Usage
Carbachol is a cholinergic agonist, specifically a parasympathomimetic or cholinomimetic agent, primarily indicated for the treatment of glaucoma and the reduction of intraocular pressure (IOP) during and after ocular surgery. It is also used to induce miosis (pupillary constriction) during surgical procedures.
Alternate Names
Carbachol is also known by the brand name Miostat. Carbastat and Carboptic are other brand names.
How It Works
Pharmacodynamics: Carbachol acts as a direct-acting cholinergic agonist, mimicking the effects of acetylcholine at muscarinic and nicotinic receptors. In the eye, its action at muscarinic receptors in the ciliary body and iris sphincter muscles causes miosis (pupil constriction) and contraction of the ciliary muscle, leading to increased outflow of aqueous humor and a subsequent reduction in IOP. It also causes cyclospasm. This effect makes Carbachol is useful in the management of glaucoma, a condition characterized by elevated IOP that can damage the optic nerve.
Pharmacokinetics: Following topical ophthalmic administration, Carbachol poorly penetrates the cornea. Benzalkonium chloride is sometimes added to formulations to enhance corneal penetration. Intraocular administration directly delivers the medication to the anterior chamber of the eye. Carbachol undergoes minimal systemic absorption after topical or intraocular administration. The drug is not metabolized significantly and its elimination pathways are not fully characterized.
Mode of Action: Carbachol binds directly to muscarinic cholinergic receptors in the eye, triggering a cascade of intracellular events that result in miosis and increased aqueous humor outflow. Its effects are more prolonged than those of acetylcholine because it is resistant to hydrolysis by acetylcholinesterase.
Dosage
Standard Dosage
Adults:
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Glaucoma: 1-2 drops of a 1.5-3% carbachol ophthalmic solution instilled into the conjunctival sac up to three times a day.
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Intraocular Hypertension/Miosis Induction during Surgery: 0.5 mL of a 0.01% carbachol intraocular solution is instilled into the anterior chamber of the eye before or after securing sutures.
Children:
The safety and efficacy of carbachol have not been established in pediatric patients.
Special Cases:
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Elderly Patients: Dose adjustments may be necessary based on individual patient response and tolerance.
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Patients with Renal Impairment: No specific dosage adjustments are typically required.
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Patients with Hepatic Dysfunction: No specific dosage adjustments are typically required.
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Patients with Comorbid Conditions: Use with caution in patients with asthma, acute heart failure, corneal abrasion, hyperthyroidism, urinary tract obstruction, Parkinson’s disease, active peptic ulcer, or gastrointestinal spasm.
Clinical Use Cases
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Intubation: Carbachol is not indicated for intubation.
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Surgical Procedures: Used intraocularly for miosis induction during ocular surgery.
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Mechanical Ventilation: Carbachol is not indicated for mechanical ventilation.
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Intensive Care Unit (ICU) Use: Limited role in the ICU, possibly for managing IOP in specific cases.
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Emergency Situations: Carbachol is not typically used in emergency situations.
Dosage Adjustments
Dosage may be adjusted based on the patient’s response to therapy, as determined by tonometric readings.
Side Effects
Common Side Effects
- Blurred vision
- Lacrimation (watering of the eyes)
- Headache
- Burning or stinging sensation in the eye
- Conjunctival vascular congestion (redness)
Rare but Serious Side Effects
- Corneal clouding
- Persistent bullous keratopathy (formation of blisters in the cornea)
- Retinal detachment
- Postoperative iritis (inflammation of the iris)
- Systemic effects (e.g., sweating, gastrointestinal distress, urinary urgency)
Long-Term Effects
Chronic complications from prolonged use are possible, including corneal endothelial damage.
Adverse Drug Reactions (ADR)
Serious ADRs, though rare, include severe bradycardia, hypotension, and bronchospasm.
Contraindications
- Hypersensitivity to carbachol or any component of the formulation
- Conditions where pupillary constriction is undesirable, such as acute iritis, acute angle-closure glaucoma
Drug Interactions
Carbachol can interact with other ophthalmic medications, particularly miotics and other cholinergic agents, potentially leading to additive effects. Interaction data with other systemic medications are limited. However, it can interact with beta-blockers, enhancing cardiac conduction abnormalities and bronchoconstriction.
Pregnancy and Breastfeeding
Pregnancy: Pregnancy Category C. Use with caution only if the potential benefit outweighs the risk to the fetus.
Breastfeeding: Excretion in breast milk is unknown. Use with caution if the benefits outweigh the risks.
Drug Profile Summary
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Mechanism of Action: Direct-acting cholinergic agonist, mimicking acetylcholine at muscarinic receptors in the eye, causing miosis and increased aqueous outflow.
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Side Effects: Blurred vision, lacrimation, headache, burning/stinging, corneal clouding, bullous keratopathy, retinal detachment.
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Contraindications: Hypersensitivity, conditions where miosis is undesirable.
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Drug Interactions: Other miotics/cholinergics, beta-blockers.
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Pregnancy & Breastfeeding: Use with caution if benefits outweigh risks.
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Dosage: Adults: Glaucoma: 1-2 drops of 1.5%-3% solution up to tid; Intraocular surgery: 0.5 mL of 0.01% solution.
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Monitoring Parameters: Intraocular pressure, visual acuity, corneal clarity.
Popular Combinations
Carbachol is not typically used in combination with other medications for long-term glaucoma management.
Precautions
- Monitor for systemic side effects, especially in patients with pre-existing cardiovascular or respiratory conditions.
- Perform regular ophthalmic examinations to monitor for corneal or retinal changes.
FAQs (Frequently Asked Questions)
Q1: What is the recommended dosage for Carbachol?
A: For glaucoma: 1-2 drops of 1.5-3% solution up to three times daily. For intraocular surgery: 0.5 mL of 0.01% solution.
Q2: How does Carbachol work to lower intraocular pressure?
A: Carbachol activates muscarinic receptors in the eye, constricting the pupil and ciliary muscle. This increases aqueous humor outflow, lowering IOP.
Q3: What are the common side effects of Carbachol?
A: Blurred vision, lacrimation, headache, and burning or stinging in the eye.
Q4: Is Carbachol safe to use during pregnancy?
A: Pregnancy Category C. Use with caution only if the potential benefit outweighs the potential risk to the fetus.
Q5: Are there any contraindications to using Carbachol?
A: Hypersensitivity to the drug, and conditions where miosis is undesirable (e.g., acute iritis, acute angle-closure glaucoma).
Q6: Can Carbachol interact with other medications?
A: It can interact with other miotics and cholinergics, and potentially beta-blockers.
Q7: How should Carbachol be administered?
A: Topically for glaucoma as eye drops, and intraocularly for surgical miosis.
Q8: What should I monitor in patients using Carbachol?
A: IOP, visual acuity, corneal clarity, and any systemic cholinergic side effects.
Q9: What is the onset of action of Carbachol?
A: Topical: 10-20 minutes. Intraocular: 2-3 minutes.
Q10: What is the duration of effect of Carbachol?
A: Topical: 4-8 hours. Intraocular: Up to 24 hours.
