Mydriasis is the dilation of the pupil caused by contraction of the iris dilator muscle. It occurs normally in response to low light, strong emotional stimuli, and pharmacologic mydriatics or cycloplegics, but may also result from ocular trauma, neurologic disorders, or systemic conditions that decrease consciousness. Certain drugs such as antihistamines and anticholinergics may cause mydriasis as an adverse effect.

Pathophysiology

Mydriasis — pupillary dilation — occurs when the iris dilator muscle contracts due to increased sympathetic activity or decreased parasympathetic input. Normally, the iris sphincter is controlled by parasympathetic fibers from the oculomotor nerve (cranial nerve III), mediating pupillary constriction (miosis), while the iris dilator is innervated by sympathetic fibers from the superior cervical ganglion, mediating dilation.

• Physiologic mydriasis: Low light, strong emotional stimuli → sympathetic activation → dilator contraction.

• Pharmacologic mydriasis: Anticholinergics, cycloplegics → parasympathetic blockade → unopposed dilation.

• Neurologic mydriasis: Lesions of cranial nerve III, Edinger–Westphal nucleus, or ciliary ganglion → loss of parasympathetic tone → unopposed dilation.

• Traumatic or surgical mydriasis: Damage to the sphincter muscle → inability to constrict the pupil.

Central lesions (brainstem, hypothalamus) and peripheral lesions (cranial nerve III, iris) can both result in mydriasis. Reflex pathways (direct and consensual light reflexes) are disrupted when parasympathetic innervation is impaired, allowing sympathetic-driven dilation to predominate. Unilateral versus bilateral mydriasis aids localization of underlying pathology.

History and Physical Examination

• History: Ask about other eye problems (pain, blurring, diplopia, visual field defects).Obtain a health history focusing on eye/head trauma, glaucoma, neurologic or vascular disorders, and drug use.

• Eye Examination:

  • Inspect pupil size, shape, and symmetry.

  • Test direct and consensual light reflexes, accommodation, and perform a swinging flashlight test for relative afferent pupillary defect (Marcus Gunn pupil).

  • Assess visual acuity with and without correction.

  • Evaluate extraocular movements through the six cardinal fields of gaze.

  • Check for ptosis, swelling, ecchymosis, and other ocular abnormalities.

Emergent Considerations

Mydriasis may signal acute angle-closure glaucoma or traumatic iridoplegia, both ocular emergencies requiring immediate attention.

Medical causes

Other Causes:

• Drugs: Anticholinergics, antihistamines, sympathomimetics, barbiturates (overdose), estrogens, tricyclic antidepressants, early anesthesia induction, topical mydriatics and cycloplegics (phenylephrine, atropine, homatropine, scopolamine, cyclopentolate, tropicamide).

• Surgery: Traumatic mydriasis from ocular surgery.

Special considerations

• Diagnostic workup may include ophthalmologic examination, neurologic evaluation, imaging (CT/MRI), and lab studies depending on suspected cause.

• Explain tests to the patient and discuss the effects of mydriatic drugs, including ways to minimize adverse effects.

• Monitor for emergent conditions such as acute angle-closure glaucoma.

Patient counseling

• Educate about drug-induced mydriasis, including expected duration and effects on vision.

• Advise protection against bright light exposure if pupils remain dilated.

• Discuss emergency symptoms, including severe eye pain or vision loss, and when to seek immediate care.

Pediatric pointers

• Mydriasis in children is often caused by ocular trauma, drug effects, Adie’s syndrome, or increased intracranial pressure (ICP).

• Early recognition is important to prevent vision loss or neurologic complications.

References

1. Conway, B. N., May, M. E., Signorello, L. B., & Blot, W. J. (2012). Mortality experience of a low-income population with young-onset diabetes. Diabetes Care, 35, 542–548.

2. Gerstenblith, A. T., & Rabinowitz, M. P. (2012). The Wills Eye Manual. Philadelphia, PA: Lippincott Williams & Wilkins.

3. Huang, J. J., & Gaudio, P. A. (2010). Ocular Inflammatory Disease and Uveitis Manual: Diagnosis and Treatment. Philadelphia, PA: Lippincott Williams & Wilkins.

4. Rhodes, E. T., Prosser, L. A., Hoerger, T. J., Lieu, T., Ludwig, D. S., & Laffel, L. M. (2012). Estimated morbidity and mortality in adolescents and young adults diagnosed with type 2 diabetes mellitus. Diabetic Medicine, 29, 453–463.

5. Roy, F. H. (2012). Ocular Differential Diagnosis. Clayton, Panama: Jaypee–Highlights Medical Publishers, Inc.