Paper I
Question
Describe the uses, adverse effects and contraindications of atropine and atropine substitutes. (4+2+2)
Answer
Atropine is a competitive antagonist at muscarinic receptors, blocking parasympathetic effects throughout the body; its synthetic substitutes reproduce this action with greater selectivity for a particular use, reducing unwanted systemic effects.
Uses of atropine
- Pre-anesthetic medication — blocks vagal reflexes and reduces salivary/bronchial secretions
- Antidote in organophosphate/anticholinesterase poisoning — blocks the muscarinic overactivity (bronchorrhoea, bradycardia, miosis), though it does not reverse nicotinic effects such as muscle weakness
- Symptomatic sinus bradycardia and AV block — raises heart rate by blocking vagal tone
- Mydriasis and cycloplegia for ophthalmic examination, and to prevent synechiae in iridocyclitis
- Antispasmodic in intestinal/renal/biliary colic
- Antidiarrhoeal adjunct (as diphenoxylate + atropine)
Adverse effects
- Dry mouth, blurred vision (cycloplegia), photophobia
- Tachycardia
- Urinary retention, constipation
- Hyperthermia — suppressed sweating, especially dangerous in children (“atropine fever”)
- Flushed, dry skin
- CNS excitement, confusion, or delirium in toxic doses (“hot as a hare, dry as a bone, red as a beet, blind as a bat, mad as a hatter”)
Contraindications
- Narrow-angle glaucoma — mydriasis can precipitate a sight-threatening acute attack
- Benign prostatic hyperplasia — worsens urinary retention
- Tachyarrhythmias
- Paralytic ileus and pyloric stenosis
Atropine substitutes — each engineered for one specific use with a narrower adverse-effect profile than atropine itself:
- Glycopyrrolate — quaternary ammonium compound, does not cross the blood-brain barrier (no CNS effects) and has a longer duration than atropine — preferred pre-anesthetic agent and for reversing neuromuscular-blocker-associated bradycardia.
- Ipratropium/Tiotropium — delivered by inhalation for bronchial asthma/COPD; quaternary structure limits systemic absorption, so bronchodilation occurs with minimal systemic antimuscarinic effect.
- Hyoscine (Scopolamine) — more prominent CNS sedative action than atropine; used for motion sickness and as an antisecretory/sedative in pre-anesthetic medication.
- Tropicamide and Homatropine — short-acting mydriatics/cycloplegics for ophthalmic examination, chosen over atropine specifically because their shorter duration avoids prolonged post-examination blurring.
- Dicyclomine — selective antispasmodic action on intestinal smooth muscle, used for infantile colic and irritable bowel syndrome.
- Oxybutynin/Tolterodine — relatively bladder-selective, used for urinary urgency/overactive bladder.