Accordingly, anisocoria (unequal pupillary diameter) is not attributable to the angle at which light strikes the face, unilateral cataracts, or an asymmetric refractive error, unless there is local disease of the anterior segment. The transmission of less pupilloconstrictor tone to each iris sphincter would result in slightly larger pupils but of equal diameter. For example, in a patient with one blind eye, the pretectal nuclei would register and transmit to each E–W nucleus only one-half the normal level of illumination. Hence, the pupils should be equal in diameter regardless of the level of vision of either eye. Axons from each pretectal nucleus pass ipsilaterally and contralaterally to the ipsilateral and contralateral Edinger–Westphal (E–W) nucleus, a subnucleus of the oculomotor nuclear complex, The hemidecussation of the pupillary fibers at the optic chiasm and between the pretectal nuclei ensures that each E–W nucleus receives information about the level of incoming light from each eye. 'Pupillary fibers' from both eyes within the optic tract pass via the superior quadrigeminal brachium and the superior colliculus to the mesencephalic pretectum and pretectal nuclei. Fibers originating from the nasal neuroreceptor cells decussate in the optic chiasm to the opposite optic tract, whereas the temporal fibers continue in the homolateral optic tract. Considerable evidence exists that the visual cells of the retina, that is, the rods and cones, also serve as light receptors controlling pupillomotor activity. The pupillary light reflex arc begins in the retina ( Figure 58.1). The iris dilator fibers contain α-adrenergic sympathetic receptors that respond to changes in sympathetic tonus and changes in the blood level of circulating catecholamines.

The size of the pupil is controlled by the activities of two muscles: the circumferential sphincter muscle found in the margin of the iris, innervated by the parasympathetic nervous system: and the iris dilator muscle, running radially from the iris root to the peripheral border of the sphincter. Record these data so that they are easy to read and recall. Next, look at the amount of pupillary constriction that occurs when the patient is forced to focus on a near object, such as a thumb held 15 to 20 cm above the eyes. Slowly move the light up to the patient's eye level and check the pupillary response to the bright light on each side several times.

To assess pupillary size in a darkened room, illuminate the face from below. For the same reasons, try not to startle or touch patients with your hands or instruments, as psychosensory stimulation induces mydriasis, hippus, and relatively hyperactive pupils. Patients should be encouraged to fixate visually on a distant object, because if they inadvertently look at your nose or the flashlight, the attempt to converge will reflexly evoke miosis, and certain signs may be overlooked (e.g., anisocoria, light-near dissociation, or a subtle Marcus Gunn sign. Because these phenomena are best tested with the pupils in a semidilated state, clinical observations should be made in a dimly lighted room. The examiner first must check the size, shape, equality, and position of the pupils, and their response to a bright light.

One pupil larger than other