Organophosphate & Carbamate Poisoning

Organophosphate (OP) compounds and carbamates are anticholinesterase poisons that cause an acute cholinergic crisis by inhibiting acetylcholinesterase. They are among the commonest causes of suicidal and accidental poisoning in India (agricultural pesticides) and an extremely high-yield NEET PG topic spanning Pharmacology, Medicine and Forensic Medicine.

Definition & Classification

Organophosphates are esters of phosphoric acid; carbamates are esters of carbamic acid. Both inhibit acetylcholinesterase (AChE), leading to accumulation of acetylcholine (ACh) at muscarinic, nicotinic and central synapses.

Common OP compounds (agricultural/insecticidal): parathion, malathion, fenthion, chlorpyrifos, dimethoate, monocrotophos, diazinon, dichlorvos (DDVP).

Nerve agents (chemical warfare OPs): sarin, soman, tabun, VX — extremely potent, rapid ageing.

Carbamates: carbaryl (Sevin), propoxur (Baygon), aldicarb. Also therapeutic carbamates — physostigmine, neostigmine, pyridostigmine, rivastigmine.

Feature	Organophosphate	Carbamate
Bond with AChE	Covalent (phosphorylation)	Carbamylation
Reversibility	Irreversible after ageing	Spontaneously reversible (hydrolysis in hours)
Duration	Prolonged (days)	Short (≤24–48 h)
CNS penetration	Marked	Poor (less central toxicity)
Ageing	Occurs	Does not age
Pralidoxime (oxime)	Useful (before ageing)	Not required / may be harmful; atropine alone usually adequate

High-yield: The single most important distinction — carbamate-AChE bond does not undergo ageing, so it reverses spontaneously and oximes are generally not indicated; OP-AChE bond ages and becomes irreversible, hence oximes must be given early.

Etiology & Pathophysiology

Routes of exposure: ingestion (commonest in suicidal cases), dermal/transcutaneous (occupational), and inhalation. OPs are highly lipid-soluble — readily cross skin, mucosa and the blood–brain barrier.

Mechanism: OPs bind the esteratic site of AChE → phosphorylation → enzyme cannot hydrolyse ACh → ACh accumulates at:

1. Muscarinic receptors (parasympathetic post-ganglionic, sweat glands)
2. Nicotinic receptors (neuromuscular junction, autonomic ganglia, adrenal medulla)
3. CNS synapses.

Ageing: After phosphorylation, one alkyl group is lost (dealkylation) → the bond becomes permanent and resistant to reactivation by oximes. Time to ageing varies: soman ~2 min (fastest), most agricultural OPs 24–48 h, dimethyl compounds (~3.7 h) age faster than diethyl (~33 h).

OPs also inhibit:

• Plasma (pseudo)cholinesterase / butyrylcholinesterase — more sensitive early marker, falls first.
• RBC (true) acetylcholinesterase — better reflects synaptic enzyme / severity.
• Neuropathy target esterase (NTE) — inhibition (>70%) causes delayed peripheral neuropathy (OPIDN).

Sequence of cholinesterase recovery: plasma ChE regenerates in days–weeks (liver synthesis); RBC AChE regenerates only as new RBCs form (~1% per day, full recovery ~90–120 days).

High-yield: Plasma cholinesterase is the most sensitive (falls earliest) but RBC AChE correlates best with clinical severity and synaptic activity.

Clinical Features

Onset is usually within minutes to a few hours (delayed with lipophilic agents like fenthion). Three overlapping toxidromes — muscarinic, nicotinic, and central.

Muscarinic features — mnemonic DUMBBELS / SLUDGE

DUMBBELS	SLUDGE +
Diarrhoea	Salivation
Urination	Lacrimation
Miosis	Urination
Bronchorrhoea	Defecation
Bradycardia	GI cramps
Emesis	Emesis
Lacrimation	(+ Bronchospasm, Bradycardia, Bronchorrhoea — the "killer Bs")
Salivation

High-yield: Miosis (pinpoint pupils) is the classic sign. Death is usually due to respiratory failure — combination of bronchorrhoea, bronchospasm (muscarinic), respiratory muscle paralysis (nicotinic) and central respiratory depression. The "killer Bs" — Bronchorrhoea, Bronchospasm, Bradycardia — kill.

Nicotinic features — mnemonic MTWtHF (days of the week)

Mydriasis, Tachycardia, Weakness, Hypertension, Fasciculations (also muscle cramps, paralysis). Nicotinic effects may produce tachycardia and hypertension — so a "mixed" picture (e.g., miosis with tachycardia) can occur and does not exclude OP poisoning.

Central (CNS) features

Anxiety, restlessness, confusion, ataxia, seizures, slurred speech, coma, central apnoea.

High-yield: A patient with garlicky/kerosene breath + pinpoint pupils + excessive secretions + muscle fasciculations is OP poisoning until proven otherwise.

Three temporally distinct syndromes

1. Acute cholinergic crisis — within minutes–hours; muscarinic + nicotinic + CNS as above.
2. Intermediate syndrome (IMS) — 24–96 h (1–4 days) after exposure, after recovery from the acute crisis and before delayed neuropathy. Proximal muscle weakness, neck flexor weakness ("broken neck sign"), cranial nerve palsies, weakness of respiratory and proximal limb muscles → respiratory failure. Due to persistent AChE inhibition at the NMJ / post-synaptic dysfunction; oximes do not reliably reverse it. Needs ventilatory support.
3. OP-induced delayed polyneuropathy (OPIDN) — 2–3 weeks later; distal sensorimotor neuropathy, glove-and-stocking, foot/wrist drop. Due to NTE inhibition, NOT cholinesterase; classically with triorthocresyl phosphate (TOCP) — historical "Ginger Jake paralysis", "Jamaica ginger palsy". Does not respond to atropine/oxime.

Syndrome	Onset	Mechanism	Key feature	Antidote response
Acute cholinergic	Minutes–hours	AChE inhibition (ACh excess)	SLUDGE, miosis, fasciculations	Atropine + oxime
Intermediate (IMS)	1–4 days	Persistent NMJ AChE inhibition	Proximal/neck/respiratory weakness	Poor; ventilate
Delayed neuropathy (OPIDN)	2–3 weeks	NTE inhibition	Distal sensorimotor neuropathy, foot drop	None

Diagnosis & Investigation of Choice

Diagnosis is largely clinical (toxidrome + history + garlicky odour). Confirm with:

• RBC acetylcholinesterase — best correlation with severity (true cholinesterase, synaptic enzyme). Investigation of choice for monitoring severity.
• Plasma/serum (butyryl)cholinesterase — most sensitive, falls earliest, used for screening; less specific (also low in liver disease, malnutrition, pregnancy).
• Atropine challenge / therapeutic trial: giving 1–2 mg atropine IV — absence of anticholinergic signs (no tachycardia, no dry mouth, no mydriasis) supports the diagnosis.
• Pseudocholinesterase levels: >50% reduction is significant; severe poisoning often <10%.

High-yield: Plasma cholinesterase = most sensitive & earliest to fall; RBC AChE = best reflects clinical severity and synaptic state.

Supportive: ABG (hypoxia, type II failure), ECG (sinus brady/tachy, QTc prolongation, torsades, AV block), blood glucose (often hyperglycaemia), serum amylase/lipase (pancreatitis), electrolytes.

Management / Drug of Choice

Resuscitation first — ABC. The priority is the airway and oxygenation, because respiratory failure kills.

Stepwise approach: Decontamination → Airway/oxygen → Atropine (titrate to atropinisation) → Pralidoxime (early, before ageing) → Benzodiazepine for seizures → Supportive/ICU care.

1. Decontamination

• Remove contaminated clothes; wash skin/hair with soap and water (staff must wear gloves/gowns — secondary contamination risk).
• Gastric lavage only if presenting early (<1 h) and airway protected; activated charcoal 1 g/kg may be given.

2. Atropine — physiological antidote (drug of choice for muscarinic effects)

• Mechanism: competitive antagonist at muscarinic receptors; reverses bronchorrhoea, bronchospasm, bradycardia, secretions. Does NOT reverse nicotinic effects (fasciculations, weakness) or CNS at usual doses.
• Dose: start 2–5 mg IV (adults), 0.05 mg/kg (children); double the dose every 5 minutes until atropinisation.
• Endpoints of atropinisation (target): clear chest on auscultation (dry secretions), heart rate >80–90/min, systolic BP >80 mmHg, dry axillae. Pupil size is NOT a reliable endpoint (slow to respond).
• Maintain with infusion; watch for atropine toxicity (delirium, hyperthermia, ileus, urinary retention, tachyarrhythmia).
• Glycopyrrolate may be used to reduce secretions with fewer central effects.

High-yield: Drying of pulmonary secretions and HR >80/min are the key targets of atropinisation, not pupillary dilatation. Bronchorrhoea is the parameter that matters most.

3. Oximes — pralidoxime (2-PAM) / obidoxime — reactivators (specific antidote)

• Mechanism: reactivate phosphorylated AChE by dephosphorylation — only effective before ageing; reverse nicotinic effects (fasciculations, weakness) which atropine cannot.
• Pralidoxime dose: loading 1–2 g IV over 15–30 min, then infusion 8 mg/kg/h (or repeated boluses). WHO suggests ≥30 mg/kg loading.
• Give early (ideally within 24–48 h, before ageing). Must be given with atropine (never alone — transient oxime-induced AChE inhibition can worsen).
• Carbamate poisoning: oximes generally not indicated (no ageing, spontaneous reversal); may worsen carbaryl toxicity — atropine alone is sufficient.

4. Seizures / agitation

• Benzodiazepines (diazepam/lorazepam/midazolam) — drug of choice; also prophylactic in nerve-agent exposure. Avoid phenytoin (ineffective).

5. Supportive

• Mechanical ventilation for respiratory failure / IMS (commonest reason for ICU admission and death).
• Avoid: morphine, aminophylline, phenothiazines, succinylcholine (prolonged paralysis as it is metabolised by pseudocholinesterase), and reserpine.
• Magnesium sulphate and newer adjuncts (sodium bicarbonate) are investigational.

High-yield: Atropine treats muscarinic; pralidoxime treats nicotinic (and regenerates enzyme). Both needed in OP. Carbamate → atropine alone, avoid oxime.

Complications

• Respiratory failure — leading cause of death (secretions + bronchospasm + muscle paralysis + central depression).
• Intermediate syndrome — delayed respiratory failure (1–4 days).
• OPIDN — delayed neuropathy (weeks).
• Cardiac: QTc prolongation, torsades de pointes, AV block, ventricular arrhythmias (poor prognostic markers).
• Aspiration pneumonia, ARDS.
• Acute pancreatitis, hyperglycaemia, hepatotoxicity.
• Extrapyramidal features, cognitive impairment (chronic).

Key Differentials

• Other cholinergic crises: carbamates, nerve agents, certain mushrooms (Inocybe/Clitocybe — muscarine), physostigmine overdose.
• Pinpoint pupils: opioid poisoning (but opioids → respiratory depression without secretions/fasciculations; respond to naloxone), pontine haemorrhage, clonidine.
• Cholinergic vs anticholinergic toxidrome: OP = wet (secretions, miosis, bradycardia); anticholinergic (datura/atropine) = dry, hot, mydriasis, delirium, tachycardia ("dry as a bone, red as a beet, mad as a hatter").

Toxidrome	Pupils	Skin/secretions	HR	Mental status
Cholinergic (OP)	Miosis	Wet (sweating, salivation)	Brady (often)	Confusion/coma
Anticholinergic (datura)	Mydriasis	Dry, flushed, hot	Tachy	Delirium, hallucinations
Opioid	Miosis	Normal/dry	Brady	Sedation/coma

Forensic / Postmortem Aspects

• Garlicky / kerosene odour from mouth, stomach contents and viscera.
• Stomach mucosa congested; froth in air passages; pulmonary oedema.
• Pupils — constricted (miosis) may persist after death.
• Cyanosis, congestion of viscera.
• Preservation of viscera: save stomach + contents, intestine, liver, kidney, blood. Preservative = saturated saline (NOT formalin — formalin destroys/inactivates OP and is also itself not used here; rectified spirit is also avoided). Blood for cholinesterase estimation can be taken; sodium fluoride preserves cholinesterase activity.
• Detection: thin-layer/gas chromatography, cholinesterase assay.

High-yield (Forensic): Preservative of choice for viscera in OP poisoning is saturated salt (sodium chloride) solution; formalin and rectified spirit are contraindicated. Pinpoint pupils + garlicky odour are classic autopsy pointers.

Recently asked / exam angle

• Antidotes pairing: "Atropine + pralidoxime" — atropine for muscarinic, oxime for nicotinic & enzyme reactivation. Frequently asked which symptom atropine does NOT reverse → fasciculations/muscle weakness (nicotinic).
• End-point of atropinisation = drying of secretions + HR >80/min (NOT pupil dilation). Repeatedly tested.
• Ageing concept & why oxime must be early; soman = fastest ageing (~2 min).
• Carbamate vs OP — oxime not needed in carbamate; carbamate has poor CNS penetration; spontaneous reversal.
• Intermediate syndrome — onset 1–4 days, neck flexor weakness, respiratory failure; does not respond to oxime.
• OPIDN / TOCP / Jake paralysis — mechanism = NTE inhibition (not AChE); foot drop after 2–3 weeks.
• Best lab marker — RBC AChE for severity, plasma ChE most sensitive.
• Drug to avoid — succinylcholine (prolonged apnoea), morphine, aminophylline.
• Forensic — preservative = saturated saline; garlicky odour; miosis.
• Cause of death — respiratory failure.
• Nerve agents (sarin/VX) as warfare; mark of high lethality and rapid ageing.

Rapid revision

1. OP = irreversible AChE inhibition with ageing; carbamate = reversible, no ageing, oxime not needed.
2. Muscarinic = SLUDGE/DUMBBELS; classic sign = miosis (pinpoint pupils); death from respiratory failure.
3. Nicotinic = fasciculations, weakness, tachycardia, hypertension — NOT reversed by atropine.
4. Atropine = physiological antidote (muscarinic); endpoint = dry chest + HR >80/min, not pupils.
5. Pralidoxime (2-PAM) = enzyme reactivator; give early before ageing, always with atropine; reverses nicotinic effects.
6. Carbamate → atropine alone; avoid oxime (esp. carbaryl).
7. Plasma ChE = most sensitive (earliest fall); RBC AChE = best severity correlation.
8. Intermediate syndrome = 1–4 days, neck flexor/respiratory weakness, ventilate; oxime unhelpful.
9. OPIDN = 2–3 weeks, distal neuropathy/foot drop, due to NTE inhibition (TOCP/Jake paralysis).
10. Avoid succinylcholine (metabolised by pseudocholinesterase → prolonged paralysis), morphine, aminophylline.
11. Soman ages fastest (~2 min); nerve agents = sarin, soman, tabun, VX.
12. Forensic: garlicky odour, miosis, saturated saline preservative (no formalin/rectified spirit).