Lecture: Decoding Astigmatism

DR ROY: Hello, everybody! I am Aravind Roy. I’m a cornea consultant at the Tej Kohli Cornea Institute, LV Prasad Eye Institute. So today’s topic of astigmatism is very interesting, because this is a very commonly encountered clinical condition. The astigmatism encountered in the eye is due to inability of rays of light to come into a point focus on the fovea. It is the sum total of the internal astigmatism of the eye, composed of the lens and the corneal astigmatism. Our discussion for today will focus on the corneal astigmatism, primarily. And when we discuss astigmatism, it is important to understand that the point rays of light through horizontal and vertical meridians come to a focus in different entities of myopia and hyperopia, either in front of or behind the eyeball, and in astigmatism, each meridian come into focus differently. This creates a beam of light, and the refractionist tries to bring them to a focus, so as to enable accurate and clear vision. As we proceed further into the talk, could you please indicate your position? Okay, thank you. So what do you think about the shape of the cornea? How would you best describe it? So I believe that all of you are correct. Because the cornea has a complex three-dimensional geometry. And probably all the points that have been mentioned here would truly describe the shape of the cornea. So the normal cornea would be more like an egg. It is both aspheric — because it is not spherical. It is prolate. We’ll come to that in a moment. It is toric, because the meridians are different, each having a different curvature. It can be aspherotoric. It’s also an ellipsoid. This is so, because when we understand that these are conical structures, and they can be described by different mathematical models. So as our understanding of the cornea improves, our ability to calculate the shape of the cornea and therefore have a better topography of the cornea, improves. With improving technology. How important do you feel — it’s important to correct the astigmatism of the cornea? Should we correct all the astigmatism? Or should we leave behind some? Okay. So it’s important to correct astigmatism, but as well it is important to understand that we should not correct all the astigmatism. This is because the astigmatism adds to the depth of focus. Studies have found that eyes which have a small amount of myopic astigmatism, especially postcataract surgery, tend to have more depth of focus, and therefore that is also used as a strategy to aid near vision in the pseudophakic eye. So to understand corneal astigmatism, it is very important to revisit the corneal topography and have a look at how the cornea in health, or the normal cornea, looks like in the topography map. The central cornea has a steep feature, which is elevated above the line, which produces the central island. The paracentral cornea is much flatter, which creates the sea in the elevation maps, and the extreme periphery is again steeper, creating the peripheral high-rises. This is the prolate shape of the cornea. It is not spherical. It is aspheric. Sometimes it’s called a toric surface. So the toric surface would have one meridian which is steeper and one meridian which is flatter. This causes the power of the cornea to be at 90 degrees to that. Therefore, if you look at the elevation and axial curvature maps, they not only look different — they look almost reversed. This is a very important concept in understanding the normal cornea. So there are several eyes which are with the rule. That means that they have a with the rule astigmatism, with a steeper vertical axis. And there are some which are against the rule. And there are other eyes which have oblique astigmatism, meaning that they do not conform to vertical or horizontal meridians, but they conform to oblique meridians. And the normal cornea can be differently shaped, and studies on the corneal topography patterns have found that there are symmetric and asymmetric bow ties, which are again a variation of the normal, in addition to the irregular topography patterns that are found. So where should you place a limbal relaxing incision in the cornea? The limbal relaxing incision often tends to change the shape of the cornea, and this is how a surgeon tries to manage astigmatism. Should you place it in the steeper axis? Or should you place it in the flatter axis of the cornea? Good. So one should always place it in the steeper axis of the cornea. Why? How do we understand limbal relaxing incisions, and how does the cornea behave to such interventions? The cornea flattens adjacent to any incision. It flattens over sutured. It flattens adjacent to loose sutures, and steepens adjacent to tight sutures. So what happens if there is a cut in the cornea? The apex actually tends to flatten down. As seen in the animation above. And if you suture that laceration, then that area of the cornea becomes flatter, and the apex tends to tent up, and also move away from the sutures. So any area that is sutured tends to flatten that area, the corresponding area of the cornea. And when you place a radial incision, then the flattening of the cornea actually occurs along the axis of the incision, as shown by the black arrows in diagram A. And if you make circumferential cuts, then the apex steepens. These are the principles that are used earlier, in doing radial keratotomy, used in the pre-LASIK or the pre-Excimer laser area. So you can manage astigmatism by making limbal relaxing incisions, you can use a rule of thumb by making use of several incisions. You can use 6 millimeter incisions for 1 to 2 diopters of astigmatism. Make two cuts for 2 diopters. And for 3 diopters, use a slightly longer LRI, and use 10 millimeter lengths for 4 diopters. These are sometimes variable. Different nomograms exist. But what is important to understand is that you have to place them at the steeper axis, and you have to place it just inside the limbus, at the 80%, 90% depth. How do you place the limbal relaxing incision? Or how do you place the astigmatic keratometry cuts? You need some basic instruments, such as fixation rings, and the markers. This is a surgery that is being performed for managing astigmatism by performing astigmatic keratotomy. It is marked with the marker, and then the diamond knife is used to make cuts along the marked areas. So you can see that the surgeon is slowly progressing to involve the entire length, and two incisions are given adjacent to each other. And once you have done that, it is important to check that the incisions are of adequate depth. Because that’s what will be a very important factor in helping you know that your surgery is going to be successful. And in the end, you have to use an operative keratoscope to look at it. So there are several common clinical scenarios that we encounter in our day-to-day practice, such as keratoconus, where there is a non-inflammatory progressive asymmetric bilateral ectasia of the cornea. This is a patient who presented with a rapid progression of his keratoconus in his right eye. And crosslinking is the only approved method that is currently available to us to arrest the progression of keratoconus. And the crosslinking was done, which led to stabilization of the region and stabilization of the keratometry. So researchers evolved a protocol where they not only did the crosslinking for tectonic stability of the keratoconus, but also advised keratectomy to ablate the surface. They recommend that you perform a phototherapeutic ablation, and they found that ablating up to 50 microns was safer. After this, the corneas were exposed to 20 seconds of mitomycin C, which was washed off, and then there was 10 minutes exposure to riboflavin, followed by 30 minutes of UVA exposure, so this led to not only improved tectonic stability of the keratoconic eye, but it also helped bring down the irregular astigmatism and improve the vision. There is another scenario, which is post-LASIK ectasia. A scenario after LASIK, when again, a cornea which is weakened by ablation tends to bend and change in shape and increase in cylinder after surgery. This can happen to any patient, though there are certain risk factors for post-LASIK ectasia. And this patient was a young medic, who had progressive increase in his cylinders in his left eye after three years of LASIK. And these are the Orbscans, which show an increasing cylinder. And in these situations, we can help them with something called the Cretan protocol, where a transepithelial PTK is performed, following which, the eyes are exposed to riboflavin, with 20% Dextran for 30 minutes, after which, UVA30 minutes under riboflavin is used to differ 5.4 joules per centimeter, therefore increasing the tectonic stability. Once you have obtained tectonic stability, then RGP lenses usually help in maintaining stability. They act on the refractive surface of the eye, and in a well fit RGP lens, you would have a small central touch with paracentral pooling, and a thin layer of fluorescein with an adequate amount of edge lift. And as you can see in this video, the lens is perfectly centered, even with eye movements and blinking. This is the endpoint. This provides excellent depth of vision, width of vision, clarity, and helps neutralize very irregular astigmatism in keratoconus, after penetrating keratoplasties. The other option that we have for treatment of keratoconus is Intacs, where rigid PMMA segments are placed in the cornea, and as opposed to Excimer laser, which ablates the cornea, these ring segments flatten the center of the optical zone without ablating any tissue, and therefore allow the light to be focused onto the fovea. So this is an Intacs surgery that is being performed. A femtosecond laser is used. To dot the cornea. And after that, the channels for placing the Intacs are made with the femtosecond laser. And a cut is made radially, to allow the rings to be passed to the cornea. There are several nomograms that are available, by which the surgeon selects the length and the number of segments that need to be placed in the cornea. And once the dissected cornea is opened up by the biprong dissector, the surgeon places the rings. These rings need to be threaded in very gently, making sure that there is no loss of resistance, which denotes that there is a full thickness entry to the anterior chamber. And they need to be completely placed in the channels that have been created, and at the end, the surgeons uses a dialer to push them into the corneal stroma. Similarly, the opposite segment is also threaded in. These need to be used in corneas which have at least 450 microns of thickness. And they need to be placed at 70% depth. In the end, the channel through which these segments have been placed need to be closed with the suture. The vision can be stabilized by 3 to 6 months after placement. And a rigid gas permeable contact lens can be used to overrefract. So this was a patient who came with a rapidly progressing keratoconus. And a steep inferior cornea. And the Intacs were placed, and the vision stabilized well. And as you can see in the previous maps, all the keratometry parameters improved vastly with the placement of the Intacs. Then this patient was stabilized with RGP lenses, which led to the restoration of vision. The other scenario that we look into is the post-keratoplasty astigmatism. Keratoplasty, while restoring the clarity of the cornea, can often lead to very bizarre patterns of astigmatism, which are very difficult to manage with glasses alone. While RGP contact lenses remain the mainstay of visual restoration, some other modalities such as the AK is also a proven modality in managing corneal astigmatism. This patient had a steeper axis in the horizontal meridian. I don’t have a video, unfortunately, for this case. But after placement of astigmatic keratotomy with the femtosecond laser, you can see the marks as denoted by the white arrows which are placed just inside the graft. It led to stabilization, and now you can see the corneal parameters are much more regular, and the vision was also reasonably restored, and the patient is doing well with glasses. The other common scenario is premium IOL surgery planning. Where patients who have more than 1 diopter of astigmatism need to be recommended for toric IOL implantation. You can use any of the online toric calculators, depending on the manufacturer from which you are using the IOL. I use this online Alcon toric IOL calculator. And for that, as a prerequisite, after you have brought your calculation, you need to mark the 0 and 90 degree. As the surgery progresses, you first take the markings, and then mark the cornea in the axis in which you intend to place the IOL. As per the calculation that is provided by the online calculators. Once that is done, the surgery progresses routinely. So I am going to advance this, because this is a routine phaco. And in the end, one needs to place the toric IOL in the bag. You can see that the toric IOL has a mark where the cylinder is to be located. So this needs to be aligned to the external mark from the cornea, which tells that the manufacturer has recommended that the IOL be placed in this axis, so as to neutralize the astigmatism of the eye. And this is the endpoint for the surgery. There are some other ways in which you can also use toric IOLs. This is the Callisto system in use, and there is an app by which you take a photograph of the cornea and feed it into the Callisto. Once that has been fed intraoperatively, the Callisto denotes the axis of astigmatism by the three blue lines that are here. And this is a toric lens that is placed, and the surgeon is guided by the three blue lines, by which he needs to place the IOL at the end of the surgery. When you are performing toric IOL cataract surgery, make sure that you do not dial the IOL completely, until the end of the procedures. Towards the end, after you have completely washed off the entire Viscoat, both in front and behind the IOL, then place the IOL in the recommended axis. So if you watch this video, you will note that the surgeon is just keeping the IOL away from the axis, until the end of the procedure. After which he tends to go and do a Viscoat wash. Once the Viscoat wash is performed, you can see at the end the surgeon will place it in the axis. And this is how it should be done, so that you do not overextend the desired axis. So with the advent of newer technology, and with greater understanding of the cornea, we have been able to map and not calculate the cornea from several parameters. So these machines give an idea of the structure of the cornea from the anterior surface, the posterior surface, and it gives us an idea of how the cornea would look like from several ways or several permutations and combinations. This has led to interesting results in understanding the cornea, and one such entity which has come up is the posterior corneal astigmatism. The posterior corneal astigmatism tends to be much more steeper in the vertical meridian. And this has been found in 80% of the eyes, causing against the rule astigmatism. How does it help? In young adults, this actually neutralizes the with the rule astigmatism, keeping the with the rule astigmatism under control. And this tends to increase as the with the rule astigmatism increases, up to a 1 diopter power. The absolute value of posterior corneal astigmatism has been found to be -0.3 to -1.1. So -0.1, I’m sorry. So the range of the astigmatism that is corrected is from -0.01 to -1.1 diopter. Now, how do these help in planning? With the premium IOLs? So our understanding is that as the cornea ages, the astigmatism shifts from the vertical to the horizontal meridian. And then the posterior corneal astigmatism actually adds to the total astigmatism of the cornea. Therefore, when we are planning for toric IOLs, it is very important to take the posterior corneal astigmatism into our consideration. So that we have more accurate calculations, and therefore have better postsurgery results, after implantation of toric IOLs. And this was found in a subsequent study by Reitblat et al., where they found different methods to retrospectively calculate the posterior astigmatism with placement of toric IOLs. And as you can see in these plots, the nomogram and vector analysis had better postoperative refractive error when they took the posterior corneal astigmatism into consideration. And this can be seen both in with the rule eyes as well as against the rule eyes. In fact, the nomogram was 80%, as opposed to analysis, which achieved 64% improvement. When you also have to use limbal relaxing incisions, you can also take the help of online LRI calculators. So these are simple and easy to use. And one needs to feed in the preoperative biometry. There are several nomograms you can select from. I used this nomogram for a patient with 3 diopters of astigmatism. This tells us what length LRI needs to be delivered, where your incision is based, and what are the clock hours of incision for LRI that has to be delivered, either before or at the end of surgery. So as you can see from this nomogram, I have to give two cuts, from 350 to 225 to correct the astigmatism in this eye. This is the surgery that is being performed, and at the outset, we’ll need to mark the clock hours, and once that is done, the phaco proceeds routinely. At the end of the procedure, after placing the IOL, I prefer to place a limbal relaxing incision, and I use a 300 micron guarded knife here. This is because this patient had scarred cornea of about 350 microns, so I did not prefer to use a 600 micron, which I would normally use in a normal thickness cornea. The other way in which we can also provide LRIs in cataract surgery is by using femtosecond limbal relaxing incisions. So femtosecond lasers tend to provide more accurate depths and sizing of the peripheral limbal cuts, and this is helpful in providing adequate depth of the incision, and therefore it helps in controlling the astigmatism in a more predictable fashion. There are several nomograms that one can use to give those LRIs, using femtosecond lasers. However, often LRIs can have inconsistency due to inaccurate assessment. The nomograms may not factor in the corneal thickness, or they may not factor in the limbal diameter, misalignment may occur, and there may be an inconsistency in performance of techniques. It is important to understand that cuts which are made at 90 degrees are most effective. Cuts which are more than 80% depth are more effective than those which are 60% or less. And it is also important to use different nomograms, as per the case. And even then, some amount of scarring may occur, which may reverse the effect of LRIs, postcataract surgery, or wherever you have used them. So there are certain caveats. Use empirical nomograms. Use the correct axis. Use the desired depth and the appropriate optic zone. So in conclusion, the uncorrected visual acuity is the single most important outcome in a refractive intervention. The mathematical models of the cornea help in improving our understanding for measuring corneal astigmatism. And the improvement in our diagnostics have significantly helped us improve treatment and manage corneal astigmatism. So thank you for your patient hearing. I would like to acknowledge colleagues who have helped me with material for this talk. Thank you very much. We’ll take some questions now.