Pupillary Reflexes
Pupillary Reflexes
Two pupillary reflexes are routinely examined in clinical practice:
- Light reflex:
The eye is illuminated directly and the reaction (constriction) observed; the consensual light reflex is observed by illuminating the contralateral eye. In an eye with poor acuity, a rela-tive afferent pupillary defect may be observed using the "swinging flashlight test." The afferent pathway subserving the light reflex is optic nerve to thalamus, brainstem, and Edinger-Westphal nucleus, with the efferent limb (pupillomotor parasympathetic fibers) in the oculomotor (III) nerve. The contralateral (consensual) response results from fibers crossing the midline in the optic chiasm and in the posterior commissure at the level of the rostral brainstem. Paradoxical constriction of the pupil in darkness (Flynn phenomenon) has been described.
- Accommodation reflex:
This is most conveniently examined by asking the patient to look into the distance, then focus on a near object (sufficiently close to necessitate convergence of the visual axes) when pupil constriction should occur (accommodation-convergence synkinesis). The afferent pathways subserving this response are less certain than for the light reflex, and may involve the occipital cortex, although the final (efferent) pathway via Edinger-Westphal nucleus and oculomotor nerve is common to both accommodation and light reflexes.
In comatose patients, fixed dilated pupils may be observed with central diencephalic herniation, whereas midbrain lesions produce fixed midposition pupils.
A dissociation between the light and accommodation reactions (light-near pupillary dissociation, q.v.) may be observed.
References
Kawasaki A. Approach to the patient with abnormal pupils. In: Biller J (ed.). Practical neurology (2nd edition). Philadelphia: Lippincott Williams & Wilkins, 2002: 135-146
Cross References
Argyll Robertson pupil; Ciliospinal response; Cortical blindness; Flynn phenomenon; Light-near pupillary dissociation; Miosis; Mydriasis; Relative afferent pupillary defect (RAPD); Swinging flashlight sign
