Walsh & Hoyt: Integrated Activities that Influence Pupil Size and Movement

Pupillary reflexes appear in the 5th month of development and are active by the 6th month. In infancy, the pupil is small; however, during the first 6 months of life, the pupil begins to widen, and in adolescence it attains its widest diameter. This increase in size may be related to an incompletely developed iris sympathetic system at birth and growth of the anterior segment of the eye during these years. From 20 to 60 years of age, the pupil steadily becomes smaller. Korczyn et al. performed pharmacologic studies that suggested that although the iris dilator muscle does not lose its sensitivity to norepinephrine with age, sympathetic tone decreases with age. These investigators postulated that the decrease in sympathetic tone that occurs in persons over 60 years of age results from either degeneration or functional inactivity of the postganglionic neurons. In addition to a decrease in peripheral sympathetic tone, there is also probably a decrease in the amount of central inhibition of the Edinger-Westphal nucleus that contributes to the pupil becoming smaller with age. Both in infancy and in old age, pupillary responses to light and near stimuli appear to be less active than in adolescence and adult life.