Mechanism and Principles

• Mechanism: high-frequency electrical stimulation of target nuclei → modulates pathological circuit activity (inhibits/disrupts abnormal oscillatory patterns)
• Does NOT simply “lesion” the target — involves complex effects on local neurons, afferent/efferent fibers, and network-level modulation
• Reversible and adjustable — key advantage over ablative procedures (thalamotomy, pallidotomy)
• Bilateral stimulation is possible (unlike ablative lesioning where bilateral lesions carry high morbidity)

Components

• Electrode (lead): implanted stereotactically into brain target; 4–8 contacts for current delivery
• Extension wire: tunneled subcutaneously from scalp to chest
• IPG (implantable pulse generator): battery/stimulator implanted in infraclavicular subcutaneous pocket; non-rechargeable (3–5 year battery) or rechargeable (~15 years)

Surgical Placement Techniques

DBS Programming Parameters

• Frequency: typically 130–185 Hz (high frequency); lower frequencies (≤60 Hz) may worsen symptoms or be used for specific indications (e.g., gait freezing)
• Amplitude (voltage/current): determines spread of stimulation field; higher amplitude = wider field but more side effects
• Pulse width: typically 60–90 μs; wider pulse width recruits more axons; narrower = more selective
• Monopolar vs. bipolar: monopolar = wider field (IPG is positive pole); bipolar = more focused stimulation between two contacts
• Directional leads: segmented contacts allow steering current toward target and away from side-effect-causing structures

DBS Targets: STN vs. GPi

Patient Selection Criteria — CAPSIT-PD

CAPSIT-PD (Core Assessment Program for Surgical Interventional Therapies in Parkinson Disease) is the standard selection framework:

• Robust levodopa response: ≥30% improvement in UPDRS-III on levodopa challenge — DBS improves what levodopa improves
• Motor fluctuations and/or dyskinesias despite optimized dopaminergic therapy
• No dementia — cognitive impairment worsens post-DBS (especially STN); screen with formal neuropsych testing
• No severe psychiatric comorbidity — active depression, psychosis, or severe impulse-control disorders are contraindications
• MRI without significant structural lesions (excludes vascular parkinsonism, NPH, atypical parkinsonism)
• Disease duration typically ≥4 years to ensure correct diagnosis (excludes atypical parkinsonism)
• Adequate general health and realistic expectations about outcomes

Symptoms — What Responds vs. What Does Not

Rule: levodopa-unresponsive symptoms are DBS-unresponsive symptoms — these are axial/midline features that reflect non-dopaminergic degeneration.

Key Trials

• EARLYSTIM (Schuepbach et al., NEJM 2013): STN-DBS in early motor complications (mean 7.5 years disease duration) → superior quality of life vs. best medical therapy; supports earlier DBS intervention in select patients
• PD SURG (Williams et al., Lancet Neurol 2010): DBS + best medical therapy superior to best medical therapy alone at 1 year; benefits sustained at 3 years
• VA Cooperative Study (Follett et al., NEJM 2010): STN and GPi showed similar motor improvement at 24 months; GPi had a more favorable neurocognitive/mood profile, while STN was associated with worsening processing speed and a trend toward worsening depression

Essential Tremor — VIM-DBS

• Target: VIM (ventral intermediate nucleus of thalamus) — cerebellar relay nucleus
• Best DBS indication for tremor — 70–90% contralateral tremor improvement
• Typically unilateral (contralateral to dominant hand); bilateral possible but carries significant risk of dysarthria, dysphagia, and gait ataxia
• Indicated for medication-refractory ET (failed propranolol + primidone)
• Tolerance/habituation: ~10–15% develop reduced efficacy over years; may require reprogramming
• Alternative target: posterior subthalamic area (PSA/caudal zona incerta) — emerging target with potentially broader tremor control

Dystonia — GPi-DBS

• Target: GPi (globus pallidus internus)
• DYT1 (TOR1A) genetic dystonia: among the best responders to GPi-DBS — typically 50–70% BFMDRS improvement (Burke-Fahn-Marsden Dystonia Rating Scale), with some patients exceeding 80%
• Also effective for cervical dystonia (50–60% improvement); tardive dystonia responds well to GPi-DBS (often 50–80% improvement)
• Critical difference from PD: response in dystonia is delayed — takes weeks to months (vs. PD which responds within hours to days)
• Acquired/secondary dystonia (post-stroke, post-hypoxic): generally poorer outcomes
• STAR trial: demonstrated GPi-DBS efficacy for isolated dystonia

Tourette Syndrome

• Investigational — no single FDA-approved target
• Multiple targets under study: centromedian-parafascicular complex (CM-Pf) of thalamus, GPi, anterior limb of internal capsule (ALIC)
• Reserved for severe, treatment-refractory cases with significant functional impairment
• Variable outcomes; small case series only

Huntington Chorea

• GPi-DBS for medically refractory chorea
• Limited evidence; considered in highly select cases
• Progressive neurodegeneration limits long-term benefit

Hemorrhagic and Infectious Complications

• Symptomatic intracranial hemorrhage: ~1–3% (asymptomatic hemorrhage on imaging up to ~5%); risk factors include hypertension, microelectrode passes, transventricular trajectory
• Infection: 3–15% — often involves the IPG pocket or hardware tract; commonly requires hardware removal, IV antibiotics, and staged reimplantation

Hardware Complications

• Lead migration or fracture — causes loss of efficacy or new stimulation-related side effects
• IPG malfunction — battery failure, circuitry failure
• Skin erosion over the IPG, extension wire, or connector — risk of secondary infection
• Hardware revision rate ~5–15% over the life of the system

Stimulation-Related Side Effects

• Target-specific: dysarthria, paresthesias, gait disturbance, mood changes, visual phenomena
• Usually reversible with reprogramming or amplitude reduction

Overview

• Incisionless transcranial thermal ablation using high-intensity focused ultrasound under real-time MRI thermometry guidance
• Creates a small thermal lesion at a stereotactically defined target — no implanted hardware, no craniotomy
• Patient is awake; lesion size and location are titrated by stepwise sublesional sonications with clinical testing

FDA-Approved Indications (Exablate Neuro)

• Unilateral VIM thalamotomy for essential tremor — FDA-approved 2016
• Unilateral VIM thalamotomy for tremor-dominant Parkinson disease — FDA-approved 2018
• Unilateral GPi pallidotomy for advanced PD motor complications (dyskinesias, motor fluctuations) — FDA-approved 2021 (NOT STN — STN is NOT an FDA-approved MRgFUS PD target)
• Staged bilateral VIM thalamotomy for ET — FDA-approved 2022 (second side performed at least ~9 months after the first)
• Staged bilateral pallidothalamic tractotomy (PTT) for selected advanced idiopathic PD with medication-refractory motor complications — FDA-approved July 3, 2025

Advantages and Limitations

• Advantage: option for patients who cannot or will not undergo DBS (anticoagulation, frailty, infection concerns, hardware aversion); no infection risk, no battery, no programming
• Limitations: irreversible lesion; not adjustable; sub-optimal skull density ratio (SDR < ~0.4) limits energy delivery; bilateral simultaneous lesioning is not approved (risk of dysarthria/ataxia/gait)
• Side effects: gait imbalance, paresthesias, dysarthria — often transient

Deep Transcranial Magnetic Stimulation (Deep TMS)

• Non-invasive repetitive magnetic stimulation using a Brainsway H-coil that reaches deeper cortical targets than standard figure-8 coils
• FDA-approved indications:
  • Treatment-resistant depression (major depressive disorder)
  • Obsessive-compulsive disorder (OCD)
  • Smoking cessation (short-term nicotine abstinence)
  • Anxious depression (depression with comorbid anxiety)
• Outpatient procedure; no anesthesia; main risk is seizure (rare)

VC/VS DBS for Refractory OCD

• Target: ventral capsule / ventral striatum (VC/VS) — modulates cortico-striato-thalamo-cortical (CSTC) loops implicated in OCD
• FDA Humanitarian Device Exemption (HDE) 2009 — Medtronic Reclaim DBS system for severe, chronic, treatment-resistant OCD
• Reserved for patients who have failed multiple SSRIs (including augmentation) and evidence-based CBT/ERP
• Investigational for treatment-resistant depression (TRD) and other psychiatric indications

SANTE Trial and Indications

• Target: anterior nucleus of thalamus (ANT) — part of the circuit of Papez; modulates limbic network excitability
• SANTE trial (Fisher et al., 2010): pivotal RCT demonstrating efficacy of bilateral ANT-DBS
• FDA-approved 2018 for adults (≥18 years) with drug-resistant focal epilepsy
• Indications: drug-resistant focal epilepsy (failed ≥2 ASMs) in patients NOT candidates for curative resective surgery

Efficacy

• Median seizure reduction: ~41% at 1 year → ~69% at 5 years → ~75% at 7 years
• Progressive improvement over time — unlike a medication where effect is immediate and stable
• Does NOT typically achieve sustained seizure freedom — goal is palliative seizure reduction
• ~16% experienced ≥6-month seizure-free periods in long-term follow-up

Side Effects

• Depression: reported in ~15% early post-implantation; usually transient
• Memory complaints: subjective memory issues; generally mild and improve over time
• Standard surgical risks: hemorrhage, infection, lead migration
• Stimulation-related: paresthesias (usually resolved with reprogramming)

Mechanism

• Intermittent electrical stimulation of the left vagus nerve in the neck
• Afferent fibers → nucleus tractus solitarius (NTS) → widespread cortical and subcortical projections (locus coeruleus, raphe nuclei, thalamus, cortex)
• Open-loop system: delivers scheduled, continuous cycles of stimulation (e.g., 30 seconds on, 5 minutes off) regardless of brain activity
• Modulates noradrenergic and serotonergic neurotransmission → raises seizure threshold

Indications

• FDA-labeled as adjunctive therapy for focal-onset seizures in appropriate patients (meeting age and medication-failure criteria) — FDA-approved 1997 for age ≥12 (later expanded to ≥4 years)
• Treatment-resistant depression — FDA-approved 2005
• Commonly used off-label as palliative therapy in generalized / developmental epileptic encephalopathy syndromes such as LGS — framed separately from the FDA label
• Suitable for patients who are NOT candidates for intracranial surgery

Why Left Vagus Nerve?

• The right vagus nerve has denser sinoatrial (SA) node innervation, so right-sided stimulation carries a higher risk of bradycardia/asystole
• The left vagus nerve is therefore preferred — the safety rationale is avoidance of SA-node stimulation, not protection conferred by AV innervation
• (The left vagus does preferentially innervate the AV node, but this is not what makes left-sided VNS safer)
• This is a classic board question — VNS is ALWAYS placed on the LEFT

Efficacy

• ~50% responder rate (>50% seizure reduction) at 1–2 years
• Improves progressively over time — efficacy continues to increase over years of therapy
• Seizure freedom rate is low (~5–8%) — palliative, not curative
• Additional benefits: improved mood, alertness, and quality of life

Magnet Activation

• Patients/caregivers can swipe a magnet over the chest generator to deliver an extra burst of stimulation
• Used at seizure onset (aura) or during a seizure to potentially abort or shorten it
• Empowers patients/families with an active intervention tool

Side Effects and Contraindications

• Hoarseness/voice change: most common side effect (~30–60%); occurs during stimulation; usually well-tolerated
• Cough, throat pain, dyspnea during stimulation cycles
• No cognitive side effects — key advantage over many ASMs
• Rare: bradycardia during intraoperative lead testing (lead placement test required)
• Contraindication: prior left cervical vagotomy
• Cautions: cardiac conduction abnormalities, obstructive sleep apnea (stimulation may worsen OSA), and pre-existing dysphagia

Mechanism — Closed-Loop System

• Only FDA-approved closed-loop neurostimulation device for epilepsy (approved 2013)
• Continuously records electrocorticography (ECoG) from seizure focus
• Detects abnormal ECoG activity patterns → delivers brief responsive electrical stimulation to abort seizures before clinical onset
• Fundamentally different from VNS and conventional DBS which are open-loop (stimulate on a schedule regardless of brain state)

Components and Placement

• Generator (neurostimulator): cranially implanted — sits flush within a craniectomy defect in the skull (cosmetically favorable)
• Leads: depth electrodes and/or cortical strip electrodes placed directly at the seizure focus (1–2 foci)
• Can target up to 2 seizure foci simultaneously
• Battery life: original RNS-300M generator ~3–4 years; current RNS-320 generator ~8–11 years (primary cell, non-rechargeable). There is no currently FDA-approved rechargeable RNS model

Indications

• Drug-resistant focal epilepsy with 1–2 identified seizure foci
• Especially valuable when seizure focus is in eloquent cortex (language, motor, visual areas) where resection would cause unacceptable deficits
• Bilateral mesial temporal foci — where bilateral resection is not possible
• Age ≥18 years

Efficacy

• Median 53% seizure reduction at 2 years
• Median 66% at 6 years
• Median 75% seizure reduction at 9 years — progressive improvement over time
• ~30% achieve ≥90% seizure reduction in long-term follow-up
• Key trials: pivotal RNS trial (Morrell et al., 2011); long-term open-label extension data

Unique Advantage — Chronic ECoG Data

• RNS provides continuous, long-term ECoG recordings from the seizure focus
• Data uploaded to cloud-based platform (PDMS — Patient Data Management System) for clinician review
• Helps refine the epileptogenic zone characterization over time
• Can guide future surgical decisions (e.g., subsequent resection) based on years of seizure data
• No other device provides this chronic ambulatory ECoG monitoring capability

Mechanism and Components

• Baclofen: GABA_B receptor agonist → inhibits spinal reflex arcs → reduces muscle tone
• Intrathecal delivery bypasses the blood-brain barrier → achieves 100× higher CSF concentrations with 1/100th of the oral dose → fewer systemic side effects
• Programmable pump: implanted in anterior abdominal wall (subcutaneous pocket)
• Intrathecal catheter: tunneled subcutaneously from pump to intrathecal space (typically lumbar)
• Pump refill every 1–6 months; battery life ~5–7 years (requires surgical pump replacement)

Indications

• Severe spasticity refractory to oral medications in:
  • Cerebral palsy (CP)
  • Multiple sclerosis (MS)
  • Spinal cord injury (SCI)
  • Traumatic brain injury (TBI)
  • Stroke-related spasticity
• Must have failed oral baclofen, tizanidine, dantrolene, or other antispasmodics

Baclofen Trial

• Screening trial required before permanent implant
• Intrathecal bolus via lumbar puncture: typically 50–100 mcg
• Positive trial = clinically meaningful reduction in spasticity (commonly ≥2 points on the Modified Ashworth Scale or significant functional improvement) sustained over 4–8 hours
• If no response at 50 mcg → can repeat at 75 mcg, then 100 mcg on separate days

Hardware Complications

• Catheter problems: kink, migration, fracture, disconnection — most common hardware complication
• Infection (wound, pump pocket, meningitis)
• CSF leak around catheter insertion site
• Pump malfunction or battery depletion
• Catheter-tip granuloma (inflammatory mass at catheter tip → can cause spinal cord compression)

Baclofen Withdrawal — Life-Threatening Emergency

Treatment of Baclofen Withdrawal

• Restore intrathecal baclofen delivery as rapidly as possible (troubleshoot pump/catheter)
• Oral/enteral baclofen at high doses (poor CNS penetration but better than nothing)
• IV benzodiazepines (diazepam, lorazepam) for seizures and spasticity
• Dantrolene for hyperthermia and rhabdomyolysis
• Cyproheptadine (serotonin antagonist) — adjunct in refractory cases
• ICU admission, aggressive hydration, cooling measures

Baclofen Overdose

• Opposite picture to withdrawal: flaccidity, hypotonia, areflexia, respiratory depression, drowsiness, coma
• Causes: programming error, catheter migration to higher spinal level, accidental bolus
• First step: empty the pump reservoir and/or turn off the pump
• Supportive care: airway protection, mechanical ventilation as needed, hemodynamic support
• CSF removal via lumbar puncture (30–50 mL) has been reported in severe cases to reduce intrathecal drug concentration
• Physostigmine has been used historically but is not routinely recommended (inconsistent benefit, risk of seizures/bradycardia)
• Flumazenil does NOT reverse baclofen (baclofen acts at GABA_B, not the benzodiazepine site of GABA_A)

Microvascular Decompression (MVD) for Trigeminal Neuralgia

• Jannetta procedure: posterior fossa craniotomy → identify neurovascular compression at trigeminal root entry zone → place Teflon pledget between vessel and nerve
• Offending vessel: most commonly superior cerebellar artery (SCA); also AICA, basilar artery, veins
• ~80–90% initial complete pain relief; ~70% pain-free at 10 years — best long-term outcomes of any TN surgery
• Non-destructive — preserves trigeminal nerve function (unlike ablative procedures)
• Ideal candidate: younger, medically fit, neurovascular compression (NVC) demonstrated on high-resolution MRI (FIESTA/CISS sequences)
• Complications: hearing loss (ipsilateral CN VIII, ~1–2%), facial numbness, CSF leak, cerebellar injury

MVD for Hemifacial Spasm (HFS) and Glossopharyngeal Neuralgia (GPN)

• Hemifacial spasm (HFS): caused by a vascular loop (most commonly AICA or PICA, occasionally vertebral artery) compressing the facial nerve (CN VII) at the root entry/exit zone (REZ) in the cerebellopontine angle
• MVD is curative for HFS — retrosigmoid approach, Teflon pledget interposed between offending vessel and CN VII REZ; ~85–95% long-term resolution
• Intraoperative lateral spread response (LSR) monitoring confirms adequate decompression
• Glossopharyngeal neuralgia (GPN): compression of CN IX (and often upper rootlets of CN X) at the REZ — offending vessel is most commonly PICA
• Surgical options: MVD of CN IX/X at the REZ, or sectioning of CN IX and the upper rootlets of CN X when no clear vessel is found or for recurrence
• GPN risks: hoarseness, dysphagia, hemodynamic instability (vagal sectioning) — cardiac monitoring essential intraoperatively

Percutaneous Procedures for Trigeminal Neuralgia

• All percutaneous procedures are ablative/destructive — damage the trigeminal ganglion/root
• Ideal for elderly patients, high surgical risk, MS-related TN, or absent NVC on MRI
• Higher recurrence rates than MVD (~50% at 5 years vs. ~70% pain-free at 10 years for MVD)
• Risk of anesthesia dolorosa (painful numbness in denervated territory) — higher than MVD
• Easily repeatable (unlike MVD)

Spinal Cord Stimulation (SCS)

• Mechanism: gate control theory (Melzack & Wall) — large-fiber (A-beta) stimulation of dorsal columns inhibits small-fiber pain transmission (A-delta, C fibers) in dorsal horn
• Epidural electrodes placed over dorsal columns at appropriate spinal level
• Indications: failed back surgery syndrome (FBSS), complex regional pain syndrome (CRPS), peripheral neuropathy, refractory angina
• ~50–60% of patients achieve ≥50% pain reduction
• PROCESS trial: SCS superior to reoperation for FBSS
• Newer modalities: high-frequency stimulation (10 kHz), burst stimulation — paresthesia-free alternatives to traditional tonic stimulation
• Trial period (5–7 days) required before permanent implant

Motor Cortex Stimulation (MCS)

• Epidural electrodes placed over primary motor cortex contralateral to pain
• Indications: central post-stroke pain (thalamic pain / Dejerine-Roussy syndrome), trigeminal neuropathic pain
• Mechanism: descending modulation of thalamic and brainstem pain processing
• Investigational/off-label; variable outcomes; no large RCTs

Ablative Pain Procedures

• Ablative procedures are irreversible — reserved for patients with limited life expectancy (cancer pain) or exhausted alternatives
• Cordotomy: typically done at C1–C2 level contralateral to pain; risk of respiratory failure if bilateral (Ondine curse); most effective for somatic nociceptive pain
• DREZ lesion: particularly effective for brachial plexus avulsion pain where deafferentation creates severe neuropathic pain