Organophosphorus (OP) Poisoning: Signs, Symptoms, ICU Management & Intermediate Syndrome

MD Anaesthesiology Long Answer

INTRODUCTION

PATHOPHYSIOLOGY

• Muscarinic receptors — postganglionic parasympathetic nerve endings, CNS
• Nicotinic receptors — autonomic ganglia, neuromuscular junctions (NMJ)
• CNS synapses

• Erythrocyte AChE (true cholinesterase) — reflects synaptic inhibition; more accurate
• Plasma butyrylcholinesterase (pseudocholinesterase) — easier to assay, decreases first

CLINICAL FEATURES

A. Muscarinic Effects (Parasympathetic overstimulation)

B. Nicotinic Effects (NMJ and ganglionic)

C. Central Nervous System Effects

• Anxiety, restlessness, emotional lability
• Headache, dizziness, ataxia
• Seizures (often refractory) — ACh-mediated
• Coma, loss of consciousness
• Central respiratory depression
• Altered mental status — present in most severely poisoned patients along with pinpoint pupils, excessive sweating, and dyspnoea

D. ECG Changes

• QT prolongation (risk of torsades de pointes)
• ST-segment changes, peaked T waves
• AV block, ventricular tachycardia/fibrillation

GRADING OF SEVERITY

DIAGNOSIS

• Clinical: Based on history of exposure + cholinergic toxidrome. Do not delay treatment awaiting laboratory confirmation.
• Odour: Hydrocarbon or garlic-like odour may be present
• Plasma butyrylcholinesterase: Decreases first; <50% of normal is significant; normalises over 28–42 days without oxime
• RBC AChE: More accurate; reduced to 10–20% in moderate, <10% in severe poisoning; full recovery takes up to 120 days
• ECG: QT prolongation, arrhythmias
• CXR: May show pulmonary oedema
• Routine labs: Hyperglycaemia or hypoglycaemia, leukocytosis, raised amylase (pancreatitis), deranged LFTs
• EMG: May help identify neuromuscular junction dysfunction; useful in intermediate syndrome

ICU MANAGEMENT

1. Decontamination (PRIORITY — Before Anything Else)

• Staff protection: Neoprene or nitrile gloves (NOT latex); protective clothing mandatory to prevent secondary poisoning
• Remove all clothing and accessories; place in sealed plastic bags (hazardous waste)
• Wash patient with copious soap and water — scalp, hair, fingernails, skin folds, conjunctivae
• Contaminated runoff water must be disposed of as hazardous material
• Instruments decontaminated with chlorine bleach

2. Airway and Respiratory Support

• Immediate 100% oxygen via non-rebreather mask
• Early intubation for altered consciousness, respiratory failure, or profuse secretions
• AVOID succinylcholine — OP poisoning inhibits pseudocholinesterase, causing prolonged neuromuscular blockade; use rocuronium instead
• Avoid ester-type local anaesthetics (also hydrolysed by pseudocholinesterase)
• Positive pressure ventilation for respiratory muscle paralysis
• Avoid β-blockers — may potentiate bradycardia
• Pulmonary oedema treated with oxygen, IPPV, atropine, and pralidoxime

3. Antidote Therapy

A. ATROPINE — First-line, Cornerstone of Treatment

• Competitive muscarinic receptor antagonist — counters muscarinic effects only (no effect on nicotinic/NMJ)
• Initial dose: 1.2–3 mg IV (adults); 0.05 mg/kg IV (children), depending on severity
• Double the dose every 5 minutes until adequate atropinisation is achieved
• Extremely large doses may be required: 200–500 mg in the first hour in severe cases

• Clear chest on auscultation (dry lung fields)
• Heart rate >80 bpm
• Systolic BP >80 mmHg
• Reduced oral and bronchial secretions
• Maintenance infusion: 10–20% of the total cumulative dose used to achieve atropinisation, per hour (typical: 0.4–4 mg/h IV in adults)
• Tachycardia is NOT a contraindication to atropine — tachycardia may result from hypoxia/bronchospasm
• Monitor for atropine toxicity: absent bowel sounds, hyperthermia, delirium

B. PRALIDOXIME (2-PAM) — Oxime Reactivator

• Binds the OP–AChE complex causing a conformational change → enzyme reactivation; also acts at NMJ (reverses nicotinic effects)
• Give as early as possible — before aging occurs; still may be given 24–48 h after exposure in most agricultural OPs
• Adult dose:
  • Bolus: 30 mg/kg (up to 1–2 g) IV over 5–10 min (or 30 min in Rosen's)
  • Maintenance infusion: 8 mg/kg/h for 24–48 h (some regimens: 500 mg/h up to 7 days)
• Paediatric dose: 25–50 mg/kg IV over 30 min
• Alternative dosing: 2 g over 20 min + 500 mg/h infusion up to 7 days
• Indications for oximes: respiratory depression/failure, fasciculations, seizures, haemodynamic instability, requirement for repeated large doses of atropine
• Other available oximes (not in India): obidoxime (Toxogonin), HI-6 (military)

C. BENZODIAZEPINES — For Seizures

• Diazepam or lorazepam IV for seizure control
• Seizures are ACh-mediated; benzodiazepines are the agents of choice
• Phenytoin is ineffective for OP-induced seizures

4. Additional ICU Interventions

• Cardiac monitoring: Continuous ECG; manage QT prolongation (avoid QT-prolonging drugs); treat arrhythmias appropriately
• Gastric lavage: No proven benefit; activated charcoal — no proven benefit
• Magnesium sulphate: Growing evidence for reduction in ICU stay; may reduce arrhythmias
• Fluids and vasopressors: For haemodynamic support
• Antiepileptics: Continue benzodiazepines; consider phenobarbitone if refractory
• Nutrition: Early enteral nutrition once haemodynamically stable
• Avoid: Morphine, theophylline, phenothiazines, reserpine (may worsen cholinergic crisis)

INTERMEDIATE SYNDROME (IMS)

Definition

Incidence

Pathophysiology

• Distinct from the acute cholinergic phase
• Involves dysfunction at the NMJ — postsynaptic in nature
• Proposed mechanisms:
  • Persistent AChE inhibition at the NMJ leading to receptor downregulation and desensitisation
  • Oxidative stress and direct myopathy
  • Inadequate treatment of acute phase (insufficient pralidoxime)

Clinical Features

1. Weakness of neck flexors (cannot lift head from bed)
2. Cranial nerve palsies — facial weakness, dysarthria, dysphagia, extraocular muscle weakness, diplopia
3. Proximal limb weakness — shoulder abductors and hip flexors; cannot lift arms/legs against gravity
4. Respiratory muscle paralysis — the most life-threatening component; may lead to sudden respiratory failure and death
5. Absent or reduced deep tendon reflexes in affected muscles
6. No features of acute cholinergic crisis — this distinguishes IMS from recurrent/inadequately treated acute poisoning

Diagnosis

• Primarily clinical — in the appropriate temporal context (1–5 days post-OP exposure, after apparent recovery from acute phase)
• EMG: Shows repetitive electrical discharges after single nerve stimulation; confirms NMJ dysfunction; decrementing response on repetitive nerve stimulation
• Cholinesterase levels: remain depressed
• Respiratory function monitoring: Serial FVC, peak flow, NIF (Negative Inspiratory Force) — critical to detect impending respiratory failure

Outcome

• Symptoms usually resolve within 4–18 days (commonly cited as 7 days for mild cases; up to several weeks for severe cases)
• Prognosis is good if respiratory failure is recognised and managed early with ventilatory support
• In developing countries with limited resources, IMS is frequently lethal due to failure to recognise respiratory involvement
• Nerve agents (sarin, soman) do NOT cause IMS

Management of IMS

• No specific pharmacological antidote; atropine and pralidoxime are ineffective in the established syndrome
• Mechanical ventilation — the cornerstone; may be required for days to weeks
• Prevention: Aggressive early antidote therapy (pralidoxime given promptly) and supportive measures during the acute phase may prevent or reduce the severity of IMS
• Careful monitoring for respiratory deterioration with daily bedside respiratory function assessment
• Nutritional support, DVT prophylaxis, and standard ICU care

DELAYED COMPLICATIONS

1. Organophosphate-Induced Delayed Neuropathy (OPIDN):
   • Occurs 2–3 weeks after acute exposure
   • Due to inhibition of neuropathy target esterase (NTE)
   • Symmetric distal sensorimotor axonopathy; begins with leg cramps → distal weakness → ascending paralysis (can mimic Guillain–Barré)
   • Cognitive dysfunction, extrapyramidal signs, autonomic dysfunction
2. Chronic OP-induced neuropsychiatric disorder (COPIND):
   • After low-level chronic exposure
   • Memory impairment, mood changes, peripheral neuropathy, autonomic dysfunction

KEY POINTS FOR EXAM

REFERENCES

• Tintinalli's Emergency Medicine: A Comprehensive Study Guide, 9th ed., Chapter 201 — Insecticides, pp. 1342–1344
• Rosen's Emergency Medicine: Concepts and Clinical Practice, 9th ed., Chapter 152 — Antidote Therapy, pp. 1492–1498
• Morgan & Mikhail's Clinical Anesthesiology, 7th ed., Chapter on Neuromuscular Pharmacology, p. 2406
• Katzung's Basic and Clinical Pharmacology, 16th ed., Chapter 56 — Organophosphorus Pesticides, pp. 1573–1574
• Rashid M et al. "Efficacy and Safety of Glycopyrrolate in the Management of Organophosphate and Carbamate Poisoning: A Systematic Review." Curr Rev Clin Exp Pharmacol, 2025. [PMID: 38797902]
• Nelakuditi M et al. "Magnesium Sulfate as a Game Changer in Reducing ICU Stays in Organophosphate Poisoning Cases." Cureus, 2024. [PMID: 39188488]