What pupillary change is commonly seen with transtentorial (uncal) herniation?

• A. Ipsilateral dilated nonreactive pupil due to oculomotor nerve compression
• B. Bilateral pinpoint pupils due to pontine involvement
• C. Bilateral dilated reactive pupils
• D. Normal pupil reactivity

Answer: (A)

Uncal herniation presses on the oculomotor nerve (CN III) on the side of the herniation. The parasympathetic fibers that constrict the pupil run on the outer part of CN III, so they are crushed first. This causes the pupil on that side to dilate and lose its light reflex (nonreactive). It’s an early, hallmark sign of transtentorial herniation and indicates impending brainstem compromise. Other patterns, like bilateral pinpoint pupils or normal reactivity, don’t fit this mechanism, since the issue here is focal CN III compression producing an ipsilateral dilated, nonreactive pupil.