The Diagnostic Power of the Pupil: A Structured Bedside Approach to Identifying Neurologic Emergencies in Acute Care

Pupillary abnormalities are clinically significant findings in emergency and urgent care settings, where subtle changes in size or reactivity may represent either benign physiologic variation or life-threatening neurologic disease. Because the pupillary light reflex integrates afferent visual pathways, midbrain processing, and autonomic efferent function, careful bedside examination provides rapid insight into brainstem and cranial nerve integrity. This review synthesizes the relevant neuroanatomy, pathophysiology, and examination techniques necessary to evaluate common abnormalities encountered in acute care, including anisocoria, Horner syndrome, Adie (tonic) pupil, Argyll Robertson pupil, relative afferent pupillary defect, Hutchinson pupil, Parinaud syndrome, and oculomotor nerve (CN III) palsy. Distinct clinical patterns allow localization along sympathetic, parasympathetic, or afferent pathways and help identify high-risk presentations that warrant urgent imaging and specialty consultation, such as painful anisocoria, fixed unilateral mydriasis, acute Horner syndrome with neck pain, or pupillary involvement in CN III palsy. A structured and consistently documented pupillary examination enhances early detection of neurologic emergencies, supports timely intervention, and strengthens clinical decision-making in the emergency setting.