Pupils can become mydriatic, or dilate, in response to potential disease, drug toxicity, trauma, increased intracranial pressure, brainstem damage, or nerve damage to cranial nerve II and/or III. While there are other reasons for variation in pupillary dilation and constriction, such as arousal leading to changes in the balance of the sympathetic and parasympathetic nervous systems, here we will focus on its relation to light exposure. Light entering the eye is processed through the pupillary light reflex, and signals directed to the iris sphincter muscle to adjust the amount of light that reaches the retina. ĭirect and consensual pupillary light reflexes test for appropriate neurological pathway connections and functioning of both cranial nerve II and III. Overall, normal pupillary response times are about one second for initial constriction and 5 seconds for dilation. Latency increases by approximately 1 millisecond per year with aging. Pupillary latency occurs when the reaction time of the pupil is inversely related to the increase in light intensity from the stimulus this can serve as a cue to a potential neurologic cause. One such condition is anisocoria, and it is estimated at 4% of the general population has anisocoria of greater than 1 millimeter, in which case neurological compromise must be ruled out. The pupillary light response exhibits varying sensitivity to the chromic spectrum, indicating that the process of light recognition is significantly complex it is not as simple as a binary response with the detection of “light” versus “no light.” While there is a baseline fluctuation in the steady-state conditions for pupillary dilation, a concern for neurological abnormalities is considered in cases of marked pupillary changes, whether with constriction or dilation. Per decade of aging that occurs, there is a 0.3 mm decrease in the standard pupil diameter that has been associated with iris stiffening.
In standard clinical testing conditions, the diameter of the pupils will usually range from two to five millimeters. Commonly, clinicians document PERRL–saying the pupils are equal, round, and reactive to light or PEARL - pupils equal and reacting to light. A 3+ grading indicates a moderate response, 2+ is a small, slowed response, 1+ represents a tiny/just visible response, and a 0 indicates unresponsive pupils. A normal, healthy adult patient is expected to have a 4+ response, which indicates a brisk, large response. Pupillary light reflexes are measured based on a 0 to 4+ gradient that considers the magnitude and speed of the light response. Of note, the pupillary dark reflex involves a separate pathway, which ends with sympathetic fibers from long ciliary nerves innervating the dilator pupillae muscle. And, because of the crossing fibers, there is not only a direct pupillary reflex but also a consensual pupillary light reflex. This extensive pathway is being tested when a light is shined in the eyes. The contraction of the iris sphincter muscles leads to pupillary constriction (miosis). Efferent parasympathetic preganglionic fibers travel on the oculomotor nerve and synapse with the ciliary ganglion, which sends postganglionic axons to directly innervate the iris sphincter muscles. There are a minority of axons that go to the hypothalamus and the olivary pretectal nucleus (OPN).
Each pretectal area sends bilateral signals to the preganglionic parasympathetic nuclei in the midbrain called Edinger-Westphal nuclei. The optic tracts join the brachium of the superior colliculus, and then signals travel to the pretectal area of the midbrain. Thus, the right optic tract will contain temporal retinal fibers from the right eye, as well as nasal retinal fibers from the left eye. At the optic chiasm, nasal retinal fibers will cross to the contralateral side of the optic tract, and the temporal retinal fibers continue on the ipsilateral side. The optic nerve then forms the optic chiasm, which diverges into a left and right optic tract. These are the first steps of the pupillary light reflex afferent pathway. The optic nerve sends impulses to the brain for further processing and image recognition. These signals are then relayed to the bipolar cells, which interact with ganglion cells, which in turn coalesce to form the optic disc and optic nerve (CN II). Photoreceptor cells in the outer layers of the retina, which are called rods and cones, convert light stimuli into neuronal impulses. Light travels through the cornea, anterior chamber, pupil, lens, and the posterior chamber, eventually reaching the retina.
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