Usage
Homatropine, often used as homatropine hydrobromide (HBr), is primarily prescribed in ophthalmology for its mydriatic (pupil-dilating) and cycloplegic (paralyzing ciliary muscle) effects. It is used to facilitate eye examinations, before and after eye surgeries, and in the treatment of uveitis.
Pharmacological Classification: Anticholinergic, antimuscarinic agent.
Mechanism of Action: Homatropine HBr competitively antagonizes acetylcholine at muscarinic receptors in the eye. This action inhibits the constriction of the pupil and paralysis of the ciliary muscle, leading to mydriasis and cycloplegia.
Alternate Names
Homatropine hydrobromide, Isopto Homatropine (brand name). The combination of hydrocodone and homatropine methylbromide is marketed under brand names such as Hycodan, Tussigon, and Hydromet.
How It Works
Pharmacodynamics: Homatropine HBr blocks the action of acetylcholine at muscarinic receptors in the eye, causing pupillary dilation (mydriasis) and paralysis of accommodation (cycloplegia). Systemic effects are less pronounced than with atropine due to its reduced ability to cross the blood-brain barrier.
Pharmacokinetics:
- Absorption: When administered as eye drops, homatropine HBr is absorbed systemically, though to a lesser extent than atropine.
- Metabolism: Hepatic metabolism plays a role in the breakdown of homatropine.
- Elimination: The elimination pathways are not fully characterized but likely involve renal and hepatic routes.
Mode of Action: Homatropine HBr competitively binds to muscarinic acetylcholine receptors (specifically M1, M2, and M3 subtypes) in the eye, preventing acetylcholine from binding and exerting its effects.
Receptor Binding, Enzyme Inhibition, or Neurotransmitter Modulation: Homatropine HBr is a muscarinic receptor antagonist. It does not inhibit enzymes or directly modulate neurotransmitter release, but its antagonism of acetylcholine receptors indirectly affects cholinergic neurotransmission in the eye.
Elimination Pathways: The primary routes of elimination are believed to be renal and hepatic.
Dosage
Standard Dosage
Adults:
- Ophthalmic: 1-2 drops of a 2% or 5% homatropine HBr solution instilled into the affected eye(s), repeated every 3-4 hours as needed for uveitis. For eye examinations or surgery, 1-2 drops are instilled just prior to the procedure, potentially repeated after 5-10 minutes.
Children:
- Ophthalmic: Dosing varies with age and concentration:
- 3 months - 2 years: 1 drop of 0.5% solution once daily or on alternate days.
-
2 years: 1 drop of 1% or 2% solution twice daily.
Special Cases:
- Elderly Patients: Use with caution. Monitor for systemic anticholinergic effects.
- Patients with Renal Impairment: No specific dosage adjustments are routinely provided; caution is advised.
- Patients with Hepatic Dysfunction: No specific dosage adjustments are routinely provided; caution is advised.
- Patients with Comorbid Conditions: Use cautiously in patients with glaucoma, prostatic hypertrophy, or cardiovascular disease.
Clinical Use Cases
The following dosage information applies to the ophthalmic use of homatropine:
- Intubation: Not typically used.
- Surgical Procedures: 1-2 drops prior to ophthalmic surgical procedures.
- Mechanical Ventilation: Not typically used.
- Intensive Care Unit (ICU) Use: Not typically used.
- Emergency Situations: Not typically used.
Dosage Adjustments
Dose adjustments are based on patient response and the presence of any adverse effects.
Side Effects
Common Side Effects
- Ophthalmic: Blurred vision, photophobia, dry eyes, increased intraocular pressure.
- Systemic: Dry mouth, constipation, urinary retention, flushing, tachycardia.
Rare but Serious Side Effects
- Ophthalmic: Angle-closure glaucoma, allergic reactions.
- Systemic: Confusion, hallucinations, delirium, cardiac arrhythmias.
Long-Term Effects
Prolonged use can increase the risk of systemic anticholinergic effects.
Adverse Drug Reactions (ADR)
Angle-closure glaucoma, allergic reactions (local and systemic), cardiac arrhythmias.
Contraindications
- Primary glaucoma or predisposition to glaucoma.
- Narrow-angle or shallow anterior chamber.
- Hypersensitivity to homatropine or other belladonna alkaloids.
Drug Interactions
- Other anticholinergics: Additive anticholinergic effects.
- Nitrates, antihistamines, tricyclic antidepressants: Increased risk of systemic anticholinergic effects.
- Medications that prolong QT interval: Increased risk of arrhythmias.
Pregnancy and Breastfeeding
- Pregnancy: Should be used only if clearly needed.
- Breastfeeding: Use with caution. Monitor infants for potential anticholinergic effects.
Drug Profile Summary
- Mechanism of Action: Muscarinic receptor antagonist, causing mydriasis and cycloplegia.
- Side Effects: Blurred vision, photophobia, dry eyes (ophthalmic); dry mouth, constipation, urinary retention, flushing (systemic).
- Contraindications: Glaucoma, narrow angle closure glaucoma, hypersensitivity to homatropine.
- Drug Interactions: Other anticholinergics, nitrates, antihistamines, tricyclic antidepressants.
- Pregnancy & Breastfeeding: Use with caution if clearly needed.
- Dosage: See detailed dosage section above.
- Monitoring Parameters: Intraocular pressure, pupil size, heart rate, respiratory rate.
Popular Combinations
Homatropine is often used in combination with hydrocodone for cough suppression (e.g., Hycodan).
Precautions
- Monitor intraocular pressure, especially in patients with glaucoma risk factors.
- Use cautiously in elderly patients and those with cardiovascular, renal, or hepatic impairment.
- Advise patients about potential blurred vision and photophobia.
- Monitor infants for anticholinergic effects if breastfeeding.
FAQs (Frequently Asked Questions)
Q1: What is the recommended dosage for Homatropine ophthalmic solution?
A: The dosage varies depending on age and indication. For adults, 1-2 drops of a 2% or 5% solution are typically used for uveitis, while 1-2 drops are used for eye examinations. Pediatric dosing varies based on age and concentration; please see the dosage section above.
Q2: How long do the effects of Homatropine last?
A: The mydriatic and cycloplegic effects can last for several days, typically 2-3 days for a 2% solution and up to a week for a 5% solution.
Q3: What are the contraindications for Homatropine?
A: Contraindications include primary glaucoma, narrow-angle or shallow anterior chamber, and hypersensitivity to homatropine or belladonna alkaloids.
Q4: Can Homatropine be used in pregnant or breastfeeding women?
A: Use with caution in pregnancy and breastfeeding. The potential benefits should outweigh the risks. Monitor infants for anticholinergic effects.
Q5: What are the common side effects of Homatropine ophthalmic solution?
A: Common side effects include blurred vision, photophobia, and dry eyes. Systemic effects such as dry mouth, constipation, and urinary retention can also occur.
Q6: How does Homatropine work in the treatment of uveitis?
A: Homatropine dilates the pupil, relieving pain and preventing posterior synechiae (adhesions between the iris and lens).
Q7: Can Homatropine be used in children?
A: Yes, but dosing must be adjusted based on age and concentration.
Q8: What should I do if a patient experiences an adverse reaction to Homatropine?
A: Discontinue the medication immediately and consult an ophthalmologist. Treatment may include supportive care and pilocarpine to reverse the mydriasis.
Q9: What are the signs of an overdose of Homatropine?
A: Signs of systemic overdose include flushing, dry skin, blurred vision, rapid/irregular pulse, fever, abdominal distention, mental aberration (hallucinosis), and loss of neuromuscular coordination.
Q10: How should Homatropine ophthalmic solution be stored?
A: Store at room temperature and protect from light. Discard discolored or cloudy solutions.
