Pupil

The pupils provide a rapid check of visual input, midbrain processing, and the parasympathetic motor pathway that constricts the eyes. Light entering the retina travels through the optic nerve (cranial nerve II) to the midbrain, where signals are sent through both oculomotor nerves (cranial nerve III). As a result, shining light into one eye should constrict that pupil directly and the opposite pupil consensually. This wiring explains why the assessment is more than a size comparison. A pupil may be abnormal because light is not reaching the brain effectively through the afferent pathway, because the efferent pathway cannot constrict the eye, or because medication or an ocular problem is altering the response. The examination can identify a dangerous change; it does not, by itself, establish the diagnosis. Lighting changes pupil size normally. In ordinary lighting, pupils measure approximately 2–4 mm; in dim lighting, they commonly dilate to 4–8 mm. These are reference ranges, not a substitute for comparing the client’s two pupils and checking the documented baseline. Assess size, shape, and equality before shining the light, because the stimulus itself...