Organophosphate poisoning — atropine and pralidoxime

Overview

Organophosphate (OP) and carbamate compounds irreversibly (OP) or reversibly (carbamate) inhibit acetylcholinesterase (AChE), causing accumulation of acetylcholine at all cholinergic synapses. The result is a life-threatening cholinergic toxidrome dominated by respiratory failure, bronchorrhoea, bronchospasm, and paralysis acting in concert. Agricultural exposure accounts for the vast majority of ICU presentations in ANZ and the Asia-Pacific; deliberate self-poisoning with concentrates (parathion, dimethoate) carries the highest mortality. Chemical warfare agents (sarin, soman, VX) share the same mechanism but with far shorter ageing times and potential for mass-casualty presentation. Carbamates (e.g. aldicarb, methomyl) produce a clinically identical but spontaneously reversible syndrome, they rarely require pralidoxime.

Agents and Exposure Routes

Exposure routes: Dermal (agricultural), inhalational (vapours, aerosols, chemical warfare), ingestion (self-poisoning concentrates), ocular.

Mechanism of Toxicity

Acetylcholinesterase Inhibition

$$\text{OP} + \text{AChE} \rightarrow \text{OP-AChE complex} \xrightarrow{\text{ageing}} \text{Irreversible OP-AChE}$$

• OP binds the serine residue at the AChE active site → phosphorylation → ACh cannot be hydrolysed
• ACh accumulates at muscarinic receptors (smooth muscle, glands, SA node), nicotinic receptors (NMJ, autonomic ganglia), and CNS synapses
• Carbamates carbamylate AChE transiently, spontaneous hydrolysis occurs within minutes to hours; no ageing

Ageing, The Critical Concept

Ageing = covalent dealkylation of the OP-AChE bond, rendering it permanently resistant to reactivation by oximes (pralidoxime).

Pitfall: Ageing is irreversible. Every hour of delay in pralidoxime administration narrows the therapeutic window. In soman poisoning, ageing is effectively complete within minutes, pralidoxime is futile.

Toxidrome Recognition

Muscarinic Effects, SLUDGE / DUMBELS

Both mnemonics describe the same syndrome; DUMBELS is more comprehensive for exam purposes.

Additional muscarinic signs: bradycardia, AV block, hypotension, diaphoresis.

The "Killer Bs"

$$\text{Bronchorrhoea} + \text{Bronchospasm} + \text{Bradycardia}$$

These three are the primary drivers of early death, airway management and atropine target these directly.

Nicotinic Effects

• Peripheral NMJ: fasciculations (early, unreliable), weakness, paralysis (depolarising block from persistent ACh)
• Sympathetic ganglia: tachycardia, hypertension, mydriasis (counteracts miosis, mixed pupils can occur)
• Adrenal medulla: catecholamine surge, pallor, tachycardia

Common mistake: Attributing tachycardia and hypertension to "resolving" toxicity, these may reflect unopposed nicotinic stimulation, particularly when coexistent with ongoing muscarinic features. Atropine does NOT reverse nicotinic manifestations.

CNS Effects

• Anxiety, agitation, confusion → seizures (status epilepticus) → coma
• Central apnoea, a major contributor to death alongside peripheral respiratory muscle paralysis
• ACh accumulation at muscarinic and nicotinic CNS receptors; atropine crosses BBB and addresses central muscarinic activity

Diagnosis

Clinical

The combination of miosis + hypersalivation/bronchorrhoea + bradycardia in a patient with pesticide exposure is virtually diagnostic. Seek: farm/crop exposure history, characteristic smell (garlic-like for some OPs), empty containers.

Cholinesterase Activity

Pitfall: Cholinesterase levels are a supporting test only, treatment must NOT be withheld pending results. Levels may be misleadingly low at baseline in some patients (pregnancy, liver disease, inherited deficiency).

Immediate Management, Priority Sequence

1. Personal Protective Equipment and Decontamination

• Staff PPE first, gloves, gown, eye protection, N95 minimum (full face shield + butyl rubber gloves for concentrated liquid or nerve agent)
• Remove ALL clothing, reduces dermal exposure by ~80%
• Copious water/soap skin decontamination; prolonged eye irrigation for ocular exposure
• Chemical warfare scenarios: HAZMAT team, full Level B/C protection before patient contact

2. Airway, Intubate Early

• Bronchorrhoea, hypersalivation, and depressed consciousness combine rapidly to cause airway loss
• Intubate before the patient deteriorates, do not wait for SpO₂ to fall
• Suxamethonium is CONTRAINDICATED, plasma pseudocholinesterase (BChE) is inhibited → failure to metabolise suxamethonium → prolonged depolarising block (potentially hours)
• Use rocuronium (1.2 mg/kg) for RSI; sugammadex reversal available if needed
• High-dose atropine should begin concurrently with airway management, do not sequence them

3. Atropine, Titrated Doubling-Dose Protocol

Atropine competes with ACh at muscarinic receptors only. It addresses bronchorrhoea, bronchospasm, bradycardia, and secretions, it has NO effect on nicotinic (NMJ, ganglionic) features.

Dosing Protocol

$$\text{Initial dose: 1.2-3 mg IV}$$

Double the dose every 5 minutes until atropinisation is achieved:

• Total loading doses of hundreds of mg may be required in severe agricultural OP poisoning, do NOT stop prematurely
• After atropinisation: maintenance infusion at 10-20% of total loading dose per hour, titrated to secretions and HR
• If IV atropine is unavailable: IM atropine (autoinjectors in mass-casualty settings)
• Glycopyrrolate (0.1-0.2 mg/kg IV): quaternary ammonium, does not cross BBB; useful if central atropine effects (tachyarrhythmia, agitation, hyperthermia) are problematic. Targets peripheral muscarinic effects. Not a first-line agent in ANZ practice but an alternative if atropine supply limited

Pitfall: Using mydriasis or tachycardia as the primary atropinisation endpoint risks over-atropinisation. The clear chest and dry secretions are the correct targets. Pupils may remain mid-position due to coexistent nicotinic effects; tachycardia may be pre-existing or nicotinic.

4. Pralidoxime (2-PAM), Oxime Reactivation

Pralidoxime nucleophilically attacks the OP-AChE bond, liberating the phosphate group and restoring AChE activity, provided ageing has not occurred.

Dosing (ANZ / WHO Recommended)

$$\text{Loading dose: } 30 \text{ mg/kg IV over 30 minutes (max ~2 g)}$$ $$\text{Maintenance infusion: } 8\text{-}10 \text{ mg/kg/h}$$

• Continue for 24-48 h from last significant exposure (or until clinical improvement and cholinesterase recovery)
• Pralidoxime addresses nicotinic features (weakness, fasciculations, paralysis) as well as reinforcing atropine's effect by reducing ACh burden at all synapses
• Given IN ADDITION TO, not instead of, atropine, both required in OP poisoning

Evidence and Controversy

• The PRO-COP trial (Asia-Pacific, 2016, Eddleston et al. Lancet 2008 Sri Lanka data; further meta-analysis ongoing) found no survival benefit from high-dose pralidoxime in agricultural OP poisoning and possible harm (increased mortality in some analyses). This has created genuine clinical uncertainty.
• Most ANZ toxicologists and CICM exam consensus: still administer pralidoxime within the first 24-48 h in moderate-severe OP poisoning, particularly when nicotinic features (weakness, fasciculations) are prominent, as the biological mechanism is sound and benefit may exist in early administration. Avoid in carbamate poisoning (unnecessary; theoretical risk of inhibiting spontaneous AChE recovery).
• Carbamates: pralidoxime generally not indicated; relies on spontaneous AChE reversal

Pitfall: Pralidoxime administered after ageing is complete is ineffective and may cause adverse effects (transient weakness, hypertension, laryngospasm if infused too rapidly). Rapid infusion (>200 mg/min) can precipitate hypertensive crises, always infuse over ≥30 minutes.

5. Seizures

• Benzodiazepines first-line: diazepam 10-20 mg IV (or lorazepam 4 mg IV); may require large doses
• Mechanism: central muscarinic + GABA-ergic disruption, phenytoin is ineffective for OP-induced seizures (avoid)
• Refractory status: phenobarbitone, propofol infusion, or general anaesthesia (midazolam infusion)
• Atropine (central muscarinic blockade) contributes to seizure control

Intermediate Syndrome

A delayed neuromuscular complication distinct from the acute cholinergic crisis:

• Onset: 24-96 hours after acute poisoning (as acute crisis resolves)
• Features: Proximal limb weakness (neck flexors, hip flexors), cranial nerve palsies (facial, abducens, oculomotor), respiratory muscle weakness → respiratory failure
• Mechanism: Not fully elucidated, post-synaptic NMJ dysfunction, persistent AChE inhibition, possibly direct myopathy
• Agents most associated: Dimethoate, fenthion, methyl parathion, chlorpyrifos
• Treatment: Supportive, prolonged mechanical ventilation; no specific antidote; cholinesterase activity monitoring
• Duration: Days to weeks; most patients recover fully if ventilated through the syndrome

Pitfall: Intermediate syndrome occurs AFTER discharge from the acute crisis phase, extubation must not be premature. The patient who looked almost normal on day 1-2 may arrest from respiratory failure on day 3 if prematurely extubated without formal assessment of respiratory muscle strength.

OP-Induced Delayed Neuropathy (OPIDN)

• Onset: 2-5 weeks after exposure
• Mechanism: phosphorylation and ageing of neuropathy target esterase (NTE) → "dying-back" axonopathy (distal sensorimotor polyneuropathy)
• Features: distal sensory loss, flaccid weakness, ataxia; may progress to upper motor neuron signs as Wallerian degeneration ascends
• Common agents: triorthocresyl phosphate (TOCP), leptophos, mipafox
• No specific antidote, physiotherapy, supportive care; partial recovery over months to years

Key Numbers

ICU Relevance

Monitoring

• Continuous ECG: QTc prolongation, bradyarrhythmias, AV block, atropine targets these; watch for atropine-induced tachyarrhythmia (sign of over-atropinisation)
• Serial chest auscultation and secretion assessment, the primary guide to atropine titration
• Cholinesterase activity (RBC AChE): trend over time guides duration of antidote therapy; recovery of RBC AChE lags clinical improvement by days to weeks
• Lung-protective ventilation: bronchospasm + secretions → high airway pressures; optimise PEEP, use regular suction, nebulised ipratropium as adjunct
• Bladder drainage: urinary incontinence, IDC and strict fluid balance
• Temperature: hyperthermia from nicotinic/central effects; hypothermia from vasodilation

Escalation Triggers

• Failure to achieve atropinisation despite >20 mg atropine, escalate dose rapidly; consider whether diagnosis is correct
• Worsening respiratory mechanics despite atropinisation, consider intermediate syndrome, pneumonia, ARDS
• Refractory seizures, escalate to propofol or barbiturate infusion
• Suspected chemical warfare agent, notify public health, activate mass-casualty protocol, contact Poisons Information Centre (13 11 26 in Australia)

Common ICU Scenarios and Pitfalls

Pitfall 1, Suxamethonium in RSI: A single dose can cause 2-6 hours of paralysis. Always use rocuronium in suspected OP poisoning, even before the diagnosis is confirmed if the clinical picture fits.

Pitfall 2, Stopping atropine too soon: When bronchorrhoea clears and HR normalises, the instinct is to stop atropine. Maintenance infusion must continue, abrupt cessation causes resurgence of muscarinic features within 30-60 min. Wean slowly.

Pitfall 3, Premature extubation and intermediate syndrome: Patients can deteriorate abruptly from respiratory failure between day 1 and day 5. Formal respiratory muscle assessment (NIF, VC, 5-second head lift) before extubation; maintain low threshold for elective ventilation through the intermediate syndrome risk period.

Pitfall 4, Staff safety: Secondary contamination from intact skin or clothing is a genuine risk, particularly with dermal agents (VX, parathion concentrates). Resuscitation should occur in a decontamination area with PPE, not in the standard resuscitation bay, until decontamination is complete.

Pitfall 5, Glycopyrrolate misconception: Glycopyrrolate does not cross the blood-brain barrier, it will not address central apnoea or seizures. It is a reasonable adjunct for peripheral secretions but cannot replace atropine's CNS actions.

Pitfall 6, Carbamates and pralidoxime: Administering pralidoxime to carbamate poisoning is not only unnecessary but may theoretically inhibit spontaneous AChE recovery. Atropine + supportive care is the mainstay.

Poisons Information and Toxicology Support

• Australian Poisons Information Centre: 13 11 26 (24/7)
• Clinical toxicology consultation via local toxicology service (available in major ANZ centres)
• For chemical warfare agent exposure: notify state health department + Australian Department of Defence CBRN response