Central Anticholinergic Syndrome: Diagnosis and Treatment

Central anticholinergic syndrome (CAS) is one of the chameleons of emergency medicine. It presents with nonspecific symptoms – ranging from agitation to somnolence to coma – and is therefore alarmingly often overlooked or misinterpreted in clinical practice. The differential diagnoses range from stroke to hypoglycemia to delirium of other etiologies. Yet the specific treatment with physostigmine would be rapidly available and highly effective. If you don't actively include CAS in your differential diagnostic considerations, you'll miss a treatable cause of altered consciousness – with potentially significant consequences for your patients.

Pathophysiology: Why Acetylcholine Deficiency Shuts Down the Brain

CAS results from a relative or absolute deficit of acetylcholine at central muscarinic receptors. Acetylcholine is one of the most important excitatory neurotransmitters in the central nervous system and plays a key role in:

• Consciousness and vigilance (ascending reticular activating system)
• Cognition and memory (cholinergic pathways in the hippocampus and cortex)
• Thermoregulation (hypothalamic control)
• Motor coordination (basal ganglia)

When muscarinic transmission at these structures is pharmacologically blocked, the result is a symptom complex that, depending on the extent of blockade, can range from mild confusion to deep coma. The crucial point is: this is a functional disturbance. Unlike structural brain lesions, CAS is in principle completely reversible – provided it is recognized and treated.

Central vs. Peripheral Effects

Anticholinergic blockade typically affects not only the CNS but also peripheral muscarinic receptors. The peripheral symptoms (tachycardia, dry mouth, mydriasis, urinary retention, decreased bowel motility) are often the diagnostic clues that lead to recognition of the central problem. However, CAS can also occur without pronounced peripheral symptoms – particularly with substances that have high CNS penetration and low peripheral activity.

Typical Triggers: More Than Just Atropine

The list of substances with anticholinergic potency is long and encompasses medications from virtually every medical specialty. In emergency medicine practice, the following triggers are particularly relevant:

Classic Anticholinergics

• Atropine – perioperatively, as an antidote for organophosphate poisoning, or for bradyarrhythmias
• Scopolamine – as a patch for motion sickness prophylaxis, intraoperatively
• Hyoscine butylbromide (Buscopan®) – ubiquitous in acute care for colicky pain

First-Generation Antihistamines

• Promethazine (Phenergan®) – as a sedative and antiemetic
• Diphenhydramine, dimenhydrinate (Dramamine®) – available over the counter and therefore frequently used in self-medication
• Clemastine

Tricyclic Antidepressants (TCAs)

• Amitriptyline, doxepin, clomipramine, imipramine
• TCAs are among the most potent anticholinergic substances and are simultaneously common causes of severe intoxications

Antipsychotics

• Low-potency antipsychotics such as chlorprothixene, levomepromazine, promethazine
• High-potency antipsychotics such as haloperidol have comparatively low anticholinergic activity

Other Relevant Substances

• Benztropine, biperiden (Akineton®) – antiparkinsonian agents
• Oxybutynin, tolterodine – urologics for overactive bladder
• Baclofen – muscle relaxant
• Anesthetics and adjuvants – particularly in combination, a cumulative anticholinergic effect develops (the so-called "anticholinergic burden")

The Concept of Anticholinergic Burden

In practice, CAS frequently results not from a single substance at a toxic dose but from the cumulation of multiple medications, each with moderate anticholinergic potency. An elderly patient taking amitriptyline, dimenhydrinate, and oxybutynin simultaneously can develop a significant anticholinergic burden, even though each substance individually is administered at a therapeutic dose. This concept is particularly important for geriatric emergency medicine, as elderly patients are especially vulnerable due to altered pharmacokinetics (reduced hepatic clearance, diminished renal elimination, increased blood-brain barrier permeability).

Clinical Presentation: Two Faces of CAS

CAS manifests clinically in two fundamentally different forms, which further complicates diagnosis:

Agitated Form (more common)

• Psychomotor agitation, restlessness, picking at objects, wandering
• Hallucinations (typically visual: insects, small animals)
• Disorientation, confabulation
• Logorrhea, incoherent speech
• Seizures (in severe cases)
• Hyperthermia (due to impaired sweating and increased metabolism)

Somnolent/Comatose Form (less common, but more frequently missed)

• Somnolence to deep coma
• Respiratory depression
• Areflexia
• Hypothermia possible

The somnolent form is particularly easy to miss, as it does not immediately suggest an anticholinergic etiology and instead points toward differential diagnoses such as sedative intoxication, stroke, or metabolic coma.

Peripheral Accompanying Symptoms

The classic mnemonic summarizes the peripheral anticholinergic signs:

• "Blind as a bat" – mydriasis, accommodation disturbance
• "Dry as a bone" – dry skin and mucous membranes, absent sweating
• "Red as a beet" – skin flushing due to cutaneous vasodilation
• "Hot as a hare" – hyperthermia
• "Mad as a hatter" – delirium, hallucinations
• "Full as a flask" – urinary retention
• "Tacky as …" – tachycardia

The presence of several of these signs in combination with altered consciousness should always raise suspicion of CAS.

Diagnostics: A Clinical Diagnosis

There is no laboratory parameter and no diagnostic test that can definitively prove CAS. The diagnosis is based on:

1. Clinical symptom constellation (central + peripheral anticholinergic signs)
2. History/medication history (exposure to anticholinergic substances)
3. Exclusion of other causes (structural CNS lesion, metabolic derangement, intoxication with other substances)
4. Therapeutic trial with physostigmine (ex juvantibus diagnosis)

Differential Diagnosis

The following differential diagnoses must be systematically considered and, if necessary, ruled out:

Differential Diagnosis	Differentiation from CAS
Hypoglycemia	Blood glucose measurement – quick and easy to rule out
Stroke	Focal neurological deficits, CT/MRI
Serotonin syndrome	Hyperreflexia, clonus, diarrhea (in CAS: areflexia, constipation)
Neuroleptic malignant syndrome	Rigidity, elevated CK, slow onset
Delirium of other etiology	Signs of infection, metabolic derangement
Opioid intoxication	Miosis (in CAS: mydriasis), naloxone test
Alcohol withdrawal delirium	Tremor, diaphoresis (in CAS: dry skin)

The differentiation from serotonin syndrome is particularly clinically relevant, as both syndromes can occur in the perioperative setting and in psychiatric patients. The key difference: in serotonin syndrome you find neuromuscular hyperactivity (clonus, hyperreflexia, myoclonus), whereas in CAS there is more neuromuscular hypoactivity. Sweating also differs diametrically – serotonin syndrome patients sweat profusely, CAS patients are typically dry.

Treatment: Physostigmine as a Specific Antidote

Physostigmine – Mechanism of Action

Physostigmine (Anticholium®) is a tertiary amine and therefore – unlike neostigmine – crosses the blood-brain barrier. It reversibly inhibits acetylcholinesterase, thereby increasing acetylcholine concentration in the synaptic cleft. The effect sets in within minutes and lasts approximately 60–90 minutes.

Dosing and Administration

Adults:

• Initial dose: 1–2 mg slow intravenous injection over 5 minutes
• Repeat dosing: If response is inadequate, an additional 1–2 mg may be administered after 10–20 minutes
• Maximum dose: The literature reports varying maximum doses; a cumulative dose of 4–6 mg should not be exceeded
• Note duration of action: Since the duration of action of physostigmine is shorter than that of most triggering substances, repeat doses or a continuous infusion (1–2 mg/h) may be necessary

Children:

• 0.02 mg/kg body weight slow IV, maximum 0.5 mg as a single dose

Administration Notes

• Slow injection (over at least 5 minutes) is essential, as rapid administration can trigger seizures and bradycardia
• Monitoring: Continuous ECG monitoring is mandatory
• Atropine must be readily available as an antagonist (dose: half the physostigmine dose)
• Administration should ideally be performed by an experienced emergency physician or under intensive care conditions

Contraindications

• Mechanical ileus or mechanical urinary tract obstruction
• Bronchospasm / severe bronchial asthma
• Second- or third-degree AV block (without pacemaker)
• Type 1 diabetes mellitus (relative contraindication)
• Concurrent intoxication with depolarizing muscle relaxants
• Known hypersensitivity

Special Caution in TCA Intoxication

The use of physostigmine in tricyclic antidepressant intoxications is controversial. TCAs block not only muscarinic receptors but also sodium channels and can cause severe cardiac complications (QRS widening, ventricular tachycardia, asystole). In this constellation, physostigmine can potentiate parasympathomimetic effects on the heart and lead to bradycardia or asystole. The current recommendation is therefore:

• In pure TCA intoxication with cardiac symptoms: use physostigmine cautiously, sodium bicarbonate as the primary cardiac therapy
• In TCA-associated CAS without cardiac symptoms and after careful risk-benefit assessment: physostigmine may be considered under intensive care monitoring

Supportive Measures

In addition to specific antidote therapy, the following supportive measures are relevant:

• Airway management in comatose patients
• Temperature management in hyperthermia (physical cooling; note: antipyretics are ineffective since the mechanism is not prostaglandin-mediated)
• Rehydration (especially in hyperthermia and reduced fluid intake)
• Urinary catheter for urinary retention
• Benzodiazepines for seizures (midazolam or diazepam)
• Monitoring: Intensive care monitoring for at least 24 hours, as the duration of action of many triggering substances exceeds that of physostigmine

CAS in the Perioperative Setting

Perioperative CAS deserves special attention, as it is a common and simultaneously underdiagnosed cause of postoperative confusion and delayed emergence. During anesthesia, anticholinergic substances are regularly administered or medications with anticholinergic side effects are used:

• Atropine for vagal blockade or as part of premedication
• Inhalational anesthetics (possess inherent anticholinergic activity)
• Opioids (indirect anticholinergic effect)
• Muscle relaxants in combination with their reversal agents

If a patient shows delayed emergence from anesthesia or unexpected postoperative agitation, and structural as well as metabolic causes have been ruled out, a therapeutic trial with physostigmine should be considered. The prompt improvement of symptoms within minutes confirms the suspected diagnosis and can spare the need for extensive further workup.

Practical Tips for the Emergency Setting

• Think of it! CAS must be actively included in your differential diagnostic considerations. If you don't think of it, you won't diagnose it.
• Obtain a thorough medication history – even seemingly harmless OTC products (antihistamines, herbal sedatives) can contribute.
• Check for dry skin – this simple clinical sign is a strong indicator. A patient in delirium who is not sweating very likely has an anticholinergic problem.
• Examine the pupils – bilateral mydriasis with sluggish light reaction is a cardinal sign.
• Use physostigmine as a diagnostic tool – when there is reasonable suspicion, the therapeutic trial simultaneously serves as diagnostic confirmation.
• Stock Anticholium® – the antidote should be available in the emergency department, the recovery room, and at emergency physician stations.

Practical Training

Diagnosing and treating central anticholinergic syndrome requires a trained clinical eye and experience in using the antidote physostigmine – precisely because CAS is so frequently missed. In the emergency physician refresher course by Simulation Tirol, you train differential diagnostic algorithms for altered consciousness and the structured approach to toxicological emergencies in realistic simulation scenarios. This way, you build the confidence you need to avoid missing rare but treatable diagnoses in real-life emergencies.