||All right, today we're going to be talking about aberrant regeneration. And, as you know, peripheral nerves when they're damaged will try to re-grow. That, we call regeneration. And, because the cranial nerves are peripheral nerves once they leave the brainstem, these cranial nerves if they're damaged-III, IV, VI-will try and regrow. Because the fourth and the sixth only go to one muscle, it can only go to one place, so when the fourth and the sixth regrow they just regenerate right back to their target organ. The fourth goes to the superior oblique muscle, and the sixth goes to the lateral rectus muscle. There really is no opportunity for these nerves to get off on the wrong track. How aboutcranial III, though. Cranial nerve number III goes to the lid and so in the acute phase, they're going to have ptosis, and then the lid is going to slowly recover. The ptosis is going to get better. And it goes to the pupil, which is the parasympathetic fiber, and that causes the patient's pupil to constrict to light and to near. And we've got the motility, which is the muscles other than the superior oblique and the lateral rectus muscle, which is the superior rectus muscle, the inferior rectus muscle, inferior oblique muscle, medical rectus muscle. So, when you have aberrant regeneration, a nerve that used to go to the lid, the levator, or the pupil, or to a muscle, is now going to something it's not supposed to go to. And so clinically what we see is, if we have a ptosis of a third nerve palsy and the nerve regrows normally the ptosis will just get better. But if the fiber that used to go to the lid is actually rerouted to the inferior rectus muscle, when they look down, the lid would retract. The brain thinks you're firing the inferior rectus muscle to pull the eye down, but it's actually hooked up to the levator half, and so the lid will retract on attempted down gaze. Or, it could try and look up with the superior rectus muscle and then the eye suddenly goes in. It adducts instead of supraducts. That means the muscle (medial rectus muscle) is firing when your brain thinks the superior rectus muscle is firing. These are examples of aberrant regeneration. It regenerated, it regrew, but it regrew to the wrong target. And, the pupil-the pupil might get smaller or bigger when you look in or up or down because you think you're firing the muscle, but really the pupil fiber is firing. And it's really important to recognize aberrant regeneration of the third nerve because ischemia cannot cause this. So, when you have an ischemic palsy to the nerve, the number three nerve, the small vessels that supply the artery have been ischemic because it's small vessel ischemia. It doesn't damage the track, so when it regrows, it just follows the same track and goes right back to the muscle or the lid or the pupil from which it came. However, if you have a third nerve that's been cut from trauma or has a tumor pressing on it or an aneurism, then when it tries to regrow, it might go down the wrong track. It's supposed to goto the inferior rectus, but it got to the lid instead. Because the tracks were disrupted by the tumor, or the aneurism, or whatever this was. Ischemia does not produce aberrant regeneration, but tumors, compressive lesions, aneurisms do. So even if the patient is better clinically-for example after having an aneurism repair, posterior communicating artery would be a good example-they might regenerate, but it might regenerate aberrantly. And if the third is hooked up to the wrong thing, the lid might go up,the pupil might constrict, or the wrong muscle might fire. And those are the clinical findings or aberrant regeneration. The clinical significance, not ischemic, and it's the natural sequel of having a compressive lesion that's been repaired. If it happens without a history of third nerve damage, we call that primary. If it happens with a third nerve palsy, we call that secondary. And primary means they never had a third nerve palsy-they just show up to the office and their lid retracts in down gaze, or their motility, the eye looks in when it's supposed to look up, and that's almost always a compressive lesion that's slow-growing with the nerve damage and the nerve repair kind of happening at the same time. And usually that means it's a cavernous sinus mass that is slow-growing. Usually cavernous sinus meningioma or injured cavernous internal carotid artery aneurism. So, primary aberrancy we're going to image that looking at the cavernous sinus. Secondary, you're going to look and see what the cause was. If you thought it was ischemic and it's aberrantly regenerating, it's not ischemic. And that's what you need to know about aberrant regeneration.