| Transcript |
"Today we're going to be talking about optic nerve sheath fenestration. And obviously, I am not an oculoplastic surgeon, and so I'm not really going to be talking to you about the mechanics of performing the procedure. I'm really only going to be talking to you about how we in neuro-ophthalmology-medical neuro-ophthalmology-deal with sheath fenestration, both in terms of when do we do it and when do we ask oculoplastics to do it, and then how does it work, and what are the limitations of them. So the main indication for optic nerve sheath fenestration is to let the fluid out from the optic nerve. That's what fenestra means. It means make a window. And so as you know, the fluid in the sheath, the cerebrospinal fluid, is surrounding the optic nerve, and that fluid from increased intracranial pressure will be increased because there's a hydrostatic gradient pushing it down the sheath, and that will cause the clinical finding, papilledema. And even though it's not really edema-there's no extracellular water here, it's causing axoplasmic flow stasis here at the disc head, and that will cause the blind spot to be bigger. The blind spot will be bigger because the nerve is bigger. And as you know, it's a blind spot because there's no photoreceptors on the optic nerve head. And so, one of the major field defects that we see in increased intracranial pressure is enlargement of the blind spot because the disc is bigger. We do not do sheath fenestration, however, for big blind spot. That's actually not even a real field, we think. It's caused by elevation of the peripapillary retina from the disc being bigger. However, papilledema, including idiopathic intracranial hypertension, which is our most common cause of field defects related to this, cause field defects that are nerve fiber layer in origin, and so that means they're going to produce field defects that are arcuate because the nerve fiber layer is arcuate in shape, and if it's more severe, it'll be altitudinal. And then over time it'll involve both the upper and lower, and the papillomacular bundle is usually spared because these are pressure-resistant fibers-it goes straight right into the optic nerve. So the stress and the strain that they are feeling at the optic nerve head is less if you've got a short papillomacular bundle-direct connection to the nerve, than if you're this long, bendy temporal fiber. So in IIH, we might consider a sheath fenestration to cut a window in the sheath and let the fluid out. Now some people believe that it's actually causing a scar here that's causing a ‘dam' phenomenon to prevent the fluid from coming forward. It's not clear which mechanism is at play in sheath fenestration. It probably doesn't matter clinically. So when would we consider doing a sheath fenestration? We would consider sheath fenestration in patients who have failed maximal medical therapy. And the first line, of course, is weight loss in IIH, followed by the water pill, acetazolamide, which has been shown in the clinical trial to be effective for reducing this papilledema and the vision loss. You could use second-line agents, furosemide-that's Lasix, third-line agents, topiramate/Topamax, steroids in the acute setting, but normally we try to avoid steroids because it can cause increased intracranial pressure during the withdrawal of the steroid phase. So if you fail, are intolerant of, or non-compliant with maximal medical therapy, you're having nerve fiber layer loss and a visual field defect, but not just a big blind spot, or you have loss of acuity, or you have a severe and acute form called fulminant IIH. All of those are going to be indications for doing a sheath fenestration. Sheath fenestration is safer and easier to do than a shunting procedure, but you have to have the right person available, you have to have oculoplastics that are willing to do it, and it only works on one side. However, a lot of studies have shown that if you just do a sheath fenestration on one side, it improves the other side as well, whether that's from psychological reasons, like improved compliance and just taking your medicine, or having a big incentive to lose weight because you don't want to have the sheath fenestration again, it's not clear. But empirically, we know that if you have a sheet fenestration on one side, it often reduces the disk edema on the contralateral side, so maybe it doesn't matter what the mechanism is, as long as it works. And so, sheath fenestration for me is the go-to procedure for patients who have papilledema-related vision loss, who have failed maximal medical therapy. The other reason to consider it is in papilledema not related to IIH. So we would consider it even in patients who have IIH-related or non-IIH-related papilledema. So if they have papilledema from whatever source, if they're losing their vision and they have failed maximum medical therapy, you could still do a sheath fenestration on that patient. Now, there was a study to look at doing sheath fenestration on non-arteritic anterior ischemic optic neuropathy. That was the ischemic optic neuropathy decompression trial. So they actually did decompressions using this same technique on ischemic optic neuropathy, and that study showed that not only did the sheath fenestration not help patients, it made their vision worse. So we do not do the sheath fenestration for non-arteritic anterior ischemic optic neuropathy anymore. So you need to know a little bit about a sheath fenestration, how it works, when we do it, and its major complications are loss of vision from damage in the nerve, central retinal artery occlusion, some people have tonic pupil afterwards, or they have diplopia from the muscle being damaged. Most of the sheath fenestration complications are local to the surgery right here. Nobody really dies from a sheath fenestration though, so its mortality is a lot lower than having a shunting procedure or even a stenting procedure. So for me, sheath fenestration is safer, more effective, can work on both eyes even if you only do it once, and it's our go-to for vision loss related to papilledema failed maximal medical therapy." |
