Mydriasis and miosis: why a pupil dilates or constricts

Mydriasis refers to an abnormally dilated pupil, miosis to an abnormally constricted one. Most of the time, these changes are physiological and simply adjust to light or emotion. But when only one pupil changes size (anisocoria), or when pain, double vision or a drooping eyelid appears, it may be a neurological or ophthalmological sign that calls for a prompt assessment.

UNDERSTANDING

What are mydriasis and miosis?

The pupil is the central opening of the iris that regulates the amount of light entering the eye, like the aperture of a camera. Its diameter normally varies between 2 and 4 mm in daylight and can reach 6 to 8 mm in the dark. Two opposing muscles control this opening:

• The iris sphincter muscle, driven by the parasympathetic system (oculomotor nerve, III), constricts the pupil: this is miosis.
• The iris dilator muscle, driven by the sympathetic system, enlarges the pupil: this is mydriasis.

We speak of mydriasis when the pupil stays dilated in an unusual way, and of miosis when it stays abnormally constricted. These terms describe a state, not a disease: the goal is always to identify the cause, which ranges from a simple reflex to a neurological disorder.

CAUSES

Mydriasis: why the pupil dilates

Physiological causes

Dilation of both pupils is normal in the dark, during a strong emotion, an effort or pain (a discharge of the sympathetic system). These forms of mydriasis are symmetrical, reactive to light and transient.

Drug-related and toxic causes

This is the most common cause of persistent mydriasis. It is usually harmless once it is recognised:

• Dilating eye drops used to examine the back of the eye (tropicamide, phenylephrine): the pupil stays dilated for a few hours after the consultation, which is expected.
• Accidental contact with a scopolamine patch (motion sickness) or certain plants containing atropine (jimson weed, deadly nightshade).
• Medications with an anticholinergic or sympathomimetic effect, and some asthma sprays that are mishandled.
• Various substances (stimulants, hallucinogens) cause bilateral mydriasis.

Drug-induced mydriasis is recognised because it does not constrict, or only slightly, after instilling low-dose pilocarpine, a simple test carried out during the consultation.

Neurological and ophthalmological causes

• Oculomotor nerve palsy (III): mydriasis associated with a drooping eyelid (ptosis) and double vision must always prompt exclusion of an intracranial aneurysm compressing the nerve. This is an emergency Trobe 1988.
• Adie’s tonic pupil: mydriasis, most often unilateral, in young women, with a slow reaction to light and to near vision. It is generally benign Kanzaria 2012.
• Ocular trauma: a contusion can injure the iris sphincter and leave a permanently dilated pupil (post-traumatic mydriasis).
• Acute angle-closure glaucoma attack: the pupil is in mid-dilation, fixed, the eye is red and painful, with blurred vision and halos. This is an ophthalmological emergency Khazaeni 2023.

CAUSES

Miosis: why the pupil constricts

Physiological causes

Miosis in both eyes is normal in bright light, during near vision (accommodation) and during sleep. With age, the pupil also tends to constrict: this is senile miosis, which has no consequences apart from difficulty seeing in dim light.

Drug-related and toxic causes

• Miotic eye drops such as pilocarpine, used in certain forms of glaucoma.
• Opioids (morphine and derivatives): tight miosis in both eyes is a classic sign of overdose.
• Poisoning by organophosphate compounds (certain insecticides).

Pathological causes

• Horner syndrome: the combination of miosis, mild ptosis and an absence of sweating on one side of the face, due to involvement of the sympathetic pathway. A Horner syndrome of sudden onset, especially with neck pain, should prompt a search for a carotid artery dissection Nguyen 2020.
• Intraocular inflammation (anterior uveitis, iritis): miosis is then accompanied by a red, painful, light-sensitive eye.
• Argyll Robertson pupil: bilateral miosis that does not react to light but reacts to near vision, described in certain neurological conditions.

Key takeaway — a dilated or constricted pupil that is isolated, painless and reactive to light is rarely serious. It is the combinations — pain, red eye, drooping eyelid, double vision, headache, sudden onset — that should prompt you to seek care without delay.

DIAGNOSIS

Anisocoria: the key is light and darkness

When the two pupils are not the same size, we speak of anisocoria. A slight difference (less than 1 mm) exists in nearly one person in five as a harmless finding: this is physiological anisocoria, identical in light and in the dark. The medical reasoning rests on a simple question: is the difference more marked in bright light or in the dark?

• The difference increases in bright light → the abnormal pupil is the larger one (it does not constrict well). This points towards a third-nerve palsy, an Adie’s pupil or a pharmacological cause.
• The difference increases in the dark → the abnormal pupil is the smaller one (it does not dilate well). This points towards Horner syndrome.

EMERGENCY

Warning signs: when to seek urgent care

Some situations call for a prompt ophthalmological or neurological assessment, sometimes in the emergency department:

• Dilated pupil + red, painful eye + blurred vision + halos + nausea: suggests an acute glaucoma attack, an absolute emergency.
• Dilated pupil + drooping eyelid + double vision: suggests oculomotor nerve involvement, to be investigated without delay (excluding an aneurysm).
• Constricted pupil + sudden-onset drooping eyelid + neck or facial pain: suggests Horner syndrome, which may reveal a carotid dissection.
• A pupillary abnormality after a head or eye injury, or associated with unusual headaches, weakness in a limb or speech difficulties.

DIAGNOSIS

Diagnosis: examination and pharmacological tests

The examination begins by observing the pupils in light and in the dark, measuring their diameter, studying the light reflex and looking for a relative afferent pupillary defect. The slit lamp is used to examine the iris and to look for inflammation, a sphincter injury or signs of glaucoma.

Pharmacological tests help to clarify the mechanism Antonio-Santos 2005:

• Low-dose pilocarpine constricts an Adie’s pupil (denervation hypersensitivity) but not pharmacological mydriasis.
• Apraclonidine reverses the anisocoria of Horner syndrome and helps to confirm it.
• Standard-concentration pilocarpine does not constrict a pupil blocked by an atropine-type eye drop, which confirms a drug-related cause.

Depending on the context, brain imaging (CT scan, MRI, MR angiography) is requested urgently when a neurological cause is suspected.

MANAGEMENT

What management is needed?

Treatment depends entirely on the cause:

• Mydriasis caused by an examination eye drop or by contact with a substance resolves on its own within a few hours to a few days; no treatment is required.
• An Adie’s pupil is usually not treated; glasses or a near correction can ease the accommodation difficulty.
• Uveitis is treated with anti-inflammatory and dilating eye drops to prevent adhesions of the iris.
• An acute glaucoma attack requires emergency treatment to lower the pressure, followed by laser treatment (iridotomy).
• A Horner syndrome or a third-nerve palsy requires a neurological work-up that determines the course of action.

FAQ

Frequently asked questions

Scientific sources

1. Antonio-Santos AA, Santo RN, Eggenberger ER. Pharmacological testing of anisocoria. Expert Opin Pharmacother. 2005;6(12):2007-2013. PMID 16197355
2. Trobe JD. Third nerve palsy and the pupil. Footnotes to the rule. Arch Ophthalmol. 1988;106(5):601-602. PMID 3358724
3. Nguyen MTB, Zafar A, Fisch S, et al. Apraclonidine for the pharmacologic confirmation of acute Horner syndrome. J Neurol Sci. 2020;419:117179. PMID 33075593
4. Kanzaria HK, Brown DF, Nadel ES. Adie’s tonic pupil. West J Emerg Med. 2012;13(6):527-528. PMID 23359653
5. Khazaeni B, Khazaeni L. Acute Angle-Closure Glaucoma. StatPearls. Treasure Island (FL): StatPearls Publishing. PMID 28613607

Further reading

• Cataract: symptoms and surgery
• Diabetes and the retina — Paris 13
• Dr Moïse Tourabaly — Ophthalmologist

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Disclaimer

This article is intended for information. A personalised ophthalmological opinion remains essential for any decision concerning your vision.

This article is intended for general information and is not a substitute for a medical consultation. Assessing a pupillary abnormality requires an ophthalmological, and sometimes neurological, examination tailored to each situation. In the event of a dilated or constricted pupil of sudden onset, associated with pain, a red eye, a drooping eyelid or double vision, seek care without delay.