Unilateral mydriasis is defined by a significant difference in pupil size, where one pupil is noticeably larger than the other. This condition of unequal pupils is technically known as anisocoria, while mydriasis specifically refers to pupil dilation. Although pupil diameter naturally changes based on ambient light, a persistently dilated pupil in one eye represents an abnormality in the control system. A fixed, unilaterally dilated pupil that does not react to light is concerning and indicates a disruption of the neurological pathways governing the eye. This asymmetry is a sign requiring immediate investigation to determine the specific cause.
How Pupil Size is Controlled
The pupil’s diameter is managed by two opposing muscles within the iris. The iris sphincter muscle constricts the pupil (miosis), while the iris dilator muscle widens the pupil (mydriasis). These muscles are regulated by the autonomic nervous system.
Pupillary constriction is commanded by the parasympathetic division, with nerve fibers traveling along the oculomotor nerve (Cranial Nerve III). This system acts on the sphincter muscle to reduce light entry, such as in bright conditions.
The opposing action of dilation is controlled by the sympathetic nervous system. This system stimulates the dilator muscle to allow more light to enter the eye, occurring in dim environments or during a stress response. Unilateral mydriasis results when one of these systems is interrupted or overstimulated on only one side.
The Range of Potential Causes
Causes of unilateral mydriasis range from localized issues to severe, life-threatening neurological events, making a thorough evaluation necessary. The underlying problem involves either a failure of the parasympathetic system to constrict the pupil or an overstimulation of the sympathetic system to dilate it.
Pharmacological and Accidental Exposure
A frequent cause of a dilated pupil in one eye is accidental exposure to certain medications or plant toxins. These substances contain anticholinergic agents, which block the parasympathetic neurotransmitter acetylcholine, paralyzing the iris sphincter muscle. This mydriasis is unilateral because the agent was applied only to one eye, perhaps from rubbing the eye after handling a patch or medication.
Examples include the accidental transfer of dilating eye drops (like atropine or scopolamine) or exposure to nebulized medications (like ipratropium bromide). Certain garden plants, such as Datura or Brugmansia species, contain powerful anticholinergic alkaloids that cause prolonged unilateral dilation if sap is transferred to the eye. Pharmacological mydriasis results in a large pupil, often 7 to 8 millimeters or greater, with a sluggish or absent light reflex.
Ocular and Traumatic
Direct physical damage to the eye causes unilateral mydriasis, known as traumatic mydriasis. A blunt force injury, such as from a ball or a fist, can directly damage the iris sphincter muscle fibers. When the constricting muscle is torn or paralyzed, the pupil remains dilated regardless of light exposure.
Following trauma, the pupil may appear irregular or oval, and the degree of non-reactivity depends on the injury extent. This injury is usually isolated, meaning it does not involve neurological symptoms like a drooping eyelid or double vision. Another ocular cause is Adie’s tonic pupil, a benign condition caused by damage to the ciliary ganglion. This results in a pupil that is larger than the other and reacts very slowly, or tonically, to light.
Neurological and Systemic
The most urgent cause involves the neurological pathway of the parasympathetic system. The fibers responsible for pupil constriction travel on the outside surface of the oculomotor nerve (Cranial Nerve III). Compression of this nerve, particularly where it exits the brainstem, can selectively damage the pupillary fibers, leading to a dilated pupil.
The most concerning cause of compression is an expanding mass lesion, such as a posterior communicating artery aneurysm or a tumor. Unilateral mydriasis in this context is a sign of impending brain herniation due to increased intracranial pressure, which is a medical emergency. If the mydriasis is due to Cranial Nerve III palsy from compression, it is frequently accompanied by a drooping eyelid (ptosis) and impaired eye movement.
Determining the Underlying Condition
Medical professionals follow a systematic process to determine the specific cause of unilateral mydriasis, as treatment depends on the correct diagnosis. The first step is a detailed patient history, focusing on recent trauma, exposure to chemicals or plants, and the use of eye drops, patches, or nebulizer medications. The examination focuses on pupil size in light and dark conditions, the presence of ptosis, and the ability to move the eye.
Pharmacological testing, most commonly using pilocarpine eye drops, helps differentiate causes. If the dilated pupil constricts significantly after applying a very dilute pilocarpine solution (0.125%), it suggests Adie’s tonic pupil due to the sphincter muscle becoming hypersensitive. If the pupil fails to constrict even after a stronger concentration (1% or 2%), it suggests pharmacological mydriasis, where the drug has paralyzed the muscle.
If a neurological cause like Cranial Nerve III palsy is suspected, especially with eye movement deficits or a drooping eyelid, imaging studies are immediately pursued. A CT scan or MRI is necessary to look for a mass, such as an aneurysm or tumor, that could be compressing the nerve and requiring urgent neurosurgical intervention.
Immediate Action and Medical Triage
The sudden onset of unilateral mydriasis requires prompt attention due to the potential for a serious neurological issue. If the dilated pupil appears suddenly and is accompanied by symptoms like a severe headache, eye pain, double vision, or a drooping eyelid, immediate emergency medical attention is necessary. These combined symptoms suggest compression of the oculomotor nerve, potentially indicating an expanding aneurysm or brain bleed.
If the mydriasis is isolated, meaning there are no other symptoms, the urgency is slightly less, but medical consultation is still required. When there is a clear history of exposure to eye drops, a scopolamine patch, or a nebulizer, the cause is likely benign and temporary, but a doctor must confirm this finding.