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 intima
