1. Clinical Overview
Atropine is a naturally occurring tropane alkaloid extracted from plants like Atropa belladonna. As a 1% w/w formulation, it is a potent, competitive, and reversible antagonist of muscarinic acetylcholine receptors (mAChRs). It acts as a parasympatholytic agent, blocking the actions of acetylcholine at postganglionic parasympathetic neuroeffector sites in smooth muscle, cardiac muscle, exocrine glands, and the central nervous system. In the Indian clinical context, its 1% strength is primarily used in ophthalmology for diagnostic and therapeutic purposes, and as a pre-anesthetic medication.
| Onset | Duration | Bioavailability |
|---|---|---|
| Ophthalmic: Mydriasis begins within 5-10 minutes, cycloplegia within 25-30 minutes. Parenteral: Effects begin within 1-2 minutes after IV administration and 15-30 minutes after IM administration. | Ophthalmic: Mydriasis lasts 7-10 days; cycloplegia lasts 6-12 days. Systemic effects after a single IV dose last 2-4 hours. | Ophthalmic: Poor systemic absorption from the eye if used correctly. Systemic: Well absorbed from the GI tract and injection sites; oral bioavailability is approximately 25-50% due to first-pass metabolism. |
2. Mechanism of Action
Atropine competitively blocks the binding of acetylcholine (ACh) to muscarinic (M) receptors (M1, M2, M3, M4, M5) at parasympathetic innervated effector organs. This blockade inhibits all muscarinic functions. It has negligible effect on nicotinic receptors. Its actions are dose-dependent: low doses inhibit salivary and bronchial secretion and sweating; moderate doses dilate pupils, increase heart rate (by blocking vagal tone); high doses decrease GI motility and urinary bladder tone.
3. Indications & Uses
- Pre-anesthetic medication to reduce salivary and bronchial secretions and to prevent reflex bradycardia
- Ophthalmic use: To produce mydriasis and cycloplegia for refractive testing and fundoscopic examination
- Treatment of symptomatic sinus bradycardia and AV block
- Antidote for organophosphate and carbamate insecticide poisoning
- Adjunct in the management of peptic ulcer disease (rarely used now)
4. Dosage & Administration
Adult Dosage: **Highly indication-specific.** Pre-anesthetic: 0.4-0.6 mg IM/IV/SC 30-60 min pre-op. Bradycardia: 0.5-1 mg IV, repeat every 3-5 min as needed, max 3 mg. Organophosphate poisoning: 2-4 mg IV initially, then double the dose every 5-10 min until full atropinization (dry skin, tachycardia, mydriasis), then maintenance infusion. **Ophthalmic (1% solution):** 1-2 drops into the conjunctival sac. For uveitis: Apply 1 drop up to 3 times daily.
Administration: **Ophthalmic:** Wash hands. Tilt head back, pull lower eyelid down to form a pouch. Instill prescribed drops. Apply gentle pressure to the lacrimal sac (inner corner of eye) for 1-2 minutes to minimize systemic absorption. Do not touch dropper tip. **Parenteral:** For IV use in emergencies, administer rapidly. For IM/SC, inject into a large muscle mass. In organophosphate poisoning, administer until secretions dry and heart rate increases.
5. Side Effects
Common side effects may include:
- Dry mouth (xerostomia)
- Blurred vision (cycloplegia)
- Photophobia (due to mydriasis)
- Tachycardia
- Constipation
- Decreased sweating
- Difficulty in micturition
6. Drug Interactions
| Drug | Effect | Severity |
|---|---|---|
| Other Anticholinergics (e.g., antihistamines, TCAs, antipsychotics) | Additive anticholinergic effects (dry mouth, constipation, urinary retention, confusion). | Major |
| Potassium Chloride (wax-matrix formulations) | Increased risk of GI lesions due to reduced motility. | Major |
| Digoxin | Increased serum digoxin levels due to reduced GI motility, increasing risk of toxicity. | Moderate |
| Antacids, Antidiarrheals (adsorbents like kaolin-pectin) | Reduced absorption of atropine from the GI tract. | Moderate |
| Metoclopramide | Atropine antagonizes the prokinetic effect of metoclopramide. | Moderate |
| Phenothiazines (e.g., Chlorpromazine) | Increased anticholinergic effects and decreased antipsychotic efficacy. | Moderate |
| Amantadine | Increased risk of anticholinergic side effects and psychosis. | Moderate |
| Nitrates, Nitrites | Increased intraocular pressure. | Moderate |
7. Patient Counselling
- **DO** apply pressure to the inner corner of the eye (for 1-2 minutes) after using eye drops to reduce systemic absorption.
- **DO** inform your doctor about all medications you are taking.
- **DO** maintain good oral hygiene if experiencing dry mouth (sugar-free gum/candies, frequent sips of water).
- **DO NOT** drive or operate machinery if vision is blurred or if you feel dizzy/drowsy.
- **DO NOT** touch the dropper tip to any surface, including the eye, to avoid contamination.
- **DO NOT** use in eyes if solution is discolored or contains particles.
- **DO NOT** engage in strenuous activity in hot weather, as you may be prone to heat stroke.
8. Toxicology & Storage
Overdose: **Symptoms progress with dose.** Early: Dry mouth, blurred vision, tachycardia, flushing, hyperthermia, urinary retention. Moderate: Restlessness, excitement, confusion, hallucinations, delirium. Severe: Hypertension followed by hypotension, circulatory collapse, respiratory failure, coma, convulsions, death. In children: Hyperthermia, seizures, coma.
Storage: Store at room temperature (15-25°C), protected from light. Do not freeze. Keep the container tightly closed. Keep out of reach of children. Do not use after the expiry date printed on the label.