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Pupillary size and movement are controlled by the autonomic nervous system, which innervates two muscles of the iris, the radial dilator (sympathetically innervated), and the circular sphincter (parasympathetically innervated). In the normal state, the distribution of efferent pupillomotor signal to the iris is symmetric between the two eyes so that pupil size is generally equal under varying lighting conditions. Similarly, reflex pupillary movements, eg, constriction and dilation, are identical between the two eyes. Asymmetry in the pupillomotor neural output or their muscular forces results in impaired pupillary movement on one side and unequal pupil size between the right and left eyes. This chapter reviews the evaluation of anisocoria, both transient and persistent, in assuming that only one side (pupil) is faulty and focuses on the neurologic causes of pupillary dysfunction.