Imaging in Cataract Surgery.

Modern cataract surgeons have several tools in their armamentarium that allow them to provide the best possible outcomes to their patients. These include high-precision optical biometers, advanced intraocular lens power formulae, femtosecond lasers, and premium intraocular lenses. We have also witnessed significant advancements in surgical microscope technology such as incorporation of digital microscope markers, intraoperative optical coherence tomography (OCT), and 3D-head up display, which have significantly enhanced the ease and safety of performing cataract surgery.[1,2] Traditionally, surgeons relied on preoperative slit-lamp examination to clinically assess the morphology and grade of cataracts while planning surgery. A thorough preoperative evaluation is particularly important in complex cases such as posterior polar cataracts wherein the status of the posterior capsule has to be determined or in white cataracts to detect raised intralenticular pressure using indirect signs such as bulge of anterior capsule or shallow anterior chamber. Today, the use of imaging modalities such as OCT has become an integral part of preoperative cataract work-up that greatly impacts surgical planning, especially in such complex scenarios. Preoperative Imaging in Cataract Surgery Preoperatively, anterior segment OCT (ASOCT) has become an indispensable tool to visualize the lenticular morphology. In mature cataracts, ASOCT imaging can reveal the stage of cataract progression, ranging from early lamellar separation visualized as hypoechoic areas within the anterior cortex to extensive lens matter liquefaction in the more advanced stages that appear as relatively echo-free dark areas underneath a bulging anterior capsule [Fig. 1a]. The presence of features suggestive of extensive lens matter liquefaction is indicative of an increased risk of intraoperative complications and guides the surgeon to adopt preventive measures to reduce the intralenticular pressure while performing capsulorhexis.[3]Figure 1: Preoperative anterior segment optical coherence tomography (ASOCT) imaging in cataract surgery. (a) ASOCT image of white cataract depicting increased convexity of the anterior capsule and intralenticular fluid clefts. (b) ASOCT image of posterior polar cataract depicting an intact posterior capsule with a clear hypoechoic space between the posterior plaque and capsuleAnother important role of preoperative ASOCT imaging is to assess the integrity of the posterior capsule and should be routinely performed in posterior polar cataracts, traumatic cataracts, vitrectomized eyes, and postintravitreal injections.[4] Several authors have highlighted the role of preoperative ASOCT imaging in identifying posterior polar cataract cases that are at high risk for intraoperative posterior capsular rupture.[5,6] Features such as an intact posterior capsule or clear hypoechoic space between the posterior plaque and capsule have been associated with a lower risk of intraoperative capsular dehiscence [Fig. 1b]. Meanwhile, certain specific morphologies on ASOCT imaging such as a conical, moth-eaten, or ectatic appearance were found to be associated with an increased risk of intraoperative posterior capsular rupture [Table 1].[5,7] However, clear visualization of the posterior capsule may not always be possible on ASOCT owing to the shadowing caused by the posterior plaque. Surgeons may need to rely on the appearance of the posterior lens contour to better understand capsular morphology in such cases.[7] Other imaging modalities described for preoperative assessment of the posterior capsule include Scheimpflug imaging and ultrasound biomicroscopy.[8,9] In addition to these, specialized modules such as the lens densitometry in Pentacam (Oculus, Inc.) and the Dysfunctional Lens Index on iTrace (Tracey Technologies Corp.) can objectively quantify the grade of the lens opacity and aid in surgical decision-making.Table 1: Abnormal/Deficient Morphologies of Posterior Capsular in Posterior Polar Cataract on Preoperative Anterior Segment Optical Coherence Tomography (Pujari et al., 2020)[ 7 ]Intraoperative Imaging in Cataract Surgery The imaging modalities described above were a significant advancement over slit-lamp examination for preoperative visualization of lenticular morphology and identification of high-risk cases. However, these tools did not provide real-time intraoperative dynamic feedback to the surgeon. The advent of microscope-integrated OCT changed this and has truly revolutionized the surgical approach, especially in complex cataract cases. Intraoperative OCT (iOCT) is a great tool for cataract surgeons as it provides real-time dynamics of the anterior segment ultrastructure during each step of surgery. It is a unique imaging modality that can be beneficial in both normal and complex cataract cases. In normal cases, iOCT imaging helps the surgeon to assess the wound morphology and distortion if any during each step, confirm whether the fluid waves are in the correct plane during hydro procedures, and identify the exact depth of trenching during nuclear emulsification. iOCT can also be used to assess the IOL position in relation to the posterior capsule and detect any residual viscoelastic beneath the IOL at the end of surgery. It is also an invaluable teaching tool that can be used by expert surgeons to three dimensionally demonstrate to trainee surgeons the surgical intricacies and ultrastructural tissue changes that occur in real time during each surgical step. Moreover, in complicated scenarios such as white cataract, posterior polar cataract, traumatic cataract, or postvitrectomy cases, the real-time feedback provided by iOCT imaging aids in risk stratification and intraoperative decision-making that enhances safety during phacoemulsification.[10] Intraoperative imaging in white cataracts Performing capsulorhexis is one of the most challenging steps during phacoemulsification in white cataracts due to the imminent risk of rhexis extension and radial tears owing to raised intralenticular pressure. We devised an iOCT-based classification system for white cataracts based on the lens morphology and intraoperative dynamics that guide the surgeon to successfully complete capsulorhexis.[11] While white cataracts comprise a wide spectrum of morphological variants with a combination and overlap of characteristics, we grouped them into four types. In addition to the static features such as the contour of the anterior capsule, arrangement of anterior cortical fibers, and intralenticular fluid clefts which can be observed on preoperative ASOCT as well, iOCT also helps visualize the dynamic changes in lens ultrastructural morphology once rhexis is initiated. These include flattening of the anterior lens capsule on injecting viscoelastic, capsular flap dynamics after making the initial nick as well as the changes in the subcapsular lens matter, such as release of milky fluid or bulging of the cortical fiber complex [Fig. 2a and b].[11] Based on these static as well as dynamic features observed on iOCT, the surgeon can identify the cases at high risk for peripheral extension of the rhexis and modify their technique accordingly to successfully complete the capsulorhexis. In fact, the enhanced understanding of the intraoperative dynamics gained from iOCT imaging in white cataracts can help the surgeon correctly interpret and anticipate the lenticular structural dynamics during rhexis even when iOCT imaging is not available to them, thus aiding in intraoperative decision-making.Figure 2: Lenticular dynamics after initiation of capsulorhexis observed on intraoperative OCT imaging in white cataracts. (a) Bulging of the cortical fiber complex (white arrow) signifying raised intralenticular pressure after rhexis initiation in type II white cataract. (b) Slow spontaneous egress of oily/turbid fluid (white arrow) with partial lowering of intralenticular pressure (depicted by simultaneous flattening of the anterior lens capsule) in type III white cataractFemtosecond laser-assisted cataract surgery (FLACS) is advantageous in white cataract owing to its ability to create precise, circular capsulorhexis in a safe manner. OCT incorporated within the femtosecond laser machine can reveal the fluid-filled spaces in the subcapsular space. In such cases, the initiation of laser-assisted rhexis is often followed by a sudden release of milky fluid that can hinder the penetration of laser to the capsule underneath, resulting in skip areas. Staining the capsule with trypan blue dye prior to anterior capsular removal helps the surgeon identify these skip areas and safe completion of rhexis.[12] Intraoperative imaging in posterior polar cataracts Phacoemulsification in posterior polar cataracts remains a challenge owing to an inherently fragile posterior capsule that increases the risk of intraoperative posterior capsular dehiscence. We proposed a classification system based on the iOCT imaging of posterior polar cataracts to identify high-risk cases for intraoperative posterior capsular dehiscence. Three morphological variants of posterior polar cataracts were described based on the visibility of the posterior capsule and clearance between the posterior polar opacity and capsule.[13] Hydrodissection is an important step in phacoemulsification that can ease subsequent cortical clean-up and reduce the surgical time. However, it is conventionally avoided in posterior polar cataracts due to the increased risk of posterior capsular dehiscence. We demonstrated that hydrodissection can be safely performed in type I opacity wherein a clear space is observed between the posterior polar opacity and capsule on iOCT imaging. Meanwhile, hydrodissection should be strictly avoided in type 2 and type 3 cases, which are characterized by a dense opacity and inability to delineate the posterior capsule status. Proper hydrodelineation is a crucial step in posterior polar cataracts and iOCT imaging helps assess its adequacy by allowing direct visualization of the passage of fluid wave during this step. The presence of a fluid wave that separates the endonucleus from the underlying epinucleus is desirable since an intact epinuclear plate acts as a protective cushion for the posterior capsule during subsequent nuclear emulsification [Fig. 3]. We have described a modified phaco-chop technique, chop and tumble nucleotomy, that allows the surgeon to safely perform nuclear emulsification within an intact epinuclear cushion in posterior polar cataracts, in the absence of hydrodissection and nuclear rotation.[14]Figure 3: Assessment of adequacy and safety of hydrodelineation using iOCT imaging. Hydrodelineation fluid wave can be visualized that separates the endonucleus from the underlying intact epinucleusiOCT imaging also helps in the visualization of any fluid misdirection leading to inadvertent hydrodissection that can lead to posterior capsular rupture. Real-time continuous assessment of the posterior capsule can be performed throughout surgery allowing the surgeon to promptly detect an intraoperative posterior capsular dehiscence and modify the subsequent steps. FLACS has been found to be a safe and effective modality for phacoemulsification in posterior polar cataracts. Intraoperative imaging with the femtosecond laser-integrated ASOCT is a useful tool to detect preexisting posterior capsule dehiscence and alert the surgeon to adopt safety measures such as increasing the posterior offset, which enables the creation of a thicker protective epinuclear cushion.[15] Such real-time dynamic visualization of posterior capsule status during surgery can be very beneficial in other scenarios associated with a fragile or dehiscent posterior capsule such as postvitrectomized eyes, posttraumatic cataract, or eyes that have previously received intravitreal injections. Recently, Dr Ronald Yeoh described the ‘intraoperative OCT scroll sign’ to differentiate a posterior capsule rupture from an anterior radial tear. The former is characterized by more scrolled edges as the posterior capsule is thinner, while an anterior capsular tear is visualized as two thick, curved lines on iOCT imaging.[16] Intraoperative imaging of corneal wound architecture Proper wound construction is crucial to reducing the incidence of incision-site complications such as Descemet membrane detachment (DMD). iOCT imaging helps in real-time assessment of clear corneal incision (CCI) morphology and DMD during surgery. We observed that a ragged morphology of the proximal opening of CCI was associated with a higher risk of incision-site DMD. Femtosecond laser CCIs had a lower incidence of incision-site DMD than microkeratome-assisted CCI.[17] In cases such as Fuchs’ endothelial dystrophy that are associated with an increased risk for intraoperative DMD during phacoemulsification, iOCT imaging is of great help in detecting a DMD during surgery which may not be visible under the microscope. Intraoperative imaging allows the surgeon to clearly visualize the size and extent of detachment and promptly adopt measures on the table itself to manage the complication. iOCT imaging is an invaluable tool for performing cataract surgery. It provides real-time visualization of each step that allows the surgeon to assess the incision architecture, intralenticular pressure, capsular dynamics during rhexis, adequacy of hydro procedures, depth of nuclear trenching, posterior capsular integrity, and effective lens position during phacoemulsification. In addition to being a useful intraoperative adjunctive modality in normal cataracts, iOCT is a highly effective training tool for demonstrating surgical steps to trainee surgeons. Furthermore, it is particularly beneficial in challenging cases such as white cataracts and posterior polar cataracts wherein it helps in identifying high-risk morphological features, allows real-time analysis of surgical planes, detects intraoperative complications, and aids in surgical decision-making. The additional visual dimension provided by iOCT imaging during surgery has greatly enhanced our understanding of the surgical dynamics that can be successfully incorporated into our surgical practice even in the absence of the availability of iOCT imaging.[2]