This patient had prior RK (Radial Keratotomy) done a few decades ago for the treatment of myopia. The patient was about -5 D of myopia before the RK and these six radial incisions were made to induce corneal flattening to bring the eye close to emmetropia. These RK incisions are deep and when we measure them with corneal OCT imaging, they are more than 90% depth and they extend from just inside the limbus to a central optical zone of 3 mm in diameter. What are the challenges that we face with cataract surgery in this patient?
We have extensively covered the issues and techniques for cataract surgery in eyes with prior RK in this article. Pre-operatively, the primary issue is measuring the central corneal power and doing the IOL calculations. Referencing that previous article, we want to make sure that we take the lowest central corneal power and then adjust it even a bit lower. This will help increase the IOL power that is chosen in an effort to decrease the risk of a post-op hyperopic surprise. For the IOL power estimation, it would be wise to use one of the online calculators specifically designed for post-RK eyes or to use the Aramberri Double-K method wherein the measured (and then adjusted) K value is used for the IOL power part of the equation, while a fixed value of about 43 to 44 D is used to determine the effective lens position.
Our patient is currently a +7 D hyperope for distance vision with corneal powers that measure in the range of 32 to 34 D. We have done our mathematics homework and determined that an IOL power of +28.0 D should give us a post-op refraction of between plano and -0.50 spherical equivalent.
Q: How do we do the complicated IOL calcs for this patient?
A: Full analysis and article coming tomorrow detailing this step by step!
During the surgery, we must ensure that our phaco incisions do not intersect any of the pre-existing RK cuts. If we mistakenly intersect one of these incisions, it will tend to open up and cause a lot of leakage which will give us anterior chamber instability and a high risk of posterior capsule rupture. In addition, multiple sutures would be required to close the incisions at the end of the case.
We decrease the infusion pressure (or lower the bottle height) so that the intra-ocular pressure (IOP) is kept modest. Too high of an IOP could result in leakage from the pre-existing RK cuts because they are 90% depth, which means that just 50 microns of corneal tissue is what is keeping the eye watertight. Finally, at the end of the surgery, we need to check all of the RK incisions to make sure that they have held up well during the surgery.
In the post-op period, give these patients plenty of time to heal. Wait until the post-op K values measure about the same as the pre-op K values before you make a decision as to the post-op refractive state of the eye. This can often take weeks or sometimes even months. The RK incisions swell during surgery and then cause further flattening of the central cornea in the first post-op week or two, thereby resulting in a hyperopic refraction. If you check this same eye a month later, it will likely have returned to the baseline K values and much closer to an emmetropic outcome.
Click below for video of narrated instructions for Cataract Surgery in RK eyes:
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