Crying or forced eyelid closure may also induce miosis. Focusing on a near object also stimulates convergence of the eyes toward each other. Because the innervation for accommodation is the same as that for the pupillary sphincter, the accommodating patient also has reflex contraction of the pupils, particularly when focusing at near. This prevents the eyes from needing to accommodate. When testing pupillary size, it is essential that the patient be instructed to look at a distant target that does not involve reading letters or numbers. Local factors can also cause physical changes in the iris or in the surrounding structures that may result in miosis or mydriasis. With the exceptions noted next, it is distinctly unusual for the parasympathetic fibers to be damaged without other evidence of third cranial nerve palsy (a deficit in the ability of the eye to adduct, look upward, and/or look downward, and/or ptosis see Chapter 23 Eye: Strabismus). Unilateral mydriasis can be caused by damage to the parasympathetic fibers anywhere along their course. Short ciliary nerves then carry the postsynaptic fibers to the pupillary sphincter muscle and the ciliary muscle (behind the iris). The latter contains the parasympathetic fibers that then pass into the ciliary ganglion, where they synapse. Just before entering the posterior orbit through the superior orbital fissure, the third cranial nerve splits into a superior and an inferior division. The nerve then runs anteriorly and enters the cavernous sinus superiorly and laterally. These neurons travel with the third cranial nerve, exiting the midbrain on its ventral aspect and passing between the posterior cerebral artery and the superior cerebellar arteries at the circle of Willis. Parasympathetic neurons originate in the Edinger–Westphal nuclei, located on the dorsal aspect of the third cranial nerve nucleus. The lower lid may be higher than the contralateral side (“upside down ptosis”). Disruption of sympathetic innervation anywhere along its course results in ipsilateral miosis (Horner syndrome) and is often accompanied by mild ptosis, enophthalmos, with or without ipsilateral anhidrosis. Fibers extend to the iris dilator via the ciliary nerves. They then travel through the ciliary ganglion in the orbit without synapse. The third-order neurons travel with the internal carotid artery into the cranial vault, where the fibers gain access to the orbit via the nasociliary branch of the first division of the trigeminal nerve. Therefore, complete unilateral anhidrosis in association with unilateral miosis suggests damage to the second-order neurons or superior cervical ganglion. Sympathetic innervation to the face departs from the superior cervical ganglion or at the bifurcation of the common carotid artery. The cervicothoracic sympathetic trunk then travels over the apex of the lungs to the superior cervical ganglion in the neck, where synapses are made with the third-order neurons. The first-order sympathetic neurons extend from the hypothalamus through the midbrain, pons, and medulla into the spinal cord, where they synapse with the second-order neurons just before exiting the cord at roots C8–T2. The pupillary sphincter receives parasympathetic innervation that also supplies the ciliary muscle of the eye that governs focusing (accommodation) of the lens. The pupillary dilator muscle receives sympathetic innervation. Figure 24.1 represents a flowchart for an approach to anisocoria. Miosis refers to an abnormally constricted pupil. An abnormally dilated pupil is called mydriasis. When the pupils are different in size, the term applied is anisocoria. Congenital or acquired (e.g., after trauma) corectopia generally requires ophthalmology consultation. The pupil can also be malpositioned ( corectopia). Pupillary disorders can be divided into two categories: Disorders in which the size of one or both pupils is abnormal and disorders in which the shape of one or both pupils are abnormal. LEVIN, MD, MHSc, FRCSCĪbnormalities of the pupils can be helpful diagnostically when assessing central nervous system, autonomic nervous system, orbital, and ocular problems.
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