4.3 Antipsychotics, Neuroleptic Malignant Syndrome, and Lithium Neurotoxicity

Antipsychotics and lithium carbonate represent two core pillars of psychiatric pharmacotherapy for schizophrenia, bipolar disorder, and major affective illnesses. In overdose and toxic accumulation states, these medications trigger severe neurological, cardiovascular, and metabolic decompensation. A comprehensive mastery of dopamine-receptor pharmacology, the distinction between acute extrapyramidal syndromes and Neuroleptic Malignant Syndrome (NMS), and the unique renal handling of lithium is fundamental for specialist practice in poison information.

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Antipsychotic Pharmacology: Typical vs. Atypical Classes

Antipsychotics are divided into first-generation (typical) and second-generation (atypical) classes, differing markedly in their receptor binding profiles and overdose manifestations.

1. First-Generation (Typical) Antipsychotics

First-generation antipsychotics—including high-potency agents (haloperidol, fluphenazine) and low-potency agents (chlorpromazine, thioridazine)—function primarily as competitive antagonists at central dopamine D2 receptors:

• High-Potency Agents (Haloperidol, Fluphenazine): Minimal anticholinergic or alpha-adrenergic activity; overdose produces intense central D2 blockade resulting in parkinsonism, acute dystonic reactions, akathisia, and obtundation, with relatively preserved blood pressure.
• Low-Potency Agents (Chlorpromazine, Thioridazine): Possess potent antagonist properties at vascular alpha-1 adrenergic receptors, muscarinic acetylcholine receptors, and cardiac ion channels. Chlorpromazine overdose causes marked orthostatic hypotension, reflex tachycardia, sedation, and miosis. Thioridazine exerts potent myocardial fast sodium channel blockade and IKr potassium channel blockade, leading to wide-complex ventricular dysrhythmias and Torsades de Pointes.

2. Second-Generation (Atypical) Antipsychotics

Second-generation agents combine 5-HT2A receptor antagonism with variable D2 receptor antagonism, along with diverse affinities for histaminic, muscarinic, and adrenergic targets:

• Quetiapine: Potent H1 and alpha-1 antagonist with weak D2 affinity. Overdose characteristically causes marked somnolence, coma, and profound sinus tachycardia (from reflex response to alpha-1 vasodilation). Quetiapine tablets can aggregate into large pharmacobezoars in the stomach, resulting in erratic, delayed peak absorption and protracted coma lasting multiple days.
• Olanzapine: Structurally related to clozapine; exhibits high affinity for H1, alpha-1, 5-HT2A, and muscarinic receptors. Overdose frequently manifests with pinpoint pupils (miosis) and central nervous system depression, closely mimicking an opioid toxidrome, but without response to naloxone.
• Clozapine: Highly toxic in overdose; causes severe sedation, central anticholinergic delirium, seizures (dose-dependent), marked orthostatic hypotension, and paradoxical sialorrhea (hypersalivation) driven by muscarinic M4 agonism and alpha-2 blockade. Clozapine also causes profound gastrointestinal hypomotility, leading to fatal anticholinergic ileus and bowel necrosis. Idiosyncratic reactions include life-threatening agranulocytosis and myocarditis.
• Ziprasidone: Moderate D2/5-HT2A antagonist; causes dose-dependent QTc interval prolongation, requiring continuous cardiac telemetry.
• Aripiprazole: Partial D2 and 5-HT1A receptor agonist; exhibits a favorable therapeutic index with mild somnolence and tremor in overdose, though pediatric accidental exposures can provoke paradoxical agitation and dystonia.

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Extrapyramidal Symptoms (EPS) and Acute Dystonia Management

Extrapyramidal symptoms arise from an acute perturbation of the balance between dopaminergic and cholinergic neurotransmission in the nigrostriatal pathway:

Acute Dystonic Reactions

Acute dystonia is an involuntary, painful spasmodic contraction of discrete muscle groups occurring within hours to days of initiating or escalating D2 antagonist therapy:

• Clinical Manifestations: Spasmodic torticollis (neck twisting), oculogyric crisis (sustained upward deviation of the eyes), trismus (jaw clenching), grimacing, glossal protrusion, opisthotonos (arching of the spine), and potentially fatal laryngeal dystonia (stridor and airway compromise).
• Pharmacological Antidote: Parenteral anticholinergic administration restores the dopamine-acetylcholine equilibrium:
  • Diphenhydramine: 25 to 50 mg IV or IM in adults (1 mg/kg in pediatrics).
  • Benztropine: 1 to 2 mg IV or IM in adults.
• Onset and Maintenance Mandate: Dramatic clinical relief typically occurs within 10 to 20 minutes of intravenous injection. Because typical antipsychotics have elimination half-lives exceeding 24 to 48 hours (much longer than diphenhydramine's 4- to 6-hour half-life), oral anticholinergic therapy must be continued for 48 to 72 hours (e.g., diphenhydramine 25–50 mg PO TID or benztropine 1–2 mg PO BID) to prevent recurrent dystonic storm.

Other Extrapyramidal Syndromes

• Akathisia: Subjective motor restlessness and compulsive pacing. Treated with centrally acting beta-blockers (propranolol 10–20 mg PO TID) or low-dose benzodiazepines.
• Parkinsonism: Bradykinesia, resting tremor, and cogwheel rigidity developing over days to weeks. Managed by reducing the neuroleptic dose or adding oral benztropine.
• Tardive Dyskinesia: Involuntary, choreiform movements of the tongue, face, and extremities resulting from chronic dopamine receptor up-regulation; does not respond to anticholinergics and is treated with VMAT2 inhibitors (e.g., deutetrabenazine, valbenazine).

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Neuroleptic Malignant Syndrome (NMS)

Pathophysiology and Precipitants

Neuroleptic Malignant Syndrome is a life-threatening, idiosyncratic reaction triggered by profound central dopamine D2 receptor blockade within the hypothalamus and basal ganglia. Hypothalamic dopamine antagonism disrupts the preoptic anterior thermoregulatory set-point and impairs peripheral heat dissipation, while striatal dopamine antagonism provokes massive, uninhibited skeletal muscle contraction.

Precipitants include any D2 antagonist (haloperidol, fluphenazine, chlorpromazine, olanzapine, risperidone, quetiapine), antiemetics with D2 blocking properties (metoclopramide, prochlorperazine, promethazine), and critically, the abrupt withdrawal of dopamine agonists (levodopa, carbidopa, bromocriptine, amantadine) in patients with Parkinson's disease.

Clinical Presentation and Diagnostic Tetrad

1. Severe Generalized Rigidity: Described as "lead-pipe" rigidity (uniform resistance throughout the entire range of passive motion) or "cogwheel" rigidity. Deep tendon reflexes are characteristically hyporeflexic or normal, in stark contrast to the hyperreflexia and clonus of Serotonin Syndrome.
2. Hyperthermia: Core body temperatures characteristically exceed 38.5°C to 40.5°C (101°F to 105°F), driven by continuous muscular rigidity and hypothalamic failure.
3. Autonomic Instability: Marked blood pressure lability (fluctuating hypertension and hypotension), severe sinus tachycardia, profuse diaphoresis, tachypnea, and cardiac dysrhythmias.
4. Altered Mental Status: Evolves from initial confusion and catatonic mutism to stupor, agitated delirium, and deep coma.

Laboratory Hallmarks

• Marked Creatine Kinase (CK) Elevation: Ranging from >1,000 IU/L in mild cases to >50,000 to 100,000 IU/L in fulminant cases, reflecting widespread rhabdomyolysis.
• Leukocytosis: White blood cell count characteristically elevated to 10,000 to 40,000/mm³ with a left shift.
• Metabolic Derangements: Hyperkalemia, hyperphosphatemia, hyperuricemia, and elevated serum creatinine from myoglobinuric acute kidney injury.

Stepwise Management of NMS

1. Discontinue Causative Agents: Stop all neuroleptics immediately. If caused by dopamine agonist withdrawal, immediately restart the dopaminergic agent.
2. Supportive Resuscitation and Cooling: Vigorous IV crystalloid hydration (targeting urine output >= 2 mL/kg/hr to flush myoglobin and protect renal tubules) combined with aggressive external cooling.
3. Dopaminergic Agonist Pharmacotherapy:
   • Bromocriptine: Direct central D2 receptor agonist. Administer 2.5 to 5 mg orally or via NG tube every 8 hours, titrating as needed (commonly to a maximum of about 40 mg/day) until rigidity and hyperthermia abate.
   • Amantadine: Facilitates dopamine release and blocks reuptake. Administer 100 mg orally or via NG tube every 12 hours, up to 400 mg/day.
4. Direct Muscle Relaxant — Dantrolene:
   • Mechanism: Ryanodine receptor (RyR1) antagonist that blocks calcium release from the sarcoplasmic reticulum of skeletal muscle, uncoupling excitation-contraction and terminating muscular thermogenesis.
   • Dosing: Administer 1 to 2.5 mg/kg IV bolus, repeated every 5 to 10 minutes as needed up to a maximum cumulative dose of 10 mg/kg/day.

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Lithium Neurotoxicity and Toxicokinetics

Lithium is a small monovalent cation (atomic weight about 7; lithium carbonate is about 74 g/mol). It has no plasma protein binding and distributes into total body water with an apparent volume of distribution of 0.7 to 0.9 L/kg.

Renal Handling and Drug Interactions

Lithium is eliminated almost exclusively by renal excretion. It is freely filtered at the glomerulus, after which 80% of the filtered load is reabsorbed in the proximal convoluted tubule, handled identically to sodium. The kidney cannot distinguish lithium from sodium. Any physiological state or medication that depletes total body sodium or decreases glomerular filtration rate prompts compensatory proximal tubular sodium reabsorption, simultaneously driving lithium reabsorption and triggering severe toxicity:

Classification of Exposure Patterns

1. Acute Overdose (Lithium-Naive): Ingestion of a large single dose. Serum concentrations rise rapidly, but lithium distributes slowly across the blood-brain barrier. Patients exhibit prominent early gastrointestinal symptoms (nausea, vomiting, profuse diarrhea), while central nervous system toxicity is delayed by 24 to 48 hours. Serum concentrations correlate poorly with early clinical severity.
2. Acute-on-Chronic Overdose: A patient on long-term lithium ingests an acute overdose. Because tissue and brain stores are already partially or fully saturated, acute elevations in serum levels rapidly translate into severe neurotoxicity alongside GI distress.
3. Chronic Toxicity (Accumulation): The most lethal form of lithium poisoning. Develops insidiously over days to weeks secondary to dehydration, intercurrent illness, or newly added medications (NSAIDs, ACE inhibitors, thiazides). Brain tissue concentrations are maximally elevated; patients present with predominantly neurological toxicity, while gastrointestinal symptoms are frequently absent.

Clinical Manifestations of Lithium Toxicity

• Neurological Hallmarks: Coarse resting and intention tremors, hyperreflexia, ankle clonus, severe cerebellar ataxia, dysarthria, nystagmus, choreoathetosis, fasciculations, confusion, delirium, seizures, and non-convulsive status epilepticus.
• Cardiovascular Effects: Sinus node dysfunction, PR prolongation, T-wave flattening or inversion, and rare conduction blocks.
• Endocrine and Renal Sequelae: Nephrogenic diabetes insipidus (NDI) from down-regulation of aquaporin-2 channels in the collecting ducts, leading to polyuria, polydipsia, and hypernatremia; chronic tubulointerstitial nephritis.
• SILENT Syndrome: Syndrome of Irreversible Lithium-Effectuated Neurotoxicity—persistent, permanent cerebellar dysfunction, dementia, extrapyramidal signs, and brainstem encephalopathy persisting for greater than 2 months following complete lithium cessation and normalization of serum levels.

Elimination Strategies and Extracorporeal Treatment (EXTRIP)

• Decontamination: Activated charcoal does not bind lithium due to its small ionic radius. Whole Bowel Irrigation (WBI) with polyethylene glycol electrolyte solution (PEG-ELS at 1.5–2 L/hr in adults) is reserved for massive acute ingestions of sustained-release lithium tablets presented within 2 to 4 hours.
• Intravenous Fluid Resuscitation: Administer 0.9% Normal Saline (200–300 mL/hr) to restore intravascular volume, establish euvolemia, and provide sodium ions to suppress compensatory proximal tubular lithium reabsorption. Forced diuresis or sodium bicarbonate diuresis is ineffective and causes fluid overload.
• Indications for Extracorporeal Treatment (EXTRIP Workgroup, lithium):
  • Recommended: impaired kidney function with serum lithium > 4.0 mEq/L; OR a decreased level of consciousness, seizures, or life-threatening dysrhythmias at any lithium concentration.
  • Suggested: serum lithium > 5.0 mEq/L; significant confusion; or an expected time of more than 36 hours to reach a lithium concentration below 1.0 mEq/L with optimal management.
  • EXTRIP does not use separate acute and chronic numeric cutoffs; the chronic pattern matters because brain lithium is already high, so neurologic signs appear at lower serum levels. Intermittent hemodialysis is the preferred modality.
• Post-Hemodialysis Rebound: Lithium follows a two-compartment kinetic model. Hemodialysis rapidly clears the intravascular compartment, but intracellular lithium redistributes slowly back into plasma over the subsequent 6 to 12 hours. Serial lithium concentrations must be followed over the 12 hours after dialysis (EXTRIP); repeat hemodialysis or continuous renal replacement therapy (CRRT) is indicated if serum concentrations rebound above toxic thresholds.