Plant Poisons: Aconite, Dhatura & Strychnine
Forensic Medicine · Toxicology · lean revision notes
Plant Poisons: Aconite, Dhatura & Strychnine
Vegetable (plant) poisons are a perennial favourite in Forensic Medicine because each has a signature toxidrome that lets you reverse-engineer the offending plant from the clinical picture. This note drills the four classics — aconite, dhatura, strychnine (nux vomica) and abrus — with their alkaloids, mechanisms, post-mortem findings, medico-legal angles and antidotes.
Classification of plant poisons (where these fit)
In forensic toxicology, plant poisons are grouped by their dominant clinical action. Knowing the bucket helps you predict the toxidrome.
| Class | Action | Key examples |
|---|---|---|
| Deliriant / Somniferous | CNS depression, delirium, anticholinergic | Dhatura (Datura), Cannabis, Belladonna |
| Spinal / Convulsant | Hyperexcitability of spinal cord | Strychnine (Nux vomica), Gelsemium |
| Cardiac | Myocardial / conduction effects | Aconite, Oleander, Digitalis, Cerbera (suicide tree) |
| Asphyxiant / Cellular | Tissue hypoxia | Cyanogenic plants (cassava, bitter almond) |
| Vesicant / Irritant | Local & systemic irritation | Abrus precatorius, Calotropis, Croton, Semecarpus |
High-yield: The single most repeated NEET PG pattern is "match the toxidrome to the plant." Memorise the one defining sign for each: aconite → tingling/numbness + bradycardia; dhatura → dry-hot-blind-mad-red (anticholinergic); strychnine → opisthotonos with intact consciousness; abrus → sterile abscess from a 'sui'.
1. Aconite (Aconitum napellus / Bachnag, Meetha vish, Vatsanabh)
The roots are the most toxic part; the dried root is the classic homicidal/abortifacient agent and was historically called the "king of poisons" and "queen of poisons."
Active principle & mechanism
- Chief alkaloid: aconitine (also pseudaconitine, mesaconitine — Aconitum ferox is more toxic than A. napellus).
- Mechanism: Aconitine binds the voltage-gated sodium (Na⁺) channels and holds them in the open state → persistent depolarisation → first stimulation then paralysis of excitable tissue (nerve, cardiac, muscle). This explains the early tingling (neuronal stimulation) followed by numbness (block) and the ventricular arrhythmias.
Clinical features (flow)
Oral burning/tingling → numbness & paraesthesia of mouth, throat, then whole body → tingling "ant-crawling" sensation → salivation, nausea, vomiting → bradycardia & hypotension → ventricular arrhythmias (bidirectional VT, VF) → death from cardiac arrest/respiratory paralysis.
- Pathognomonic early symptom: tingling and numbness of tongue, lips and extremities.
- Cardiac: bradycardia, hypotension, ventricular ectopics, bidirectional ventricular tachycardia (classic ECG hint), VF.
- Consciousness is typically preserved until the terminal phase.
Diagnosis & confirmation
- Largely clinical (history of indigenous medicine/aphrodisiac + toxidrome).
- Pheasant's eye test / Tingling test: chemically, a portion of suspected material applied to the tongue produces tingling then numbness — a classic forensic bedside identification.
- Detection: chromatography (LC-MS) of aconitine.
Management & antidote
- No specific antidote; supportive is the mainstay.
- Atropine for bradycardia; amiodarone or flecainide (Na-channel effects) for ventricular arrhythmias; magnesium; charcoal if early; cardiac monitoring.
- Avoid drugs that worsen arrhythmia.
Medico-legal points
- Used as a homicidal poison (called the "poison of antiquity"), an arrow/dart poison (A. ferox), and as an aphrodisiac/abortifacient in indigenous medicine.
- No characteristic post-mortem finding — asphyxial signs only; diagnosis rests on history + chemical analysis. This "absence of specific PM findings" is itself a tested fact.
High-yield: Aconite = tingling → numbness + bradycardia + bidirectional VT, consciousness retained, NO specific autopsy finding. Antidote concept = atropine for the heart-rate, none specific for the alkaloid.
2. Dhatura (Datura stramonium / D. fastuosa — Thorn apple, Jimson weed, Devil's trumpet)
The seeds (which resemble chilli/capsicum seeds but are flattened and kidney-shaped) and fruit (a spiny capsule) are the toxic parts.
Active principles & mechanism
- Belladonna alkaloids: atropine (hyoscyamine), scopolamine (hyoscine).
- Mechanism: Competitive antimuscarinic (anticholinergic) blockade of M-receptors → classic anticholinergic toxidrome.
The anticholinergic toxidrome — mnemonic
Mnemonic: "Blind as a bat, Mad as a hatter, Red as a beet, Hot as a hare, Dry as a bone, Full as a flask, The bowel and bladder lose their tone, and the heart runs alone."
| Sign | Mechanism |
|---|---|
| Mydriasis (blind as a bat) | Pupillary muscle paralysis → blurred vision, photophobia |
| Delirium, hallucinations (mad as a hatter) | Central muscarinic block |
| Flushed skin (red as a beet) | Cutaneous vasodilation |
| Hyperthermia (hot as a hare) | Loss of sweating |
| Dry mouth/skin (dry as a bone) | Secretory blockade |
| Urinary retention, ileus | Smooth-muscle atony |
| Tachycardia | Vagal block |
Clinical features (flow)
Dry mouth & dysphagia → mydriasis, blurred vision → flushing, hyperthermia → tachycardia → ataxia, slurred speech → delirium, hallucinations ("picking at clothes", muttering) → drowsiness → coma & death.
High-yield: Dhatura causes mydriasis (dilated pupils) — the opposite of opioids/organophosphates (miosis). It is the favourite "road-poison"/stupefying agent used by criminals to rob travellers (mixed in food/sweets/tea) — hence called the "poison of the highwaymen."
Diagnosis & confirmation
- Clinical anticholinergic syndrome + history.
- Vitali's test — the classic chemical test for atropine/belladonna alkaloids: residue + fuming nitric acid, evaporated → add alcoholic KOH → violet/purple colour (positive). High-yield named test.
- Seeds recovered from vomit/stomach: flattened, reniform, yellowish-brown.
Management & antidote
- Supportive: cooling, sedation with benzodiazepines for agitation, urinary catheter.
- Specific antidote: PHYSOSTIGMINE (a tertiary, CNS-penetrant cholinesterase inhibitor) — used for severe central anticholinergic delirium/arrhythmia. Use cautiously (can cause seizures/bradyasystole).
- Activated charcoal if early.
Medico-legal points
- Most common stupefying / "drug-facilitated crime" poison in India.
- Also accidental (children eating seeds), and seeds added to country liquor.
- A weak homicidal poison (bitter taste limits dose) but powerful as a stupefacient.
High-yield: Antidote for dhatura/atropine poisoning = physostigmine. Confirmatory test = Vitali's test (violet colour). Pupil = dilated.
3. Strychnine — Nux vomica (Strychnos nux-vomica — Kuchla)
The disc-shaped, grey, velvety seeds are intensely bitter. Strychnine is the classic spinal/convulsant poison.
Active principles & mechanism
- Alkaloids: strychnine and brucine (brucine is less toxic, gives the bitter taste).
- Mechanism: Strychnine is a competitive antagonist at glycine receptors (the chief inhibitory neurotransmitter in the spinal cord, especially on Renshaw cells). Loss of inhibition → unchecked reflex motor excitation → simultaneous contraction of agonist + antagonist muscles → generalised convulsions with consciousness fully retained.
Clinical features (flow)
Bitter taste, restlessness, stiffness of neck/face → heightened reflexes & hyperacusis → sudden symmetrical tonic convulsions (triggered by light/touch/sound) → opisthotonos with risus sardonicus → muscle relaxation between spasms (consciousness intact, patient terrified) → death from asphyxia/exhaustion during a spasm.
Hallmark signs:
- Risus sardonicus — the "sardonic grin" from facial spasm.
- Opisthotonos — arched back; in extreme cases body rests on heel and head (also emprosthotonos, pleurosthotonos variants).
- Convulsions are intermittent, triggered by stimuli, with complete relaxation between fits, and patient remains conscious throughout — these features separate it from epilepsy/tetanus.
High-yield: Strychnine death is essentially "tetanus-like fits with a clear sensorium." Body may show cadaveric spasm / instant rigor, and rigor mortis sets in early and is marked.
Strychnine vs Tetanus — a classic comparison (very frequently asked)
| Feature | Strychnine poisoning | Tetanus |
|---|---|---|
| Onset | Rapid (within minutes–½ hr) | Slow (days; incubation) |
| First muscles | Extremities/whole body together | Jaw first → trismus (lockjaw) then spreads |
| Spasm pattern | Intermittent, complete relaxation between | Persistent rigidity with super-added spasms |
| Trismus | Late/absent | Early & prominent |
| Consciousness | Retained | Retained |
| Triggered by stimuli | Yes (light, sound, touch) | Yes |
| History | Ingestion | Wound/injury |
Diagnosis & confirmation
- Clinical + history of seed/rodenticide ingestion.
- Chemical: bitter taste; colour tests and chromatography (LC-MS) for strychnine.
Management & antidote
- No specific antidote.
- Control convulsions with benzodiazepines (diazepam/midazolam) ± barbiturates; severe cases → muscle paralysis (vecuronium) + mechanical ventilation.
- Minimise stimulation (dark, quiet room), supportive care, treat hyperthermia/rhabdomyolysis/acidosis.
Medico-legal points
- Used as a rodenticide (rat poison) and historically as a homicidal poison (intense bitterness limits its practicality).
- Also an old "tonic/aphrodisiac" adulterant.
High-yield: Strychnine acts on glycine receptors (spinal cord) → convulsant; antidote = diazepam (symptomatic). Risus sardonicus + opisthotonos + retained consciousness = strychnine until proven otherwise.
4. Abrus precatorius (Jequirity, Rosary pea, Gunja, Rati)
Bright scarlet seeds with a black spot at the hilum (like a ladybird). Classic cattle-poison and a forensic curiosity.
Active principle & mechanism
- Abrin — a toxalbumin (ribosome-inactivating lectin, mechanism similar to ricin) → inhibits protein synthesis → cellular death.
- Prepared into a needle-shaped spike called "sui" (or "sutari") — a paste of crushed seeds dried around a sharp point.
Clinical / medico-legal features
- Used to kill cattle (and occasionally humans) by stabbing with the sui → produces a localised sterile abscess / swelling, oedema, then systemic collapse, haemolysis, multi-organ failure.
- At the puncture site, a small mark of needle with surrounding induration; the sui may dissolve leaving little trace.
- Abrin is also a haemagglutinin → agglutination of RBCs.
High-yield: Abrus = "sui" needle, scarlet seed with black spot, toxalbumin (abrin), kills cattle, leaves a localised sterile swelling. A favourite one-liner MCQ.
Pattern-recognition table — "Which plant poison?"
| Toxidrome clue in stem | Poison | Mechanism | Antidote |
|---|---|---|---|
| Tingling/numbness of tongue + bradycardia, bidirectional VT | Aconite | Na⁺ channel (held open) | Atropine + antiarrhythmic (none specific) |
| Dilated pupils, dry hot red skin, delirium, tachycardia | Dhatura | Antimuscarinic | Physostigmine |
| Risus sardonicus, opisthotonos, conscious, stimulus-triggered fits | Strychnine | Glycine antagonist (spinal) | Diazepam (symptomatic) |
| Cattle death, scarlet seed/black spot, "sui" puncture | Abrus (abrin) | Toxalbumin (RIP) | Supportive |
High-yield: A "bidirectional ventricular tachycardia" stem points to aconite (or digoxin). Pupil status is the fastest discriminator: mydriasis = dhatura; normal/preserved consciousness with fits = strychnine.
Key differentials & related distractors
- Miosis (constricted pupils): opioids, organophosphates, pontine haemorrhage — opposite of dhatura. Watch for stems that swap pupil findings.
- Convulsant differentials of strychnine: tetanus (jaw-first, slow), epilepsy (loss of consciousness, no relaxation pattern), and hypocalcaemia/tetany.
- Cardiac plant poisons to keep apart from aconite: Cerbera odollam (suicide tree) and oleander → cardiac glycoside picture (hyperkalaemia, AV block) → managed like digoxin (consider digoxin-Fab).
- Other deliriants: Cannabis (run amok, but no anticholinergic dryness/mydriasis), Belladonna (same alkaloids as dhatura).
Recently asked / exam angle
- Single-best-toxidrome MCQs: "A traveller is found stuporous after sharing food with a stranger; pupils dilated, skin dry and flushed, tachycardic — poison?" → Dhatura (stupefacient/road poison).
- Mechanism MCQs: "Strychnine acts by antagonising which neurotransmitter?" → Glycine (spinal inhibitory). Aconite → "holds Na⁺ channels open."
- Antidote matching: Physostigmine ↔ atropine/dhatura; atropine ↔ aconite bradycardia/organophosphates; diazepam ↔ strychnine.
- Named-test MCQs: Vitali's test → atropine/belladonna (violet colour). Tingling/"pheasant's eye" link → aconite.
- Eponym/sign MCQs: Risus sardonicus + opisthotonos → strychnine; "king of poisons" → aconite; "sui" + scarlet seed → abrus.
- PM-finding MCQs: "Plant poison with NO characteristic autopsy finding" → aconite. "Early/marked rigor mortis with cadaveric spasm" → strychnine.
- Pupil-discriminator reasoning: stems pairing pupil size with HR/temperature/sweat to force aconite vs dhatura vs opioid differentiation.
Rapid revision
- Aconite (Bachnag/Vatsanabh): alkaloid aconitine, holds Na⁺ channels open; tingling→numbness, bradycardia + bidirectional VT, consciousness retained, no specific PM finding.
- Aconite was the "king of poisons"; A. ferox used as arrow poison; manage with atropine + antiarrhythmics, no specific antidote.
- Dhatura (Thorn apple): alkaloids atropine, hyoscine → anticholinergic toxidrome (dry, hot, red, blind/mydriasis, mad, tachycardia).
- Dhatura is the classic stupefying "road poison"; antidote = physostigmine; confirmatory chemical test = Vitali's test (violet).
- Dhatura seeds = flattened, reniform, resemble chilli seeds; remember mydriasis distinguishes it from opioids/OP (miosis).
- Strychnine (Nux vomica/Kuchla): glycine-receptor antagonist in spinal cord → convulsant; risus sardonicus + opisthotonos, fits stimulus-triggered, consciousness fully intact.
- Strychnine vs tetanus: poison = rapid onset, whole-body, relaxation between spasms; tetanus = jaw-first (trismus), slow, persistent rigidity. No specific antidote → diazepam + quiet dark room ± ventilation.
- Strychnine death → early, marked rigor mortis / cadaveric spasm; used as rodenticide.
- Abrus precatorius (Gunja/Jequirity): toxalbumin abrin (like ricin, inhibits protein synthesis); scarlet seed with black spot; the "sui" needle kills cattle → localised sterile swelling.
- Pupil rule: mydriasis → dhatura; miosis → opioids/OP; aconite/strychnine → pupils not the discriminator (use HR & sensorium).
- Bidirectional VT in a stem = think aconite (or digoxin/oleander-cerbera among plants).
- Antidote cheat-sheet: Dhatura → physostigmine; Aconite → atropine (rate) only; Strychnine → benzodiazepine; Abrus → supportive.