| Description |
As opposed to structures in the central nervous system, nerves of the peripheral nervous system are capable of regeneration. Regeneration is the mechanism by which patients recover from ischemic palsies of CN III, IV, or VI, which often improve with time. During the regeneration process of ischemic palsies, the nerve path is usually not disrupted, thus retaining the nerve's function. However, if there is a compressive lesion against a cranial nerve that is removed, the nerve path may be disrupted due to mass effect, resulting in aberrant regeneration. Aberrant regeneration is a particular problem for CN III, as it innervates multiple muscles (lid, pupil, and motility muscles), and can cause nerve fibers to regrow towards and ultimately stimulate muscles other than the original target muscle. For example, CN III nerve fibers originally approaching the inferior rectus may be diverted to the levator palpebrae superioris after aberrant regeneration; therefore, firing of the inferior rectus could result in lid retraction. Any combination of inappropriate firing and/or movement of the lid, pupil, or motility muscles is suggestive of aberrant regeneration of CN III, which cannot be caused by ischemia and needs to be scanned for an underlying etiology. |
| Transcript |
"So, today we're going to be talking about aberrant regeneration of the third nerve, and it's a very important topic for ophthalmologists who are dealing with third nerve palsies. So, when you have a nerve that is damaged for any reason, because it's a peripheral nerve, it can regrow, and that means nerves can regenerate. As opposed to the central nervous system, where regeneration pretty much doesn't happen very well, if at all, the peripheral nervous system can sprout and then get back to the target organ. This regeneration is what's responsible for recovery in ischemic palsies. If you have an ischemic third or fourth or sixth, we often tell patients who have ischemic palsies that it'll just go away in four to six weeks or six to eight weeks. It just gets better because it takes time to regrow this nerve. But it does get better, and that's like, ninety percent of the patients just get better after having ischemic palsies because it regenerates. However, there are other lesions that are not ischemic that can damage the nerve. For example, if you have a compressive lesion, like an aneurysm, on your third nerve, that will press on the nerve. And, after you treat the aneurysm with endovascular coil, or a clip, the mass effect is relieved on the third nerve, and the person will get better. However, during the regeneration of a compressive lesion, it might not regenerate normally. It might regenerate aberrantly, and what that means is the branch that's going on the third nerve that goes normally to your lid, or your pupil, or to your eye movement muscles might take a wrong turn at "Albuquerque". So, here we go - we're trying to go to the lid, but we got diverted to the pupil. Or, we were trying to go to the muscle, and it went to the lid, or any of these combinations. This does not occur in ischemic lesions because the path was not disrupted. The ischemia was to the tiny blood vessels to the nerve itself, but the track on the train track was not broken. So, if you don't disrupt the track, the nerve wasn't really damaged - in the sense of "moved" - and so when it regrows, it just stays on the same track. So, when you have an ischemic lesion (ischemic third), it'll regenerate, but it cannot regenerate aberrantly because the ischemia just caused the localized infarct but did not move the tract. And so, when the thing regrows, it grows right back to where it's supposed to go. In contrast, compressive lesions - like aneurysms and tumors - push the track, and when you get the track pushed to the new track, the train will run down that new track. So, just by pressing on the track, it pushed the train onto the wrong track, and that we call aberrant regeneration. And that aberrant regeneration can be the lid, the pupil, or the motility in the third nerve palsy. And so, what that means is, when you look down, normally the levator is inhibited. So if you just look down, there is no depressor, so the only way we get the lid to go down is by not elevating. In the levator, if your muscle is being fired, you think that you're firing the muscle, but instead the lid is firing. So for example, the inferior rectus muscle - when you're looking down your brain thinks it's firing the inferior rectus muscle, but the track was diverted to the levator. And so now, when you look down, the inferior rectus muscle fires, but also, the lid retracts. That lid retraction - in down gaze - is triggered by the eye movement. It's not there all the time. It only comes when you fire the inferior rectus muscle, and the same thing could happen in the pupil. The pupil might constrict when you're trying to look up or look down, or it could be the motility. You're trying to look up, but the medial rectus fires instead. So any combination of wrong muscles - lid when you're supposed to be firing pupil, pupil when you're supposed to be firing lid, motility when you're supposed to be firing lid, lid when you're supposed to be firing motility - any of those combinations represent aberrant regeneration. And the most important part is that it means it's not ischemic. So if you see a third nerve palsy in this vasculopathic patient, you're going to tell them, "look it's going to get better", which it does. But what if it gets better by regenerating aberrantly? You need to scan this, because ischemia cannot cause aberrant regeneration." |
