Dilated eye exam: what the drops are for and can you drive afterwards?

‘I’ll put in a few drops, please take a seat in the waiting room.’ Twenty minutes later the light in the corridor seems stronger, the phone screen is hard to read, and in the bathroom mirror your pupils are two enormous black circles. Then the ophthalmologist calls you in, brings a lens close to your eye and looks inside, for a long time.

This is the examination of the back of the eye with the pupil dilated, one of the most common in ophthalmology and also one of the most misunderstood. What is it for, how long do the drops last and, above all, can you drive afterwards? Here is what the studies say.

In this article

1. Dilated eye exam: what the test is for
2. Why dilate the pupil
3. How long the effect of the drops lasts
4. Can you drive after the drops? What studies say
5. The risk of acute glaucoma: rare, but worth knowing about
6. Can it be done without dilating the pupil?
7. Frequently asked questions
8. Sources

Dilated eye exam: what the test is for

The back of the eye is the part at the rear, opposite the pupil: the retina, the macula at its centre, the optic nerve and the blood vessels that nourish them. Looking at it helps to recognise many diseases. In age-related macular degeneration, for example, a review indicates that diagnosis requires precisely an examination of the back of the eye with the pupil dilated; after 75 the risk is 25% for the early form and 8% for the advanced one¹.

It is also a key test when floaters or flashes of light appear suddenly. A review of 17 studies found that, among people who come to the ophthalmologist with these symptoms, 14% have a retinal tear, which if not treated can lead to a detachment². We discuss this in the article on floaters.

And it is needed from birth onwards: according to a French review, babies born before the 32nd week or weighing less than 1,500 grams should routinely have an examination of the back of the eye with the pupil dilated, to look for retinopathy of prematurity¹⁰.

Why dilate the pupil

The pupil is the window through which the ophthalmologist looks inside the eye. When it is small, the window is narrow: the centre can be seen, but little of the periphery of the retina. The drops, called mydriatics, widen the pupil for the duration of the examination.

The difference can be measured. In a randomised trial of 3,650 people with diabetes, photographs of the retina taken without drops were impossible to assess in 19.7% of cases; with the pupil dilated the share fell to 3.7%³. Unreadable images increased with age and the most frequent cause was a marked cataract³. We also discuss this in the article on diabetes and eyes.

How long the effect of the drops lasts

The studies we have read measure the duration in young, healthy people. In a study of 30 young adults, after the drops the pupil diameter went from 4.1 to 7.8 millimetres in 30 minutes, stayed at that size for up to two hours and returned to normal within six hours⁵. The duration can change with the type of eye drops and with the person: it is a question to ask the ophthalmologist.

Can you drive after the drops? What studies say

It is the most frequent question, and the studies’ answer is less clear-cut than one would like.

The study of the 30 young adults looked instead at dynamic visual acuity, the ability to read moving objects: with the pupil dilated it turned out to be even better and returned to its initial values in six hours, in parallel with the pupil⁵. The authors present it as useful information for assessing the effects of dilation on driving⁵: a single aspect of vision, measured in the laboratory, is not enough, however, to say whether driving is safe.

What can be drawn from this? That the data are few, collected on small groups of young people and in laboratory conditions, and that they measure different aspects of vision^5,6. They say nothing precise about older people, night driving or people who already have an eye disease. How to organise the journey home is therefore a question to ask the ophthalmologist, who knows the eye drops used and your eyesight.

The risk of acute glaucoma: rare, but worth knowing about

In a small proportion of people the angle through which the fluid leaves the eye is narrow. When the pupil is dilated, the iris thickens at its base and can close it: this is one of the mechanisms of drug-induced acute glaucoma⁸. Among the anatomical and personal risk factors are a shallow anterior chamber, a short eye, female sex, Asian descent, family history and advanced age⁸.

How frequent is it? A study in Singapore dilated the pupils of 471 people over 50 with a narrow angle, that is, at higher risk, after giving them a preventive drug: an acute attack appeared in 3 people within six hours, 0.64%⁷. The authors judge the risk to be low; according to them the only clinical parameter linked to a significant rise in pressure was precisely the narrowness of the angle⁷. You will find the differences between the forms of glaucoma in the article on open-angle and angle-closure glaucoma.

The signs of an acute attack are an eye that suddenly turns red, pain, headache, nausea, blurred vision and halos around lights^8,9. It is an emergency: it needs to be recognised and treated at once⁹.

Can it be done without dilating the pupil?

In part, yes. There are cameras that photograph the retina without drops, and new technologies, such as ultra-widefield imaging and portable devices, are changing screening for diabetic retinopathy⁴. But without dilation a higher share of images cannot be assessed, especially in older people³. OCT, which studies the retina in cross-section, is instead a different and complementary test.

Frequently asked questions

How long does the effect of the drops for the dilated eye exam last?

It depends on the eye drops and on the person. In a study of young adults the pupil stayed dilated for up to two hours and returned to normal within six⁵.

Can you drive after a dilated eye exam?

The studies are few and small. In one, of 12 young drivers, dilation worsened visual acuity and contrast sensitivity, and many slowed down in the simulator; the authors consider larger studies necessary before giving recommendations⁶. It is a question to ask the ophthalmologist.

Are the drops that dilate the pupil dangerous?

Their effect is temporary: in a study of young adults the pupil returned to normal within six hours⁵. Rarely, in people with a narrow angle, they can trigger acute glaucoma: in a study of people with a narrow angle it happened to 3 out of 471⁷.

Why doesn’t the ophthalmologist always examine the back of the eye without drops?

Because with a small pupil less of the retina can be seen and the images are more often unusable: in one study 19.7% against 3.7% with the pupil dilated³.

The examination of the back of the eye with drops means a few hours of blurred vision, but it allows the ophthalmologist to see what would otherwise remain hidden. The studies on driving after dilation are still few and do not give an answer that is valid for everyone: this is why the most useful thing is to talk about it beforehand, when you book.