25.2 ) ( Box 25.2 ) consist of (1) pupil movement as an objective indicator of afferent input, (2) pupil diameter as an indicator of wakefulness, (3) pupil inequality as a reflection of autonomic nerve output to each iris, (4) the influence of pupil diameter on the optical properties of the eye, and (5) the pupil response to drugs as a means of monitoring pharmacologic effects. The clinical aspects of pupil function ( Fig. When a subject views objects at near, not only the accommodation power of the eye changes but the near response of pupil contraction helps bring objects into better focus by increasing the depth of focus afforded by the smaller aperture size.īesides the physiologic functions of the pupil explained earlier and outlined in Figure 25.1, the pupil diameter and its movement under different conditions also provide an important indicator used for clinical assessment of a patient. Third, a small pupil increases the depth of focus of the eye’s optical system, similar to the known pinhole effect of camera lenses used for photography. Therefore there exists an optimal range of pupil diameter for vision, and this size may vary somewhat, depending on the individual optical characteristics of a person’s eye. There is a limit to the beneficial optical effects of a small pupil because constriction beyond a certain diameter results in image degradation as a result of increased diffraction and reduction of retinal illumination below an optimal level. Topical drops that produce miosis may reduce pupil diameter enough in dim lighting conditions (without affecting accommodation) to alleviate the symptoms of aberration. Most refractive surgeons attempt to address this problem by carefully measuring the pupil diameter in dim lighting conditions preoperatively and then adjusting the optical zone of corneal refractive surgery according to pupil diameter. The area of the pupil is large enough in these patients to exceed the corneal optical zone of refractive surgery. Part of the reason is that a smaller aperture size limits the light rays entering the optical system to the central cornea and lens, avoiding more peripheral portions of the cornea and lens, where aberrations are greater ( Box 25.1 ).Īfter refractive surgery, younger patients, who usually have larger pupils in dim light compared with older individuals, often experience bothersome symptoms of glare and image degradation, especially at night, as a result of optical aberrations. A small pupil reduces the degree of chromatic and spherical aberration. Second, the diameter of the pupil can also contribute to improving (up to a point) the image quality of the retina when the steady-state pupil diameter is small. This emphasizes the important role of the pupil in optimizing visual perception in a timely fashion over a wide range of lighting conditions of the environment. These symptoms are described by patients with an immobile pupil because compensatory retinal photoreceptor adaptation is not fast enough. Patients with a fixed immobile pupil are usually symptomatic during an abrupt change in illumination they may be photophobic when they are subjected to sudden increases in light, and they may not be able to discern objects in their environment when they first enter dim lighting conditions. Although this reduction in retinal illumination is only a portion of the 12 log unit range of light sensitivity of the retina, it provides an important and immediate means for light adaptation. With exposure to bright light, pupil constriction can reduce retinal illumination by up to 1.5 log units within 0.5 seconds. In dim light, dilation of the pupil provides an immediate means for maximizing the number of photons reaching the retina, which in turn supplements the slower dark adaptive mechanisms involving retinal gain control at the photoreceptor and bipolar cell level. Functions of the pupil include control of retinal illumination, reduction in optical aberrations, and improved depth of focus.įirst, pupil movement in response to changing light intensity aids in optimizing retinal illumination to maximize visual perception.
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