What’s the significance of pupil involvement with oculomotor nerve paralysis?
When faced with a case of third-nerve palsy, evaluation of the pupil is a crucial step. The parasympathetic fibers that control pupil constriction run on the outside of the nerve. If the pupil is affected, most likely the nerve is being compressed, by an aneurysm or tumor. On the other hand, less dangerous paralysis of the nerve from ischemia (diabetic or hypertensive) tends to occur deep in the nerve, not affecting the pupil.
This also explains why the pupil blows with uncal herniation – the nerve gets compressed from the outside.
This day and age, however, you should probably image all cases of oculomotor palsy to rule out life-threatening mass.
don’t forget about CSF leaks and consequent brain sag; this will tract on the CN3’s just dandy.
The parasympathetic fibres of the occulomotor nerve run in the outer side of the nerve & more specifically in the superior medial perimeter of the nerve.
The Pcom. aneurysm causes pupillary dilatation as it is medial to the third nerve.