Pupil

Pupil assessment is a rapid bedside test of the visual afferent pathway and the oculomotor efferent pathway. Light entering the eye is carried through cranial nerve II (CN II) to the midbrain. The response then travels through parasympathetic fibres in cranial nerve III (CN III) to constrict the pupil. Because the midbrain signal is sent to both sides, shining light into one eye should produce a direct response in that eye and a consensual response in the opposite eye. Pupil size reflects the balance between constricting parasympathetic activity, dilating sympathetic activity, ambient light, medications, and disease. A normal adult pupil may be about 2 mm in bright light and about 8 mm in darkness. The number alone is not the finding to memorize: pupils should be round, similar in size, and react briskly to light, with constriction occurring within approximately 1–2 seconds. The light reflex and accommodation are related but different tests. When a patient shifts focus from a distant object to a near object, the pupils normally constrict as part of the near response. A light response may be...