Damage to cranial nerve III will cause what to happen?

• A. Excessive pupillary constriction in response to light.
• B. Loss of pupillary constriction with abnormal dilation and poor light response.
• C. No change in pupil size.
• D. Ptosis with preserved pupil response.

Answer: (B)

Damage to the oculomotor nerve disrupts both the motor control of most eye muscles and the parasympathetic input to the pupil. The parasympathetic fibers that travel with this nerve normally cause the pupil to constrict in response to light and to accommodate. When these fibers are damaged, the pupil can no longer constrict, leaving unopposed sympathetic activity that makes the pupil dilate (a dilated, or blown, pupil) and reducing the pupil’s response to light. Ptosis can also occur because the nerve also raises the eyelid via the levator palpebrae superioris. So you end up with a pupil that fails to constrict and a pupil that is dilated with a poor light response, which matches the described scenario. The other patterns—excessive constriction, no change in pupil size, or ptosis with a preserved pupil response—don’t fit the typical outcome of full CN III damage affecting both motor and parasympathetic fibers.