| Affiliation |
(AGL) Chairman, Department of Ophthalmology, The Methodist Hospital, Houston, Texas; Professor of Ophthalmology, Weill Cornell Medicine, New York City, New York; (AM) Class of 2023, Baylor College of Medicine, Houston, Texas |
| Description |
Summary: • Autoregulation o Within threshold, blood flow is maintained across various blood pressures, even too low or two high pressures > Mediated via changing size of blood vessels o Outside threshold, blood flow is not able to be maintained and there is rapid decompensation causing ischemia • Hypertensive disease o Autoregulatory threshold is shifted right > Lowering blood pressure to a "normal" level is past their lowest threshold point, causing decompensation in these patients o Decreased blood pressure slowly > 20-25% decrease in first hour to eight hours > Requires ICU monitoring • Intraocular pressure (IOP) o Similar to autoregulation in hypertensive disease o Decrease in IOP can lead to decreased blood pressure, translaminar perfusion pressure and ischemic optic neuropathy > Called "snuffing" or "wipeout" of the optic nerve head o Occurs in glaucoma patients following trabeculectomy surgery • Intracranial pressure (ICP) o Autoregulation controls intracranial pressure and cerebral blood flow > Rapid decrease in ICP impairs cerebral blood flow and vision, transiently or permanently o Idiopathic intracranial hypertension (IIH) with papilledema > Transient visual obscurations (TVO) occur with standing before autoregulation returns blood flow to the optic nerve head o Surgical (tumor resection or shunt) > Acute loss of ICP causes ischemic optic neuropathy and permanent vision loss > Persistent elevated ICP following surgery may be due to venous sinus thrombosis, venous sinus removal, or elevated CSF protein. |
| Transcript |
So today we're going to be talking about auto, which means self, and regulation. And autoregulation allows control of certain parameters within a range, but once you get above or below the autoregulatory thresholds, you'll have rapid decompensation. So, the best example we have is blood flow against blood pressure. So normally, as you increase the blood pressure, the blood flow increases, but then over a very wide range of blood pressures we have stable blood flow even on the too low or too high side. And that is mediated by autoregulatory phenomenon where we can either increase or decrease the size of the blood vessel. And so, for hypertension, the thing that we're looking for is staying within the autoregulatory zone. In patients who have chronic disease, however, the curve is shifted over to the right and the reason that's important to know is if someone has markedly elevated blood pressure, if we drop their blood pressure, even if it's within what we would consider the normal range, if it gets too close to the threshold, they might get decompensation and ischemia. And so that's why in most recommendations for malignant hypertension or hypertensive retinopathy related to the hypertensive emergency, we don't want to drop their blood pressure too fast. And so, we're usually recommending something like 20-25% reduction in the first hour to eight hours, and they're going to have to be monitored in the ICU. So, we really want their blood pressure to go down over a two day period instead of a two hour period of time. And the same thing happens in intraocular pressure as well. So, when we have markedly elevated intraocular pressure, that's also going to affect the translaminar perfusion pressure. And so sometimes dropping the intraocular pressure down too low can cause a snuffing out of the optic nerve or a wipeout of the ocular nerve. And so this has been reported in the literature as snuff or wipeout optic neuropathy. So the patient has glaucoma and they have a severe cup to disc increase and a bad visual field. And then they go into a surgery that is supposed to be helping them, so they have the trabeculectomy, and the acute lowering of the intraocular pressure too low drops them into the steep side of the autoregulatory curve, and they end up with ischemic optic neuropathy after having the ‘trab. And finally, the last pressure that we need to consider is intracranial pressure. So instead of it being blood pressure or IOP, it's ICP. And instead of blood flow alone, it's going to be cerebral blood flow and cerebral blood pressure (the mean arterial pressure). And so again, under normal conditions, we have autoregulatory control of the intracranial pressure and the mean arterial pressure. At the optic nerve head, because that's central nervous system, we also have autoregulatory mechanisms. So for example, in patients who have IIH, who have papilledema, they often have TVOs, which is transient visual obscurations. So if they get up too quickly, they get a few seconds of losing their vision. But it's that autoregulatory mechanism that allows the perfusion to return to the optic nerve head and so they only have a transient visual obscuration, they don't fall down the ischemic slope of this line. However, if it's very, very high intracranial pressure and we lower it too quickly either with surgery, by taking out a tumor, or if we put a shunt in, that acute loss of the pressure, that delta (the change in the ICP) can lead to an equivalent phenomenon to glaucoma, where we have snuff or wipeout of the optic nerve function from superimposed ischemic optic neuropathy. And the setting that we see this in is in big brain tumors, like frontal lobe lesions that are big and producing increased ICP papilledema, and they have surgery, and they lose their vision. And so that is probably an analogous circumstance. The other thing that you need to know is even if you take out a brain tumor, if you have venous sinus thrombosis or if the surgery took the sinus, the venous sinus, from surgery or if they have elevated CSF protein, that can still leave the ICP high even though you've taken out the tumor. And so, these are the mechanisms that we have to be thinking about when we are confronted of pressure related phenomenon causing an acute ischemic event after lowering the pressure. Whether it's lowering the blood pressure, lowering the intraocular pressure, or lowering the intracranial pressure, we should not do that too fast. If we do it too fast, the delta will be too great, that will drop the perfusion pressure of the optic nerve and lead to vision loss from ischemic optic neuropathy. |
