Iris heterochromia has been reported in association with Hirschsprung disease. Iris nevi are almost always benign in children, but evaluation may be required if they change (similar to nevi on the skin). The size is variable, and large lesions may mimic heterochromia. Iris nevi are relatively common in children, and present as focal areas of increased pigment against the lighter normal iris stroma ( Figure 29–13). Ocular abnormalities include heterochromia, cataracts, and abnormal retinal pigmentation. Central nervous system and limb anomalies occur in some patients. Hypomelanosis of Ito is a sporadic disorder characterized by an unusual whorl-like pattern of skin pigmentation ( Figure 29–12). Waardenburg syndrome is also associated with focal areas of decreased hair pigmentation and hearing loss. These include Waardenburg syndrome ( Figure 29–11), hypomelanosis of Ito, and congenital Horner syndrome. This may occur in association with a number of ocular or systemic conditions ( Table 29–1). Heterochromia is present if the color of the 2 irises is different ( Figure 29–10). In most people, the 2 pupils have the same color. During the first year of life, pigment accumulation may change the color. The color of the iris is determined by pigment cells within the stroma. These pathways mediate pupil constriction to light and near. These neurons originate in the Edinger-Westphal subnucleus of the third cranial nerve and travel along the inferior division of the nerve to the ciliary ganglion (preganglionic fibers), then to the iris sphincter through the short ciliary nerves (postganglionic fibers). The iris sphincter muscle is innervated by the parasympathetic system. The chain of neurons responsible for dilation begin in the hypothalamus and synapses in the thoracic vertebra (first-order neuron), then passes out of the spinal column, across the pulmonary apex to synapse in the superior cervical ganglion (second-order neuron), then along the internal carotid plexus and through the cavernous sinus to join with the ophthalmic division of cranial nerve V and travel to the dilator muscle (third-order neuron) ( Figure 29–1). The dilator muscle of the iris is stimulated by sympathetic pathways. It changes in response to neural input to the smooth muscles within the iris.
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