An Optometrist’s Guide to Pupils: APD Testing, Unequal Pupils, and What They Can Mean

The pupil exam is one of the fastest ways to screen the visual pathway, optic nerve, retina, brainstem pathways, and autonomic nervous system. A careful pupil check can reveal subtle but important findings such as a relative afferent pupillary defect, Horner syndrome, third nerve palsy, pharmacologic dilation, trauma, inflammation, or benign physiologic anisocoria.

For optometrists, pupils should not be treated as a quick “PERRLA” checkbox. They are a vital sign of the eye and nervous system.

Doctor and patient in eye clinic review a pupil exam guide on a monitor, with pupil clues charts, plants, and exam tools nearby.

Why Pupils Matter in an Eye Exam

The pupils respond to light through a complex pathway involving the retina, optic nerve, midbrain, parasympathetic system, sympathetic system, iris sphincter, and iris dilator muscles. Because of this, abnormal pupils can help localize disease.

A pupil problem may point toward:

• Optic nerve disease

• Significant asymmetric retinal disease

• Neurologic disease

• Third cranial nerve palsy

• Horner syndrome

• Medication or chemical exposure

• Eye trauma or prior eye surgery

• Uveitis or angle-closure glaucoma

• Benign physiologic anisocoria

A relative afferent pupillary defect, or RAPD/APD, is most commonly associated with asymmetric optic nerve disease, but it may also occur with severe asymmetric retinal disease.

Infographic titled Clinical Pupil Exam Basics showing five steps: pupil size, light response, near response, shape, and documentation.

Step 1: Observe Before You Shine the Light

Before testing reactivity, look at the patient naturally.

Ask yourself:

• Are the pupils equal in size?

• Is one pupil larger or smaller?

• Is there ptosis?

• Are the eyes aligned?

• Are extraocular movements full?

• Is there pain, diplopia, headache, trauma, or recent surgery?

• Is the anisocoria greater in light or in dark?

This first observation helps separate a potentially benign finding from something that needs urgent workup.

Infographic titled Anisocoria Workup shows 5 steps for unequal pupils, with eye icons, blue/pink panels, and myoptism.com.

Step 2: Check Pupil Size in Light and Dark

Unequal pupils are called anisocoria. The key clinical question is whether the anisocoria is greater in bright light or greater in darkness.

If anisocoria is worse in bright light

The larger pupil is usually the abnormal pupil because it is not constricting properly.

Possible causes include:

• Third nerve palsy

• Adie tonic pupil

• Pharmacologic dilation

• Traumatic mydriasis

• Iris sphincter damage

• Prior eye surgery

• Acute angle closure

• Severe inflammation or posterior synechiae

A third nerve palsy can cause a dilated poorly reactive pupil, ptosis, and abnormal eye movements. A pupil-involving third nerve palsy is concerning because a posterior communicating artery aneurysm is one possible life-threatening cause.

If anisocoria is worse in the dark

The smaller pupil is usually the abnormal pupil because it is not dilating properly.

Possible causes include:

• Horner syndrome

• Physiologic anisocoria

• Pharmacologic constriction, such as pilocarpine exposure

• Old trauma or iris abnormality

In Horner syndrome, the classic pattern is miosis, mild ptosis, and sometimes decreased sweating on the affected side. It results from disruption of the sympathetic pathway to the eye.

If anisocoria is similar in light and dark

This may be physiologic anisocoria, especially if it is longstanding, mild, and not associated with ptosis, diplopia, pain, trauma, or neurologic symptoms. Physiologic anisocoria is common, but it should be a diagnosis of exclusion after ruling out concerning causes.Step 3: How to Check for an APD

An APD, also called a relative afferent pupillary defect or Marcus Gunn pupil, is tested with the swinging flashlight test. It compares how well each eye detects light.

This is not primarily a test of pupil size. It is a test of the afferent visual pathway, meaning the retina and optic nerve input into the brain.

How to perform the swinging flashlight test

1. Dim the room lights.

2. Ask the patient to look at a distant target.

3. Use a bright, steady light.

4. Shine the light into one eye for about 2–3 seconds.

5. Quickly swing the light to the other eye.

6. Repeat several times.

7. Watch the direct and consensual response.

Normal response

Both pupils constrict similarly when light is directed into either eye.

APD response

When the light is moved from the normal eye to the affected eye, both pupils paradoxically dilate or constrict less. This means the affected eye is sending a weaker light signal to the brain.

Example:

• Light in right eye: both pupils constrict well.

• Light in left eye: both pupils dilate or constrict less.

• Interpretation: left APD.

Infographic titled RAPD: Swinging Flashlight Test with eye and flashlight icons, step-by-step instructions on detecting afferent defect.

What an APD Can Mean

An APD suggests asymmetric disease affecting the retina, optic nerve, or anterior visual pathway.

Common causes include:

Optic nerve causes

• Optic neuritis

• Ischemic optic neuropathy

• Compressive optic neuropathy

• Traumatic optic neuropathy

• Advanced asymmetric glaucoma

• Optic nerve tumor or infiltration

Retinal causes

• Large retinal detachment

• Central retinal artery occlusion

• Central retinal vein occlusion

• Severe asymmetric macular or retinal disease

Important clinical note

A patient can have blurry vision without an APD if the problem is refractive, corneal, lenticular, macular but mild/symmetric, or bilateral and symmetric. APD is most helpful when the disease is asymmetric between the two eyes.

Infographic on small vs large pupils with eye icons and 5 numbered clues for miosis, mydriasis, ptosis, and CN III.

Common Pupil Patterns and What They Suggest

1. Large pupil, poor reaction to light

Possible causes:

• Third nerve palsy

• Pharmacologic dilation

• Adie tonic pupil

• Traumatic iris damage

• Acute angle closure

• Iris sphincter injury

A large pupil with ptosis, double vision, eye movement restriction, or pain should be treated as urgent until proven otherwise.

2. Small pupil with mild ptosis

Possible causes:

• Horner syndrome

• Physiologic anisocoria

• Medication effect

• Prior surgery or trauma

Horner syndrome may be subtle. The affected pupil is often smaller and may show dilation lag in the dark. Because the sympathetic pathway travels from the brain through the neck and chest before reaching the eye, acquired Horner syndrome can occasionally reflect serious pathology.

Infographic titled Light-Near Dissociation from OPT-ISM Eye Care Blog, with eye icons and numbered steps about pupil responses.

3. Poor near response or light-near dissociation

Possible causes:

• Adie tonic pupil

• Dorsal midbrain syndrome

• Neurosyphilis/Argyll Robertson pupil

• Diabetes-related autonomic dysfunction

• Prior inflammation or trauma

Adie tonic pupil usually involves a larger pupil with poor light reaction and better near response. It is related to damage of the postganglionic parasympathetic fibers supplying the iris and ciliary body.

4. Irregular pupil shape

Possible causes:

• Iris trauma

• Posterior synechiae from uveitis

• Prior intraocular surgery

• Iris atrophy

• Angle-closure attack

• Congenital iris abnormality

Irregular pupils should prompt a slit-lamp exam to evaluate the iris, anterior chamber, inflammation, intraocular pressure, and surgical history.

5. Bilaterally small pupils

Possible causes:

• Opioid use

• Pilocarpine or miotic drops

• Pontine lesion

• Aging-related miosis

• Bright light exposure

• Certain systemic medications

Context matters. Bilateral pinpoint pupils with neurologic symptoms, altered mental status, or respiratory depression is very different from mild age-related miosis.

6. Bilaterally large pupils

Possible causes:

• Anticholinergic medication exposure

• Sympathomimetic drugs

• Pharmacologic dilation

• Trauma

• Severe neurologic injury

• Anxiety or adrenergic stimulation

Ask about recent eye drops, scopolamine patches, inhalers, nebulizers, motion sickness medication, plant exposure, and accidental chemical contact.

Infographic titled Medications & Neuro Red Flags about pupil changes, listing causes like drops, opiates, optic neuritis, and CN III palsy.

Red Flags: When Unequal Pupils Need Urgent Evaluation

Anisocoria is more concerning when it is new, symptomatic, or associated with neurologic findings.

Urgent referral or emergency evaluation should be considered with:

• New anisocoria with severe headache

• Eye pain or brow pain

• Double vision

• Ptosis

• Abnormal eye movements

• New neurologic symptoms

• Recent head, neck, or eye trauma

• Suspected third nerve palsy

• Suspected acute Horner syndrome

• Decreased vision with APD

• Acute angle-closure signs: pain, redness, nausea, halos, high IOP

A pupil-involving third nerve palsy is especially concerning because compressive lesions, including aneurysm, must be considered.

Practical Optometry Workflow for Pupils

A strong pupil exam can be documented in a consistent sequence:

1. Size

Record pupil size in light and dark.

Example:Pupils 4 mm light / 6 mm dark OD, 4 mm light / 6 mm dark OS

2. Shape

Round or irregular.

Example:Round OU or irregular OS secondary to prior trauma

3. Reactivity

Brisk, sluggish, fixed, or minimally reactive.

Example:Briskly reactive OU

4. APD

Always document presence or absence.

Example:No RAPDor1+ RAPD OS

5. Associated findings

Document:

• Ptosis

• EOM restriction

• Diplopia

• Pain

• Trauma

• Vision loss

• IOP

• Slit-lamp findings

• Optic nerve appearance

• Retinal findings

Infographic titled OPT-ISM Eye Care Blog: Classic Pupil Diagnoses, listing Horner’s syndrome, Adie’s tonic pupil, CN III palsy, RAPD.

Example Documentation

Normal pupil exam:Pupils equal, round, reactive to light, no RAPD. No anisocoria in light or dark. EOMs full. No ptosis.

Possible APD:Pupils round and reactive OU. 1+ RAPD OS by swinging flashlight test. Reduced VA OS with optic nerve pallor noted. Recommend further optic nerve workup with OCT RNFL/GCC, visual field, color vision, and neuro-ophthalmology referral as indicated.

Possible Horner pattern:Anisocoria greater in dark with smaller pupil OS and mild upper lid ptosis OS. Dilation lag suspected. No EOM restriction. Discussed concern for possible Horner syndrome and need for further evaluation depending on acuity and associated symptoms.

Possible third nerve palsy pattern:Dilated poorly reactive pupil OD with ptosis and limited adduction/elevation/depression OD. Patient reports new diplopia and headache. Urgent emergency evaluation recommended.

Testing Beyond the Flashlight

Depending on the findings, additional testing may include:

• Visual acuity

• Color vision

• Confrontation visual fields

• Automated visual field testing

• OCT RNFL and ganglion cell analysis

• Dilated retinal exam

• Optic nerve photos

• IOP measurement

• Slit-lamp evaluation for iris trauma, inflammation, or angle closure

• Extraocular motility testing

• Eyelid measurements

• Pharmacologic pupil testing when appropriate

• Neuroimaging or emergency referral when indicated

Patient-Friendly Explanation

Patients often worry when they notice one pupil looks bigger than the other. Sometimes this is harmless and longstanding. Other times, it can be a clue that the eye or nervous system needs closer evaluation.

A simple way to explain it:

“The pupils are controlled by both the eyes and the nervous system. When we check them, we are looking for signs that both eyes are sending equal light signals to the brain and that the nerves controlling pupil size are working properly.”

Final Thoughts

The pupil exam is quick, but it can reveal important disease. For optometrists, the key is to slow down, compare pupils in light and dark, perform a careful swinging flashlight test, and look for associated signs such as ptosis, diplopia, pain, trauma, optic nerve changes, or vision loss.

A normal pupil exam is reassuring. An abnormal pupil exam can be the first clue to optic nerve disease, retinal disease, Horner syndrome, third nerve palsy, medication exposure, trauma, or other serious conditions.

In short: pupils are small, but they can tell a big story.