Neurology & Anesthetic Pharmacology

Selecting the optimal antiseizure medication (ASM) depends on accurate electroclinical classification. For focal-onset seizures (including focal aware, focal impaired awareness [formerly complex partial arising from the temporal lobe], and focal-to-bilateral tonic-clonic), first-line narrow/broad options are lamotrigine, levetiracetam, carbamazepine, oxcarbazepine, and lacosamide. For primary generalized tonic-clonic and juvenile myoclonic epilepsy, broad-spectrum ASMs—valproate (most efficacious, but strictly avoided in women of childbearing potential due to a 10% risk of neural tube defects/spina bifida via folate antagonism and histone deacetylase inhibition), levetiracetam, lamotrigine, and topiramate—are first-line (note that narrow-spectrum Na⁺ channel blockers like carbamazepine and phenytoin can paradoxically worsen absence and myoclonic seizures!). For childhood absence seizures (3-Hz generalized spike-and-wave discharges on EEG with brief staring spells and no postictal confusion), ethosuximide is the unequivocal first-line drug of choice because it selectively blocks thalamic T-type Ca²⁺ channels (if generalized tonic-clonic seizures coexist with absence seizures, valproate is preferred because ethosuximide lacks tonic-clonic coverage). In status epilepticus (seizure ≥5 minutes or recurrent seizures without recovery of consciousness), immediate first-line therapy is an intravenous benzodiazepine (lorazepam IV preferred; midazolam IM if no IV access), followed immediately by a second-line IV loading infusion of levetiracetam, fosphenytoin, or valproate.

Every licensing examination tests the signature adverse effect profiles of classic ASMs. Phenytoin (and its water-soluble IV prodrug fosphenytoin) exhibits zero-order saturation kinetics at therapeutic levels: acute dose-dependent cerebellar toxicity presents with nystagmus, diplopia, and ataxia, while chronic therapy causes gingival hyperplasia (via PDGF-driven fibroblast proliferation), hirsutism, coarsened facial features, megaloblastic anemia, osteopenia (accelerated vitamin D catabolism), peripheral neuropathy, and fetal hydantoin syndrome (cleft lip/palate, microcephaly, hypoplastic nails/distal phalanges). Carbamazepine is the first-line medical therapy for trigeminal neuralgia, auto-induces its own CYP3A4 metabolism, and causes SIADH (euvolemic hyponatremia), idiosyncratic agranulocytosis/aplastic anemia, teratogenic neural tube defects, and Stevens-Johnson syndrome / toxic epidermal necrolysis (SJS/TEN) in patients carrying the HLA-B*1502 allele (mandatory screening in Asian ancestry). Valproate causes fatal idiosyncratic hepatotoxicity (microvesicular steatosis, especially in children <2 years), acute hemorrhagic pancreatitis, hyperammonemic encephalopathy (treated with L-carnitine), thrombocytopenia, weight gain, alopecia, and tremor. Topiramate blocks Na⁺ channels, potentiates GABA, antagonizes AMPA/kainate receptors, and weakly inhibits carbonic anhydrase—causing cognitive word-finding difficulty ("Dopamax"), weight loss, calcium phosphate/oxalate renal stones, and secondary acute angle-closure glaucoma (ciliary body choroidal effusion). Vigabatrin irreversibly inhibits GABA transaminase (used in infantile spasms / West syndrome with tuberous sclerosis) and causes permanent concentric bilateral visual field constriction.

Local anesthetics consist of a lipophilic aromatic ring connected via either an ester linkage (procaine, cocaine, tetracaine, benzocaine, chloroprocaine—rapidly hydrolyzed by plasma pseudocholinesterase into para-aminobenzoic acid [PABA], which accounts for IgE allergic reactions) or an amide linkage (lidocaine, bupivacaine, ropivacaine, mepivacaine, prilocaine—Contain two "i"s in their generic spelling and are metabolized by hepatic CYP450 amidases). Weakly basic local anesthetics (pK_a 7.6–8.9) exist in equilibrium between an uncharged base (B) and a cationic acid (BH⁺): the uncharged form (B) is required to penetrate the lipid axonal membrane, whereas the ionized cationic form (BH⁺) binds the intracellular S6 segment of open/inactivated voltage-gated Na⁺ channels. In infected or abscessed tissue (acidic pH ~6.0–6.5), excess protons shift the Henderson-Hasselbalch equilibrium toward the charged BH⁺ state extracellularly, preventing membrane penetration and rendering local anesthetics ineffective. Co-injection of epinephrine (α₁ vasoconstrictor) reduces systemic vascular absorption—prolonging anesthetic duration, reducing surgical bleeding, and lowering systemic CNS/cardiac toxicity—but epinephrine must be avoided in end-arteriolar sites (fingers, toes, nose, pinna, penis) if vascular disease is present. Small, rapidly firing axons are blocked before large fibers (type B autonomic > type C/Aδ pain and temperature > Aβ touch/pressure > Aα motor); benzocaine and prilocaine oxidize hemoglobin Fe²⁺ to Fe³⁺, causing methemoglobinemia (chocolate-brown blood, SpO₂ ~85% refractory to 100% O₂, treated with IV methylene blue).

Intravenous general anesthetics achieve rapid induction within one arm-to-brain circulation time due to high lipid solubility, and awakening from a single bolus results from rapid redistribution from the highly perfused brain into skeletal muscle and adipose tissue—not hepatic metabolism. Propofol potentiates GABA_A Cl⁻ currents to produce smooth, rapid anesthesia and antiemesis, making it the standard agent for outpatient endoscopy and ICU mechanical ventilation sedation; however, it causes profound arterial and venous vasodilation and myocardial depression (↓↓ BP), respiratory apnea, and rare Propofol Infusion Syndrome (rhabdomyolysis, severe lactic acidosis, hyperkalemia, lipemia, and refractory bradyarrhythmia during prolonged high-dose infusions >48 hours). Etomidate is a GABA_A agonist with neutral cardiovascular hemodynamics (minimal change in BP, HR, or cardiac output), making it the induction agent of choice for rapid sequence intubation in hemodynamically unstable trauma or hypovolemic shock patients; it causes myoclonus and transiently inhibits adrenal cortical 11β-hydroxylase (suppressing cortisol synthesis in septic shock). Ketamine is a non-competitive NMDA glutamate receptor antagonist that induces "dissociative anesthesia" (cataleptic state with open eyes, nystagmus, intact airway reflexes, and profound analgesia) while centrally stimulating sympathetic outflow (↑ BP, ↑ HR, and potent bronchodilation—ideal for status asthmaticus or hemorrhagic shock, but causes postoperative emergence hallucinations attenuated by midazolam). Dexmedetomidine is a selective central α₂-adrenergic agonist in the locus coeruleus that produces cooperative sedation and analgesia without respiratory depression (side effects: bradycardia and hypotension).

Parkinson disease results from selective degeneration of pigmented dopaminergic neurons in the substantia nigra pars compacta projecting to the striatum, creating a deficiency of dopamine (which normally excites the D₁ direct pathway and inhibits the D₂ indirect pathway) alongside relative striatal cholinergic excess. Because dopamine itself cannot cross the blood-brain barrier, its metabolic precursor levodopa (L-DOPA) is co-administered with carbidopa—a peripheral DOPA decarboxylase inhibitor that cannot cross the blood-brain barrier. Carbidopa prevents peripheral conversion of L-DOPA to dopamine, increasing central L-DOPA bioavailability while preventing peripheral dopamine-mediated nausea/vomiting (chemoreceptor trigger zone outside the BBB) and cardiac arrhythmias/orthostasis; however, central adverse effects—specifically peak-dose choreiform dyskinesias, visual hallucinations, psychosis, and "on-off" motor fluctuations—are actually increased because more L-DOPA reaches the brain (Parkinson psychosis is treated specifically with pimavanserin [a 5-HT_2A inverse agonist without D₂ blockade] or low-dose quetiapine/clozapine). Adjunctive classes include COMT inhibitors (entacapone [peripheral COMT only] and tolcapone [central + peripheral COMT; causes fatal hepatotoxicity]), selective MAO-B inhibitors (selegiline, rasagiline, safinamide), direct dopamine D₂/D₃ agonists (pramipexole, ropinirole, rotigotine) (associated with impulse control disorders: pathologic gambling, hypersexuality, compulsive shopping, and sleep attacks), and amantadine (useful for levodopa-induced dyskinesias; causes livedo reticularis and ankle edema).

In Alzheimer disease, progressive degeneration of cholinergic neurons in the nucleus basalis of Meynert impairs hippocampal memory consolidation. First-line symptomatic therapy utilizes central tertiary/reversible acetylcholinesterase inhibitors (donepezil, rivastigmine, galantamine), which modestly slow cognitive decline (side effects: nausea, diarrhea, insomnia, and sinus bradycardia / AV nodal block / syncope). For moderate-to-severe Alzheimer dementia, memantine—an uncompetitive, low-affinity NMDA glutamate receptor antagonist—blocks pathologic tonic Ca²⁺ influx and excitotoxic neuronal apoptosis while preserving physiological synaptic plasticity. In Multiple Sclerosis (MS), acute demyelinating relapses are treated with high-dose IV methylprednisolone (or plasmapheresis if steroid-refractory), while chronic disease-modifying therapies (DMTs) include anti-CD20 B-cell-depleting antibodies (ocrelizumab, ofatumumab), natalizumab (anti-α₄-integrin antibody blocking leukocyte VCAM-1 transmigration across the blood-brain barrier; carries high risk of JC virus reactivation causing Progressive Multifocal Leukoencephalopathy [PML]), and fingolimod/siponimod (sphingosine-1-phosphate [S1P] receptor modulators that sequester lymphocytes in lymph nodes and cause first-dose bradycardia/macular edema). In migraine, acute abortive triptans (sumatriptan, rizatriptan) act as 5-HT_1B/1D agonists to constrict dilated meningeal vessels and inhibit trigeminal CGRP release (contraindicated in coronary artery disease, Prinzmetal angina, uncontrolled hypertension, and prior stroke—where gepants [ubrogepant, rimegepant; CGRP receptor antagonists] or lasmiditan [5-HT_1F agonist] are safe non-vasoconstrictive alternatives).

Neuromuscular blocking agents (NMBAs) paralyze skeletal muscle without providing any analgesia, amnesia, or sedation. Succinylcholine (consists of two acetylcholine molecules linked together) is the only clinical depolarizing NMBA: in Phase I, it persistently agonizes postsynaptic nicotinic N_M channels, producing transient generalized muscle fasciculations followed within 45–60 seconds by flaccid paralysis (ideal for rapid sequence intubation) because perijunctional voltage-gated Na⁺ channels become trapped in the inactivated state; it is rapidly hydrolyzed in 5–10 minutes by plasma butyrylcholinesterase (pseudocholinesterase) (so administering neostigmine during Phase I prolongs and worsens paralysis!). Because every depolarizing channel opening releases ~0.5 mEq/L of intracellular potassium into plasma, and denervated or damaged skeletal muscle upregulates extrajunctional fetal nicotinic receptors across the entire myofiber surface, succinylcholine is strictly contraindicated (>24–48 hours after injury) in major thermal burns, crush trauma, spinal cord transection/stroke, upper or lower motor neuron denervation, Guillain-Barré syndrome, muscular dystrophies (Duchenne/Becker), rhabdomyolysis, and baseline hyperkalemia—as it precipitates fatal hyperkalemic cardiac arrest. Among competitive non-depolarizing NMBAs (rocuronium, vecuronium, pancuronium, atracurium, cisatracurium), atracurium and cisatracurium undergo spontaneous non-enzymatic Hofmann elimination at physiological temperature and pH, making them completely independent of both renal and hepatic clearance (making cisatracurium the NMBA of choice in multi-organ liver/kidney failure).

Peripheral nerve stimulation using a Train-of-Four (TOF) ulnar nerve twitch monitor (four supramaximal 2-Hz stimuli over 2 seconds) distinguishes depolarizing from non-depolarizing blockade: Succinylcholine Phase I blockade reduces all four twitches equally without fade (TOF ratio = 1.0) and shows no post-tetanic potentiation, whereas non-depolarizing blockade (and Succinylcholine Phase II desensitization blockade) exhibits progressive fade (T₄ < T₃ < T₂ < T₁) and post-tetanic potentiation. Non-depolarizing blockade can be reversed by two mechanisms: (1) Neostigmine (an AChE inhibitor that increases synaptic ACh to outcompete the blocker at N_M receptors), which must always be co-administered with an antimuscarinic (glycopyrrolate—preferred because it shares neostigmine's onset speed and quaternary lack of CNS effects—or atropine) to prevent lethal vagal bradycardia/asystole and bronchorrhea; or (2) Sugammadex, a modified γ-cyclodextrin ring that directly chelates and encapsulates aminosteroid NMBAs (rocuronium > vecuronium) in plasma in a 1:1 host-guest complex (note: sugammadex also encapsulates steroidal hormonal contraceptives, requiring 7 days of backup non-hormonal contraception). Finally, exposure to volatile inhaled anesthetics (sevoflurane, isoflurane, desflurane, halothane) or succinylcholine in genetically susceptible patients (autosomal dominant RYR1 mutation) triggers Malignant Hyperthermia: uncontrolled sarcoplasmic reticulum Ca²⁺ release causes masseter spasm, generalized rigidity, a rapid spike in end-tidal CO₂ (earliest sign on capnography!), tachycardia, hyperthermia, and rhabdomyolysis/hyperkalemia—treated immediately by stopping the trigger and infusing IV dantrolene (blocks RyR1).

Two independent physical constants govern inhaled volatile anesthetics (desflurane, sevoflurane, isoflurane, halothane, enflurane) and nitrous oxide (N₂O): (1) The Blood:Gas Partition Coefficient measures solubility in blood and is inversely proportional to the speed of induction and recovery. An agent with a LOW blood:gas partition coefficient (Nitrous oxide [0.47], Desflurane [0.42], Sevoflurane [0.65]) dissolves poorly in blood; therefore, alveolar partial pressure (P_A) equilibrates rapidly with inspired partial pressure (P_I) and brain partial pressure, producing rapid induction and rapid emergence. Conversely, an agent with a HIGH blood:gas partition coefficient (Halothane [2.5], Isoflurane [1.4]) treats the blood as a massive sink that must be saturated before brain partial pressure rises, causing slow induction and slow recovery.

(2) The Oil:Gas Partition Coefficient measures lipid solubility in neuronal membranes (Meyer-Overton rule) and is directly proportional to potency, which is inversely expressed as the Minimum Alveolar Concentration (MAC)—the alveolar concentration at 1 atm that prevents movement in 50% of patients exposed to a noxious surgical incision (analogous to ED₅₀). High lipid solubility (halothane) yields a low MAC (0.75% = high potency), whereas low lipid solubility (nitrous oxide) yields a high MAC (>104% = low potency, cannot achieve surgical anesthesia alone at 1 atm). Remember that MAC values are additive, decrease with advancing age (~6% per decade), hypothermia, pregnancy, and acute ethanol/opioid/alpha₂-agonist co-administration, increases with chronic ethanol abuse and hyperthermia, and is unaffected by gender, height/weight, or duration of anesthesia. Signature organ toxicities include: Halothane (centrilobular hepatic necrosis via trifluoroacetylated neoantigens + catecholamine sensitization arrhythmias), Sevoflurane (degrades into nephrotoxic Compound A in desiccated soda-lime CO₂ absorbers; sweet-smelling ideal for pediatric mask induction), Desflurane (pungent airway irritant causing laryngospasm/sympathetic surge), Enflurane (epileptogenic spike-and-wave seizures), and Nitrous oxide (diffuses rapidly into closed gas-filled cavities—contraindicated in pneumothorax, bowel obstruction, middle ear surgery, or intraocular SF₆ gas—and oxidizes the cobalt atom of Vitamin B₁₂, inhibiting methionine synthase to cause megaloblastic anemia and subacute combined degeneration).

Intravenous Anesthetics: Propofol (GABA_A agonist formulated in soybean oil/egg lecithin emulsion) provides rapid induction, antiemetic properties, and cerebral metabolic protection but causes profound vasodilatory hypotension, respiratory apnea, and Propofol Infusion Syndrome (rhabdomyolysis, refractory lactic acidosis, hyperkalemia, lipemia, and cardiac failure with prolonged ICU infusions). Etomidate preserves cardiovascular hemodynamics (no change in BP or HR) making it ideal for rapid sequence intubation in hypotensive/trauma patients, but causes myoclonus, PONV, and transient 11β-hydroxylase adrenal suppression. Ketamine (phencyclidine derivative, non-competitive NMDA receptor antagonist) induces "dissociative anesthesia" (cataleptic state with eyes open and preserved pharyngeal/laryngeal reflexes), stimulates sympathetic outflow (increasing BP, HR, and intracranial pressure), and causes direct bronchodilation—making it the induction agent of choice in status asthmaticus and hemorrhagic/septic shock (co-administer midazolam to blunt emergence hallucinations).

Local Anesthetics are weak bases split into Esters (one "i" in the generic name: procaine, cocaine, tetracaine, benzocaine, chloroprocaine—hydrolyzed rapidly by plasma pseudocholinesterase into para-aminobenzoic acid [PABA], accounting for higher IgE allergic reactions) and Amides (two "i"s in the name: lidocaine, bupivacaine, ropivacaine, mepivacaine, prilocaine—metabolized hepatically by CYP450 enzymes). Their mechanism follows a strict two-step sequence: the uncharged (non-ionized, free base) form must first diffuse across the hydrophobic axonal lipid bilayer; once inside the axoplasm, it becomes re-protonated into the charged (cationic, BH⁺) form, which binds the intracellular vestibule of voltage-gated fast Na⁺ channels in their inactivated/activated states (use-dependent / state-dependent blockade).

Clinical corollary: In infected or abscessed tissue (acidic pH ~5.5–6.0), the Henderson-Hasselbalch equilibrium shifts toward the extracellular ionized (BH⁺) form, which cannot cross the neuronal membrane—rendering local anesthetics ineffective! Furthermore, speed of onset is determined by pKa (drugs with a pKa closer to physiological pH 7.4, like lidocaine [pKa 7.9], have a higher fraction of uncharged base and faster onset than bupivacaine [pKa 8.1]). Nerve fibers are blocked in order of small diameter and rapid firing rate (type B autonomic and unmyelinated C / myelinated Aδ fibers before large Aα fibers; myelinated fibers are blocked before unmyelinated fibers of the exact same diameter because only 3 consecutive nodes of Ranvier must be blocked). Clinical sequence of loss of function: (1) Autonomic sympathetic vasomotor tone (vasodilation/warmth) → (2) Pain and Temperature → (3) Touch and Pressure → (4) Somatic Motor function (recovery occurs in the exact reverse order). Co-administering epinephrine causes alpha₁ vasoconstriction, decreasing systemic vascular absorption, prolonging local anesthesia duration, and reducing peak plasma toxicity (except cocaine, which intrinsically blocks NET reuptake and causes severe vasoconstriction—never add epinephrine to cocaine, and avoid epinephrine in end-arteriolar fields where vasospasm is risky).

Mild-to-moderate migraines respond to NSAIDs (ibuprofen, naproxen, ketorolac) and antiemetics (metoclopramide, prochlorperazine—which both treat nausea and antagonize central dopamine receptors involved in migraine prodrome). Moderate-to-severe migraines are aborted first-line with Triptans (sumatriptan, rizatriptan, zolmitriptan, eletriptan), which act as selective 5-HT_1B/1D receptor agonists: 5-HT_1B activation constricts dilated meningeal blood vessels, while presynaptic 5-HT_1D activation on trigeminal sensory nerve endings inhibits the release of vasoactive peptides (CGRP and substance P). Because 5-HT_1B receptors are also expressed on coronary arteries, triptans (and dihydroergotamine) can induce coronary vasospasm and are strictly contraindicated in coronary artery disease, Prinzmetal angina, prior stroke/TIA, uncontrolled hypertension, peripheral vascular disease, and hemiplegic/basilar migraine. In patients with cardiovascular disease, two newer non-vasoconstrictive abortive classes are safe: (1) Lasmiditan, a selective 5-HT_1F receptor agonist ("ditan"—inhibits trigeminal CGRP release without 5-HT_1B vascular constriction; causes dizziness/sedation, no driving for 8 hours), and (2) oral small-molecule CGRP receptor antagonists ("gepants": ubrogepant, rimegepant, zavegepant).

Migraine Prophylaxis utilizes five major evidence-based classes: (1) Beta-blockers (propranolol, metoprolol, timolol—avoid in asthma, bradycardia, or Raynaud phenomenon), (2) Anticonvulsants (topiramate [causes weight loss, paresthesias, cognitive word-finding difficulty, calcium phosphate kidney stones, and acute secondary angle-closure glaucoma] and valproate [teratogenic]), (3) Tricyclic antidepressants / SNRIs (amitriptyline, venlafaxine—ideal when comorbid insomnia, depression, or neuropathic pain is present), (4) CGRP Monoclonal Antibodies (erenumab [binds the CGRP receptor directly; causes constipation] and fremanezumab, galcanezumab, eptinezumab [bind the CGRP ligand]) or oral preventive gepants (atogepant, rimegepant), and (5) OnabotulinumtoxinA injections for chronic migraine (≥15 headache days/month). By contrast, for Cluster Headache, the #1 acute abortive treatment is 100% high-flow non-rebreather oxygen (12–15 L/min) or subcutaneous sumatriptan, while the #1 preventive agent is verapamil (requires baseline and periodic ECG monitoring for PR prolongation / AV block).

In Parkinson Disease (loss of pigmented dopaminergic neurons in the substantia nigra pars compacta), therapy restores striatal dopamine-acetylcholine balance: (1) Levodopa + Carbidopa (carbidopa inhibits peripheral DOPA decarboxylase so L-DOPA crosses the blood-brain barrier without causing peripheral dopamine-induced nausea/orthostasis or arrhythmias; central dopamine still stimulates the area postrema [outside the BBB] to cause nausea; chronic therapy leads to "wearing-off" motor fluctuations and peak-dose choreiform dyskinesias via nigrostriatal neurodegeneration), (2) Dopamine Receptor Agonists (pramipexole, ropinirole, rotigotine)—also first-line along with alpha₂delta ligands (gabapentin/pregabalin) for Restless Legs Syndrome; cause impulse control disorders (pathological gambling, hypersexuality, compulsive shopping), sleep attacks, and hallucinations, (3) MAO-B Inhibitors (selegiline, rasagiline, safinamide), (4) COMT Inhibitors (entacapone, tolcapone)—block 3-O-methyldopa formation to extend L-DOPA duration (tolcapone acts centrally and peripherally but causes fatal hepatotoxicity), (5) Amantadine (NMDA antagonist that reduces L-DOPA-induced dyskinesias; causes livedo reticularis and ankle edema), and (6) Benztropine / Trihexyphenidyl (central M₁ antagonists that improve resting tremor ONLY; avoided in patients >65 years due to urinary retention and delirium).

In Alzheimer Disease (degeneration of cholinergic neurons in the nucleus basalis of Meynert), centrally acting acetylcholinesterase inhibitors (donepezil, rivastigmine, galantamine) modestly slow cognitive decline (causing nausea, diarrhea, vivid dreams, and vagotonic bradycardia/syncope), while memantine acts as a non-competitive NMDA receptor antagonist in moderate-to-severe dementia to block Ca²⁺-mediated glutamate excitotoxicity, and anti-amyloid-beta monoclonal antibodies (lecanemab, donanemab) clear plaques in early Alzheimer disease (requiring brain MRI monitoring for Amyloid-Related Imaging Abnormalities [ARIA-E cerebral edema and ARIA-H microhemorrhages], especially in APOE ε4 homozygotes). In Amyotrophic Lateral Sclerosis (ALS), riluzole inhibits presynaptic glutamate release via Na⁺ channel blockade (modestly extending tracheostomy-free survival) while edaravone scavenges free radicals.