Anticholinergic agents such as diphenhydramine can precipitate an acute attack of narrow‑angle glaucoma by dilating the pupil and impairing aqueous humor outflow, leading to a rapid rise in intra‑ocular pressure (IOP). This interaction is considered a medical emergency.
Mydriasis from anticholinergics can trigger an acute angle‑closure crisis.
The reaction can develop within minutes to a few hours after ingestion.
Prompt ophthalmic emergency care is required to prevent permanent vision loss.
Prefer alternative antihistamines without anticholinergic activity for these patients.
Diphenhydramine blocks muscarinic receptors in the iris sphincter muscle, causing mydriasis (pupil dilation). In eyes with a predisposed anatomically narrow anterior chamber angle, dilation narrows the angle further, obstructing trabecular meshwork drainage of aqueous humor and causing a sudden increase in IOP.
Acute angle‑closure glaucoma can present with severe ocular pain, blurred vision, halos around lights, headache, nausea, and vomiting. If untreated, it can lead to irreversible optic nerve damage and permanent vision loss within hours.
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