Accreditation and Course Overview Details
Optic Nerve Head Biomechanics
February 28, 2014
February 28, 2017
Ophthalmologists, primary care physicians
Learning Objective / Overview
J. Crawford Downs, PhD, Professor and Vice-Chair Basic Science Research in the Department of Ophthalmology and Director of the Ocular Biomechanics and Biotransport Program at UAB, discuss new findings in basic science research on ocular biomechanics to glaucoma pathogenesis. Emerging data on age and race disparities of current glaucoma patients will be presented along with new research related to the dynamics of intraocular pressure – a known risk factor for glaucoma. This research is relevant to anyone in the medical field who works with glaucoma patients.
Dr. Downs has no relationships with proprietary entities producing health care goods or services.
The University of Alabama School of Medicine is accredited by the Accreditation Council for Continuing Medical Education (ACCME) to provide continuing medical education for physicians.
The University of Alabama School of Medicine designates this enduring material for a maximum of 1 AMA PRA Category 1 Credits. Physicians should only claim credit commensurate with the extent of their participation in the activity. TM DR. CRAWFORD DOWNS: Hi, I'm Dr. Crawford Downs, Professor and Vice Chair of Basic Science Research and Director of the Ocular Biomechanics and Biotransport Program at the University of Alabama School of Medicine, department of optimology. Ocular biomechanics are an interesting area of study. And the reason people are interested in it is that there's a wide range of sensitivity to Intraocular Pressure, or IOP. So in the general population, there are some sub-population that have high intraocular pressure-- so ocular hypertension. That's pressure above 21 millimeters of mercury, which is considered the normal range. There's a subset of people who get glaucoma, develop glaucoma with high IOP, but also a subset of persons who develop glaucoma at what is extensively normal levels of intraocular pressure. And what we want to know is the difference between what is a frail eye and a robust eye. So an eye that's very resistant to the effects of intraocular pressure and an eye that is very susceptible to the effects of intraocular pressure even if it's in the normal range. So when you talk about glaucoma patients, we talk about the issue that there's normal tension and high tension glaucoma. But we believe it's all one disease. And really, it's about individual susceptibility to the level of IOP that is in a particular patient's eye. So one of the things we'd like to know is, why do some people develop glaucoma and some don't even at the same level of intraocular pressure? And we believe it's perhaps due to differences in ocular biomechanics, specifically anatomy and geometry of the optic nerve head and the surrounding sclera, tissue composition, mechanical properties, stiffnesses, optic nerve head blood flow, cellular reactivity, and intraocular pressure and ocular profusion pressure fluctuations. So you can think about the eye as a pressure vessel. It's very much like a basketball. It's much more complex than that. But it is a pressurized soft vessel in the body. The reason that your eye has its round shape is because of the intraocular pressure inside of it. And you can think about the stress, mechanical stress, in the wall of the vessel as being a function of the diameter of the vessel, the pressure inside it, and the thickness of the wall. So many people use what they call Laplace's Law, which is a mathematical relationship, to define the stress in the wall. The problem with using Laplace's Law is that it requires a fully spherical vessel-- the eye is not-- uniform wall thickness-- the eye doesn't have that either-- and very uniform material properties or stiffnesses. And the eye has very anisotropic, inhomogeneous material properties. So the problem with Laplace's Law is the eye doesn't conform to the assumptions that are associated with that law. So one of the reasons that this doesn't work is because stress concentrates around the scleral canal. So you have what is an otherwise very, very strong corneascleral envelope-- so the cornea, the sclera, the white of your eye. And then you have a small hole through which all the retinal ganglion cell axons have to pass on their way from the retina to the brain. So if you interrupt that transmission of those electrical signals from the retina to the brain, it results vision loss. So the problem with Laplace's Law is that it breaks down in the eye because stress concentrates around the scleral canal and then the lamina cribrosa because it is a soft spot in what is otherwise a very strong pressure vessel. So what you see here is you see a hoop stress concentration that occurs right around the edge of the scleral canal. Hoop stress is named for the hoops around a barrel. So even though the pressure of the fluid is pushing directly on the wall, perpendicular to the wall, it gets transmitted into a stress that lies in the plane of the wall. So it's called a hoop stress. So you can see here that there are several main drivers of that hoop stress, the main one being intraocular pressure. But also on the backside of the lamina cribrosa, the connective tissues that span the scleral canal, there is cerebrospinal fluid pressure. So that's a bit of a back pressure. So people talk in terms of-- or the community talks in terms of the difference between intraocular pressure and cerebrospinal fluid pressure bathing the orbital optic nerve. And so we talk in terms of the translaminer pressure difference. So the difference between intraocular pressure and cerebrospinal spinal fluid pressure in the sub [INAUDIBLE] space. So you could think of this in terms of how these pressures all interact with this structure and this geometry to generate what is a very complex stress concentration gradient, strained gradient-- meaning a mechanical strain-- in the optic nerve head region. And the interesting thing about that is that from the perspective of pathology, glaucoma is a disease of the optic nerve head. So that's where in every animal model and humans, it's been shown that damage to the retinal ganglion cell axons, those axons that are taking visual information from the retina to the brain, occurs at the optic nerve head. It occurs as those neurons, those axons, leave the eye through the lamina crimbrosa. So we study that. I want to talk a little bit about scleral material property. So this is the white of the eye. And the reason the sclera is important is because even though the damage happens at the optic nerve head, the sclera around the optic nerve head, the peripapillary sclera, actually forms the mechanical boundary condition for that very susceptible region. So we study the sclera. Things to know about ocular soft tissues are that they're both anisotropic and non-linear. So nonlinearity means that collagin fibrils that make up the tissues is it gives them their stiffness, they start off, they're coiled like a spring. And as you stretch them, they uncoil. And as they straighten, they get very stiff. So as intraocular pressure goes up initially at low IOPs, you get a lot of strain, a lot of stretch. And then as those fibers stretch out, they become very stiff. And the eye stiffens quite a bit. And you get very little deformation with subsequent move an IOP up from a moderate range to a high rate. Then you have the issues of anisotropy. So this is collagen fiber alignment. So this is very much like reinforced concrete or ropes in jello is the way we like to talk about it. And basically, this is the concept that collagin fibrils sit in an otherwise gel like matrix. And it's stiffer in the direction of those fibrils. So if the fibrils are randomly aligned, you have stiffness that is randomly aligned. In other words, the tissues are stiff in every direction the same amount. If you have them perfectly aligned like a tendon or a ligament, then the tissues are only stiff in the direction that those collagin fibrils are aligned and they're not very stiff in the directions that they're not aligned. So the sclera is kind of a mix of this. There is some preferential alignment anisotropy, especially around the optic nerve head. We're going to talk about that a little bit later. So if you look at the effects of fibril orientation, we do some computational models. So what you'll see is the effects of fibril orientation on a computational model, just a spherical model of an eye. And what we did here was do sort of three experiments, computational experiments where we computationally aligned the collagen fibrils circumferencially way around the optic nerve head helicoital-- so sort of in a sort of curved pattern away from the optic nerve head, and then the longitudinal, which is sort of radial or meridional away from the optic nerve head. And you can see the three different types of displacement that those fibral orientations engender in what is otherwise a round vessel. So it turns out that the eye has some mixture of all three of these different anisotropy patterns. And those all concentrate so it's more circumference around the optic nerve. And that keeps the scleral canal from expanding when IOP is elevated. And you also have some more longitudinal fibers around the equator. And that keep the eye from dimpling out like you see in the top figure. The problem is that the eye is not of uniform-- it's not spherical and it's not of uniform thickness. And so you have very varying thickness properties around the optic nerve head, especially. They're very complex. And the lamina cribrosa is even more complicated. So if you look at these two models, what you see is a circular optic nerve head embed in a perfectly spherical pressure vessel with uniform thickness. And that's what gives you the stress patterns that you see in the left photo. And so high stress is red. It's concentrated around the optic nerve head so we get the concentration that we've talked about earlier. But it's a very nice pattern. Now, if you in induce the variable thickness and the variable optic nerve head geometry from circular to ellipsoidal, what you end up with is a very complex stress pattern. And it's very regional. And this is the way the eye really is. So again, not easy to use to figure out what the stress patterns are without knowing the individual characteristics of a particular eye. And we're all different. So what we see here is a picture of an electronic speckle pattern interferometer. And this is my colleagues Luigi Bruno and Massimo Fatsio. And this is a laser based speckle pattern interferometer. And what that means is it casts a speckle pattern onto an inflatable scleral shell. You'll see that down in the photo. And with that speckle pattern, we can measure the three dimensional deformations of a scleral shell under inflation at very, very high resolution. So 30 nanometer resolution, which is extraordinarily difficult. When you do that sort of experiment and you inflate with this interference pattern, what you see is this interference pattern generates fringes. And as you-- these are four camera shots of the different fringes that we're acquiring at 30 shots per second as we inflate the eye. And you count the distance between the fringes. And that actually tells you how inflated the eye actually is and where those inflation, where those deformations are actually happening in three dimensions. And you can generate from those for defamation patterns, you can generate scleral strain maps. Now, strain happens-- strain is mechanical strain. So that's a percent stretch or percent compression or percent sheer which is a deformational strain. And you can see the pattern here of the strain in a single optic nerve head around a single optic nerve head in a single posterior scleral shell from a single human donor. So these are all the different strain patterns. But we combine those patterns just to-- you know, that's sort of hard to get in your head all these different components of the strain. So we combine that into a maximum principal strain, which is sort of one map that gives you an idea of the sectorial variability in the strain in the peripapillary region. So in the bottom right figure, you can see the maximum principle strain, which is basically a percentage stretch. And so you think about the red areas, the scalar is stretching 2.5%. And in the blue area, it's stretching something below 1%. So what you can see here is the peripapillary region, which is that region within the dotted line, and then the mid posterior region, which is the region in between the peripapillary sclera, the dotted line and the solid line. And then we've split that up into superior, inferior, nasal, and temporal, and also the off quadrants as well. So we have eight regions that we look at. If you look at what happens with age-- so glaucoma is a disease of the elderly. Onset is typically 60 to 65 years old and increases as age increases. And so what we did was we did a study where we looked at the distribution of peripapillary scleral strains and how those changed with age in human donor eyes. And it's a very interesting pattern emerges. So two things to note on this figure. One is that overall, the strains are lower in an eye that is 90 years old versus an eye that is 20 years old. So a 20-year-old is much more compliant than a 90-year-old eye. But one thing to note is the only place where the old eye was more compliant then the young is in the inferior temporal quadrant. And that's a really interesting spot in glaucoma. And the reason it's interesting is because even in normal eyes, if you look at the left hand diagram, what you see is the rate of neural retina rim area measurement change over time in a normal control population. These are in vivo clinical measurements. This was published a while back. And then on the right, what you see is our map of age-related strain change as a function of quadrant in human donor eyes. So what this tells you is that the pattern of neural retinal rim area measurement change with age matches exactly the pattern of peripapillary scleral compliance change with age in normal eyes in a human population. The other interesting thing is is that glaucoma is a focal disease and the inferior temporal quadrant is usually associated with wedge defects and arcuit changes in the visual field and glaucoma, especially early onset. And some of that is driven by optic disc hemorrhages. And the prevalence of optic disk hemorrhages is also much more prevalent in the inferior temporal quadrant than any other quadrant. So these are all what we call associative results in the sense that we don't have causation. We don't know why. It's just very interesting that the peripapillary scleral strain changes we see match other things that are happening inside the optic nerve head with age. So, another thing that we've been working on is changes with these parameters with race. So, African Americans, or persons of African descent-- don't have to be American-- get glaucoma or develop glaucoma at twice the rate-- so have double the prevalence of glaucoma-- then whites. Given all of the other things including access to health care, diabetes, all the other comorbidities, it still sort of pops out that African Americans have double the prevalence of glaucoma than whites. So if we look at the change in the peripapillary scleral strain with age, what we see is the solid blue line, which is how the peripapillary sclera changes in persons of European descent. So it stiffens with age just as we would expect from our previous result. But the peripapillary square from African Americans actually stiffens at a much quicker rate then that in persons of European descent. So another sort of clue that these variables are changing. And not only are they changing with age. They're also changing differently or differentially with racial origin. OK, so once we have the deformational changes that we know happen with age and race, how do we get to the mechanisms underlying those changes? So we have observational changes. Now we need to understand what happens with the sclera within the sclera that's driving those changes with age and race. So we do what we call scleral material property thing. So we need to know-- we don't just need to know about its behavior. We need to know what's underlying that behavior. So what we do is we fit computational models-- and this is an iterative process and we do this in the computer-- where we take the experimental deformations-- and you can see those radial, circumferencial, and meridional displacements on the top row on the right. And then we fit, iteratively, a computational model. And we change the parameters until the model matches the patterns that we see in the experiment. And then we also are at the same time predicting the collagen fibril anisotropy-- the architecture of the collagen fibrils especially as it relates to the optic nerve head. And you can see the ring of collagen fibrials, the circumpapilary scleral ring, there. And that's the red ring around the optic nerve head in the left hand photo. And what that tells you is that we have that ring that everybody knows exists from histology. And that's reinforcing the optic nerve head endings in these scleral shells. So once we have that, then we can pull out the parameters from those scleral material properties, the best fit properties, and look at how those change with age and race. And when you do that, we see two things sort of pop out of this analysis. One is the sheer modulus, which is the stiffness of the matrix not including the collagen fibril-- so just the glue around the fibrils-- how that changes with age. So you can see in the top left graph that the red diamonds are persons of African descent. Black circles are those of European descent. So those are both climbing with age. So as the sheer modulus goes up, the sclera stiffens. The matrix of the sclera stiffens. So the glue between the fibrils are stiffening. And there is a significant difference between the European population and the African population. So we know that those are sort of two different things that happen here. And what that really means is that with age in a European population, you see a change in the IOP versus strain curve. So the amount of stretch in the sclera versus interocular pressure changes with time. So that goes down. So as humans age, basically the sclera gets stiffer. So we knew that. But we know now that some of that is due to a change in the stiffness of the glue binding the collagen fibers in the sclera. And the second thing that changes is the collagen fibril crimp angle. And a crimp angle is a measure of how coiled the collagen fibrils are at a particular point in time. So if they're very coiled, then you can stretch them a whole lot before they stretch out and stiffen. And if they're not very coiled up, so the crimp angle is pretty low, what that means is that you only have to stretch them a little bit before they straighten out and get very stiff. And so you can see in the lower right graph that the crimp angle decreases with age. So we have sort of a lower level of compliance before things get really stiff. And that happens with age. And also, again, a change in the difference between persons of European descent and persons of African descent. And African descent folks have a lower crimp angle to start. And they also have this age related effect. Kind of the overall message here is that we think of the scelra as a very uniform scleral shell much like a basketball. It's not. It's very, very complex. Its mechanics are complex. But it's also a moving target. So we know that from studies in animal models that the sclera thins in response to chronic IOP elevations. We know that the material properties of the sclera change in response to chronic IOP elevations in an animal model. And those changes can be individual specific. So we've done some work there. We know that as we age, the sclera stiffens. And the sectorial pattern of peripapillary scleral strain reverses to match the sectoral pattern of highest nuero retinal rim loss rate and the highest incidence of disc hemorrhage, which are-- it's again, associative result, not a causative result. But still very interesting. And the whole point here is that the sclera is really a living tissue. And we need to think of it in terms of the tissue that's remodeling in response to both physiologic-- so like an aging response-- and pathophysiologic-- so more like an elevated intraocular pressure stimuli. And that changes it's biomechanical response over time. So now we're going to move on to the optic nerve head, which is really the question. We all know that the damage to the axons happens in the optic nerve. And what we really need to know is what's driving that damage. So we know that the sclera provides the mechanical boundary condition for the optic nerve head. So let's talk about the optic nerve head itself. It's an extraordinarily complex structure. And what you have is a hole in the scleral shell. And then spanning that hole, the neural canal, is a fenestrated, sort of mesh like connective tissue structure called the lamina cribrosa. And the lamina cribriosa is a very interesting structure. And it's the only endorgan capillary bed under the kind of strain that these tissues experience in the entire body. So it's really unique. So each of the those load bearing lamina cribrosa beams, most of them contain a capillary. That's where the axons that are coursing through that region and going through this very complex pore network, that's how they are getting their nutrients and blood supply and their metabolic demands met is through these capillaries that are providing nutrients through the load bearing tissues them selves, which is a really interesting thing. And the interesting thing about glaucoma is one of the hallmarks of the pathophysiology is what they call glaucomatous cupping. So you get to the point where you have, if you look at the cross section of an electron micrograph of an eye that's been-- all the neural tissue has been digest away so all you see is the connective tissue structure-- you see this nice connective tissue structure. You have the normal eye. The lamina cribrosa sits just under the ledge of the sclera. But when you get glaucoma, that structure gets all cupped out so it looks like a big bean, like a half of a bean pot. And you get the feeling that you could kind of stick your fingers up underneath the edge of the sclera into this pot of the sclera. So that's called glaucomatous cupping. And it's a hallmark of glaucoma that is very different and distinct from the things you see with other optic neuropathy such as an anterior ischemic optic neuropathy or arteritic schemic optic neuropathy. So there are other optic neuropathies that cause nerve loss, but they don't have the same morphology. So that's a very glaucoma-specific thing. So to study the morphometry, what we do is we use a device that we call the cutter. And it's really a three dimensional florescent episcopic image capturing device that's automated. And what that allows us to do is to reconstruct the tissues at very, very high resolutions in the optic nerve head. So this is very similar to like an MRI scan or a CAT scan. But our resolution, instead of being five millimeters, is 1.5 micrometers per voxel. So it's a volumetric, three dimensional, histologic reconstruction. And we use this episcopic fluorescent image capture device to do that. So here, you see an image of the device. We chuck basically a paraffin embedded optic nerve head into a microtome. That microtome slices the optic nerve tissues away at 1.5 micron thickness. And we image what's left in the block face using a very high resolution camera at 1.5 microns per voxel. So this is what you get. And this is a very interesting movie. So this is a three dimensional reconstruction of an optic nerve head And what you see here is the retinal layers. You see the choroid. Now in the outside of the image, you see the sclera coming in. And then we'll run into the lamina cribrosa. And so this is basically a 3D movie fly-through through a serial histologic reconstruction of the lamina cabrosa itself. And you see how beautiful of a structure it really is and how complicated it really is. So if you then take those tissues and you separate out the lamina cabrosa from the rest of the tissues, then we can start to study it. So we've developed this technique. So this is a digital slice through that stack of images. So it's about 1,200 images that we've stacked and aligned. And you can see this is a digital slice through that stack. And you see the sclera, the lamina cribrosa, the retina, its layers, the prelaminar tissues, retro laminar optic nerve. Very beautiful structure, and it can be very accurately reconstructed using this technique. And much more so than you could get with, say, an in vivo image such as optical coherence tomography or something like that. So this is a three dimensional reconstruction of a human donor lamina cribrosa. And what you see here is is that through this movie as you see, it's extraordinarily complex. Lots of regional differences in the laminar microstructure structure itself. And what that tells us is we need to be paying attention to the regional differences in the laminar microstructure. And this next image is basically to make the point that within a single three dimensionally reconstructed optic nerve head, you have huge regional variations in the laminar microarchitecture itself. So these are areas in the top right. You see a sort of a call out box that shows you a region of the optic nerve that has relatively kind of an even distribution of pores and beams-- no particular orientation to those beams. Then in that central right-hand region, you see beams that are clearly oriented left and right and a lot more dense compared to the pores. And then the bottom region which is on the bottom left, the [INAUDIBLE] there, you see beams that are clearly oriented top to bottom and also extraordinarily dense compared to the other regions in the eye. And what that means is in this bottom region, for instance, that the strain, the mechanical strain in that area, is going to be-- it's going to be much stiffer in the top to bottom direction and much stiffer than the other parts of the eye that have less connective tissues to bear the load. So one of the ways that we attack solving the mechanical problem that is the lamina cribrosa-- so I've talked about how extraordinarily complex the micro architecture is. We can build computer models of these. So these are things that are very difficult to see in vivo. So it's very difficult to do clinical imaging that's going to let you know anything about the laminar micro architecture itself. But what we can do is build computer models. So in this image, what you see is sort of our pipeline for building what we call fine ailment models. It's a computer mechanical engineering model of individual optic nerve heads, individual eyes. So we take the optic nerve head and we insert it from an individual eye that we've reconstructed. We insert it into a more generic but still anatomic scleral shell. Within that optic nerve head is the lamina cribrosa. And we've assigned stiffnesses based on how much tissue is in each region. That's what you see in the bottom right. CTVF which is Connective Tissue Volume Fraction. So laminar density. How dense is the lamina in that region? And anisotropy. Which direction are the beams going and how preferential are those beams? And the stiffnesses are then directional and density based. Put that all in a big computer model of the posterior half of the eye and then pressurize that computer model and see what happens to it. So we do this for a bunch of different eyes. And you get some really interesting results. The first thing to notice is on the top thing. So this is contour map of the particular variable shown on the left and the call out of the optic nerve head itself. So what you see here is the displacements are concentrated in the optic nerve head itself. And that leads to very high strains. So very low strains, very low stretch in the sclera surrounding the optic nerve head. And that stretch is concentrated at the optic nerve head. That said, the optic nerve head is a relatively protected from stress. So most of the sclera is bearing the load. So intraocular pressur is coming in there. The force is distributed. The sclera is really bearing that load. But the optic nerve head is so weak that it still has lots of stress and lots of deformation, lots of displacement. So the eye is doing what it can structurally to protect the optic nerve head. But it's still not enough to not have it have a lot of mechanical strain, a lot of mechanical stretch. So if we want to get to what really happens and why that's important is that we can correlate the local laminar density with mechanical strain and stress. And the reason this is important is we are coming into the age where we're going to be able to measure laminar density with in vivo clinical imaging methods. And what this means is that for those regions where there's lots of laminar beams, you have the lower strain. And where there's fewer laminar beams, you have higher strain. So these are imaging targets. We know that glaucoma's a focal, sort of sectorial disease. And if you can look into someone's eye and say, oh, Mrs. Jones, you have a particularly sparse laminar cribrosa in the inferior temporal quadrant of your left eye. We're going to have to really watch that. That gives you additional information as a clinician to make decisions about managing Mrs. Jones' care. And that's also true with stress. Stress concentrates in those areas and are borne by those areas with higher laminar density. So even though the higher density laminar beam regions are carrying most of the load, most of the strain is still occurring in those regions and have lower laminar densities. If you want to know what's really going on at the level of the lamina cribrosa beams, now that's really the question that I think all of us as engineers want to know. So we can do these sort of more macro scale models that I've showed previously of the whole posterior scleral shell and what happens sort of within a general sort of macro level homogenized sense inside the optic nerve head. And that can even be regional. But we really want to know is what's happening at the level of the laminar beams. So what we can do is take out a single section or a single finite element from that parent model and build a child model. And that child model has the actual architecture of the individual laminar beams within it. So we use the deformations from the parent model to impose upon the child model. And then we look and see what happens to the beams in the child model when we impose those deformations. And what you see here is a movie of a section of laminar beams, just about 1/50 of the laminar cribrosa. And this is associated with an intraocular pressure increase from 10 to 45 millimeters of mercury very much in what we would consider the normal range. And what-- the color code in the movie is the strain. So higher strains are reds, lower strains are blues. Point here is, what you can see is that the laminar cribrosa, first of all, deforms a lot as an overall structure. Secondly, those deformation and strains are very local to certain parts of the beam. So you can imagine in those areas where we have high strains, that's where the cells on that beam are going to be upset. That's where the capillary going through that laminar been is going to be disrupted and have a lower blood flow. And that's where the axon going through the pore adjacent to that high strain area is potentially going to be disrupted. So these disruptions-- the interesting thing is they really cause a huge feedback in terms of remodeling of the laminar cribrosa tissues themselves. So what you see here is a pair of [INAUDIBLE] eyes in an animal model where one eye of the animal was given to glaucoma-- that's the eye on the left-- and the other eye serves as a normal control. So this is never a study you could do in a human patient. But we can do this in animal models. And so what we've done here is induce glaucoma in one eye leaving the other eye as a normal control as I mentioned. And after just a few months of elevated intraocular pressure, this is what the laminar cribrosa reconstruction looks like from the glaucoma eye compared to the normal eye. What you see here-- it's hugely different. The curvature has changed. The size has changed. The density of the tissue is changed. And in fact, the early glaucoma eye in this particular animal generated-- had 55% lamina cribrosa by volume than the normal eye. And the curvature was very different, as was the overall diameter. So things are really changing in the laminar cribrosa very, very early in response to chronic intraocular pressure elevation exposure. So we don't know, again, what is driving these things. What are the mechanisms behind this laminar modeling that we've measured experimentally? What I showed you was an experimental observation. So we turn back to the modeling. So we know that the laminar cribrosa thickens initially in early experimental glaucoma. We know that the laminar cribrosa migrates outward in early glaucoma. We know that there's posterior deformations of the lamina in early glaucoma-- connective tissue volume increase. And we know that that mechanism happens by the eye adding more laminar beams through the laminar thickness by recruiting retro-laminar recepta into the 3D load bearing laminar structure itself. So how do we figure out what's really driving this? And again, we're turning to computer models to do this. So my colleague, Raphael Grytz, who's an assistant professor here at the University of Alabama Birmingham School of Medicine in the department of ophthalmology, very talented engineer, has come up with a remodeling rule that he can apply to finding element models in which basically the model changes itself in response to strain. And so what happens is if you overstress the collegan fibrils, his models want to add tissue in that region, add collagen in that region. And if you use this as a feedback loop, then you can get into a situation where you have an overstretch of the collagen fibrils. The model will add collagen fibrils in that region and then bring the overall strain state back into what we consider the homeostatic range. And so again, Raphael run these models. And the results are really extraordinary. It's the first time we've really tried to understand the mechanisms underlying these experimental observations we're seeing in experimental glaucoma. He starts out with a model that model the neural canal tissues with a constant collagen fibril density throughout the tissue. So no laminar cribrosa-- just a sclera and the neural canal issues. And then we look at the homeostatic strain. We inflate those eyes to 15 millimeters of mercury. And you see a huge strain field develop in what would be the area of the lamina. And once that strain field develops, his model remodels the tissues in the model itself, lays down more collagen to create a more homeostatic strain state. And so what his models predict is mammals-- humans-- need a laminar cribrosa to have a homeostatic strain environment in the optic nerve head. So his models predict the formation of a lamina-like structure at normal levels of intraocular pressure. So if then we take those normal eyes and we elevate the pressure to 25 millimeters of mercury-- so somewhat elevated-- what you can see is the collagen fibril strain goes way up, right? And that's what we would expect. Get to mechanical deformation due to IOP, strength goes up. Well, it lays down more tissue. And what do we see? His models predict that the lamina thickens just like we see in the experiment. So one of the things we're thinking is overstretch of the collagen fibrils actives the cells. Those cells lay down additional tissues. And that serves to sort of balance-- try to rebalance the homeostatic environment in the optic nerve head. So a lot of this remodeling is driven by a mechanical signal. So one of the other things that has recently come to pass is there's been a few studies that have shown with imaging that when you elevate intraocular pressure in a human patient and you image both before and after the pressure was applied, what you see is the lamina cribrosa doesn't deform rearward, right? So you're thinking, oh, well, we elevate intraocular pressure and the lamina should move back. We should see the optic nerve head deform rearward. It's the soft spot in the eye. That's where the pressure should go. That's not exactly what happens. What we've seen experimentally is that there are two simultaneous actions that are happening and they balance each other out. So the first thing that happens is you elevate intraocular pressure. And that would serve to push the lamina posterially. But at the same time, when you're doing that, you get a slight scleral canal expansion. And that pulls the lamina back taut. So it's been thrown out there in several publications, several studies, that, well, that must mean that the lamina itself, the lamina cribrosa, is not under mechanical strain. Well, that's not true. In fact, the laminar strain is very large. And you just can't see it in these in vivo images. What's happening is the lamina is getting stretched by an expansion of the scleral canal. And that stretch is very significant even at normal levels of intraocular pressure. So final sort of wrap up from the laminar side is to understand that the lamina is also a moving target. It responds-- it changes with age. So we know that it remodels with age. We know that it remodels in response to chronic IOP elevations and that that remodeling is quick and very large. And so what we're getting is the idea that both the sclera and the lamina are really changing a lot in response to both physiologic and pathophysiologic signals. What's really driving the bus, though, and what we haven't really talked about, is intraocular pressure. So everybody assumes that-- or it has been assumed to date-- that in general, intraocular pressure is a relatively steady state variable-- that it's highest at night, lowest during the day. But in general, it's a thing that changes very slowly. It's relatively static. Now, there is some change-- two to three millimeters of mercury at night, sometimes up to five or six depending on the patient. But in general, we think of this as sort of a circadian rhythm-- very slow changing variable. So what we've done here at UAB is develope an intraocular pressure telemetry system for an animal model. And what we can do is implant this system into the orbit of an animal and put a tube into the anterior chamber and read their intraocular pressure in both eyes as well their aortic pressure at the same time. So we can get intraocular pressure bilaterally, aortic pressure-- blood pressure-- by having a transducer in the aorta. And from that, we can calculate ocular profusion pressure well. And this system reads these variables at 500 times per second, radios that out to an antenna, and we collect those data 24 hours a day for about two years. This is a photo of the transducer and how we do it. Just give you an idea about how we do this, it's very non invasive in terms of the eye in general. We put-- the only thing that's invasive to the eye is a small tube very much like a bar belt around that shunt tube into the anterior chamber. And that goes to a transducer that's affixed in the orbit. And that's where the pressure is read. And we also have electrooculogram electrodes above the eyes as well. This is a video of the signal from our new system. So this is, you see, electrooculogram left and right on the top two traces, the aortic blood pressure in the middle, and intraocular pressure left and right in the bottom two traces. And what this gives you an idea of-- so pressure here is 10 to 12 millimeters of mercury baseline. You can see the ocular pulse amplitude. So intraocular pressure changes in both eyes associated with systolic blood filling at every heartbeat. You can see these peaks which are due to blinks and sicods. And we've actually gone to the trouble of videoing these research subjects and correlating the video to the intraocular pressure signal that we're seeing here. And every time you move your eye, every time you blink, you get a huge pressure spike. And those pressure spikes are very frequent and also very large. So they don't last very long. In other words, we're talking about presures spikes that last somewhere between 100, 300 milliseconds. But there's lots of them. And they're about 100% of the total baseline IOP signal. So about double. So if you quantify those signals over time-- so we've got a few things. We've looked at this in multiple time scales. So most people come into the clinic. They get their pressure read with the new myotonometer, snapshot measurement. Sit there, Mr. Jones. Don't blink. Don't move your eye. We're going to take your intraocular pressure, blow a puff of air on your eye or touch you with a tonometer. They read your pressure and they send you on your way. And you come back even as a glaucoma patient maybe three months if you're like and probably more like six months later. And the clinician assumes that your intraocular pressure is the same or relatively static in that timeframe between those visits. Not true. So what we've seen in these animal models is intraocular pressure is extraordinarily dynamic. And even when you take out those short term pressure spikes that I showed in the previous video and you're averaging data for every 10 minutes-- so average it-- and then you plot that for a 24 hour period, it's extraordinarily dynamic. It's changing all the time. And so if you do a histogram, what you realize is that, yes, in this particular animal, IOP was 11 to 12 millimeters or mercury for most of the time, but spent a significant amount of time at seven or eight millimeters of mercury and some significant amount of time at 15 or 16 millimeters of mercury within this single 24 hour period. And if you look at days over time in two hour time windows, what you realize is that on Monday, an animal might be at 10 millimeters of mercury on Monday on average throughout the day, be at 20 millimeters of mercury on Wednesday, and then back down to 10 millimeters of mercury on Friday. And there's no repeatable pattern to this at all, which means that snapshot intraocular pressure measurements are not very good if these data are to be believed and they translate well into the human condition. If you then average all those data across a single animal and you look at it, there is a nickname or a rhythm. So we do see elevations in pressure at bed time for the animal-- so 7 PM. So at night, their pressure is a little higher. They fall during the day, which is very similar to the human. But that rhythm is not real, real strong. So let's talk a little bit about the eye as a pressure vessel. So now that we know that intraocular pressure is extraordinarily dynamic, we need to talk about how the eye is responding to those dynamic peaks in intraocular pressure. So we need to start thinking about the eye as sort of a dynamic, energy absorbing pressure vessel. And in this particular graph, what I'm showing here is that as intraocular pressure is raised, so this is about six seconds of data all taken within about five minutes, as we're raising intraocular pressure within an eye that's been instrumented, what we see is that intraocular pressure goes from 3.5 to 54 millimeters of mercury and the ocular pulse amplitude, which is very tightly regulated in terms of ocular blood flow, goes from zero to about 1.8 millimeters of mercury. And what this tells us is that as intraocular pressure goes up and the eye stretches, it's less able to absorb these perturbations and therefore less able to have lower IOP spikes. So as the eye loses its elastic ability to expand and absorb pressure elevations-- expand energy, absorb energy-- it results in higher IOP spikes. So the energy absorption ability of the eye decreases as the ocular coat stiffness increases. And this happens as we age. It also happens as the eye stretches when we have elevated IOP. So the basal level of IOP influences the amplitude of these IOP changes that are generated by external forces like blinks and eye movements. So if we want to quantify how that really works, what we need to do is figure out a way to quantify these high frequency intraocular pressure fluctuations. So if we use an engineering technique, a signal analysis technique, we could use a finite impulse response filter, find all the peaks and valleys, and then count them-- count the peaks-- and then also figure out the energy that's within just those peaks compared to baseline. So we can calculate the area under the IOP curve over time just associated with the peaks and sort of subtract that out or add that to what is the sort of more baseline that what we traditionally think of in intraocular pressure. And what you see here is that if you look at that peak histogram of one hour of data, you can see that there were over 4,000 peaks between one and two millimeters that hit the eye in a single hour. And if you add up all the peaks that happened over-- that are over five millimeters of mercury above baseline, what you see-- there's several thousand of them. So your optic nerve head an your sclera are getting hit 3,000, 4,000, 5,000 times an hour during waking hours by these IOP spikes. And if you look at the energy associated with those spikes, I mean, most people say, oh, those spikes, they're short. Yes, they're big. But they can't really mean anything. And if you look at the energy associated-- so this is the total IOP energy that the eye has to absorb-- these are three different research subjects. You're looking at left and right eyes by color. And you calculate the number of spikes. And the percentage of the IOP, the total IOP signal due to spikes versus steady state IOP, what you see here is the hourly energy associated with IOP spikes as a percentage of total IOP. So what you see here is that IOP spikes alone, just those little spikes I was showing you, account for up to 15% of the IOP energy that the eye must absorb during waking hours. Now, that's a huge amount. Most clinicians would say if they can reduce IOP with a surgery or a pharmacological intervention, a 30% reduction is a great clinical outcome. And we're talking about 16% of the signal that's not even on anybody's radar. And this is what might be, we think, clinically relevant. So IOP dynamics. What do we know now? We know that IOP is much more dynamic than has previously been appreciated. And that occurs on the second, minute, and hourly time scales. And the pattern of intraocular pressure fluctuation varies from day to day. So a measurement you're going to take Wednesday at 2 o'clock is not going to be measured like a measurement you'd take Wednesday at 2 o'clock in the following week in a human patient. So what this means really clinically is snapshot intraocular pressure measurements are wholly inadequate to characterize true intraocular pressure or it's fluctuations on any time scale, which means that I don't think we really know what intraocular pressure really is in glaucoma patients. And that may underlie the reason why the glaucoma intraocular pressure relationship is a bit fuzzy. We just don't-- we're not very good at diagnosing glaucoma, and we don't know much about intraocular pressure. So that leaves us in this conundrum of not being able to understand how IOP is really related to glaucoma in any real, detailed way. So we also know that the magnitude of intraocular pressure fluctuations are in part determined by the structural stiffness of the eye. And we know that the structural stiffness of the eye increases as it stretches with IOP increases. So if you have a high level of IOP, you're going to have high spikes. We also know that the corneoscleral shell stiffens with age. And as a result, the sclera is much stiffer. And therefore, IOP spikes are going to be bigger in the elderly. We also know that the sclera has been shown to stiffen in eyes exposed to chronically elevated IOP in an animal model. So what that means is if you have a history of high IOP, then the amplitude of those IOP spikes due to blinks and sicads is probably larger in your eye as well. So the real question is, for us, is high frequency IOP fluctuations should be higher at higher basal IOPs. They're higher in the elderly, and they're higher in patients with a history of chronically elevated IOP. So there is a IOP telemetry system out on the market for human patients. It's in clinical trials in the US. And it's called the Triggerfish. And it's from a company called Sensimed AG out of Switzerland. And what this is is a contact lens that has a stretch sensor embedded in the circumference of it. And it measures circumferencial stretch that are presumably due to changes in intraocular pressure. And this is a great device except that it doesn't really tell you what intraocular pressure really is. All it can tell you is what it's high and when it's low. So in terms of clinical value, there's not really a system right now available for clinical human use that gives you true intraocular pressure, number one, and with any measurement frequency that can tell you anything about these fluctuations. So we need to sort of think about how we might develop a system that would be useful in humans to measure these sorts of things. So let's talk about just a wrap up of biomechanics and glaucoma. And it's basically just a really complicated story. We think that scleral and optic nerve head biomechanics are central to the development and progress of glaucoma and they likely play a critical role in individual susceptibility to the level of IOP in a particular patient's eye. So it has to do with individual susceptibility to intraocular pressure. We also know that the sclera and lamina exhibit regional densities, thicknesses, and material properties. And hence, the mechanical stress and strain are also local. And they're very individual specific in different eyes. But we know that the structural properties of the lamina cribrosa and the sclera change with age. And they're actively remodeled beginning very early in glaucoma. So we have really a very difficult task in terms of trying to assess this disease. So we need better clinical instruments that are going to get at some of these issues. We know that intraocular pressure is much more dynamic than is currently appreciated. And we know that high frequency IOP fluctuations are larger at higher basal IOP, higher in stiffer eyes-- so the elderly and those with a history of high IOP. And what we want to know really is, why isn't the IOP glaucoma relationship better? And one potential explanation is that the cumulative effects of larger high frequency IOP and ocular profusion pressure fluctuations play a role in glaucomatous damage. And this is just not something that we appreciate right now. We have no way of measuring it. And so one open question is, could this explain in part why the elderly are more susceptible to glaucoma and those with existing glaucomatous damage tend to progress more rapidly? I'm going to leave you with that. Sort of the wrap up here is that we've got a very complex relationship between ocular biomechanics, IOP, optic nerve head susceptibility, and laminar biomechanics at the cellular, mechanical, and schemic level-- so vascular levels-- and that this is also wrapped up together. There is no separation between the mechanical theory of glaucoma and the vascular theory glaucoma or the cellular theory of glaucoma. They're all really one pathophysiology. And those things are all interacting through intraocular pressure and the biomechanics of the eye to influence its pathology and its homeostasis.