Just as optical coherence tomography (OCT) has revolutionized the clinical management of retinal diseases, I believe intraoperative OCT has the potential to revolutionize lamellar corneal surgery and facilitate other types of ocular surgery because it excels at visualizing ocular structures that may otherwise be difficult for the surgeon or clinician to discern. This discussion will be limited to the use of intraoperative OCT with lamellar corneal surgery using a standard operating microscope with oculars. Examples of additional uses include assessing corneal incisions, intralenticular pressure, and posterior capsule integrity during phacoemulsification; visualizing angle structures during the placement of aqueous shunts; creating precise partial-thickness scleral flaps; and establishing a diagnosis and performing surgery in pediatric patients who are not cooperative during examination.1 Other vitreoretinal surgical conditions and procedures that can benefit from intraoperative OCT include epiretinal membranes, retinal detachments, retinopathy of prematurity, subretinal gene therapy treatments, and macular hole surgery (ie, confirming the release of vitreomacular traction and identifying occult residual membranes).2–4
Results of studies from around the world as well as my own personal experience suggest that ophthalmic surgeons doing complex corneal surgeries, such as endothelial keratoplasty, anterior lamellar keratoplasty, or deep anterior lamellar keratoplasty (DALK), could benefit from the additional information that intraoperative OCT provides.5–11 Intraoperative OCT has been available in the United States for several years, either commercially or for investigational use, with 3 manufacturers in the market, Carl Zeiss Meditec USA, Dublin, CA; Leica, Wetzlar, Germany; and Haag-Streit USA, Mason, OH. These devices provide 3 ways to view the OCT image: on a monitor specifically for the OCT unit, as an inset in the video recording, and as an image injected into the oculars. All 3 devices have good monitor and video recording images. However, in my experience, only the newest A3 version of the Haag-Streit iOCT superimposes an image into a portion of the view through both oculars that is robust enough to allow the surgeon to operate with it and see sufficient detail without either looking at the monitor or having someone else watch the monitor for them.
Having a high-quality image, on demand, in the oculars of the microscope is a game-changing technology that makes the intraoperative OCT a valuable surgical tool and more than just a cool gadget, research device, or something to enhance presentations. The surgeon does not have to look away at a monitor and instead can see everything happening real time in the field of view. The superimposed image turns on and off with the click of a button on the microscope foot pedal, providing exceptional ocular anatomical detail during the most pivotal steps of the procedure. The video recording that you will see in presentations has an inserted image on the monitor that is on continuously, whether or not the surgeon has elected to view the OCT image in the oculars. The problem with only having a high-quality, usable image on a monitor is that either the surgeon has to look away, while hoping the patient, the eye, or the tissue does not move while they are looking away, or, alternatively, the surgeon can rely on someone else to watch the monitor and relay what is happening. So, in my experience, viewing only with a monitor has limited utility but is great for teaching, giving talks, or doing research.
To understand intraoperative OCT, it is important to appreciate the different information available in the 3 ways the images can be viewed. Figure 1A is an iPhone image taken through one ocular with an iOCT (the OCT image actually shows up in both oculars), Figure 1B shows the image from an attached monitor on the scope that also has the programming portion of the iOCT, and Figure 1C shows the large-screen TV on the wall of the operating room. Typically, the images an audience will see in surgical videos are those shown in Figures 1B and C. What the surgeon sees with the image that is superimposed in the oculars and that is not visible in the other images is where the “slice” of the intraoperative OCT image is actually being taken and its orientation (ie, the line across the cornea in Fig. 1A). Recall that the iOCT image is only superimposed in the oculars if the surgeon chooses to activate this feature by stepping on a button located on the traditional microscope foot pedal. The orientation line can rotate 360 degrees with a second button on the foot pedal, or it can be changed to 2 cross sections 90 degrees apart, for example, vertical and horizontal.
In-office OCT has become indispensable in our practice because it allows the clinician to quickly discern detail that otherwise would be hard to see. If we suspect cystoid macular edema or other macular problems, we can noninvasively find what we need to see and no longer need to do fluorescein angiography. Anterior segment OCT helps show the depth of corneal scars, anterior chamber angle morphology, and the extent of endothelial keratoplasty detachment in patients with cloudy corneas. Likewise, intraoperative OCT allows surgeons to better perceive details in ocular tissues, especially the cornea.
For example, intraoperative OCT is helpful for assessing graft attachment and the presence of residual interface fluid with Descemet stripping endothelial keratoplasty.8 We used to make venting incisions to preemptively drain any potential fluid in the interface, but now we just use intraoperative OCT to see real time whether any areas are detached, and if so, we massage the cornea to milk the fluid from the interface until the donor tissue is in tight apposition with the recipient cornea. If any abnormal tissue fragments are present and impeding attachment, they also can be seen with the intraoperative OCT and removed. If the posterior corneal surface is seen to be focally irregular, such as the protuberant posterior lip of a failed penetrating keratoplasty trephination margin, the graft can be recentered away from that area.
Intraoperative OCT also facilitates Descemet membrane endothelial keratoplasty (DMEK),5,9–11 which is our most common keratoplasty procedure. With intraoperative OCT, we do not need to mark the donor tissue, thereby preserving endothelial cells. We just inject the tissue either scrolled naturally with the endothelium facing outward or folded with the endothelium facing inward using the trifold technique. Either way, we use the intraoperative OCT to determine whether the tissue is oriented correctly with the endothelium facing the host anterior chamber. Even in patients with very cloudy corneas, we can discern the DMEK tissue orientation, as shown in Figure 2.
In the United States, Fuchs dystrophy is the leading reason for DMEK, and we often operate on corneas with minimal edema, so it is easy to see different marking methods used on the donor tissue. However, many cases around the world are like the one in Figure 2, where it is difficult to see corneal marks through a swollen, cloudy cornea, and the intraoperative OCT can be especially helpful in such cases.10 Also, if our eye bank has only suitable tissue from a donor around or even younger than 40 years, we can still use it. DMEK tissue from younger donors often scrolls up tightly, and a very tight scroll could make it difficult to discern an orientation mark, but an intraoperative OCT allows us to verify tissue orientation even if the tissue is tightly curled. Operating without the intraoperative OCT has now become a chore, just as one would miss the detail and lack of control if one was to perform cataract surgery without a microscope.
Although DMEK is the most common procedure we do with the intraoperative OCT, it has been a real game changer for DALK, and this is where it has a huge potential to improve the adoption of a surgical technique.7 At least in the United States, DALK has had limited utilization. The big-bubble technique is the gold standard, but sometimes one does not want to attempt a big bubble, such as in cases with previous hydrops in which the scar involves Descemet membrane. In some cases, one cannot achieve a big bubble, so it is very helpful to have alternative methods of performing DALK. Even a peel technique can be difficult in some cases, leaving a residual recipient bed that is too thick. Use of the intraoperative OCT enables much better control of the incision depth. Because the coaxial microscope does not allow good determination of the depth of a corneal incision, I typically use the intraoperative OCT to determine when I have a sufficiently deep dissection level to either start the manual dissection or use a peel technique. Melles described the injection of air into the anterior chamber to judge incision depth,12 but that too can be difficult, and a slit beam does not provide enough resolution to effectively judge depth.
The intraoperative OCT can be used to evaluate the uniformity as well as the depth of a DALK dissection plane and thereby enable the surgeon to obtain a uniformly thin residual bed that provides visual outcomes similar to a big bubble. Figures 3A and B show an irregular corneal bed that would lead to a poor visual result; an intraoperative OCT allows the surgeon to see where the tissue needs to be dissected further. Figures 3C and D show the intraoperative OCT image of a big bubble. The image shows what looks like a thick residual bed, but in reality, this is the typical thin big-bubble bed as seen with the iOCT, and it is the depth we usually aim for with manual or peel dissections.
Figures 3E and F demonstrate something that I have now seen multiple times: the previous DALK dissection bed separated during subsequent phacoemulsification when the wounds were hydrated. It can be difficult to fully appreciate the extent of the separation when only using a coaxial microscope. The intraoperative OCT allowed me to assess the extent of the detachment, plan where to make my incision to drain the detachment, and determine when the detachment was resolved. In that particular case, I drained through the previous trephination incision, whereas in other cases I have made stab incisions through the graft similar to the venting incisions we used for Descemet stripping endothelial keratoplasty. All cases resolved and were completely attached the next day. This complication during subsequent cataract surgery may become more common as the use of DALK increases.
I have also used intraoperative OCT to judge the depth of a scar for a lamellar dissection, to evaluate the IOL positioning in the capsular bag, and to locate and remove retained nuclear fragments that could not be localized and removed with the view through a cloudy cornea. We are also using it to intraoperatively evaluate the vault of an implantable collamer lens (Visian ICL) and found that although this is helpful, the vault observed intraoperatively does not perfectly correlate with the vault observed postoperatively.
As you may have guessed, the biggest limitation to intraoperative OCT is the cost. All 3 manufacturers selling units in the United States have intraoperative OCTs that only work with the same company's microscope. So, in addition to the cost of the intraoperative OCT, there is often an additional cost to purchase the specific brand of the operating microscope for which that particular intraoperative OCT was designed. The iOCT by Haag-Streit is attached to a beam splitter, so it can come off of the microscope (for instance, to upgrade it), and it works on different microscope models; however, it only works with Haag-Streit microscopes. The Zeiss intraoperative OCT is an integral part of the microscope, so it comes as one complete unit. Leica's new intraoperative OCT is also an integral part of the microscope. For all these, the cost of a new microscope plus an intraoperative OCT can be a barrier to entry, and currently, few are in use. It is hoped that the cost of intraoperative OCT will drop as the benefits of utilization are more fully appreciated and unit sales increase.
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