The pupil is the dark spot in the centre of the eye. Pupils change in size to control the amount of light entering the eye. Pupils are an indicator of the communication pathway between the brain and the eye. In some cases, you may have a condition known as a dilated pupil (mydriasis).
The healthy pupil size in adults averages from 2 to 4mm in bright light and 4 to 8mm in dim light . Pupils are usually the same size in both eyes.
Dilated pupil. (Source: Ophthalmology Times)
Pupils naturally widen or dilate in dim environments to allow more light into the eye. On the contrary, in bright light, pupils will constrict or get smaller to prevent light from entering the eye. Tiny muscles in the iris (coloured tissue of the eye) control the size of the pupil.
Mydriasis is the term used to describe pupils that widen or dilate without any change in lighting conditions. A pupil with mydriasis does not respond to light, as such, mydriasis is commonly referred to as a “fixed pupil”.
Here are some common causes of dilated pupils:
For a better view and thorough evaluation of the retina (light-sensitive tissue at the back of the eye), the eye care professional may use dilating eye drops to temporarily make your pupils larger. This dilating eye drops usually take about 20 to 30 minutes to start working. Depending on the strength of eye drops used, the dilated effect typically wears off within 4 to 6 hours but can last up to 24 hours. Dilating eye drops are also used by ophthalmologists before an eye surgery.
When used at higher concentrations and/or increased frequency, atropine eye drops which are commonly used to slow down the rate of short-sightedness or myopia progression in children can cause pupils to dilate.
Atropine eye drops used for myopia progression in children (Source: Vanderbilt University Medical Center)
A sharp or blunt trauma to the eye can damage structures in the iris (coloured tissue of the eye), causing mydriasis in one or both pupils.
An injury inside the head can cause a build-up of pressure damaging the nerves in the iris. This can cause mydriasis in one or both pupils. Head injury, stroke and/or a tumour can all cause changes in pupil size.
Adie’s syndrome or Holmes-Adie syndrome, is a rare neurological condition that can result in abnormal pupil dilation in one or both eyes. In most people affected by this condition, the pupil is dilated and reacts to light slowly .
Blocked blood flow to nerves entering the eye, caused by a microvascular cranial nerve palsy, can cause pupil dilation and impair vision.
Benign episodic unilateral mydriasis (BEUM), associated with migraines, is a rare condition that can cause unequal pupil sizes or anisocoria.
Pupil dilation can be caused by certain prescription and/or over-the-counter medications. Among others, here is a list of such types of medications:
If you are concerned about the side effect of pupil dilation from these medications, it’s best to visit your eye care professional and/or primary care doctor.
Dilated pupils may occur after using recreational banned drugs such as cocaine, ecstasy or psychedelic mushrooms.
Stress can trigger adrenaline hormones to react, causing dilated pupils among other reactions.
Consumption of alcohol can cause muscles to relax, resulting in expansion of iris muscles leading to pupil dilation.
Dilated pupils commonly cause headaches, blurred vision and/or sensitivity to light.
Treating dilated pupils depends on the cause. If it is triggered by certain medications, your primary care doctor may recommend medication changes. If it is triggered by an injury to the eye and/or brain, you should seek immediate care for a timely intervention.
To decrease the glare and discomfort caused by pupil dilation, your eye care professional may recommend sunglasses.
Subtle changes may go unnoticed, making regular comprehensive eye checks all the more important for timely identification, possible interventions and/or referrals to the respective health care professional when dilated pupils are observed.
 H.K Walker, W.D Hall and J.W Hurst. “Clinical Methods, 3rd Edition”, 1990.
 “Adie Syndrome”, Rare Diseases. [Online]. Available: https://rarediseases.org/rare-diseases/adie-syndrome/#:~:text=Adie%20syndrome%2C%20or%20Holmes%2DAdie,also%20associated%20with%20this%20disorder. [Accessed: 27-June-2022]
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