How does intraoperative Microscope integrated optical coherence tomography help confirm correct air cannula placement in DALK? Dr JK: Microscope-integrated optical coherence tomography (MI-OCT) provides real-time visualization of corneal layers, allowing precise confirmation of cannula placement in the deep stroma just above Descemet’s membrane (DM). This is critical for successful big-bubble formation, seen as a distinct air pocket separating the stroma from DM Figure 1a. Immediate feedback enables adjustment of cannula depth, improving safety and reducing the risk of DM perforation.Figure 1: (a) Presence of an air bubble separating stroma and Descemet seen on integrated optical coherence tomography (I-OCT). (b) Bare Descemet is seen as a glistening band on I-OCT. (c) Good graft-host tissue apposition alignment seen on I-OCTDr MK: Intraoperative OCT (I-OCT), or MI-OCT, can help assess the leading edge of the air cannula in real time and assess its distance from the endothelium. The metallic instruments cause shadowing, so it is not possible to see the tissue directly beneath the cannula; however, the leading edge of the cannula gives you an accurate assessment of the depth of placement and the distance from DM. There is also a conversion scale at the corner of most MI-OCT display screens if you need to measure the exact depth of placement of cannulas. DR RM: MI-OCT can act like a safety check before the surgeon commits to the air injection step in DALK. The cross-sectional images make it easier to see exactly where the cannula tip is in relation to the deep stroma and DM. When the cannula is placed at an appropriate depth, the OCT typically shows the tip sitting just in front of the bright DM line, with only a very thin layer of stroma left underneath. If the entry is too shallow, there is a noticeably thicker stromal bed below the tip, which usually means that a classic big bubble is unlikely to form. On the other hand, if the tip has been advanced too far, the scan may reveal subtle tenting or a small indentation of DM, prompting the surgeon to pull back slightly before injecting air. By offering this immediate feedback, MI-OCT helps reduce guesswork, cuts down on repeated passes with the cannula, and lowers the overall risk of DM perforation, which is particularly reassuring for surgeons who are still gaining confidence with DALK. 2. What intraoperative optical coherence tomography findings suggest a micro-perforation of Descemet’s membrane? Dr JK: MI-OCT allows assessment of various surgical parameters, including the depth of initial trephination, the level of stromal dissection, and the visualization of bare DM after dissection Figure 1b, ensuring a safe and accurate procedure. MI-OCT may reveal subtle interface fluid accumulation, focal discontinuity or irregularity of the DM line, abnormal reflectivity, or undulating DM contours. These changes may precede clinically visible aqueous leakage, allowing early recognition and timely intraoperative intervention. Dr MK: A discontinuity in the DM can be visualized on MI-OCT in cases of perforation; however, if the perforation is very small, you will need to carefully scan in the areas of maximum thinning. A subtle forward movement of the lens-iris diaphragm with shallowing of the AC may be seen in minute perforations on OCT during dissection. The configuration of DM can change from concave to more convex in cases wherein a big-bubble had been previously formed, but in these cases, you will need to differentiate between the escape of the big-bubble or DM perforation. The clinical acumen and stage of surgery need to be taken into account along with the MI-OCT findings. DR RM: Micro-perforations of DM can be subtle and may not show obvious clinical signs at the start, which is why I-OCT is particularly useful in this setting. Surgeons often rely on the scan to pick up early structural changes that would otherwise be missed. On OCT, a small micro-perforation may appear as a localized break or irregularity in the normally smooth, hyper-reflective DM line. There may also be tiny fluid pockets just beneath DM or a suggestion of air tracking posteriorly rather than remaining in the intended stromal plane. Another clue is an abrupt thinning of the posterior stroma with associated flattening or distortion of the usual DM contour. In some cases, the surgeon may also notice that a big bubble does not form or collapses unexpectedly, even though the air injection technique and volume seem adequate. Recognizing these early signs allows the surgeon to adjust the plan – often by shifting to a more cautious manual dissection, using viscoelastic to tamponade the area, or reducing further air injection to avoid turning a small defect into a larger perforation. 3. How does microscope-integrated optical coherence tomography guide decision-making in advanced keratoconus or corneal thinning? Dr JK: MI-OCT helps quantify stromal thickness, identify the thinnest area, and locate the cone apex, allowing safer trephination depth and cannula placement. Intraoperatively, it enables real-time monitoring of dissection planes, reducing perforation risk and guiding whether to proceed with big-bubble, manual dissection, or modify the surgical plan. Dr MK: MI-OCT gives a cross-sectional view of the cornea, and cases with variable corneal thickness with areas of extreme thinning (<250 μ) have a higher incidence of perforation during bib-bubble formation and are more suited for manual DALK. In addition, any areas of scarring extending into deeper stromal layers preclude a big-bubble DALK. Healed hydrops can be visualized on OCT as a discontinuity in DM, and again, these cases are more suited for manual DALK. To summarize, OCT will give an ultrastructural view of the cornea and help visualize factors that are likely to make a case unsuitable for big-bubble DALK. While performing manual DALK, iOCT helps assess the depth of dissection in real time-this enhances the safety of the procedure and prevents perforations. DR RM: In advanced keratoconus or markedly thinned corneas, MI-OCT becomes less of an optional add-on and more of a practical safety tool during DALK. It helps the surgeon understand where it is genuinely safe to work and how close the dissection is getting to DM at each step. Pre- and intraoperatively, the scan can map small focal areas of severe thinning so that cannulation is planned away from these high-risk zones. It can also highlight asymmetric stromal thickness, which in turn guides how deep and where to place the trephination, rather than relying purely on clinical impression. During dissection, MI-OCT shows the proximity of the surgical plane to DM in real time, which helps the surgeon avoid unintentionally entering the anterior chamber. When the cornea is extremely thin, these findings often encourage a more cautious, mixed strategy: a limited bigbubble attempt, followed by careful layer-by-layer manual dissection instead of a single, aggressive air injection. This hybrid approach aims to balance the benefits of a deep dissection with a lower risk of frank perforation. 4. How does microscope integrated optical coherence tomography differentiate big-bubble types and influence surgical decisions? Dr JK: Type-1 bubbles appear as a dome-shaped separation between stroma and Dua’s layer, while type-2 bubbles are thinner and form between Dua’s layer and DM. Mixed bubbles show features of both and carry a higher DM detachment risk. MI-OCT identification guides the depth of stromal excision and alerts the surgeon to proceed cautiously or modify the technique. Dr MK: For Type I and Type II bubbles, the extent of separation of DM/predescemetic layer from the stroma in the periphery can help guide as to the type of big-bubble. MI-OCT is invaluable in the diagnosis and management of the Type III big-bubble, with separation between stroma and predescemetic membrane, as well as the predescemetic layer and DM. The two planes of separation can be clearly visualized, and the predescemetic layer can be removed under direct visualization without rupturing the DM. This helps prevent a double AC in the postoperative period. DR RM: Real-time MI-OCT helps the surgeon distinguish the different big-bubble patterns more reliably than with the operating microscope alone. This visual distinction then directly shapes how boldly or cautiously the next steps are taken during DALK. A classic type 1 big bubble appears as a dome-shaped separation between the deep stroma and the predescemetic layer, with a thin, continuous hyper-reflective DM line still visible behind it. Because DM remains covered, this configuration is generally considered more forgiving and is usually safe to open in preparation for a standard DALK. In contrast, a type 2 bubble shows a larger, more delicate space created directly between the stroma and bare DM, with no residual stromal cushion and DM looking extremely thin and smooth. This pattern carries a higher risk of perforation, so surgeons often favor partial stromal removal or a very conservative opening rather than fully entering the bubble. Sometimes the OCT reveals mixed or incomplete bubbles, with irregular cleavage planes or more than one interface rather than a single clean separation. In that situation, many surgeons prefer to back off from aggressive bubble manipulation and instead transition to a controlled manual deep dissection. 5. Which microscope integrated optical coherence tomography findings help decide whether to continue DALK or convert to PKP? DR JK: MI-OCT assesses trephination depth, residual stromal thickness, proximity to DM, intrastromal emphysema, and integrity of DM. Failure of big-bubble formation, extremely thin residual stroma, or signs of impending DM perforation favor early conversion to PKP rather than unsafe continuation of DALK. Also, the graft-host relationship with regards to tissue apposition alignment can be continually assessed during surgery and help prevent over-or under-riding of the graft Figure 1c. Dr MK: The reason to convert a case planned for DALK to PKP can be twofold: a DM perforation which is not salvageable, or residual scarring in the visual axis. In both cases, the decision to convert will mainly be clinical rather than based on OCT findings. That being said, when a surgeon is trying to salvage a case in which micro-perforation has occurred during dissection, repeated AC shallowing with anterior movement of the iris-lens diaphragm on OCT and an increase in the area of DM discontinuity may indicate a need to convert to PKP. OCT can at most act as an adjunct to clinical decision-making in these scenarios. Dr RM: During DALK, MI-OCT can give the surgeon more grounded, visual information when choosing between continuing with manual dissection and converting to PKP. Instead of relying only on intraoperative “feel,” the decision is guided by what the scan shows at the DM–stroma interface. A continued manual dissection is usually supported when DM appears intact with a reasonably uniform residual stromal thickness and a stable interface. Small, well-controlled microperforations can sometimes be managed conservatively if the anterior chamber remains formed and there is no sign of progressive DM detachment. On the other hand, the OCT may reveal larger DM tears with a clear communication to the anterior chamber, increasing the risk of further complications if dissection is continued. Worsening or scrolling DM detachment, or repeated loss of a stable posterior plane despite adjustments, often pushes the surgeon toward an earlier, planned conversion to PKP rather than a late, forced one. ENDOTHELIAL KERATOPLASTY (DSEK)– PANEL QUESTIONS 1. How does intraoperative microscope integrated optical coherence tomography help assess graft dynamics in routine cases and in eyes with severe corneal edema? Dr JK: MI-OCT provides real-time visualization of donor lenticule thickness, dissection plane, graft orientation, unfolding, and final apposition Figure 2a. In eyes with severe corneal edema or poor visibility, MI-OCT allows confirmation of correct graft positioning and attachment even when the surgical view is compromised. DR MK: MI-OCT is invaluable to assess graft dynamics in both DSAEK and DMEK. In DSAEK, the completion of graft unfolding can be directly visualized, even in cases with corneal edema or scarring. OCT can help assess the correct orientation of the graft by assessing the angle formed between the host and the donor lenticule at the edge of the graft. We have described the acute-angled bevel sign on OCT to assess donor lenticule orientation in cases where the graft is very thin or the visibility is impaired (Titiyal JS, Kaur M, Shaikh F, Bari A. Acute-angled bevel sign to assess donor lenticule orientation in ultra-thin descemet stripping automated endothelial keratoplasty. BMJ Case Rep 2019;12:e227927.) Interface fluid or air bubbles can be visualized, and additional manoeuvres can be performed to promote complete graft adhesion. In DMEK, the orientation of the graph can be assessed using I-OCT. The DMEK scroll can be observed in real time with the MI-OCT, and the endothelial aspect can be determined by the configuration of the scroll (scrolls facing upward towards the corneal stroma on MI-OCT being the correct orientation). In case an inverse graft is observed (spectacle configuration or scrolls facing downward on MI-OCT), fluid maneuvers can help flip the graft to the correct orientation under MI-OCT guidance. In addition, any interface space or scrolled edges can be determined, and further maneuvers can help promote complete graft adhesion. DR RM: MI-OCT helps the surgeon follow graft behavior both in straightforward endothelial keratoplasty and in eyes where the view is significantly compromised. In a clear cornea, it mainly serves as a confirmation tool, while in severe edema or poor visibility, it can become the main way to understand what the graft is doing. The scan can show graft orientation, which is particularly important in DMEK, where an upside-down graft can severely compromise the outcome. It also allows the surgeon to judge how the scroll is opening, whether the tissue is centered, and how well it is unfolding across the posterior surface. Even when the cornea looks almost opaque at the slit lamp, MI-OCT can confirm whether there is good graft–host apposition or whether fluid or gaps remain at the interface. In markedly edematous corneas, this information reduces guesswork and lessens dependence on corneal clarity alone, making intraoperative adjustments more deliberate and less speculative. 2. What microscope-integrated optical coherence tomography signs indicate good graft adhesion, and what corrective maneuvers can optimize adhesion? Dr JK: Good graft adhesion is indicated by complete graft–host apposition without interface fluid and a well-formed anterior chamber Figure 2b. MI-OCT detects residual interface fluid, incomplete unfolding, or peripheral detachments Figure 2c. Corrective maneuvers include corneal massage, internal air tamponade, venting incisions, and adjustment of air-fill duration to optimize adhesion.Figure 2: (a) Real-time assessment of the donor lenticule during manual dissection, verifying the appropriate dissection plane and thickness on microscope-integrated optical coherence tomography (MI-OCT). (b) Graft-host apposition confirmed on MI-OCT with well-formed Anterior chamber. (c) Detached host Descemet seen on MI-OCT after scoringDr MK: Complete graft apposition may be difficult to confirm in the absence of an MI-OCT, and a prolonged air tamponade with corneal surface massage is conventionally employed to ensure graft adherence. MI-OCT is invaluable in these cases as it can help directly visualize complete graft-host apposition. Interface fluid is visualized as a hypo-reflective space in between the host stroma and the graft, and this signifies the need for additional maneuvers such as corneal surface massage or positive-pressure air tamponade to help promote complete graft adhesion. In DMEK, even if the center is attached, a peripheral DM edge may be scrolled or not attached, and this can lead to graft detachment in the postoperative period. So if you see peripheral scrolls or pockets of interface fluid on MI-OCT, it’s a sign that you require injection of more intracameral air and/or further graft unscrolling maneuvers to help promote complete graft adhesion. When you are performing DSAEK in complex case scenarios, such as a failed penetrating keratoplasty, there may be curvature malapposition as well. Irregular host stromal interface due to prior keratoplasty may lead to a mismatch between the curvature of the DSAEK lenticule and the curvature of the host stroma that may preclude a proper apposition. This can be directly visualized on MI-OCT and may signify that you require positive intracameral air pressure for prolonged periods, and the case is a high risk for postoperative graft location. DR RM: MI-OCT provides clear visual clues about graft adhesion quality in DSAEK and DMEK, helping surgeons spot issues right away rather than waiting for postoperative problems. Reliable adhesion typically shows up as no visible fluid at the graft-host interface, even thickness across the graft where it sits flush against the host stroma, and a smooth edge-to-edge connection without any lifting. If the scan picks up poor adhesion – such as pockets of fluid, uneven apposition, or edge lift – surgeons can address it on the spot with targeted fixes. These often include a focused rebubble or air push in the problem areas, gentle directed massage over the fluid spots through the cornea, or repositioning the graft if it has scrolled or shifted off-center. Making these adjustments intraoperatively tends to cut down on later detachments and the need for return trips to the OR. 3. How does microscope integrated optical coherence tomography guide air fluid dynamics while preventing pupillary block? Dr JK: MI-OCT enables cross-sectional visualization of anterior chamber air fill, iris and fluid This allows precise of air volume to ensure graft tamponade while air that lead to pupillary or pressure Dr MK: a waiting time of at has been to ensure complete graft apposition with a complete air-fill in the anterior chamber. prolonged can be in cases with We have observed that it for complete donor apposition with the technique of positive intracameral pressure and corneal This the waiting time for graft apposition and prevents prolonged (Titiyal JS, Kaur M, of time to donor lenticule apposition using intraoperative optical coherence tomography in descemet stripping automated endothelial keratoplasty. In addition, the air bubble can be at the of and a intracameral air bubble can be left while ensuring that the graft is on This the incidence of postoperative pupillary and the need for the intracameral air on the slit DR RM: MI-OCT surgeons air and fluid management during endothelial keratoplasty, a balance between graft tamponade and complications like pupillary than the real-time shows exactly how the air is on the graft and The scan whether there is air to the graft in place against the host It also that any interface fluid has fully before up the which is a step to prevent early detachment. By anterior chamber depth and how the iris is OCT helps spot allowing a of air if OCT shows graft many surgeons just a partial air rather than a which lowers the of pupillary while still good 4. In eyes with prior surgery or how does microscope-integrated optical coherence tomography help avoid Dr JK: MI-OCT in safe graft unfolding and positioning ensuring the graft does not or It also helps optimize air tamponade to avoid postoperative pressure reducing endothelial loss and graft Dr MK: MI-OCT can help visualize the distance between the and the graft, and ensure correct positioning while a safe distance between the In addition, air tamponade is often to in these cases, and MI-OCT is a useful adjunct to ensure complete apposition. DR RM: with surgery or a for endothelial keratoplasty, where MI-OCT provides on graft placement. The reveals that are to assess through the microscope in these complex OCT clearly shows the relationship between the graft and the helping the surgeon understand if the is with tissue It can if the graft edge is being or by the which otherwise after on these surgeons may for a or of the graft to optimize and avoid This approach helps endothelial reduces the of graft detachment, and in these 5. How can microscope-integrated optical coherence tomography be integrated into the surgical without Dr JK: Microscope-integrated OCT allows continuous real-time visualization through the operating microscope without and of the surgical improving without increasing Dr MK: In many MI-OCT is the of view of one of the In addition, the depth of as well as alignment can be the microscope the helps OCT into the surgical without display are very useful if as it is easier to view the OCT on the rather than as an a can change the depth of or make adjustments while the surgeon is operating and information to the DR RM: Anterior optical coherence tomography into the surgical have to cases if surgeons it rather than The is to check rather than making it a steps include up clear – such as right after air before opening the or the air is – so the into the procedure. 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