Antihistamines are commonly used medications that alleviate allergy symptoms. Eye pressure, also known as intraocular pressure (IOP), refers to the fluid pressure within the eye. This article explores the relationship between antihistamines and eye pressure, detailing how certain types of these medications can influence it and identifying individuals who may be at a higher risk.
Understanding Antihistamines and Eye Pressure
Antihistamines work by blocking histamine, a chemical the body releases during an allergic reaction. Histamine causes common allergy symptoms such as sneezing, itching, watery eyes, and a runny or blocked nose. Antihistamines help reduce these symptoms by preventing histamine from binding to its receptors.
Intraocular pressure (IOP) is the measure of fluid pressure inside the eye, specifically the aqueous humor. The eye continuously produces and drains this fluid to maintain a stable internal pressure. Maintaining a healthy IOP is important because high pressure can damage the optic nerve, potentially leading to glaucoma and vision loss. Normal eye pressure typically ranges between 10 and 21 millimeters of mercury (mmHg).
How Antihistamines Can Influence Eye Pressure
Certain antihistamines can influence eye pressure primarily through anticholinergic effects. These effects are particularly associated with older, first-generation antihistamines, such as diphenhydramine (Benadryl) and chlorpheniramine. These medications can impact the eye’s natural drainage system, which maintains normal intraocular pressure.
Anticholinergic properties can lead to the dilation of the pupil. This pupil dilation can cause the iris to push forward. In individuals with a naturally narrow angle between the iris and the cornea, this forward movement can obstruct the trabecular meshwork, the primary drainage pathway for the aqueous humor. When the drainage pathway is blocked, the aqueous humor cannot exit the eye effectively, leading to a rapid buildup of fluid and a significant increase in intraocular pressure. This mechanism is linked to acute angle-closure glaucoma.
Newer, second-generation antihistamines, including loratadine (Claritin) and cetirizine (Zyrtec), generally pose less risk to eye pressure. These medications have reduced anticholinergic activity compared to their first-generation counterparts. They also typically do not cross the blood-brain barrier as readily, meaning their effects are more localized to peripheral histamine receptors and less likely to influence eye structures. While some studies suggest minimal impact on IOP with these newer drugs, individual responses can vary.
Who Is At Risk and What to Do
Individuals with pre-existing eye conditions, particularly narrow-angle glaucoma or those anatomically predisposed, face the highest risk of increased eye pressure from certain antihistamines. Narrow angles mean the space between the iris and the cornea is already restricted, making it easier for the iris to block the eye’s drainage system when dilated. While open-angle glaucoma is more common, antihistamines generally do not affect eye pressure in these cases.
An acute rise in eye pressure, often associated with angle-closure glaucoma, can present with several warning signs. These symptoms include severe eye pain, a throbbing headache, blurred vision, and seeing halos or colored rings around lights. Nausea and vomiting can also accompany these ocular symptoms. If any of these symptoms appear suddenly, it indicates a medical emergency requiring immediate attention from an ophthalmologist.
Consulting an eye care professional or doctor before taking antihistamines is important if you have an existing eye condition, especially any form of glaucoma, or concerns about your eye pressure. Discuss all medications you are currently taking, including over-the-counter drugs, with your healthcare provider. Your doctor can assess your individual risk and recommend the most appropriate and safest allergy relief options. This might involve choosing second-generation antihistamines or exploring alternative allergy treatments, such as topical eye drops, which may have fewer systemic effects.