Mechanism of Action on Ocular Structures
The action of ophthalmic antimuscarinics is based on the competitive blockade of specific muscarinic receptors in the smooth muscle of the eye. This leads to two key pharmacodynamic effects:
- Mydriasis (pupil dilation). Occurs due to relaxation of the sphincter pupillae muscle of the iris.
- Cycloplegia (paralysis of accommodation). Arises from paralysis of the ciliary muscle, causing the lens to tighten and flatten, temporarily losing the ability to focus on near objects.
Both of these effects are critical for performing a comprehensive ophthalmic examination and determining the true optical parameters of the eye.
Tropicamide: Standard for Rapid Diagnostics
Tropicamide is one of the most in-demand agents due to its favorable pharmacokinetics. Its key advantage is the extremely short duration of cycloplegia.
Key parameters of action:
- Onset of effect: Very rapid, occurring within 5–10 minutes.
- Duration of cycloplegia: Only 1–2 hours (peaking at 20–40 minutes).
- Duration of mydriasis: Persists for up to 6 hours.
This short duration makes tropicamide an ideal agent for routine fundus examination. The patient recovers normal vision relatively quickly after the doctor's visit.
Cyclopentolate and Homatropine
In situations requiring a more prolonged effect, other agents from this group are used:
- Cyclopentolate. Produces effects similar to tropicamide (pronounced mydriasis and cycloplegia), but has a significantly longer duration of action—from 20 to 24 hours. This makes it the drug of choice for refraction testing in young children, as well as for various short-term diagnostic procedures.
- Homatropine. A synthetic compound (a tertiary amine, ester of tropine and mandelic acid). Pharmacologically very close to atropine, but with lower potency and a shortened duration of action—about 15–20 hours. In medical practice, it is used for pupil dilation predominantly as homatropine methylbromide.
Side Effects and Contraindications
Despite local application as eye drops, these agents can cause systemic side effects due to absorption into the systemic circulation through the mucosa of the lacrimal drainage system.
Typical systemic manifestations include:
- Headache;
- Tachycardia;
- Marked dry mouth;
- Photophobia due to the dilated pupil.
Special attention should be paid to pediatric practice: children have a high risk of developing hyperthermia with these drugs.
Absolute Contraindication! All anticholinergics are strictly prohibited in glaucoma. The resulting mydriasis inevitably disrupts aqueous humor outflow from the anterior chamber, provoking a dangerous spike in intraocular pressure.