The common eye exam procedure of pupil dilation is a standard part of a comprehensive eye examination, but patients often wonder about its effect on the fluid pressure inside the eye. Understanding how dilating drops influence the eye’s internal fluid dynamics helps clarify this relationship. The procedure is generally safe, but a rare anatomical structure can make it dangerous for a small subset of the population.
Understanding Intraocular Pressure and Pupil Dilation
Intraocular pressure (IOP) is the fluid pressure maintained by the aqueous humor, a clear liquid filling the front part of the eye. This fluid is continuously produced and drained, which helps the eye maintain its shape. Measuring IOP is a routine part of an eye exam because elevated pressure is a primary indicator for glaucoma, a disease that can damage the optic nerve.
Pupil dilation, known medically as mydriasis, uses special eye drops to temporarily widen the pupil. The drops contain medications like tropicamide or phenylephrine that relax the iris muscles. This widening provides the eye doctor with a much clearer view of the internal structures, including the retina, optic nerve, and macula. A thorough inspection of these posterior structures is necessary to detect early signs of diseases like glaucoma, diabetic retinopathy, and macular degeneration.
The Direct Relationship Between Dilation and Eye Pressure
For the vast majority of people with normal, open drainage structures, dilation does not cause a clinically significant rise in intraocular pressure. Studies show that in eyes without glaucoma, the average rise in IOP after dilation is minor, often around 2.5 mmHg. This is a negligible change within the normal pressure range of 10 to 20 mmHg and is not considered harmful to the optic nerve.
The slight, temporary increase observed in some people is related to the effect of the dilating drops on the eye’s internal drainage system. The medications can cause a reduction in aqueous outflow by decreasing the tension on the trabecular meshwork, the sponge-like tissue where the fluid drains. However, this effect is transient, and the pressure usually returns to the pre-dilation level within a few hours.
The Risk of Angle Closure
While dilation is safe for most, it can trigger a sudden, dangerous spike in IOP in individuals with a specific anatomical condition: a narrow or shallow anterior chamber angle. This condition puts them at risk for acute angle-closure glaucoma. The angle is the junction where the cornea and iris meet, containing the drainage system for the aqueous humor.
In eyes with narrow angles, the iris is positioned close to the drainage angle. When dilating drops are applied, the iris bunches up as the pupil widens, causing the peripheral iris tissue to physically obstruct the trabecular meshwork. This blockage prevents the aqueous fluid from exiting the eye entirely. The resulting rapid accumulation of fluid can cause a severe, abrupt rise in intraocular pressure, often exceeding 40 mmHg, which is an ophthalmic emergency.
This acute pressure spike, known as an angle-closure attack, requires immediate medical attention. Patients may experience intense eye pain, redness, blurred vision, a headache, and seeing colored halos around lights. If the blockage is not quickly relieved, the high pressure can cause irreversible damage to the optic nerve and lead to permanent vision loss within hours.
Protecting Patients During Dilation
To prevent the rare but serious complication of acute angle closure, eye care professionals screen patients for narrow angles before administering dilating drops. This initial examination helps identify individuals at risk, such as those who are farsighted, of advanced age, or of specific ethnic backgrounds where narrow angles are more prevalent. Techniques like the penlight test offer a quick, non-invasive way to estimate the depth of the anterior chamber.
If a narrow angle is suspected, a more precise diagnostic tool, such as gonioscopy, is performed to directly visualize the drainage angle. Gonioscopy uses a special mirrored lens placed gently on the eye to check if the angle is open enough for safe dilation. If the risk is determined to be too high, the doctor may forgo dilation entirely or recommend a preventative laser procedure called a laser peripheral iridotomy (LPI).
The LPI procedure creates a small, permanent opening in the iris. This equalizes the pressure between the front and back parts of the eye, preventing the iris from bowing forward and blocking the drainage angle. Patients should seek immediate care if they develop severe eye pain, persistent blurred vision, or halos hours after their eye exam, as these symptoms could indicate a potential angle-closure event.