Impaired pupillary response: what is important to know

Pupillary reflexes are automatic changes in pupil diameter in response to light, darkness, and shifting gaze to a nearby object. The light reflex begins in the retina, travels along the optic nerve and midbrain, and then, via the parasympathetic fibers of the third pair of cranial nerves, reaches the iris sphincter. Therefore, abnormal pupillary reflexes are not a disease in themselves, but a sign of a malfunction somewhere along this pathway: from the retina and optic nerve to the sympathetic and parasympathetic fibers, the iris, and the oculomotor nerve.

In a healthy person, both pupils are usually round, nearly equal in size, and quickly constrict in bright light and dilate in darkness. When looking at a nearby object, the pupils should also constrict. For clinical evaluation, not only the absolute pupil size is important, but also the response speed, symmetry of the reaction, the shape of the pupillary edge, and the response in different lighting conditions.

A slight difference between the pupils isn't always dangerous in itself. Some people experience physiological anisocoria, a stable, slight difference in size without other symptoms, ptosis, double vision, or visual impairment. This difference can be noticeable for years and only accidentally discovered during an examination or in old photographs. [1]

The key principle of primary localization is very simple. If the difference between the pupils is more noticeable in the dark, the smaller pupil, which dilates poorly, is usually pathological, and then the physician first considers sympathetic damage, such as Horner's syndrome. If the difference is greater in the light, the larger pupil, which constricts poorly, is more often pathological, and then parasympathetic causes are considered, such as damage to the third pair of cranial nerves, Adie's pupil, mechanical damage to the iris, or drug-induced mydriasis. [2]

The shape of the pupil is also very important. An uneven, jagged, oval, or deformed pupil suggests iris trauma, post-inflammatory adhesions, surgical changes, or other mechanical causes. In such cases, the reaction disorder may be less neurological and more ophthalmological. This is why pupil assessment without examining the anterior segment of the eye is often incomplete.

Table 1. What in the pupillary reaction is most often considered normal and what is considered abnormal

Sign	Closer to normal	Looks more like a pathology
Pupil size	Almost the same, the difference is minimal	A noticeable new or increasing difference
Reaction to light	Fast and symmetrical	Slow, sluggish, or absent
Reaction to a nearby object	Saved on both sides	Weak, asymmetrical or dissociated
Behavior in the dark	Both pupils dilate similarly.	One pupil lags behind in dilation
Form	Round	Uneven, oval, deformed
Associated symptoms	No pain, ptosis, double vision	There is pain, ptosis, diplopia, decreased vision

Sources for the table.

The main types of pupillary response disorders

The first major type is afferent pupillary defects. The most well-known variant here is the relative afferent pupillary defect, or Marcus Gunn pupil. In this case, the problem lies not in the iris muscles, but in the incoming visual signal, most often from the optic nerve or retina. Typically, anisocoria may not be present at rest: the pupils are equal in size, but their behavior in the pendulum test becomes abnormal. [3]

A relative afferent pupillary defect is particularly important because it often indicates asymmetric damage to the optic nerve or retina. Causes include optic neuritis, ischemic optic neuropathy, severe retinal lesions, asymmetric glaucoma, and other conditions. This defect does not explain resting anisocoria, but it may be the earliest clue to serious visual pathway pathology. [4]

The second major type involves sympathetic pathway disorders. A classic example is Horner's syndrome, characterized by miosis, mild ptosis, and sometimes decreased sweating on the same side. Anisocoria is particularly noticeable in the dark because the affected small pupil dilates poorly. Sometimes the physician notices a so-called delayed dilation, where the pupil lags behind in the first few seconds after turning off the light. [5]

The third type is parasympathetic disorders, in which a large pupil is usually abnormal. The most dangerous variant is damage to the third pair of cranial nerves. This can be accompanied by a large, sluggish pupil, pronounced ptosis, limited eye movement, and double vision. The most worrisome cause is nerve compression by an aneurysm of the posterior communicating artery. This is why a large pupil, along with ptosis and ophthalmoparesis, is considered a potential neurovascular emergency. [6]

The fourth type is the tonic Adie pupil. This is typically a unilaterally dilated pupil with a poor light reactivity, but a more intact and slow-tonic response to a near object. Some patients also have decreased tendon reflexes. This condition is often benign, but is clinically important because it mimics dangerous causes of severe anisocoria, especially on initial examination. [7]

The fifth type includes drug-induced, mechanical, and rare neurological causes. Pharmacological anisocoria can be caused by atropine, tropicamide, scopolamine, some inhalers, antihyperhidrosis medications, and other drugs accidentally injected into the eye. Mechanical mydriasis occurs with trauma and rupture of the iris sphincter. A separate condition is the Argyll Robertson pupil: small, bilateral pupils that react poorly to light but retain a near response, classically associated with late forms of syphilis. [8]

Table 2. Main types of disorders and their clinical clues

Option	What does it look like?	Where is the source of the problem most often?
Relative afferent pupillary defect	No mandatory anisocoria at rest, pathological pendulum test	Retina or optic nerve
Horner's syndrome	Small pupil, slight ptosis, anisocoria more in the dark	Sympathetic pathway
Damage to the 3rd pair of cranial nerves	Large pupil, ptosis, diplopia, impaired eye movement	Parasympathetic fibers and the oculomotor nerve
Adi's tonic pupil	Large pupil, weak light response, better near response	Lesion of the ciliary ganglion and parasympathetic arc
Pharmacological mydriasis or miosis	Association with drugs or chemicals	Superficial action on the iris muscles
Traumatic or mechanical anisocoria	Uneven or deformed pupil	Iris and anterior segment of the eye
Argyll Robertson's pupil	Small pupils, poor response to light, response to near vision preserved	Damage to midline structures, classically seen in neurosyphilis

Sources for the table. [9]

When pupillary dysfunction is dangerous and requires immediate assistance

The most important red flag is the sudden onset of a large pupil, accompanied by ptosis, diplopia, and headache. This may indicate damage to the third cranial nerve due to compression by the aneurysm. This is a potentially life-threatening condition, as the aneurysm can rupture, causing a subarachnoid hemorrhage. This combination of symptoms requires urgent neuroimaging, usually computed tomography angiography or magnetic resonance angiography. [10]

The second critical scenario is a small pupil with mild ptosis accompanied by pain in the neck, face, or around the eye. This combination suggests Horner's syndrome due to dissection of the internal carotid artery. This condition is important to recognize quickly, as dissection can cause ischemic stroke. Pain in new Horner's syndrome is not a detail, but an important vascular alarm signal. [11]

The third dangerous variant is an acute, painful, red eye with blurred vision, rainbow-like halos around lights, nausea, vomiting, and a moderately dilated, poorly reactive pupil. This presentation is characteristic of acute angle closure. This is an ophthalmologic emergency in which intraocular pressure rapidly increases, and without prompt treatment, irreversible damage to the optic nerve and vision loss are possible. [12]

The fourth alarming scenario is any eye injury resulting in an irregularly shaped pupil, severe photophobia, blood in the anterior chamber, decreased vision, or severe pain. Traumatic mydriasis is often associated with a rupture of the iris sphincter and can be associated with other injuries to the anterior segment, the angle of the anterior chamber, the lens, and even glaucoma at a later stage. A chemical burn of the eye is also considered an absolute emergency, regardless of the pupil's shape. [13]

A fifth possibility that should not be ignored is marked visual impairment or pain behind the eye, combined with a relative afferent pupillary defect. Although this defect does not appear as dramatic as severe anisocoria, it may indicate optic neuritis, ischemic optic neuropathy, or severe retinal damage. This is not always a reason to call an ambulance, but it is almost always a reason for a very prompt ophthalmological or neurological examination. [14]

Table 3. Red flags for abnormal pupillary responses

Situation	What to suspect first	Why is this urgent?
Large pupil, ptosis, double vision, headache	Compression lesion of the 3rd pair of cranial nerves, including aneurysm	Risk of intracranial hemorrhage
Small pupil, mild ptosis, pain in the neck or face	Horner's syndrome in carotid artery dissection	Risk of stroke
Red, painful eye, rainbow-colored circles, nausea, moderately dilated pupil	Acute angle closure	Risk of rapid irreversible vision loss
Eye injury and new uneven pupil	Rupture of the iris sphincter, hyphema, angular injury	Risk of permanent visual impairment
Marked visual impairment and relative afferent pupillary defect	Optic neuritis, ischemia, retinal damage	A quick diagnosis of the cause is needed.

Sources for the table. [15]

Diagnostics

The first step is a thorough medical history. The doctor will determine when exactly the pupil changed, whether it was sudden or gradual, whether there are old photographs of the same change, and whether any neck pain, headache, double vision, nausea, redness, decreased vision, trauma, eye surgery, inhalers, eye drops, scopolamine patches, or other medications have developed. At this stage, it's already possible to narrow down the possible causes.

The second step is examination in bright light and in the dark. This is one of the most valuable and, at the same time, the simplest techniques. It helps determine which pupil is abnormal: a small, non-dilating one, or a large, non-constricting one. The pupil's shape, eyelid position, eye movements, reflexes to near objects, and the presence of accompanying signs, such as ptosis or ophthalmoparesis, are simultaneously assessed.

The third step involves specialized functional tests. If an afferent defect is suspected, a pendulum test is performed, in which light is rapidly shifted from one eye to the other. For the Adie pupil, a test with diluted pilocarpine may be used, and if Horner's syndrome is suspected, pharmacological tests with apraclonidine or cocaine are used in specialized practice. Although, in the case of acute painful Horner's syndrome, modern logic increasingly dictates not to delay visualization for the sake of pharmacological localization. [16]

The fourth step is a full ophthalmological examination. This includes visual acuity, color vision, anterior segment examination with a slit lamp, fundus examination, and, if indicated, tonometry. A slit lamp is especially important if a mechanical or traumatic cause is suspected, and tonometry is necessary if there is a painful red eye and acute angle closure is suspected. [17]

The fifth step is targeted imaging and additional studies. If an aneurysm or other lesion of the third cranial nerve is suspected, urgent vascular neuroimaging is required. In the case of new Horner's syndrome with neck pain, visualization of the carotid artery is required. If optic nerve or retinal damage is suspected, ophthalmoscopy, optical coherence tomography, visual field imaging, and, if indicated, magnetic resonance imaging are used. In neurocritical care and certain complex cases, quantitative pupillometry is increasingly being used: it is more objective than subjective assessment and better captures subtle changes, but it does not yet replace a full clinical examination. [18]

Table 4. Step-by-step examination algorithm

Stage	What does a doctor do?	What does this give?
Anamnesis	Specifies onset, pain, diplopia, medications, trauma, old photos	Helps to differentiate an acute dangerous condition from a chronic condition
Inspection in light and in the dark	Looks when anisocoria is stronger	Helps to understand whether a small or large pupil is pathological
Reaction to a nearby object	Tests light and near reaction	Reveals dissociation of light and nearness
Pendulum test	Compares afferent input from both eyes	Reveals relative afferent pupillary defect
Slit lamp and tonometry	Examines the iris, anterior chamber, measures pressure	Helps to find trauma, adhesions, acute angle closure
Neuroimaging	Performs vascular and neurological examinations as indicated	Rules out aneurysm, dissection, stroke, tumor
Quantitative pupillometry	Objectively measures pupil dynamics	Increases reproducibility and sensitivity of observation

Sources for the table.

Treatment: Why there is no one-size-fits-all approach

Treatment for pupillary dysfunction always depends on the underlying cause. It is impossible to treat physiological anisocoria, acute angle closure, Horner's syndrome, Adie pupil, and damage to the third pair of cranial nerves in the same way. Therefore, the main principle of modern tactics is to first localize the problem and exclude dangerous causes, and only then decide on symptomatic or causal therapy. [19]

Physiological anisocoria requires no treatment. If the difference is small and stable, there is no pain, ptosis, double vision, or decreased vision, and old photographs confirm the long-standing nature of the condition, observation and proper documentation of the diagnosis are sufficient. This condition itself does not damage the eye or lead to vision loss. [20]

In Horner's syndrome, the treatment is not for the small pupil itself, but for the underlying sympathetic pathway damage. In one case, this may be a vascular dissection, in another, a tumor at the apex of the lung, or in a third, postoperative or traumatic injury. If Horner's syndrome is acute and accompanied by neck or facial pain, urgent vascular diagnostics are the priority, rather than long-term outpatient observation. [21]

In cases of damage to the third pair of cranial nerves involving the pupil, treatment is determined by the results of urgent neuroimaging. If an aneurysm is detected, emergency neurosurgical or endovascular treatment is required. If a microvascular cause is reliably excluded and compression is reliably ruled out, observation, monitoring of vascular risk factors, and dynamic evaluation are possible. However, until dangerous compression is excluded, such a presentation cannot be considered "benign." [22]

With Adie's pupil, treatment is often not required, especially if symptoms are minimal. Reading glasses may help in some patients, and weak cholinergic eye drops are sometimes used for severe photophobia or accommodation disorders. With pharmacological anisocoria, the primary step is to discontinue or stop contact with the substance and observe until recovery. With traumatic mydriasis and other mechanical causes, the consequences of the injury, inflammation, intraocular pressure, and iris defects are treated. Acute angle closure, on the other hand, requires immediate reduction of intraocular pressure and urgent ophthalmological care, as delay can be dangerous to vision. [23]

Table 5. Treatment for the established cause

Cause	What do they usually do?
Physiological anisocoria	Observation, comparison with old photographs, no treatment required
Relative afferent pupillary defect	They look for and treat diseases of the optic nerve or retina
Horner's syndrome	Urgently look for the cause, especially if there is pain in the neck or face
Damage to the 3rd pair of cranial nerves	Urgent neuroimaging and treatment of the cause, including aneurysm
Adi's tonic pupil	Frequent observation, sometimes symptomatic drops and reading glasses
Pharmacological anisocoria	Substance withdrawal and observation
Traumatic or mechanical cause	Examination of the anterior segment, treatment of injury and complications
Acute angle closure	Emergency ophthalmologic treatment to reduce intraocular pressure

Sources for the table. [24]

Prognosis, observation, and what is important for the patient to remember

The prognosis for abnormal pupillary responses is highly variable. In some patients, it is a benign physiological feature or a tonic Adie pupil, which has little impact on long-term outcome. In others, abnormal pupillary responses become an early external marker of aneurysm, carotid artery dissection, acute angle closure, or severe optic nerve damage. Therefore, both calmness in the presence of benign findings and a rapid response to red flags are equally important. [25]

In cases of safe and stable causes, the goal of observation is to ensure there is no progression and to properly document the finding. Old photographs are very useful here: if the pupillary difference was the same many years ago and the person had no vision complaints, this greatly reduces the likelihood of a new, dangerous cause. This is why neuro-ophthalmologists often request old close-up facial photographs. [26]

In cases of optic nerve damage, vascular causes, and acute elevation of intraocular pressure, delayed diagnosis can cost vision and sometimes even life. Untreated acute angle closure threatens irreversible damage to the optic nerve. Carotid artery dissection carries the risk of stroke. An aneurysm due to painful damage to the third pair of cranial nerves carries the risk of intracranial hemorrhage. The pupil is a small structure, but in clinical terms, it often becomes a quick external indicator of a larger problem. [27]

It's important for patients not to attempt to self-diagnose based solely on pupil size. The same external symptom can have completely different causes, ranging from accidental eye contact with scopolamine or an inhaler to an aneurysm. Therefore, the correct sequence is to assess whether there is pain, ptosis, double vision, red eye, nausea, decreased vision, trauma, neck or facial pain, and then decide on the urgency of treatment based on these factors. [28]

The main practical conclusion is that abnormal pupillary responses are not a single diagnosis, but a whole group of clinical scenarios. The most dangerous of these are recognized by the combination of a new symptom with pain, ptosis, diplopia, decreased vision, red eye, or neck pain. The sooner the case is determined whether it belongs to the afferent, sympathetic, parasympathetic, mechanical, or pharmacological group, the more accurate and safer the treatment will be. [29]

Table 6. What is more often urgent and what is more often not urgent

Situation	More often urgently	You can have more routine examinations
New large pupil with ptosis and diplopia	Yes	No
Small pupil with pain in the neck or face	Yes	No
Red, painful eye and blurred vision	Yes	No
New uneven pupil after injury	Yes	No
Long-standing slight difference without symptoms	No	Yes
Suspected Adi's pupil without pain and ophthalmoparesis	Usually no	Yes
Drug or chemical association without other symptoms	Most often no, but requires contextual assessment.	Yes
Relative afferent pupillary defect without general urgency	It's not always an emergency, but a quick examination is needed.	Conditionally, without delay

Sources for the table. [30]

FAQ

Can a slight difference in pupil size be normal?
Yes. Mild, stable anisocoria without pain, ptosis, double vision, or decreased vision may be a physiological feature and may not require treatment. [31]

If your pupils are different sizes, is it always a neurological problem?
No. The cause can be neurological, ophthalmological, mechanical, traumatic, or drug-related. Sometimes the problem isn't in the brain at all, but in the iris or accidental drug ingestion. [32]

What does it mean if the difference is greater in the dark?
This often indicates a small, poorly dilated pupil, and Horner's syndrome and other sympathetic pathway disorders are then considered. [33]

What does it mean if the difference is greater in the light?
This usually indicates a large, poorly constricting pupil, in which case the diagnosis is Adie's pupil, damage to the 3rd pair of cranial nerves, mechanical mydriasis, or drug exposure. [34]

Why can pupils be equal in size when the optic nerve is damaged?
Because the relative afferent pupillary defect is not related to the iris muscles, but to a disruption in the incoming signal from the eye to the midbrain. While the pupils may be equal in size at rest, the problem only manifests itself in the pendulum test. [35]

When is urgent neuroimaging needed for a large pupil?
When a large pupil appears acutely and is accompanied by ptosis, double vision, limited eye movement, or severe headache. This presentation requires the exclusion of an aneurysm or other compression of the third cranial nerve. [36]

Can an inhaler or patch cause anisocoria?
Yes. Unilateral pharmacological anisocoria has been described with ocular contact with scopolamine, some bronchodilators, and other drugs. Therefore, a history of medications and household exposures is very important. [37]

What is Adie's pupil and is it dangerous?
It is a tonic, dilated pupil with a poor reaction to light and a more preserved reaction to near vision. In most cases, the condition is benign, but in the early stages, it is important to distinguish it from more serious causes of severe anisocoria. [38]

Why might trauma cause the pupil to become irregular?
Because blunt or penetrating trauma can damage the iris sphincter, causing tears at the pupillary margin, iridodialysis, and other changes in the anterior segment. Such conditions require an in-person examination. [39]

Can pupillary abnormalities indicate syphilis?
Yes, but this is a rare and specific condition. An Argyll Robertson pupil is classically associated with late syphilis and appears as small pupils that are poorly reactive to light but with a normal reaction to near objects. [40]

Key points from experts

1. Different pupils are a symptom, not a diagnosis; it is first necessary to determine which pupil is pathological and under what conditions the difference becomes more noticeable. [41]

2. A new large pupil together with ptosis and diplopia should be considered as a potential vascular urgency until aneurysm is excluded.[42]

3. A new small pupil with neck or facial pain requires exclusion of carotid artery dissection.[43]

4. Relative afferent pupillary defect is not necessarily accompanied by anisocoria at rest, but often indicates an important lesion of the optic nerve or retina.[44]

5. An uneven pupil shape more often indicates a local mechanical or traumatic cause, rather than a purely neurological one. [45]

6. Adie's tonic pupil is usually benign, but it must be distinguished from 3rd cranial nerve involvement and drug-induced mydriasis.[46]

7. Quantitative pupillometry makes the assessment of pupillary responses more objective, but does not replace a full ophthalmological and neurological examination. [47]

8. The most common practical mistake is to be reassured simply because a person sees anisocoria without severe pain; not only pain is important, but also the presence of ptosis, diplopia, neck pain, red eye and decreased vision. [48]