identified in one patient. One patient died of aspiration pneumonia more than 30 days after device implantation.

CONCLUSIONS

Vagus nerve stimulation remains a viable option for improving seizure control in difficult to treat pediatric patients with epilepsy. Surgical complications such as hardware failure (2.7%) or deep infection (3.5%) occurred, resulting in device removal or revision. Occasional stimulation-induced symptoms such as hoarseness, dysphagia, or torticollis may be expected (5.4%).

Beyond the treatment of epilepsy: new applications of vagus nerve stimulation in psychiatry

Vagus nerve stimulation (VNS) in humans generally refers to stimulation of the left vagus nerve at the cervical level VNS is an established treatment largely devoid of severe side effect for medically refractory partial onset seizures and has been used in more than 16,000 patients. Over the past 5 years, applications in other neuropsychiatric disorders have been investigated with a special emphasis on depression. Recent data from an open-label, multi-center pilot study involving 60 patients suggest a potential clinical usefulness in the acute and maintenance treatment of drug-resistant depressive disorder. The perspective of VNS as along-term treatment with the advantage of assured compliance makes it an interesting technique to potentially treat drug-resistant depression. However, definite therapeutic effects of clinical significance remain to be confirmed in large placebo-controlled trial. Results of clinical pilot studies involving patients suffering from obesity and Alzheimer's disease indicate that VNS might induce weight loss and improve cognition. Besides its clinical usefulness, VNS can be used as a research tool, allowing neurophysiologic investigations of the parasympathetic system and its interactions with other parts of the central nervous system.