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

• Motor fluctuations and/or dyskinesias despite optimized dopaminergic therapy
• Good levodopa response: ≥30% improvement in UPDRS-III on levodopa challenge — DBS improves what levodopa improves
• No significant dementia: cognitive impairment worsens post-DBS (especially STN); screen with neuropsych testing
• No untreated psychiatric illness: active depression, psychosis, or severe impulsivity are contraindications
• Disease duration typically ≥4 years: ensures 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 better depression and visuomotor outcomes

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: best responders — ≥80% improvement in Burke-Fahn-Marsden Dystonia Rating Scale
• Also effective for cervical dystonia (50–60% improvement) and tardive dystonia (>70% 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

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: 40% at 1 year → 56% at 2 years → 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 seizure freedom — goal is palliative seizure reduction
• ~16% achieve ≥90% seizure reduction 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

• Drug-resistant epilepsy (focal OR generalized) — FDA-approved 1997 for age ≥12 (later expanded to ≥4 years)
• Treatment-resistant depression — FDA-approved 2005
• Can be used with ANY seizure type (focal, generalized, or combined) — broader applicability than RNS or ANT-DBS
• Suitable for patients who are NOT candidates for intracranial surgery

Why Left Vagus Nerve?

• Right vagus nerve has more sinoatrial node innervation → higher risk of cardiac arrhythmia (bradycardia, asystole)
• Left vagus nerve preferentially innervates the AV node → lower cardiac risk
• 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)
• Contraindications: bilateral or left cervical vagotomy; caution with obstructive sleep apnea (may worsen)

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: ~3–4 years (non-rechargeable); newer rechargeable models ~8 years

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 = significant reduction in Modified Ashworth Scale spasticity score (≥1 point decrease)
• Response assessed over 4–8 hours post-injection
• 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
• Treatment: supportive care, respiratory support, consider CSF drainage to reduce intrathecal drug concentration

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

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