Mechanism of Action

• Target: Left cervical vagus nerve (CN X) — ~80% afferent fibers
• Left vagus chosen: Less cardiac innervation than the right vagus → reduced arrhythmia risk
• Pathway: Vagal afferents → nucleus tractus solitarius (NTS) → widespread brainstem/cortical projections
• Noradrenergic: NTS → locus coeruleus → cortical norepinephrine release (antiseizure)
• GABAergic: Enhanced thalamic inhibitory tone via thalamic reticular nucleus
• Thalamocortical: Desynchronization of thalamocortical circuits → reduces seizure propagation
• Anti-inflammatory: Cholinergic anti-inflammatory pathway → reduces TNF-α, IL-6

FDA Approval & Indications

• FDA approved 1997 for drug-resistant focal epilepsy, age ≥4 years
• Also FDA-approved for treatment-resistant depression (2005)
• Off-label: generalized epilepsies, Lennox-Gastaut syndrome, Dravet syndrome

Efficacy

• 3 months: 36% median seizure reduction
• 1 year: 51% median seizure reduction
• 50% responder rate: ~50% achieve ≥50% reduction by 1–2 years
• Seizure freedom: Only 2–8% — VNS is palliative, not curative
• Progressive improvement: Efficacy increases over first 1–2 years

Standard Programming Parameters

Autostimulation (Closed-Loop Feature)

• Newer generators (AspireSR, SenTiva) detect ictal tachycardia → extra stimulation burst
• Ictal tachycardia occurs in 80–90% of seizures, often preceding clinical onset by seconds
• Threshold set 20–40% above resting heart rate; operates alongside standard cycling
• Limitation: Exercise/anxiety can trigger false detections

Side Effects

• Hoarseness: Most common (62%) — during ON phase only; improves with acclimatization
• Cough, throat pain, dyspnea: Common during titration
• OSA aggravation: Laryngeal narrowing; screen at-risk patients with polysomnography
• Surgical: Infection 3–6%, vocal cord paralysis ~1%, lead fracture 1–3%

VNS & SUDEP

• SUDEP in drug-resistant epilepsy: 6–9 per 1,000 patient-years
• VNS-treated patients: 2–4 per 1,000 patient-years (observational data)
• Proposed: improved autonomic regulation, enhanced postictal arousal
• Caution: observational data with selection bias; no RCT powered for SUDEP

Mechanism & Design

• Closed-loop: Continuously monitors intracranial ECoG via 2 leads (4 contacts each)
• Detects patient-specific seizure-onset patterns → delivers brief stimulation to abort seizure
• Neurostimulator embedded in skull-recessed ferrule (craniotomy)
• Dual mechanism: Acute seizure disruption + long-term neuromodulatory plasticity

FDA Approval & Indications

• FDA approved 2013 for adults (≥18 years) with drug-resistant focal epilepsy
• Requires 1–2 identifiable seizure foci
• Especially suited for: eloquent cortex onset, bilateral MTLE, prior failed resection

Efficacy

Diagnostic Value — Chronic Ambulatory ECoG

• Unique among all neuromodulation devices — continuous intracranial EEG under real-world conditions
• Objective seizure quantification (diaries undercount by ≥50%)
• Bilateral MTLE: Can identify dominant focus (≥90% from one side) → enables curative resection
• Reveals circadian and multidien seizure patterns; monitors medication response
• Diagnostic-therapeutic bridge: RNS first (treat + diagnose), then guided resection

Complications

• Infection: 3.7% at 3 months; up to 12% long-term
• Lead revision: 4.7%
• Intracranial hemorrhage: ~3% at implantation
• Battery replacement: Every 3–4 years (cranial procedure)
• Neuropsychological testing: no decline over 9 years; some improvement

Mechanism

• Target: Bilateral anterior nuclei of thalamus (ANT)
• Circuit of Papez: Hippocampus → fornix → mammillary bodies → anterior thalamus → cingulate → hippocampus
• Open-loop scheduled cycling; desynchronizes thalamocortical circuits; raises seizure threshold
• FDA approved 2018 (US); 2010 in Europe; drug-resistant focal epilepsy in adults

SANTE Trial & Long-Term Data

MORE Registry (Real-World Data)

• 170 patients, 25 sites, 13 countries
• 2-year: 33% reduction (more modest than SANTE — real-world population)
• 5-year: 56% reduction; confirms long-term progressive improvement

Standard Programming

• Frequency: 145 Hz | Pulse width: 90 μs | Cycling: 1 min ON / 5 min OFF
• Voltage: 2–5 V, gradually increased; reduce at night to mitigate sleep disruption

Side Effects

• Depression: 15% — ANT stimulation of limbic circuitry; one suicide in SANTE; screen/monitor
• Memory symptoms: 13% (subjective; formal testing often normal)
• Paresthesias: 18% (usually transient)
• Sleep disruption: Common — mitigated by night-time programming
• Infection: ~9% | Hemorrhage: ~5% at implantation (most asymptomatic)

• Target: Centromedian nucleus — intralaminar thalamus with widespread cortical projections
• Logical target for generalized seizure networks (especially LGS)
• ESTEL trial: Double-blind, sham-controlled RCT in Lennox-Gastaut syndrome
• Significant reduction in tonic-clonic and tonic seizures vs. sham
• Not yet FDA-approved for epilepsy — larger confirmatory trials needed

VNS — Choose When:

• Not a surgical candidate (non-localizable, generalized, or declined intracranial surgery)
• Generalized epilepsy or LGS (broadest indication among the three)
• First-line neuromodulation — simplest, lowest invasiveness, most established
• Pediatric patients (≥4 years FDA-approved)

RNS — Choose When:

• Identifiable focus in eloquent cortex (resection would cause deficit)
• Bilateral MTLE — treatment + diagnostic clarification of dominant focus
• Want chronic ECoG diagnostic data to guide future surgery
• 1–2 localizable foci with failed or declined resection

DBS — Choose When:

• Multifocal or diffuse onset within focal epilepsy networks
• Failed VNS after 2+ years of optimized therapy
• Generalized epilepsy: CMN-DBS for LGS (when available)
• Temporal lobe epilepsy not amenable to resection or RNS

Key Principles

• VNS is generally first-line neuromodulation due to lowest invasiveness
• Escalate to intracranial neuromodulation if VNS fails after optimized therapy
• VNS can remain active even if intracranial device is added
• None of these devices precludes future resective surgery