Autonomic Pharmacology
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Direct answer
Atropine dries secretions but cannot reverse muscle weakness — that single sentence separates the two arms of autonomic pharmacology that FMGE tests hardest. Muscarinic effects (bronchorrhoea, salivation, miosis, bradycardia, gut cramps) respond to atropine, whereas nicotinic effects at the neuromuscular junction (weakness, respiratory paralysis) respond only to an oxime such as pralidoxime, and only if given before the phosphorylated enzyme "ages". The rest of the syllabus hangs off receptor logic: quaternary drugs such as neostigmine stay out of the brain, tertiary drugs such as physostigmine enter it, and selectivity (beta-1 versus beta-2, alpha-1A in the prostate) is what makes modern drugs safer than their ancestors.
What you must remember
- Cholinesterase inhibitors ranked by duration: edrophonium (seconds — diagnostic), neostigmine (quaternary, cannot cross the blood–brain barrier; myasthenia gravis and reversal of neuromuscular blockade), physostigmine (tertiary, crosses into the brain; belladonna/anticholinergic poisoning).
- Organophosphate poisoning produces DUMBELS (diarrhoea, urination, miosis, bronchorrhoea, excitation, lacrimation, secretions, sweating); atropine 2–5 mg IV is doubled every few minutes until chest secretions dry — the endpoint is dry secretions, not pupil size.
- Pralidoxime 1–2 g IV reactivates cholinesterase before ageing occurs; carbamate poisoning (shorter-acting, reversible) needs atropine alone because oximes add toxicity without benefit.
- Atropine toxicity in the traditional rhyme: blind as a mole, hot as a hare, dry as a bone, red as a beet, mad as a hen — cycloplegia, anhidrosis, xerostomia, flushing, delirium.
- Ipratropium and tiotropium are inhaled muscarinic antagonists for COPD that achieve bronchodilation without systemic atropine effects; tiotropium is once daily.
- Beta-2 agonists (salbutamol) cause tremor, tachycardia and hypokalaemia — the potassium shift is exploited therapeutically in hyperkalaemia; tolerance and hypokalaemia both matter in overdosed inhalers.
- Propranolol is non-selective and contraindicated in asthma; it also masks the adrenergic warning symptoms of hypoglycaemia in insulin-treated diabetics. Cardioselective metoprolol and atenolol are safer choices.
- Alpha-1 blockade: prazosin causes first-dose syncope (start at night), whereas tamsulosin is relatively prostate-selective — less hypotension, but retrograde ejaculation occurs.
A typical exam case
A 30-year-old farmer reaches the emergency department drenched in sweat, pinpoint pupils, wheezing, with fasciculations he cannot control. The sequence the examiner wants runs: airway and oxygen first, then atropine 2 mg IV doubling every five minutes until his chest audibly dries and his heart rate rises above 80 — many cycles and hundreds of milligrams may be needed, and pupils may stay constricted throughout. Pralidoxime 1–2 g infused over 15–30 minutes follows, most valuable within the first 24 hours, to strip the phosphate off neuromuscular cholinesterase before ageing fixes it. Decontamination — strip the clothes, wash the skin, protect the staff — runs in parallel.
Three contraindicated drugs hide in the options: suxamethonium (its hydrolysing plasma cholinesterase is already consumed, so apnoea is prolonged), morphine, and aminoglycosides (additive neuromuscular blockade). Watch for the intermediate syndrome of neck flexor and respiratory weakness at 24–96 hours, which is treated with ventilatory support, not more atropine.
Where students slip
Neostigmine and physostigmine are swapped almost every session — remember that the tertiary amine crosses into the central nervous system and the quaternary one cannot, so neostigmine reverses a neuromuscular block while physostigmine reverses an atropine-like delirium. The second slip is calling a persistently constricted pupil "failed atropinisation" and chasing it with toxic doses; the pupil is not the target organ. Third, the myasthenic crisis versus cholinergic crisis question: edrophinium improves a myasthenic crisis and worsens a cholinergic one, but in modern practice the safest answer is supportive care with ventilation while the distinction is sorted, because giving more anticholinesterase to a cholinergic crisis can be fatal.
Frequently asked questions
Which cholinesterase inhibitor crosses the blood–brain barrier?
Physostigmine, being tertiary and lipophilic — it is used for central anticholinergic (belladonna) poisoning, whereas neostigmine's quaternary charge confines it to the periphery.
What is the endpoint of atropinisation in organophosphate poisoning?
Drying of pulmonary and skin secretions with a heart rate above about 80 per minute; pupil size is not a reliable endpoint.
Why is suxamethonium dangerous after organophosphate exposure?
Organophosphates inhibit plasma pseudocholinesterase, so the depolarising block of suxamethonium is prolonged and the patient may stay apnoeic for hours.
How is a dopamine infusion dose-dependent?
At roughly 1–2 microgram/kg/min it acts on renal dopaminergic receptors, 2–5 on cardiac beta-1, and above 5 on alpha-1 vessels — the vasoconstrictor range that limits its use as a renal-protective drug.
Why is tamsulosin preferred for benign prostatic hyperplasia?
It is selective for the alpha-1A receptors of the prostate and bladder neck, relaxing smooth muscle with less systemic hypotension than prazosin.
Which inhaled anticholinergic is used in COPD and why is it safe?
Ipratropium (or once-daily tiotropium) — poorly absorbed and topically active, so bronchodilation occurs without the systemic atropine effects of tachycardia, dry mouth and confusion.