Causes of Unilateral Mydriasis
Unilateral mydriasis most commonly results from inadvertent topical exposure to anticholinergic or sympathomimetic agents, though serious neurological causes must be excluded in appropriate clinical contexts.
Iatrogenic and Pharmacological Causes
Anticholinergic Medications
- Ipratropium bromide inhalation is a well-documented cause when the medication leaks from nebulizer masks and makes direct contact with the eye, particularly in patients with facial weakness or poor mask fit 1, 2
- The mydriasis typically resolves within 24-48 hours after discontinuing the offending agent 1
- This occurs through muscarinic receptor antagonism causing local pupillary dilation without systemic effects 1
Topical Contamination
- Phenylephrine/lidocaine spray used for airway anesthesia can inadvertently contact the eye during procedures, causing unilateral mydriasis 3
- Parasympatholytic agents including atropine and scopolamine from various sources can produce mydriasis through direct ocular exposure 4
- Antihistamines with antimuscarinic properties may cause pupillary dilation when topically contaminated 4
Plant Exposures
- Angel's trumpet (Brugmansia) and other Datura species contain tropane alkaloids that cause mydriasis through simple topical contact with flowers, without requiring plant sap instillation 5
- The anisocoria typically resolves within 48 hours after exposure 5
- These plants have glandular hairs rich in parasympatholytic alkaloids that can transfer to the eye through handling 5
Ophthalmological Causes
Acute Angle-Closure Crisis (AACC)
- Mid-dilated, asymmetric, or oval pupil is characteristic during or following an acute angle-closure attack 6
- Pupillary reactivity is typically poor during AACC or may become nonreactive following the event 6
- Associated findings include pressure-induced corneal edema, conjunctival hyperemia, eye pain, and markedly elevated intraocular pressure 6
- The pupil becomes unresponsive to miotic therapy when IOP is markedly elevated due to pressure-induced ischemia of the pupillary sphincter 6
Iris Abnormalities
- Diffuse or focal iris atrophy, posterior synechiae, and irregular pupil shape may cause unilateral mydriasis in the context of angle-closure disease 6
Neurological Causes
Uncal Herniation
- Fixed and dilated pupil with associated mental status changes, ptosis, and extraocular movement impairment suggests uncal herniation in patients with intracranial hemorrhage or tumor 2
- This represents a neurological emergency requiring immediate intervention 2
- Distinguished from benign causes by the presence of progressive neurological deterioration and laterality consistent with the intracranial pathology 2
Other Neurological Considerations
- Stellate ganglion involvement, impaired venous return from head and neck, or acute intracranial mass lesions may produce unilateral mydriasis 3
- Direct eye trauma during procedures or pre-existing surgical conditions should be considered 3
Clinical Approach
Key Distinguishing Features
- Isolated mydriasis without other neurological signs strongly suggests iatrogenic or topical exposure, particularly with recent medication use or plant contact 1, 5
- Mydriasis with systemic symptoms (pain, nausea, blurred vision, halos) points toward acute angle-closure crisis 6
- Mydriasis with progressive neurological deterioration requires urgent neuroimaging to exclude herniation 2
Critical Pitfall to Avoid
- In intensive care patients with facial weakness receiving nebulized ipratropium, unilateral mydriasis on the side of weakness is likely benign medication effect rather than herniation, distinguished by lack of mental status change and absence of ptosis 2
- A detailed exposure history prevents unnecessary and expensive diagnostic testing, particularly in otherwise healthy children with isolated anisocoria 5