Pathophysiology
Clinical meaning
Pupil assessment evaluates the integrity of cranial nerves II (optic nerve — afferent limb) and III (oculomotor nerve — efferent limb) and provides critical information about neurological status, intracranial pressure, and brainstem function. The pupillary light reflex pathway: light enters the eye → optic nerve (CN II) transmits the signal to the pretectal nuclei in the midbrain → bilateral signal to Edinger-Westphal nuclei → parasympathetic fibers travel via the oculomotor nerve (CN III) → synapse at the ciliary ganglion → short ciliary nerves constrict the pupillary sphincter muscle (pupil constriction/miosis). Because the signal crosses to both sides, shining light in one eye causes both the direct response (ipsilateral pupil constricts) and the consensual response (contralateral pupil constricts). Sympathetic innervation from the hypothalamus through the cervical sympathetic chain causes pupil dilation (mydriasis) by contracting the pupillary dilator muscle. Unilateral fixed, dilated pupil (blown pupil) indicates compression of CN III, most commonly from transtentorial (uncal) herniation due to rising intracranial pressure — this is a neurological emergency. Other abnormal findings include: pinpoint pupils (bilateral miosis from opioid overdose or pontine hemorrhage), bilateral...