| Transcript |
Post-operative vision loss has particular applications to spine surgery in the face-down position. As you know, if you're operating on the spine, the patient has to be in the face-down position and, often, we're using a specialized table or device to get the spine up into the surgical field. This thing that they, neurosurgeons, use is called the Wilson Frame. The Wilson Frame by itself is a risk factor for post-operative vision loss related usually to ischemic optic neuropathy; the ischemic optic neuropathy that follows spine surgery can either be anterior ischemic optic neuropathy (AION), where the disc is swollen, or posterior ischemic optic neuropathy (PION), where the disc is not swollen (nothing to see, disc is normal). So normally, AION and PION that occur post-operatively causing vision loss after spine surgery are from a combination of factors. Even though we don't know what the proximate cause is in spine surgery, I'm now going to go over with you the number of hypothesized associated risk factors with this particular surgery that lead to ischemic optic neuropathy. The first is the positioning of the patient. So with the Wilson Frame alone, you can see that you're going to have impaired venous return and, potentially, you could have increased intraocular pressure because you have cephalic fluid shift toward the head in the face down position, as the patients are often awake with a big puffy face. This positioning is something that's super important. Number two is anemia and that can be compounded be pre-existing pre-surgical anemia. We'd like to treat that, if we can, pre-operatively. The blood loss that occurs from spine surgery can also lead to anemia and transfusion is becoming less and less common now because we're afraid of infectious diseases and transfusion reactions; they might let the patient run low on purpose. The third is hypotension. Sometimes we have deliberate hypotension to reduce the bleeding to treat red cell loss. Hypotension is a normal thing that occurs just from induction of anesthesia, from this type of surgery, and also in the post-operative period, so it's not surprising that these are the risk factors for ischemia in spine surgery. In addition, the replacement of the fluid can be with colloid, which is albumin and big molecules, versus crystalloid, which is sodium and D5W-the electrolyte-based solutions. It turns out that colloid is better for preventing ischemic optic neuropathy and the risk of having this ischemic event goes up the more crystalloid you've been given. So, we prefer to have colloid, which holds the intravascular volume better and is less likely to go cause the edema and third space, etcetera. So, the major factors are positioning, anemia, blood pressure, and the use of colloid. Obviously, the duration of the surgery is a risk factor: the longer your surgery, the more complex the surgery, the more likely you are to get the event. The countermeasures for ischemic optic neuropathy are to: number one, recognize preoperatively who's at risk-men, vascular pathway vectors, obese patients, patients who have had stroke risk factors. They're going to be at higher risk; so, in those patients, we'd like to counsel them about the potential of vision loss after spine surgery. Can't really do anything about positioning, but you might consider alternatives to the Wilson Frame. For the anemia, we'd like to make sure pre-operatively that their blood counts are good, and we'd like to transfuse them at a more generous threshold than we normally would. We'd like to keep them from getting too low a blood pressure because the perfusion pressure of the optic nerve equals the mean arterial pressure minus the intraoptic pressure (IOP). So, we have venous return that might be increasing the IOP, but we also have hypotension, either deliberate or inadvertent hypotension. And we prefer to use colloid over crystalloid and, if possible, we'd like to split the procedure up and do it in a staged manner, even if that means two anesthesia's rather than having a six- or ten-hour spine surgery in a person who's predisposed. Because we really don't know which of the risk factors is proximate, we probably should be addressing all of them. Sometimes not addressing one, for example not controlling the blood pressure, might lead to more blood loss and worsen the anemia; so, we really have to be careful which of these risk factors we're going to choose to treat, because we really don't know which one is actually causal and it's probably multifactorial. The use of corticosteroids is probably not that helpful. We do image the patients, because sometimes the post-op for vision loss is from apoplexy or stroke in the brain, so we want to make sure that it really is ischemic optic neuropathy, especially the posterior forms. And then, lastly, sometimes the vision loss is from a central retinal artery occlusion, a CRAO. Usually, that's not surgery related. That's usually from the positioning of the patient in the face-down position, because the eye might come into contact with the headrest. So, in the face-down position, if the headrest is mispositioned and is actually on the eye instead of their head, the headrest will press on the eye, which will lead to elevation of the intraocular pressure and blockage of the retinal artery. If it's a central retinal artery occlusion, if no embolus is seen, that's usually from a positioning problem in the face-down position. You should recognize post-operative vision loss after spine surgery, the multiple factors that could be contributing, and be looking for the key and differentiating features of AION, PION, and CRAO. |
