12/21/2023 0 Comments Pupil reaction to light![]() The afferent pathway is shown in red, and the efferent pathway is shown in green.Īfferent pupillary fibers leave the optic tract before the lateral geniculate nucleus via the brachium of the superior colliculus to reach the pretectal nuclei (explaining why lesions of the geniculate nucleus, the optic radiations, or the visual cortex do not affect pupillary size or pupillary reactivity, and why lesions of the brachium of the superior colliculus can cause a relative afferent pupillary defect without visual loss).īoth pretectal nuclei receive input from both eyes, and each sends axons to both Edinger–Westphal nuclei (connections are bilateral but predominantly from the contralateral nucleus). 12.3 Pupillary light reflex (parasympathetic pathway). 12.2 shows the normal pupil and iris.įig. These changes vary based on the patient’s age, emotional state (adrenergic tone), state of arousal, and intraocular pressure. The size changes are reflex mechanisms in response to the amount of ambient light. Pupillary size results from the balance of actions of two opposing muscle groups of the iris: the dilator of the iris (responsible for dilation) and the sphincter pupillae (responsible for constriction). Additionally, local problems such as lesions of the iris (tumor, iris synechiae on the lens, iris tears from trauma, and postsurgical pupil) may change the shape or alter the reactivity of the pupil. Lesion of the sympathetic system: Horner syndromeĬhanges in the pupil innervation will produce dilation or constriction of the pupil. Parasympathetic (cholinergic) stimulation Lesion of the parasympathetic system: third nerve palsy Adie pupil Tropicamide, cyclopentolate, homatropine, atropine Phenylephrine, adrenaline, cocaine, amphetaminesĮndogenous increase of adrenaline (pain, fear, pheochromocytoma, etc.) Table 12.2 Pharmacologic effect of the sympathetic and parasympathetic systems on the pupil size Both pupils oscillate in synchrony, and the amplitude and frequency vary ( ▶ Table 12.2). Hippus is the normal rhythmic pupillary oscillation commonly seen when light stimulates either eye. This balance is in constant flux, so that pupil sizes change symmetrically from moment to moment. The size of the pupils at any one moment is determined by the balance of the parasympathetic tone of the iris sphincter and the sympathetic tone of the iris dilator. Light directed into either eye normally produces bilateral pupillary constriction.Įach pupil receives both sympathetic (dilator muscle: active dilation) and parasympathetic (sphincter muscle: active constriction) innervation. The pupillary response in the eye that is not being illuminated is called the consensual response.ġ2.2 Clinical Anatomy and Physiology of the Pupils The pupillary response in the illuminated eye is called the direct response. Shining a light in one eye of a normal subject causes both pupils to constrict equally. When pupillary reactions are abnormal, slit lamp examination of the anterior segment and the iris may demonstrate abnormalities that may affect pupillary size and shape, such as synechiae, uveitis, iris tear, segmental contraction of the iris, iris tumor, and lens subluxation ( ▶ Fig. Presence of a relative afferent pupillary defect (RAPD) Response to light (direct and consensual response) When examining the pupils, you should record the following: Pupils should be tested in the dark with a bright light and with the patient fixating at distance (see Chapter ▶ 1, ▶ Fig. Pupils are usually symmetrical in size, although physiologic anisocoria (the difference in size between the two pupils) of 0.4 mm or greater is seen in about 20% of individuals. The normal pupil varies in size, depending on the ambient illumination.
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