Toxicology – Cholinergic Syndrome

Definition

Cholinergic syndrome is a toxidrome caused by excessive stimulation of acetylcholine receptors.

The classic syndrome includes:

• Excessive salivation and lacrimation
• Sweating
• Bronchorrhea and bronchospasm
• Miosis
• Vomiting and diarrhea
• Urination
• Bradycardia

Severe poisoning may additionally cause:

• Fasciculations
• Muscle weakness or paralysis
• Altered mental status
• Seizures
• Respiratory failure

Pathophysiology

Many important cholinergic poisons inhibit acetylcholinesterase (AChE), the enzyme responsible for breaking down acetylcholine.

This produces:

AChE inhibition → acetylcholine accumulation → excessive muscarinic + nicotinic + CNS stimulation

The two major peripheral receptor groups produce different manifestations.

Muscarinic Effects

Muscarinic overstimulation produces:

• Miosis
• Salivation
• Lacrimation
• Sweating
• Bronchorrhea
• Bronchospasm
• Bradycardia
• Vomiting
• Abdominal cramping
• Diarrhea
• Urination

A useful mnemonic is DUMBELS:

• D – Defecation/Diarrhea, Diaphoresis
• U – Urination
• M – Miosis
• B – Bronchorrhea, Bronchospasm, Bradycardia
• E – Emesis
• L – Lacrimation
• S – Salivation

Nicotinic Effects

Nicotinic receptor overstimulation at autonomic ganglia and the neuromuscular junction can cause:

• Muscle fasciculations
• Muscle weakness
• Paralysis
• Tachycardia
• Hypertension

Severe neuromuscular weakness can contribute substantially to respiratory failure.

Central Nervous System Effects

Because some cholinesterase inhibitors enter the CNS, severe poisoning may cause:

• Anxiety or agitation
• Confusion
• Altered consciousness
• Seizures
• Coma
• Central respiratory depression

Organophosphates

Organophosphate insecticides are major causes of severe cholinergic poisoning.

They inhibit acetylcholinesterase by phosphorylating the enzyme.

Over time, the organophosphate–AChE complex can undergo “aging,” after which reactivation of the enzyme becomes increasingly difficult.

Recovery then depends substantially on synthesis of new functional enzyme.

Carbamates

Carbamates also inhibit acetylcholinesterase but generally form a reversible carbamylated enzyme complex.

Their toxicity is often shorter-lived than organophosphate poisoning, although severe poisoning can still occur.

Other Causes

Cholinergic manifestations can also occur with:

• Physostigmine
• Neostigmine
• Pyridostigmine
• Bethanechol
• Pilocarpine
• Certain muscarine-containing mushrooms
• Nicotine and related agents, although their receptor effects differ from classic AChE inhibition

Not every cholinergic syndrome is therefore caused by an organophosphate.

Clinical Features

Eyes/HEENT

• Miosis
• Lacrimation
• Salivation
• Blurred vision

Skin

• Profuse sweating

Cardiovascular

• Bradycardia from muscarinic activity
• Tachycardia and hypertension from nicotinic ganglionic activity
• Hypotension
• Dysrhythmias in severe poisoning

The heart rate can therefore be either slow or fast.

Gastrointestinal

• Nausea
• Vomiting
• Abdominal cramping
• Diarrhea
• Increased bowel activity

Neuromuscular

• Fasciculations
• Weakness
• Paralysis

Neurologic

• Confusion
• Seizures
• CNS depression
• Coma

Respiratory Failure

Respiratory compromise is the major immediate threat.

It may result simultaneously from:

• Bronchorrhea
• Bronchospasm
• Central respiratory depression
• Neuromuscular weakness/paralysis
• Aspiration
• Pulmonary edema in severe cases

This combination can cause rapid hypoxemia and respiratory arrest.

Differential Diagnosis

Conditions that can partially resemble cholinergic poisoning include:

Nicotine toxicity

• Nausea/vomiting
• Sweating
• Fasciculations
• Autonomic instability

Opioid poisoning

• Miosis and respiratory depression, but usually without profuse secretions or fasciculations

Sympathomimetic poisoning

• Sweating, tachycardia, hypertension and agitation, but usually mydriasis rather than miosis and without the characteristic bronchorrhea

Beta-blocker, calcium-channel blocker, or digoxin toxicity

• Can cause bradycardia and hypotension but generally lack the characteristic secretory syndrome.

Botulism

• Produces weakness/paralysis but typically causes dryness and descending paralysis, rather than a wet cholinergic syndrome.

Diagnosis

Diagnosis is primarily clinical.

The combination of:

miosis + profuse secretions + bronchorrhea + fasciculations/weakness

strongly suggests significant cholinergic poisoning.

Exposure history should include:

• Pesticides
• Occupational exposure
• Medications
• Plants or mushrooms
• Possible dermal or inhalational exposure

Cholinesterase Testing

Laboratory measurements may include:

Plasma butyrylcholinesterase

• Often readily available
• Sensitive to exposure
• Less closely related to neuromuscular AChE activity

Red blood cell acetylcholinesterase

• More closely reflects neuronal/neuromuscular acetylcholinesterase activity
• May correlate better with significant organophosphate effects

However, treatment of a severely symptomatic patient should not wait for cholinesterase results.

Additional Evaluation

Depending on severity:

• ECG and continuous cardiac monitoring
• Oxygen saturation
• Blood gas when respiratory failure is suspected
• Electrolytes
• Glucose
• Renal function
• Chest imaging when pulmonary complications are suspected

Management

Immediate priorities are:

• Airway management
• Oxygenation and ventilation
• Suctioning of excessive secretions
• Cardiac monitoring
• Treatment of seizures
• Removal from ongoing exposure

Severe poisoning may require early endotracheal intubation and mechanical ventilation.

Decontamination

For significant dermal contamination:

• Remove contaminated clothing.
• Prevent secondary contamination of healthcare personnel.
• Thoroughly wash exposed skin.

Ocular exposures require irrigation.

Do not induce vomiting.

Older recommendations for routine gastric lavage are no longer standard practice.

Activated charcoal may be considered after selected ingestions when clinically appropriate and when the airway can be safely protected.

Atropine

Atropine is a competitive muscarinic acetylcholine receptor antagonist.

It treats the life-threatening muscarinic manifestations, particularly:

• Bronchorrhea
• Bronchospasm
• Bradycardia

The major therapeutic endpoint in severe poisoning is improvement in pulmonary secretions and ventilation, rather than normalization of pupil size or heart rate.

Severe organophosphate poisoning can require very large cumulative amounts of atropine.

Importantly, atropine does not directly reverse nicotinic neuromuscular weakness.

Pralidoxime (2-PAM)

Pralidoxime is an oxime cholinesterase reactivator.

In organophosphate poisoning, it can reactivate phosphorylated acetylcholinesterase before aging occurs.

It is particularly important for:

• Fasciculations
• Muscle weakness
• Respiratory muscle dysfunction

Its role is strongest in clinically significant organophosphate poisoning. The benefit in carbamate poisoning is less clearly established and depends on the specific exposure.

Seizures

Benzodiazepines are generally used for toxin-induced seizures.

Control of seizures is especially important because prolonged seizures increase:

• Oxygen demand
• Hyperthermia
• Rhabdomyolysis
• Secondary neurologic injury

Delayed Neurologic Syndromes

Organophosphate poisoning can occasionally produce neurologic complications after the initial cholinergic crisis.

Intermediate syndrome

• Develops after the acute cholinergic phase
• Characterized by proximal, neck, cranial, and respiratory muscle weakness

Organophosphate-induced delayed neuropathy

• Occurs later after selected organophosphate exposures
• May produce distal weakness and sensory abnormalities

Thus, apparent resolution of the initial secretory syndrome does not always mean neurologic risk has completely ended.

Key Points

• Cholinergic syndrome results from excess acetylcholine activity.
• Think “wet patient”: salivation, lacrimation, sweating, bronchorrhea, vomiting, diarrhea, and urination.
• Muscarinic effects → secretions, miosis, bronchospasm, bradycardia.
• Nicotinic effects → fasciculations, weakness/paralysis, tachycardia and hypertension.
• Respiratory failure results from a dangerous combination of bronchorrhea + bronchospasm + central depression + neuromuscular weakness.
• Diagnosis is primarily clinical; do not delay treatment while waiting for cholinesterase testing.
• Atropine treats muscarinic toxicity, particularly dangerous pulmonary secretions.
• Pralidoxime reactivates AChE before aging and is especially important in significant organophosphate poisoning.
• Atropine does not reverse nicotinic paralysis.
• Dermal decontamination is particularly important after pesticide exposure because continued skin absorption can prolong toxicity.