Anticholinergic toxicity results from blockade of muscarinic acetylcholine receptors, producing a classic toxidrome summarized by the mnemonic: "Red as a beet, dry as a bone, blind as a bat, hot as a hare, mad as a hatter." Over 600 compounds can cause this syndrome, and recognition is primarily clinical. [1-3]
1. History
- What substance(s) were ingested/exposed to? Obtain pill bottles, photos of medications, and amounts
- Time of ingestion — critical for anticipating peak effects (especially with sustained-release formulations)
- Intentional vs. accidental exposure; suicidal intent assessment
- Symptom progression: onset of confusion, hallucinations (often visual — "picking at the air"), dry mouth, blurred vision, inability to urinate
- Ask about polypharmacy — anticholinergic burden is cumulative across multiple medications with modest individual activity [4-5]
- Important negatives: absence of diaphoresis (present in serotonin syndrome, absent in anticholinergic toxicity), absence of clonus/hyperreflexia [6]
2. Alarm Features
- Seizures, coma, or respiratory failure — indicate severe/life-threatening toxicity [3][7]
- Hyperthermia >40°C (104°F) — risk of end-organ damage
- Wide-complex tachycardia or QRS prolongation (especially with TCA co-ingestion)
- Cardiovascular collapse or hemodynamic instability
- GCS ≤8 or rapidly declining mental status [8]
- Rhabdomyolysis from prolonged agitation or hyperthermia
3. Medications
Common causative agents: [4][9]
- First-generation antihistamines: diphenhydramine (most common cause), hydroxyzine, doxylamine, chlorpheniramine
- Tricyclic antidepressants: amitriptyline, nortriptyline, imipramine, doxepin
- Antipsychotics: olanzapine, clozapine, chlorpromazine, quetiapine
- Antispasmodics: dicyclomine, hyoscyamine, scopolamine, atropine
- Bladder antimuscarinics: oxybutynin, tolterodine, solifenacin
- Antiparkinsonian agents: benztropine, trihexyphenidyl
- Skeletal muscle relaxants: cyclobenzaprine, orphenadrine
- Plants: jimsonweed (Datura stramonium), deadly nightshade (Atropa belladonna)
Antidotal therapy
- Physostigmine 0.5–1 mg IV slow push (no faster than 1 mg/min); may repeat at 10–30 min intervals. Currently subject to national shortage in the US [3][10-12]
- Rivastigmine (oral 4.5–6 mg or transdermal 9.5 mg/24h patch) as an alternative when physostigmine is unavailable [8][11][13]
Contraindicated medications
- Avoid physostigmine in patients with asthma, cardiac conduction delays (QRS >100 ms), mechanical GI/GU obstruction, or co-ingestion with depolarizing neuromuscular blockers [3]
- Avoid benzodiazepines as primary treatment — a randomized trial showed physostigmine was superior to lorazepam for delirium control (delirium resolved in 56% vs 0% after bolus, p=0.01) [14]
4. Diet
- NPO if altered mental status or risk of aspiration
- Aggressive IV fluid hydration for hyperthermia, rhabdomyolysis, and tachycardia
- No specific dietary triggers; however, some food supplements may have anticholinergic activity and contribute to cumulative burden [5]
5. Review of Systems
- Neuro: confusion, agitation, hallucinations (especially visual), mumbling/garbled speech, seizures, coma
- Eyes: blurred vision, photophobia
- GI: dry mouth, dysphagia, nausea, constipation, absent bowel sounds
- GU: urinary retention, dysuria
- Skin: flushed, hot, dry (anhidrosis is a key distinguishing feature)
- CV: palpitations, chest pain
- Psych: paranoia, picking/grasping behaviors, disorientation
6. Collateral History and Family History
- Contact EMS, family, friends for medication access, pill bottles, empty containers
- Psychiatric history — intentional overdose is common, especially with diphenhydramine and TCAs [15-16]
- Access to anticholinergic medications in the household (especially elderly patients on multiple prescriptions)
- Family history is generally not contributory unless narrow-angle glaucoma is a concern
7. Risk Factors
- Elderly patients — most vulnerable due to reduced cholinergic reserve, polypharmacy, and impaired drug clearance [1][4]
- Psychiatric illness — intentional overdose with antipsychotics, TCAs, or OTC antihistamines [16]
- Polypharmacy — cumulative anticholinergic burden from multiple low-potency agents [5][17]
- Cognitive impairment or dementia (baseline cholinergic deficit)
- Renal or hepatic impairment (reduced drug clearance)
- Children — accidental ingestion of medications or plants
8. Differential Diagnosis
- Sympathomimetic toxidrome: shares tachycardia, mydriasis, agitation, and hyperthermia, but patients are diaphoretic (sweating) rather than dry [6][18]
- Serotonin syndrome: clonus, hyperreflexia, diaphoresis, and hyperactive bowel sounds distinguish it from anticholinergic toxicity, which has normal reflexes, dry skin, and absent bowel sounds [6]
- Neuroleptic malignant syndrome: slow onset over days, "lead pipe" rigidity, bradykinesia, and autonomic instability [6]
- Malignant hyperthermia: occurs after inhalational anesthetics, with rigidity and metabolic acidosis [6]
- Thyroid storm: tachycardia and hyperthermia but with diaphoresis and goiter
- CNS infection (meningitis/encephalitis): fever and altered mental status but with meningeal signs
- Sedative-hypnotic withdrawal (alcohol, benzodiazepines): tremor, diaphoresis, seizures
9. Past Medical History
- Prior overdose attempts or psychiatric admissions
- Glaucoma (narrow-angle) — anticholinergics can precipitate acute angle-closure crisis [1]
- BPH or urinary retention — worsened by anticholinergic effects
- Constipation or GI dysmotility — risk of ileus
- Cardiac disease — tachycardia may be poorly tolerated; TCA co-ingestion raises risk of arrhythmia
- Epilepsy — seizure threshold may be lowered
10. Physical Exam
Classic findings (the full toxidrome is present in a minority; at least one of tachycardia, decreased secretions, or mydriasis is found in 94% of cases): [19]
- Vitals: Tachycardia (68%), hyperthermia, hypertension
- Eyes: Mydriasis (dilated, poorly reactive pupils)
- Skin: Flushed, hot, dry (anhidrosis) — key distinguishing feature
- Oral: Dry mucous membranes
- Abdomen: Decreased or absent bowel sounds, distension
- GU: Bladder distension/urinary retention (80% in one series required catheterization) [13]
- Neuro: Agitated delirium, picking/grasping at invisible objects, mumbling speech, ataxia; in severe cases, obtundation or coma
- Reflexes: Normal (unlike serotonin syndrome, which has hyperreflexia/clonus) [6]
11. Lab Studies
- BMP/CMP: Electrolytes, renal function (rhabdomyolysis risk), glucose
- CK: If prolonged agitation or hyperthermia — evaluate for rhabdomyolysis
- Acetaminophen and salicylate levels: Standard in all intentional ingestions
- Serum ethanol level
- Urine drug screen: Low utility for anticholinergics specifically, but helps identify co-ingestants
- Lactate: If hemodynamically unstable
- LFTs: If hepatotoxic co-ingestant suspected
- Urinalysis: Myoglobinuria if rhabdomyolysis suspected
- Specific drug levels (e.g., TCA levels) if indicated by history
12. Imaging
- Not routinely required for isolated anticholinergic toxicity
- Chest X-ray: If aspiration suspected or intubated
- CT head: Consider if altered mental status is atypical, prolonged, or does not improve with treatment — to rule out structural pathology
- KUB/abdominal X-ray: If concern for ileus or body packing
13. Special Tests
- Physostigmine diagnostic challenge: Resolution of delirium after physostigmine administration confirms anticholinergic etiology. Dramatic reversal within minutes supports the diagnosis [3][7]
- Bladder scan: Point-of-care ultrasound to assess urinary retention
- Poison center consultation (1-800-222-1222): Recommended for all significant anticholinergic exposures [18]
- Anticholinergic Burden Scale (ACB score): Useful for assessing cumulative anticholinergic load in polypharmacy patients [5]
14. ECG
- Obtain on all patients with suspected anticholinergic toxicity [18]
- Sinus tachycardia: Most common finding
- QRS prolongation (>100 ms): Suggests sodium channel blockade — classic for TCA toxicity; contraindication to physostigmine [3]
- QTc prolongation: Reported in ~1% of physostigmine-treated patients; monitor for Torsades de Pointes [20]
- Terminal R wave in aVR >3 mm: Suggestive of TCA poisoning
- Repeat ECG after treatment and before physostigmine administration
15. Assessment
Anticholinergic toxicity is a clinical diagnosis based on the constellation of central (delirium, hallucinations, agitation) and peripheral (mydriasis, dry skin/mucosa, tachycardia, urinary retention, decreased bowel sounds, hyperthermia) findings. [3][7][19] The presentation is variable — the full classic toxidrome is present in fewer than 30% of cases, but at least one cardinal sign is present in >90%. [19]
Severity stratification
- Mild: Dry mouth, blurred vision, mild tachycardia, flushing — supportive care sufficient
- Moderate: Agitated delirium, urinary retention, significant tachycardia — consider physostigmine
- Severe: Seizures, coma, hyperthermia >40°C, cardiovascular collapse — ICU admission, aggressive cooling, antidotal therapy [7]
Complications: Rhabdomyolysis, aspiration pneumonia, hyperthermia-related organ damage, seizures, respiratory failure, and death [7][21]
16. Treatment Plan
Initial stabilization
- ABCs; intubation if GCS ≤8 or airway compromise
- Continuous cardiac monitoring and IV access
- Active cooling for hyperthermia (evaporative cooling, ice packs) — avoid antipyretics (ineffective for drug-induced hyperthermia) [21]
- Foley catheter for urinary retention
Antidotal therapy
- Physostigmine (first-line antidote): 0.5–1 mg IV over 5 min; may repeat at 10–15 min intervals; typical effective dose <2 mg. Duration of action is 45–60 minutes, so re-dosing or infusion may be needed. A physostigmine infusion (0.02 mg/kg/h) has shown sustained delirium control over 4 hours in a randomized trial [3][10][14][20]
- Rivastigmine (alternative if physostigmine unavailable): Oral 4.5–6 mg (may repeat hourly until symptom resolution) or transdermal 9.5 mg/24h patch. Oral appears more effective than transdermal [13]
- Benzodiazepines: For seizures (first-line for toxin-associated seizures) and as adjunct for severe agitation if physostigmine is unavailable or contraindicated [18][22]
GI decontamination
- Activated charcoal (1 g/kg, max 50 g) if within 1–2 hours of ingestion and airway is protected. Consider later administration given slowed GI motility [22]
- Whole bowel irrigation for sustained-release formulations
Sodium bicarbonate: If QRS >100 ms (suggesting sodium channel blockade, e.g., TCA) — bolus 1–2 mEq/kg IV, target serum pH 7.45–7.55
17. Disposition
- ICU admission: Seizures, coma, hemodynamic instability, hyperthermia, need for intubation, QRS prolongation, or recurrent delirium requiring repeated physostigmine dosing (57.6% of physostigmine-treated patients in one series were admitted to ICU) [20]
- Monitored bed/observation: Moderate toxicity with delirium responding to treatment; cardiac monitoring for at least 6–8 hours
- ED discharge: Mild symptoms that resolve completely, normal ECG, at least 6 hours of observation with no recurrence. In one large series, 18.8% of patients were discharged directly from the ED [20]
- Toxicology consultation: Recommended for all moderate-to-severe cases, unknown ingestions, or when physostigmine is being considered
- Psychiatry consultation: Mandatory for all intentional ingestions before discharge
18. Follow Up / Return Precautions
- Return immediately for recurrence of confusion, hallucinations, inability to urinate, fever, seizures, or chest pain
- Symptoms may recur as physostigmine wears off (duration ~45–60 min) while the offending agent persists [3]
- Counsel on medication safety: secure anticholinergic medications, avoid polypharmacy with multiple anticholinergic agents [4]
- Follow-up within 24–48 hours with PCP or toxicology if discharged from ED
- Psychiatric follow-up if intentional ingestion
- Expected recovery: most patients return to baseline within 24–48 hours with supportive care [8]
- Elderly patients: review and reduce anticholinergic medication burden at follow-up [1][9]
References
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2. The Syndrome of Anticholinergic Intoxication. — Feldman MD. American Family Physician. 1986.
3. FDA Drug Label. — Updated date: 2022-09-13. Food and Drug Administration.
4. Drugs With Anticholinergic Properties: A Current Perspective on Use and Safety. — Gerretsen P, Pollock BG. Expert Opinion on Drug Safety. 2011.
5. Anticholinergic Accumulation: A Slumbering Interaction Between Drugs and Food Supplements. — Vrolijk MF, Opperhuizen A, Jansen EH, Bast A, Haenen GR. Basic & Clinical Pharmacology & Toxicology. 2015.
6. The Serotonin Syndrome. — Boyer EW, Shannon M. The New England Journal of Medicine. 2005.
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13. Rivastigmine for the Management of Anticholinergic Delirium. — Chiew AL, Holford AG, Chan BSH, Isoardi KZ. Clinical Toxicology. 2024.
14. A Randomized Trial Comparing Physostigmine vs Lorazepam for Treatment of Antimuscarinic (Anticholinergic) Toxidrome. — Wang GS, Baker K, Ng P, et al. Clinical Toxicology. 2021.
15. Physostigmine Is Superior to Non-Antidote Therapy in the Management of Antimuscarinic Delirium: A Prospective Study From a Regional Poison Center. — Boley SP, Olives TD, Bangh SA, Fahrner S, Cole JB. Clinical Toxicology. 2019.
16. A Narrative Review on Toxidromes in the Psychiatric Population: Implications for Overdose Prevention. — Dutta S, Buciuc AG, Barry P, Padilla V. Journal of Clinical Medicine. 2025.
17. Anticholinergic Activity of 107 Medications Commonly Used by Older Adults. — Chew ML, Mulsant BH, Pollock BG, et al. Journal of the American Geriatrics Society. 2008.
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19. Prevalence of Autonomic Signs and Symptoms in Antimuscarinic Drug Poisonings. — Patel RJ, Saylor T, Williams SR, Clark RF. The Journal of Emergency Medicine. 2004.
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21. FDA Drug Label. — Updated date: 2024-10-11. Food and Drug Administration.
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