Pupillary Assessment

At the assistant level the pupil exam is a checklist: sizes, round, reactive, no defect. At the technician level it is a two-channel measurement in which you must know which channel a finding belongs to before anything you write means anything. Every pupil abnormality resolves into one of three buckets — an afferent problem, an efferent problem, or a mechanical problem in the iris itself — and the whole examination is designed to sort them.

The light reflex, afferent limb. Light striking the retina generates a signal that travels the optic nerve, partially decussates at the chiasm, and leaves the visual pathway before the lateral geniculate body to reach the pretectal nuclei of the midbrain. Each pretectal nucleus projects to both Edinger-Westphal nuclei. That double crossing is the anatomic fact underneath everything: one eye's light input drives both pupils equally. Which is why an afferent lesion, however severe, never produces anisocoria — it changes how the pupils respond, not how big they are.

The light reflex, efferent limb. Parasympathetic fibers leave the Edinger-Westphal nucleus, ride the third cranial nerve into the orbit, synapse in the ciliary ganglion, and reach the iris sphincter through the short ciliary nerves. A lesion anywhere along this limb produces a pupil that is larger and reacts poorly regardless of which eye is illuminated — the signature that separates efferent from afferent disease at the bedside.