Miotics, also known as cholinergic agonists, are a group of drugs that mimic the action of acetylcholine, a neurotransmitter in the parasympathetic nervous system. These drugs primarily act on the iris sphincter muscle and the ciliary muscle, leading to pupillary constriction (miosis) and accommodation (focusing on near objects).
Classification
Miotics can be classified into two main categories
a. Direct-Acting Miotics
Healthcare professionals use Direct-Acting Miotics as medications in ophthalmology to constrict the pupil and reduce intraocular pressure. These drugs directly stimulate muscarinic receptors in the eye, leading to pupillary constriction (miosis) and increased aqueous humor outflow. Here are some examples of direct-acting miotics
1. Pilocarpine (Pilocar) Pilocarpine is one of the most commonly used direct-acting miotic drugs. Healthcare professionals use it in various formulations, including eye drops and gel, to treat conditions such as open-angle glaucoma, angle-closure glaucoma, and to manage intraocular pressure.
2. Carbachol (Carbastat, Miostat) Carbachol is another direct-acting miotic used in ophthalmology. Doctors often prescribe it in eye drop form to reduce intraocular pressure in glaucoma and after certain eye surgeries.
These direct-acting miotics work by directly stimulating muscarinic receptors in the eye, causing the iris sphincter muscle to contract and the pupil to constrict. This pupillary constriction enhances the drainage of aqueous humor from the eye, which can help lower intraocular pressure, a crucial factor in managing various eye conditions. The choice of which direct-acting miotic to use and the specific dosage will depend on the patient’s condition and the healthcare provider’s recommendation.
b. Indirect-Acting Miotics (Cholinesterase Inhibitors)
Ophthalmologists use indirect-acting miotics, also known as cholinesterase inhibitors, as a class of medications to constrict the pupil and lower intraocular pressure by inhibiting the enzyme acetylcholinesterase, which breaks down acetylcholine.This results in an increased concentration of acetylcholine at the muscarinic receptors in the eye. Here are a few examples of indirect-acting miotic drugs
1. Echothiophate (Phospholine Iodide) Echothiophate is an indirect-acting miotic used in the treatment of open-angle glaucoma. It is available in the form of eye drops and is typically administered once a day.
2. Physostigmine (Antilirium) Physostigmine is another indirect-acting miotic used in ophthalmology. It is available in various forms, including eye drops, ointments, and systemic preparations. It can be used to treat certain types of glaucoma and for diagnostic purposes in ophthalmology.
These drugs work by inhibiting acetylcholinesterase, which leads to an accumulation of acetylcholine at the muscarinic receptors in the eye. This increased acetylcholine concentration results in pupillary constriction and increased drainage of aqueous humor from the eye, ultimately reducing intraocular pressure.
Pharmacological Actions
The pharmacological actions of miotics are as follows
Pupillary Constriction (Miosis) Miotics cause the constriction of the pupil by stimulating the iris sphincter muscle, which the parasympathetic nervous system controls. This effect can be useful in conditions such as angle-closure glaucoma.
Accommodation Miotics stimulate the ciliary muscle, enabling the eye to focus on near objects. This is important in conditions like presbyopia.
Reduction in Intraocular Pressure Miotics enhance the drainage of aqueous humor by constricting the pupil and facilitating the flow through the trabecular meshwork, which helps lower intraocular pressure.
Dose
The specific dosage of miotics can vary depending on the drug, the patient’s condition, and the healthcare provider’s prescription. However, here are some general guidelines for common miotic drugs
Pilocarpine Typically, healthcare professionals instill 1-2 drops of 1% or 2% pilocarpine into the affected eye(s) 2 to 4 times a day. The exact frequency and concentration will depend on the patient’s condition.
Carbachol The usual dose is 1-2 drops of 1.5% carbachol solution in the affected eye(s) 2 to 4 times a day.
Echothiophate The dosing schedule for echothiophate can be less frequent, often requiring only once daily application.
It’s essential to follow the healthcare provider’s instructions and not self-administer these drugs without a prescription.
Indications
Miotics are indicated for various ophthalmic conditions, including
Open-Angle Glaucoma Healthcare professionals use miotics to lower intraocular pressure and prevent optic nerve damage in patients with open-angle glaucoma.
Angle-Closure Glaucoma Miotics can help open the drainage angle and reduce intraocular pressure in acute angle-closure glaucoma.
Postoperative Management Healthcare professionals may use them postoperatively after cataract surgery to prevent pupillary block and manage intraocular pressure.
Presbyopia Miotics can be used to improve near vision in individuals with presbyopia.
Contraindications
Miotics are contraindicated in certain situations, including
Angle-Closure Glaucoma with Pupillary Block Patients with angle-closure glaucoma who have pupillary block should avoid miotics because miotics can exacerbate the condition.
Inflammatory Eye Conditions Patients should use miotics with caution in the presence of inflammatory eye conditions, as miotics can exacerbate inflammation.
Cardiovascular Issues Patients with a history of cardiovascular problems should use miotics cautiously due to potential systemic side effects.
Hypersensitivity Individuals with known hypersensitivity to miotic drugs or their components should avoid their use.
Systemic Cholinergic Effects Miotics can cause systemic cholinergic effects, so healthcare professionals should use them cautiously in individuals with conditions that cholinergic stimulation may exacerbate, such as asthma, chronic obstructive pulmonary disease (COPD), and bradycardia.
Before using miotics, it is crucial for patients to discuss their medical history and any medications they are taking with their healthcare provider to ensure the safety and efficacy of the treatment.
