Intraoperative OCT is becoming an established technology in ophthalmic surgeries. Its detailed cross-sectional images provide valuable morphological information during surgery, and several studies proved that it influences greatly the surgeon’s intraoperative judgment and decisions. In this mini review, we are going to address anterior segment OCT in ophthalmic surgeries, mainly corneal surgeries.
Abbreviations: OCT: Optical Coherence Tomography; AS-OCT: Anterior Segment Optical Coherence Tomography; AC: Anterior Chamber; PKP: Penetrating Keratoplasty; iOCT: Intra-Operative Optical Coherence Tomography; DSAEK: Descemet Stripping Automated Endothelial Keratoplasty; DALK: Deep Anterior Lamellar Keratoplasty; DMEK: Descemet Endothelial Keratoplasty; DM: Descemet Membrane
Optical Coherence Tomography (OCT) is a non-contact imaging technology that produces detailed cross-sectional images, using low-coherence interferometry in biological tissues.  Anterior Segment OCT (AS-OCT) is an innovative tool for evaluation of the cornea, conjunctiva, sclera, anterior chamber (AC), anterior chamber angle, intraocular lens, and adjacent anterior segment structures. It has been helpful for the diagnosis and management of conjunctival diseases, anterior segment tumors, Corneal diseases, and AC inflammation.  Standard clinical OCT systems are large and stationary. In the past few years, the OCT became an important tool for selected ophthalmic surgeries, and adaptations had to be made to convert the OCT to a tabletop system. Today there are several commercial systems of microscope-integrated intraoperative OCT (iOCT), which focuses on OCT video visualization in highresolution, maximum integration with the microscope, and some of them integrate a three-dimensional heads-up display system for maximum convenience of the surgeon while visualizing the surgical field. 
The iOCT is a rather new technology and it is currently applied
in a wide variety of ophthalmic surgeries. The AS-OCT is used in
lamellar keratoplasty, Penetrating keratoplasty (PKP), cataract
surgeries, glaucoma surgeries, and other corneal procedures
(post-trauma, epithelial/fibrous growth). The posterior segment
OCT aids in cases of optic pit-related maculopathy, retinopathy of
premature, macular hole, retinal detachment, proliferative diabetic
retinopathy, epiretinal membrane, and posterior uveitis.  Over
the years, two major studies were conducted to test the benefits
of the intra-operative OCT (iOCT)- the PIONEER study and the
DISCOVER study. Both studies collected Clinical characteristics and
iOCT imaging was obtained during surgical milestones as directed
by the operating surgeon. A surgeon questionnaire was issued to
each surgeon and completed after each case. The PIONEER study
was published in 2014 and enrolled 531 eyes- 275 anterior segment
cases and 256 posterior segment cases. In the anterior segment
surgeries, the most common procedure was Descemet stripping
automated endothelial keratoplasty (DSAEK, n=135), and the second most common was cataract extraction with an intraocular
Immediately after the surgery, the surgeon was required to fill a feedback form. Overall following iOCT, 48% of the eyes revealed persistent interface fluid requiring additional manipulations. In deep anterior lamellar keratoplasty (DALK), 1 of 3 cases where the surgeon did not believe the trephination was deep enough, iOCT revealed the depth was optimal and did not require further deepening. In 56% of the cases of DALK, iOCT prompted further manual dissection to deepen the initial trephination.  The DISCOVER study was published in 2015, enrolled 227 eyes, 91 of them were anterior segment cases. The most common procedure was DSAEK (43%), following DALK (9%). In this study, 8% of the cases were Descemet endothelial keratoplasty (DMEK) cases (in the PIONEER study there were no DMEK cases at all). According to surgeons in the study, 44% of the total anterior segment cases were changed or modified due to the iOCT findings.  Regarding the influence of iOCT on surgical time, the DISCOVER study concluded that in 47% of the cases, the iOCT minimized the surgical time by eliminating unnecessary manipulations but did not measure the minutes that were spared. Recent studies of iOCT use in anterior segment procedures (Figure 1).
The DALK procedure gained popularity as an alternative to PKP. The “big-bubble” technique uses a forceful injection of air into the deep stroma, to create cleavage and to separate the Descemet membrane (DM) from the overlying stroma. The reported rate of successfully achieving a big bubble is 66% to 90%. For novice surgeons, the step of big-bubble generation is the most common surgical step at which perforation of DM occurs.  According to Myerscough et al, there are two main advantages of iOCT in DALK procedure - the first is assessing the depth of the cannula tip before performing pneumatic dissection, and the second is confirming that pneumatic dissection has indeed occurred.  Liu et al presented a new approach for DALK surgery in which a low-energy FSL created an anterior stromal dissection and a pre-Descemet intrastromal tunnel for the air injection in one step, to a preprogrammed depth, with the guidance of the iOCT and to facilitate big-bubble creation. 14 eyes were included: 11 eyes with keratoconus and no evidence of a history of acute hydrops and 3 eyes with corneal scars. In all cases, a big bubble was successfully achieved without intraoperative complications related to lasers (Figure 2).
Most of the data about the DSAEK procedure still relies on the data drawn from the PIONEER study, due to its relatively large cohort. The two main surgical techniques of DSAEK differ in the air infused to the anterior chamber- one uses an active air infusion system while the other manually introduce air into the anterior chamber for graft positioning. Hallahan, et al analysed the fluid dynamics and clinical outcomes for iOCT-assisted DSAEK from the PIONEER study. They used a few features measured with the iOCT and discovered that high amounts of interface fluid significantly correlated with graft non-adherence rates within the first postoperative week, following placement and optimization of intraoperative lenticule adherence. Also, the iOCT revealed a significant difference between the area, volume, and thickness of maximum fluid pockets between the two surgical techniques- the manual technique had higher values, but in both techniques, there was a significant reduction of interface fluid during the procedure .
DMEK is currently the preferred procedure to replace diseased endothelium and is superior to DSAEK and PKP for visual recovery and a lower graft rejection rate. On the other hand, the surgeon must be skilled to perform this procedure. The difficult steps are preparation of the donor lamella, transfer of the graft into the AC, unfolding and orientation of the graft, and successful attachment after air filling. Another major challenge is operating through a hazy and/or scarred cornea, which makes the visibility obscure. [9-10] Muijzer, et al. examined 38 eyes which underwent DMEK procedure. iOCT was used during the surgery and its main steps. In 42% of the cases, the iOCT altered the surgical decision-making process. In 21% of the surgeries, OCT revealed interface fluid or a minor detachment of the graft, findings that were not noted using the en face surgical microscope view; In 32% of surgeries, the iOCT image provided crucial information regarding the graft orientation  (Figure 3).
Epithelial downgrowth is a rare complication of intraocular surgery or trauma characterized by the invasion of surface epithelial cells into the AC of the eye. Fibrous downgrowth is a similar but somewhat less aggressive condition characterized by fibrovascular connective tissue invading into the eye. Those pathologies may have a devastating sequela. There are a few case reports published that emphasized the utility of iOCT in those uneventful cases. Shazly, et al. discussed a case of a woman that underwent a DSAEK procedure in an eye with fibrous ingrowth and 2 glaucoma shunt devices. The iOCT provided a clear dissection plane of the fibrous membranes and a clear view of their relation to the iris and corneal endothelium. This proved to be valuable given poor visibility through the opacified cornea. In addition, it allowed determining the extent and location of the interface fluid gap between the DSAEK graft and the host cornea.  Ruland, et al. published a case of a woman with primary open angle glaucoma and corneal decompensation of the right eye secondary to tube shunt presented for a 3-month followup of PKP. The patient had a membrane connecting the iris to the host cornea and underwent a biopsy and excision of the membrane assisted by iOCT. The poor view of the peripheral anterior chamber secondary to recent corneal transplantation was aided using iOCT, especially in the manipulation and acquisition of material for biopsy  (Figure 4).
Anterior segment IOCtl in ophthalmic surgeries is proven to be an efficient and important tool, which can impact surgeon decision in many cases, and perhaps surgical outcomes. Cases of opaque corneas which were almost impossible to operate are given hope due to this technology. Further prospective studies should be conducted on this issue.
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- Muijzer MB, Soeters N, Godefrooij DA, Chantal M van Luijk, Robert P L Wisse, et al. (2020) Intraoperative Optical Coherence Tomography-Assisted Descemet Membrane Endothelial Keratoplasty: Toward More Efficient, Safer Surgery. Cornea 39(6): 674-679.
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