Anticholinergics such as atropine, homatropine, tropicamide, scopolamine, and cyclopentolate lead to mydriasis and cycloplegia by inhibiting parasympathetic M3 receptors of the pupillary sphincter and ciliary muscles. Pharmacologic anisocoria can present as mydriasis or miosis following administration of agents that act on the pupillary dilator or sphincter muscles. Causes include physical injury from ocular trauma or surgery, inflammatory conditions such as uveitis, angle closure glaucoma leading to iris occlusion of the trabecular meshwork, or intraocular tumors causing physical distortion of the iris. Mechanical anisocoria results from damage to the iris or its supporting structures. Examples include aniridia, coloboma, and ectopic pupil. Physiologic anisocoria may be intermittent, persistent, or even self-resolving.Ĭongenital anomalies in the structure of the iris may contribute to abnormal pupillary sizes and shapes that present in childhood. ![]() Light and near responses are intact, and the degree of anisocoria is typically equal in light and dark. The exact cause is unknown, but it is thought to be due to transient asymmetric supranuclear inhibition of the Edinger-Westphal nucleus that controls the pupillary sphincter. It is a benign condition with a difference in pupil size of less than or equal to 1 mm. Physiologic (also known as simple or essential) anisocoria is the most common cause of unequal pupil sizes, affecting up to 20% of the population. ![]() An injury or lesion in either pathway may result in changes in pupil size. Generally, anisocoria is caused by impaired dilation (a sympathetic response) or impaired constriction (a parasympathetic response) of pupils. Thus, thorough clinical evaluation is important for appropriate diagnosis and management of the underlying cause. It is relatively common, and causes vary from benign physiologic anisocoria to potentially life-threatening emergencies. © 2019 Neuro-ophthalmology Virtual Education Library: NOVEL Īnisocoria indicates unequal pupil sizes. In addition to regulating the amount of light entering through the pupil, altering its size also affects depth of focus and reduces optical aberrations.Anisocoria. The dilator muscle radiates out from the pupil margin towards the iris periphery in a similar pattern to bicycle spokes. The dilator muscle is innervated by sympathetic fibres (which run with the ophthalmic branch of the trigeminal nerve (cranial nerve V)). Constriction of the pupil is mediated via parasympathetic nerve fibres travelling in the oculomotor nerve (cranial nerve III). Other species such as horses, sheep and reptiles have different arrangements of this muscle which contribute to pupil shape. The sphincter muscle circles the iris in the dog while in the cat the fibres also criss-cross above and below the pupil allowing it to constrict to a narrow vertical slit. Movement of the iris muscles controls the quantity of light entering the posterior segment through the pupil. Normally the iris rests lightly against the anterior surface of the lens. ![]() The pupil is the central hole in the iris. The main blood vessels entering the iris are the long ciliary arteries (temporal and nasal) which together form the major arterial circle visible in most animals as an undulating ring within the anterior iris stroma. In addition to muscle fibres and melanocytes, the stroma contains numerous blood vessels, nerve endings, fibroblasts and collagen fibres. The colour of the iris depends on both the number of melanocytes present within the stroma and the thickness of the anterior layer itself. It is made up of an anterior layer of stroma and iris sphincter muscle, and a posterior layer made up of an epithelial layer (which is pigmented) and dilator muscle (Figure 32.1). ![]() The iris is the visible coloured layer seen within the eye. The anterior uvea includes the iris and ciliary body while the choroid comprises the posterior uveal tract. It is usually pigmented and is the vascular layer of the globe. The uveal tract makes up the middle layer of the globe, and consists of the iris, ciliary body and choroid.
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