What pupil response may indicate a lesion in cranial nerve III?

A dilated and non-reactive pupil can indicate a lesion in cranial nerve III, which is also known as the oculomotor nerve. The oculomotor nerve is responsible for controlling most of the eye's movements, as well as the constriction of the pupil. When there is a lesion affecting this nerve, it can disrupt its ability to constrict the pupil, leading to dilation. Additionally, this dilation often comes hand-in-hand with a lack of reactivity to light, meaning the affected pupil does not constrict appropriately in response to bright light stimuli.

In contrast, a constricted and reactive pupil typically suggests normal function or the influence of the parasympathetic nervous system, which is not compromised. A balanced pupil reaction indicates that both pupils are responding uniformly, which would suggest there is no significant nerve injury. Fluctuating pupil size is not a typical sign seen with cranial nerve III lesions and may occur due to other reasons such as neurological conditions or drug effects.