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Anisocoria in a young child is a frequent cause of concern among parents and is a common reason for referral to pediatric neurology/ophthalmology clinics. Many of these patients are eventually diagnosed with physiologic anisocoria, requiring no further evaluation. However, the differential diagnosis of anisocoria in a child contains more life-threatening conditions that should not be missed. These can include an acquired Horner syndrome secondary to a neuroblastoma of the paravertebral sympathetic chain or a third nerve palsy from a compressive intracranial process. Often, patients are referred to both a neurologist and a neuro-ophthalmologist to undergo an appropriate investigation for potential underlying problems that require further attention. The focus of this chapter is to guide the general pediatrician and pediatric neurologist through the clinical approach to a child presenting with anisocoria and to help inform decision making.


Because anisocoria represents either impaired dilation or constriction of one pupil, it is important to review the sympathetic and parasympathetic pathways that control pupil size:

  • Constriction of the pupil is achieved by the pupillary sphincter muscles, which are under parasympathetic control. The cholinergic fibers that innervate the pupillary sphincter muscle travel along the third cranial nerve.

  • Dilation of the pupil occurs through the action of the iris dilator muscle, which is under sympathetic control. These fibers originate in the hypothalamus, descend to the sympathetic trunk, and reascend to the superior cervical ganglion before arriving in the orbit. Besides the iris dilator muscle, these oculosympathetic fibers also innervate Müller’s muscle (also known as the superior tarsal muscle), which is a smooth muscle that assists with upper eyelid elevation.


Physiologic Anisocoria

The prevalence of physiologic anisocoria in the general population has been estimated to be approximately 20%, or around 1 in 5 clinic patients.1 The anisocoria is usually more pronounced under dim light.2 The major features that distinguish physiologic from pathologic anisocoria are as follows:

  • Both pupils react briskly to light.

  • There is no dilation lag.

  • The degree of anisocoria is 1 mm or less.

Reviewing past photographs can help provide reassuring evidence that the child’s anisocoria is potentially a chronic, likely benign condition. Photographs of parents and siblings may also help, as the anisocoria can be familial. Overall, physiologic anisocoria is asymptomatic, does not cause any problems with the development of vision in the child, and requires no further workup.

Horner Syndrome

Horner syndrome occurs from the disruption of the oculosympathetic chain, resulting in loss of pupillary dilation on the affected side (the affected pupil will be smaller). In children, Horner syndrome is classified as congenital or acquired, with acquired causes warranting higher concern for life-threatening etiologies. The degree of anisocoria is more pronounced, with studies in children showing an average pupillary difference ...

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