Unilateral mydriasis, a single dilated pupil while the other remains normal, can signal anything from a neurological emergency to harmless contact with a garden plant. The causes span a remarkably wide range: rising pressure inside the skull, a compressed nerve, a stray medication, direct eye trauma, or a benign episode tied to migraine. Because a blown pupil on one side is one of the classic early signs of brain herniation, clinicians treat it seriously until they can rule out the dangerous possibilities. Understanding what drives unilateral mydriasis and how doctors sort through the possibilities can help you make sense of what is otherwise a genuinely alarming symptom.
Brain Herniation and Rising Intracranial Pressure
The most feared cause of a single dilated pupil is uncal herniation, a condition in which swelling or bleeding inside the skull pushes part of the brain’s temporal lobe downward through a gap in the rigid membrane separating the brain’s compartments. As the tissue shifts, it compresses the oculomotor nerve (the third cranial nerve) on the same side. That nerve carries the parasympathetic fibers responsible for constricting the pupil, so compression knocks them out, and the pupil blows wide open. This pattern was first described in the 19th century, and the resulting finding is still sometimes called a “Hutchinson pupil.”1StatPearls Publishing. Uncal Herniation
The clinical picture unfolds in a predictable sequence: the pupil on the affected side dilates and stops reacting to light, the eyelid may droop, and weakness develops on the opposite side of the body as the brain’s motor pathways get squeezed against a rigid edge of dura. Left untreated, the compression progresses into the brainstem, leading to coma and death. This is why any patient with a new, fixed, dilated pupil and altered consciousness gets urgent brain imaging. A head injury, a stroke, or a ruptured aneurysm can all generate enough pressure to trigger this chain of events.
Posterior Communicating Artery Aneurysms
A posterior communicating artery (PcomA) aneurysm is one of the most clinically important vascular causes of unilateral mydriasis. This artery runs very close to the third cranial nerve as it exits the brainstem, so even a small aneurysm that bulges outward can press directly on the nerve and paralyze it. The typical presentation is a sudden drooping eyelid, an eye that drifts outward and downward, and a dilated pupil that won’t constrict to light, often accompanied by headache or pain around the eye. The absence of other neurological deficits besides headache is characteristic of an unruptured PcomA aneurysm compressing the nerve.2PubMed Central. Oculomotor Nerve Palsy Secondary to Posterior Communicating Artery Aneurysm: A Narrative Review and Proposed Treatment Algorithm
This matters because a PcomA aneurysm can rupture, causing a devastating subarachnoid hemorrhage. A dilated pupil in this context is therefore treated as a warning sign that demands immediate investigation, usually with CT angiography or conventional angiography. When caught before rupture and treated surgically (typically by clipping the aneurysm), recovery of the oculomotor nerve palsy has been well documented.3Journal of Neurosurgery. Oculomotor nerve palsy by posterior communicating artery aneurysms: influence of surgical strategy on recovery Timing matters: the sooner the compression is relieved, the better the chance the nerve bounces back.
Internal Carotid Artery Dissection
A tear in the wall of the internal carotid artery can occasionally produce a dilated pupil, though this is less common and can be diagnostically confusing. In one reported case, a 43-year-old woman with a history of headaches presented with a dilated pupil and sudden blurring of vision in one eye. She was initially treated for optic neuritis before imaging revealed an internal carotid artery dissection with widespread involvement of the same-side hemisphere.4PubMed Central. An Internal Carotid Artery Dissection Masquerading as Optic Neuritis: A Case Report
Carotid dissection is worth knowing about because it can masquerade as other conditions. It may produce a dilated pupil through direct compression of nearby nerve fibers, or it may cause a Horner syndrome (the opposite problem, a small pupil) depending on which part of the sympathetic chain is affected. The key takeaway is that sudden pupil asymmetry combined with headache, neck pain, or visual changes deserves vascular imaging, not just an eye exam.
Pharmacological Mydriasis from Plants
Not every blown pupil means something is wrong inside the brain. One of the most common benign explanations is accidental exposure to a plant or substance that paralyzes the pupil’s constricting muscle. This is colloquially known as “gardener’s pupil,” and it happens when tropane alkaloids found in certain plants come into contact with the eye. These alkaloids, including atropine, scopolamine, and hyoscyamine, block the receptors that normally tell the iris to constrict.5PubMed Central. Mydriasis due to Datura inoxia
Angel’s trumpet (Brugmansia) and thorn apple (Datura) are the usual culprits. You don’t need to drip sap directly into the eye; simple topical contact with the flowers can produce self-limited unilateral mydriasis that resolves on its own.6PubMed. Unilateral mydriasis due to Angel’s trumpet In one pediatric case, a boy developed anisocoria after spending a morning gardening around a trumpet plant. Further investigation confirmed that the tropane alkaloids in the Datura and Brugmansia species were responsible.7PubMed Central. An unusual case of anisocoria by vegetal intoxication: a case report
The challenge for clinicians is that a patient who shows up with one fixed, dilated pupil and doesn’t mention the gardening looks a lot like someone with a serious neurological problem. A careful history is the fastest way to avoid unnecessary imaging, though pharmacological testing can also help distinguish this from more dangerous causes.
Medications and Aerosol Exposures
Prescription drugs can produce the same effect as plant alkaloids, and accidental contamination is more common than you’d expect. Scopolamine transdermal patches, used for motion sickness, are a classic offender. If you touch the patch and then rub your eye, the scopolamine transfers to the conjunctiva and dilates that pupil. Case reports describe healthy people presenting to emergency departments with acute unilateral mydriasis from exactly this kind of inadvertent contact.8PubMed Central. Anisocoria secondary to inadvertent contact with scopolamine patch
The list of reported drug exposures extends beyond scopolamine patches to include bronchodilator nebulizers containing ipratropium bromide and even antiperspirant agents containing glycopyrrolate.9PubMed Central. Anisocoria after scopolamine transdermal patch contamination: A case report Ipratropium is especially relevant in hospital settings, where nebulized medications can drift toward the eyes if a face mask fits poorly. In one pediatric case involving a child in cardiac arrest who was being nebulized with ipratropium, the resulting unilateral mydriasis triggered prophylactic treatment for cerebral herniation before imaging showed no neurological abnormality at all; the mydriasis resolved on its own.10PubMed. Unilateral Mydriasis in Helicopter Emergency Medical Services: A Case of Ophthalmic Ipratropium Exposure in Pediatric Asthmatic Cardiac Arrest Prompting Cerebral Herniation Prophylaxis That case illustrates how high the stakes feel in the moment, and how easily a pharmacological cause can mimic a neurological emergency.
Adie’s Tonic Pupil
Adie’s tonic pupil is a condition in which one pupil becomes chronically dilated and reacts sluggishly, or not at all, to light, but constricts slowly and strongly when you focus on something close up. It results from damage to the ciliary ganglion, a small nerve cluster behind the eye that relays parasympathetic signals to the iris and the focusing muscle. Once those neurons are damaged, the pupil loses its normal light reflex but develops a peculiar hypersensitivity to acetylcholine, the neurotransmitter that normally drives constriction.
This hypersensitivity is actually what clinicians use to diagnose it. A very dilute concentration of pilocarpine (a drug that mimics acetylcholine) will cause a pupil with Adie’s to constrict while leaving a normal pupil unchanged. In one reported case, a man with a right pupil that failed to react to light but constricted strongly to a near target was diagnosed after dilute pilocarpine constricted only the affected eye.11BMJ Case Reports. Adie’s tonic pupil presenting with unilateral photophobia successfully treated with dilute pilocarpine Research has shown that using very dilute pilocarpine (0.0625%) can detect unilateral Adie’s tonic pupil with high accuracy.12Scientific Reports. Dilute pilocarpine test for diagnosis of Adie’s tonic pupil
The condition is generally benign, though the affected pupil tends to stay abnormal permanently. Some people notice glare and difficulty with near vision, which can be managed with low-dose pilocarpine drops. Viral infections, including herpes zoster ophthalmicus, can trigger Adie’s pupil by damaging the ciliary ganglion.13Indian Journal of Neurology. A Case of Isolated Adie’s Pupil in Herpes Zoster Ophthalmicus
Trauma and Surgery
A direct blow to the eye can tear the iris sphincter muscle, the tiny ring of muscle that constricts the pupil. When that muscle is damaged, the pupil stays wide open and may not react to light at all. In one documented case, an object struck a patient’s eye, causing a corneal laceration, traumatic mydriasis, and loss of the eye’s ability to focus. The pupil measured 7 mm in the injured eye compared to 4 mm in the unaffected eye, with no pupillary reactivity. Remarkably, in this particular case, both the mydriasis and the focusing dysfunction resolved over eight years, eventually returning to near-normal diameters and function.14PubMed Central. Resolution of traumatic mydriasis and accommodative dysfunction eight years after sweetgum ball ocular injury That kind of recovery is not typical, though. Most cases of traumatic mydriasis with iris sphincter tears leave the pupil permanently enlarged to some degree.
Intraocular surgery can produce a similar result. Urrets-Zavalia syndrome is a rare complication in which the pupil, dilated pharmacologically for the procedure, simply stays fixed and dilated afterward, refusing to respond to constricting drops. It has been reported after corneal transplants and lens implant procedures. In one case, a patient undergoing placement of a scleral-sutured lens implant was noted at the end of surgery to have a dilated pupil unresponsive to carbachol, and it remained fixed afterward.15PubMed Central. Urrets-Zavalia syndrome following placement of scleral-sutured intraocular lens The exact mechanism isn’t fully understood, but ischemia of the iris during surgery is thought to play a role.
Benign Episodic Unilateral Mydriasis and Tadpole Pupils
Some people experience episodes of unilateral mydriasis that come and go without any underlying structural problem. Benign episodic unilateral mydriasis is an isolated, intermittent dilation of one pupil that appears to reflect a temporary imbalance between the sympathetic and parasympathetic nervous systems. It tends to show a female predominance and often occurs in people with a personal or family history of migraine.16PubMed Central. Benign Episodic Mydriasis as a Cause of Isolated Anisocoria Some researchers consider it a limited form of ophthalmoplegic migraine.17PubMed Central. Migraine with benign episodic unilateral mydriasis
The diagnosis is made after extensive neurological evaluation comes back negative. In practice, this means a trip to the emergency department, brain imaging, and sometimes a lumbar puncture, all of which show nothing abnormal. Frustrating for the patient, but reassuring in the end.
A related oddity is the “tadpole-shaped pupil,” caused by a brief segmental spasm of the iris dilator muscle. Rather than the whole pupil dilating uniformly, one section of the iris pulls outward, creating a teardrop or tadpole shape. A study of 26 cases found that these episodes are extremely brief: less than five minutes in the vast majority, under two minutes in most, and under one minute in about a third. They can recur several times a day.18PubMed. Tadpole-shaped pupils caused by segmental spasm of the iris dilator muscle The phenomenon is benign and usually self-limiting, but it can be startling if you happen to catch it in the mirror.
How Clinicians Sort Through the Possibilities
Figuring out why one pupil is larger than the other follows a fairly systematic approach. The first question is whether the dilated pupil reacts to light. If it does, even sluggishly, many of the most dangerous causes become less likely. If it doesn’t react at all, clinicians need to determine whether the problem is in the nerve pathway (a third nerve palsy, herniation, or aneurysm), the iris itself (trauma or surgery), or the result of a drug blocking the muscle.
Pharmacological testing is a core part of this process. Different eye drops help narrow the diagnosis. Dilute pilocarpine constricts an Adie’s pupil but not a pharmacologically dilated one. Cocaine drops can help identify Horner syndrome (the opposite scenario, where one pupil is abnormally small) by testing whether the sympathetic pathway is intact.19PubMed. Pharmacological testing of anisocoria Newer work suggests that automated pupillometry can differentiate Horner syndrome from normal variation without requiring drops at all, by measuring how the pupils recover from light with high precision.20PubMed Central. Differentiation of Horner Syndrome and Physiological Anisocoria by Automated Pupillometry
In emergency settings, imaging takes priority. A CT scan or CT angiogram can quickly identify a hemorrhage, a mass, or an aneurysm. If the imaging is clean and the history points toward a drug exposure, the clinical picture becomes much less urgent, and clinicians can afford to observe.
Quantitative Pupillometry in Critical Care
In intensive care units, where patients may be sedated and unable to report symptoms, detecting subtle pupil changes early can be the difference between catching rising intracranial pressure and missing it. Automated quantitative pupillometry uses a handheld digital device to measure pupil size and reactivity with far greater precision than a penlight and a clinician’s subjective assessment. These devices can pick up minimal changes in pupil size, prolonged latency in the light response, or subtle asymmetry that wouldn’t be visible to the naked eye in a dimly lit ICU room.
Research into smartphone-based pupillometry is pushing the technology further. One system, called PupilScreen, uses a smartphone’s flash to stimulate the eye and its camera to record the response, then processes the video through neural networks to track pupil diameter over time and derive clinically relevant measurements.21Proceedings of the ACM on Interactive, Mobile, Wearable and Ubiquitous Technologies. PupilScreen The goal is to put the accuracy of a clinical pupillometry device into the hands of first responders, athletic trainers, or field medics who need to make quick assessments of possible traumatic brain injury without specialized equipment. The technology is still in development, but it represents a shift toward making pupil assessment more objective and accessible outside of hospitals.
Physiological Anisocoria and When Not to Worry
Roughly one in five people has a slight natural difference in pupil size, called physiological anisocoria, where one pupil is a fraction of a millimeter larger than the other. The asymmetry stays consistent in light and dark conditions, both pupils react normally to light, and there are no other symptoms. This is completely normal and requires no treatment. It tends to be stable over time, and most people never notice it until someone points it out or a clinician measures it during an exam.
The practical distinction between physiological anisocoria and pathological unilateral mydriasis comes down to a few features. In physiological anisocoria, the difference is usually small (around 1 mm or less), it doesn’t change dramatically with lighting, both pupils react briskly, and there is no ptosis, eye-movement limitation, or pain. If any of those additional findings are present, the asymmetry stops being “just a normal variant” and starts demanding evaluation. Old photographs can be surprisingly useful: if someone’s pupils have always been slightly unequal, that baseline information can spare them an expensive and anxiety-provoking workup.
Why History Matters More Than You’d Think
The single most powerful diagnostic tool in unilateral mydriasis is a detailed patient history, and it is frequently the thing that gets shortchanged in a busy emergency department. Was the patient gardening? Wearing a motion-sickness patch? Using a nebulizer? Hit in the eye? Experiencing a headache? Each of those answers points down a completely different diagnostic pathway. The physical exam and pharmacological testing narrow things further, but the history often gives clinicians their first and best hypothesis.
This is also where patient awareness can genuinely help. If you notice one of your pupils is larger than the other, pay attention to context. Did it coincide with handling plants, applying medication, or the onset of a headache? Is there pain, blurry vision, drooping of the eyelid, or double vision? Those additional symptoms change the urgency dramatically. A dilated pupil with a drooping lid and double vision after a sudden headache warrants an immediate trip to the emergency room. A dilated pupil that appeared after you transplanted Brugmansia in the garden is almost certainly going to resolve on its own within a day or two, though having a clinician confirm that is still worthwhile if you’re uncertain. In either case, the story surrounding the pupil is often more informative than the pupil itself.