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Why do neurosurgeons get worried when a pupil loses its light reactivity?

Recall that the dilation/constriction of a pupil is
based on a dynamic equilibrium (the old push-pull) of:

• the dilators, sympathetic nerves which originate in the hypothalamus, synapse on the RAS in the brainstem, synapse again on the superior cervical ganglion, then ascend up the carotid sheath to follow the blood vessels out to the iris
• the constrictors, parasympathetic fibres originating in the Edinger-Westphal nucleus and projecting to the ciliary ganglion via the third cranial nerve (the oculomotor nerve)

Recall further that the oculomotor nerve emerges from the anterior midbrain and courses anteriorly and laterally to enter the cavernous sinus (we neurosurgeons see this thing all the time...). And that the mesial part of the temporal lobe, the uncus, usually touches this nerve.

Here is an axial MR through the midbrain. Note that the water in this image is white, so this is a T2-weighted MR (tWo-Water-White is the mnemonic). Note also that the uncus is just anterior and intimately applied to the cerebral peduncle of the anterior midbrain, as well as the subarachnoid space just anterior to the midbrain, through which the third nerve runs.

Here's a closeup of the central part of the above MR. At surgery, these structures are often touching and connected with thin strands of arachnoid.

Now, dear physician, imagine a mass shoving the entire cerebral hemisphere over towards the other side. The following CT scan shows just this happening to an individual who had a brain tumor causing rostrocaudal deterioration. Note that the uncus is indenting the midbrain.

A closeup of the above CT scan, with the midbrain indentation outlined.

The uncus gets shoved right into the oculomotor nerve, and will bother it until it quits working. The parasympathetics will quit first, causing the ipsilateral pupil to refuse to constrict when a light shines in the ipsilateral eye. The ipsilateral pupil then dilates and becomes "fixed," i.e. it will not constrict to light.

This anatomical and physiological deterioration is not irreversible yet, but will be if you let the sequence continue to develop. You must intervene NOW to diagnose and treat whatever problem is causing the deterioration.

Now do you understand why your neurosurgeon goes into high gear when a pupil begins to blow?