Pupil Dilation

Enlarging the Pupil for Eye Examination

Dilating the pupil facilitates the view of the internal structures of the eye including the lens, optic nerve, blood vessels and retina in greater detail. Dilation is a key component of a comprehensive eye examination, as it sometimes leads to the detection and diagnosis of certain eye diseases.

A young man's face with dilated pupils

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Enlarging the Pupil

The pupil of the eye is similar to a camera aperture that can become bigger or smaller depending on how much light is needed to enter the eye. The pupil can undergo miosis, a constriction of the size of the pupil to become smaller, or it can undergo mydriasis, a dilation of the pupil or enlargement of the size of the pupil. The pupil size is controlled by both the sympathetic nervous system and the parasympathetic nervous system.

Pupil size changes depending on the amount of light that enters the eye. Pupil size may change as a response to sexual stimuli. Pupil size also changes in times of stress. The “fight or flight” stress reaction is thought to cause an enlargement of pupil size to allow more light into the eye so the body can react quicker to potentially harmful enemies.

Dilating Eye Drops

There are two types of ophthalmic eye drops. One type stimulates contraction of the muscles in the iris that enlarge the pupil and the other type relaxes the muscles that cause pupillary constriction and also relaxes the muscle that allows the eye to focus the lens inside the eye. Some of these eye drops are given together to make the pupil larger.

Dilating the eye as a part of an eye examination involves administering medicinal eye drops, sometimes administered two or three times directly into the eye. Depending on what the doctor is trying to achieve and how large the pupil needs to be, different types of eye drops can be given.

Examples of eye drops that cause the pupil to dilate:

  • Phenylephrine
  • Tropicamide
  • Hydroxyamphetamine
  • Cyclopentolate
  • Atropine


Most dilating drops cause a side effect of cycloplegia. Cycloplegia is the temporary paralysis of the ciliary body that allows the eye to focus on near objects. Most dilation drops may cause a cycloplegic effect anywhere from a couple hours to a few days. Dilation of the pupil and cycloplegia are two distinct processes but are connected because most medications that cause cycloplegia, also enlarge the pupil.

Sometimes a doctor may desire a patient to have cycloplegia. For example, a very young child’s focusing mechanism is so strong that it makes it very difficult to measure their vision or refractive error to see if vision correction is needed. When a drug is administered to cause significant cycloplegia, the child’s focusing system is temporarily paralyzed. In this manner, the doctor can comfortably measure the visual system of a child completely. In essence, the “raw data” can be measured only and the child cannot influence the measurement at all.

Another time when a doctor may want to cause a patient to have extended cycloplegia is when they have an eye disease, condition or trauma that causes acute pain and light sensitivity. For a condition called uveitis, cycloplegic eye drops serve to reduce pain and sensitivity. However, sometimes the cycloplegic side effect is unwanted. For example, when a patient is dilated as part of a comprehensive eye examination, they typically suffer some level of cycloplegic effect. For a few hours, their near vision may be blurry.

Often, the doctor is more interested in viewing the internal structures of the eye to determine eye health and is not interested in causing cycloplegia. Because of this unwanted side effect, this part of the eye examination is considered the “unpleasant” part of the eye examination and a patient may put off coming in because of it.

Light Sensitivity

When a dilating drop is administered, the pupil becomes very large. Because the pupil cannot constrict normally because of these drops, people are very light sensitive. This temporary light sensitivity is another reason why people may put off coming in for an eye examination. Because dilation of the pupils often causes near or close-up vision to be blurred and sensitivity to light, disposable sunglasses are often provided after the exam to reduce discomfort caused by sunlight. Dilation generally lasts for three to six hours, but it is not uncommon for it to last up to 24 hours for some patients, depending on the eye drops used.

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