My last #tweetorial focused on the GCS. Now let's look at another hugely important clinical window into the brain: the pupil. What does a 'blown pupil' really imply? What about small fixed pupils? To understand this, we need to see how pupil size is governed. #FOAMed 1/20

The pupil has two functions, constriction and dilation, both under autonomic control. Sympathetics dilate the pupil and open the eyelid - easy to remember, because in 'fight or flight' mode, maximum information needs to get in to keep you alive. 2/20

By contrast, constriction is parasympathetic. Its main function is to regulate the amount of light on the retina, so in response to high light intensity (like a pen torch), both pupils constrict. 3/20

These parasympathetics have a straightforward course (literally). They hitchhike on the third nerve, which runs forward from the midbrain to the orbit to supply most of the extraocular muscles and eyelid. As they're superficial, they're vulnerable to extrinsic compression. 4/20

Plenty of things can compress the third nerve and produce mydriasis (a dilated pupil) - most classically a posterior communicating artery aneurysm. But in the context of an acutely comatose patient, it suggests something worrying: 'uncal herniation'. Why is this? 5/20