Cataract Surgery in the Setting of Fuchs Dystrophy
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Fuchs’ endothelial corneal dystrophy (FECD) is a condition that affects the corneal endothelium, resulting in a reduction in the number of Na+/K+ ATPase pumps . Clinically, this manifests as corneal edema, which can lead to symptomatic glare and halos, and ultimately decreased visual acuity. Clinical signs of FECD often precede symptoms and, therefore, early recognition is important when evaluating these patients for cataract extraction. Many studies have shown significant endothelial cell loss in these patients as a result of cataract surgery, placing them at higher risk for subsequent corneal decompensation. The concomitant presence of a cataract in a patient with FECD warrants meticulous preoperative, intraoperative and postoperative considerations to achieve the best visual outcomes.
Preoperative Considerations and Patient Selection
All patients with FECD should be educated on the progressive nature of the disease and the possibility of needing a corneal transplant, either at the time of cataract surgery, or at some point in the future. Once a cataract is considered visually significant in a patient with FECD, a decision must be made on proceeding with cataract surgery alone or proceeding with cataract surgery combined with a corneal transplant, also known as a triple procedure. If the visual impairment is caused more by the corneal pathology related to FECD rather than a cataract, it may still be prudent to remove the cataract at the time of corneal surgery. The cataract will likely progress and become visually significant in the future as a result of natural aging or from postoperative long-term topical steroid use. Proceeding in this manner would also prevent further endothelial cell damage associated with a subsequent surgery.
Historically, ultrasound pachymetry and endothelial cell density, measured by specular microscopy, are used as clinical thresholds when making a surgical plan. These parameters serve as surrogate markers for corneal endothelial cell health. Pachymetry measurements greater than 640 µm and/or endothelial cell density of less than 1000/mm2 place a patient at increased risk for corneal decompensation following cataract surgery. In this situation, where a patient with FECD meets one or both of these parameters, it may be prudent to undergo a triple procedure rather than cataract surgery alone. Benefits of a triple procedure include avoiding a second surgery, speeding up visual recovery, and reducing the costs and risks associated with sequential surgery.
Recent technology has allowed for objective quantification of corneal haze, also known as backscatter, to serve as a more sensitive parameter for corneal endothelial cell health. This value can be deduced from images acquired by corneal tomography, anterior segment optical coherence tomography (OCT), and confocal microscopy. Backscatter, specifically as measured by confocal microscopy, has been shown to be more precise than corneal pachymetry in predicting the need for corneal transplant following cataract surgery. This is likely due to the wide range of pachymetry values in the general population, with abnormal pachymetry readings still falling within the range of normal. This problem is circumvented in backscatter measurements, as abnormal backscatter falls outside of the normal range. However, confocal microscopy is not as widely available as ultrasound pachymetry for use in most practices and institutions.
When deciding to perform a corneal transplant on a patient with visually significant corneal disease, there are a few options. Classically, full thickness penetrating keratoplasty (PK) was the mainstay of corneal transplantation and was the only option available for patients with FECD. However, with the development of endothelial keratoplasty (EK), more specifically Descemet stripping endothelial keratoplasty (DSEK) around 2003 and more recently Descemet membrane endothelial keratoplasty (DMEK) around 2006, trends have changed such that EK is now the most commonly performed corneal transplant for patients with FECD. As of 2016, according to the Eye Bank Association of America, of the patients that underwent a corneal transplant for FECD, 93% received an EK while 7% received a PK. Of those that underwent EK, DSEK is still the most commonly performed procedure with 78% receiving a DSEK procedure versus 22% receiving a DMEK procedure, though the trend has been increasing for DMEK every year since 2012.
The steps in cataract surgery in patients with FECD are not significantly different than cataract surgery in patients without FECD. However, there are special considerations in cataract surgery to minimize intraoperative corneal endothelial cell loss and optimizing visual outcomes.
Intraocular Lens (IOL) Selection
The selection of an IOL always requires a thorough preoperative discussion with the patient. In addition, patients with FECD should also be informed about the progressive nature of their disease, reduced contrast sensitivity, and compromised vision in glare conditions. In general, multifocal IOLs are associated with reduced contrast sensitivity and increased risk for glare and halos. Given the underlying predisposition for these same issues in patients with FECD, the implantation of a multifocal IOL can compound these effects and is therefore considered a relative contraindication. In general, a standard monofocal IOL is recommended for these patients. If the patient desires spectacle independence, a multifocal, extended range or accommodating IOL may still be considered, but a careful discussion with the patient should take place to minimize patient dissatisfaction.
In patients undergoing a triple procedure, a similar discussion should be held with the patient regarding IOL options. Due to the complexity of the procedure, possible IOL decentration, and less accuracy in IOL power calculations, a monofocal IOL has traditionally been recommended.
There are a few examples in the literature of successful implantation of multifocal IOLs in patients with FECD, more specifically in patients that have undergone DMEK. In one retrospective study by Pereira et al., DMEK was performed in 9 eyes with previously implanted multifocal IOLs and good visual outcomes were reported. With the exclusion of one patient, all eyes achieved best corrected distance visual acuity (BCDVA) of 20/30 or better, with 62.5% of the eyes achieving 20/25 BCDVA or better and 25% of the eyes achieving 20/20 BCDVA or better. In a case report by Nanavaty et al., a patient with FECD underwent clear lens extraction with trifocal IOL implantation, combined with a DMEK, in both eyes two months apart achieved uncorrected distance visual acuity of 20/16 in each eye and uncorrected near and intermediate vision of 20/20 in both eyes. To our knowledge, there are no examples in the literature of the use of multifocal IOLs in patients that have undergone DSEK. In terms of astigmatism correction, the use of toric IOLs in triple DMEK procedures has not been substantiated by the current literature, although some studies report good outcomes in these patients.
An additional consideration for eyes undergoing a triple procedure with EK is IOL power selection. A hyperopic shift is expected in eyes that undergo EK, whether DMEK or DSEK, with higher magnitudes of hyperopia reported post DSEK compared to post DMEK .
The hyperopic shift observed after DSEK stems mainly from changes in posterior corneal curvature. The added donor tissue results in increased negative refractive power at the posterior corneal surface, which ultimately leads to decreased total corneal power. This change in total corneal power translates into a hyperopic shift. One study by Hwang et al. developed a mathematical model for estimating the magnitude of hyperopic shift after DSEK. The model factored in preoperative corneal thickness, graft thickness, central to peripheral graft thickness ratio, and recipient posterior corneal curvature. Of the factors considered, thicker donor tissue was associated with larger hyperopic shift. Using this study’s mathematical model, 90% of the measured hyperopic shift was able to be predicted.
Given that DMEK simply replaces endothelium rather than adding tissue, the etiology of the hyperopic shift is speculated to differ from that of DSEK. One study by Ham et al., determined that in eyes with FECD, the posterior cornea swells as a result of natural progression of the disease. This swelling leads to posterior corneal flattening and a myopic shift. Shortly after DMEK, the cornea shows central thinning, while the periphery remains swollen. This creates posterior corneal steepening, resulting in a hyperopic shift. Once the cornea achieves normal hydration status, it continues to shows a hyperopic shift relative to the preoperative power.
As a result of this known phenomenon, most surgeons target slight myopia, typically -0.75 to -1.00 for DMEK and -1.00 to -1.25 for DSAEK, to counteract this effect. Additionally, in eyes with FECD undergoing cataract surgery alone, this expected phenomenon is typically factored into the IOL calculation in anticipation of the eventual need for EK.
Ophthalmic Viscoelastic Devices (OVDs)
The two main types of OVDs are cohesive and dispersive, both of which can be utilized in eyes undergoing cataract surgery. The soft-shell technique, first described by Steve Arshinoff, is a method that maximizes the advantages of cohesive and dispersive OVDs, while minimizing their disadvantages. This technique consists of first injecting a dispersive OVD onto the surface of the lens, creating a mound, followed by injection of a cohesive OVD posterior to the mound. Injection of the cohesive OVD pushes and lifts the dispersive OVD anteriorly, thereby coating the corneal endothelium. Once phacoemulsification begins, the cohesive OVD leaves the eye, while the dispersive OVD remains in place on the posterior corneal surface and intact through phacoemulsification and irrigation/aspiration steps. This method is used to protect the corneal endothelial cells and minimize intraoperative endothelial cell damage. Various studies have examined this technique, specifically in eyes with dense cataracts and FECD, and found favorable outcomes including reduced postoperative central corneal thickness and reduced endothelial cell loss when compared with the use of a single OVD.
One of the key parts of performing cataract extraction using phacoemulsification is the capsulorrhexis. A special consideration should be given to the size of the capsulorrhexis. When performing a triple procedure, there is significant tissue manipulation required to properly position the corneal graft. Therefore, the implanted IOL may be prone to unintended decentration and even dislocation. It is advisable to create a capsulorrhexis that is smaller than the IOL optic, to prevent movement of the IOL after implantation. Visibility is crucial to creating an optimally sized capsulorrhexis and it can be compromised in cases of dense or white cataract, vitreous hemorrhage, corneal haze or other corneal pathology. In cases with poor visualization of the anterior capsule, there is an increased risk of complications from a poor capsulorrhexis- such as capsular rupture or radial tears leading to vitreous loss and dropped nuclear fragments. Trypan blue dye has been used to aid in visualization in these cases and can help reduce the risk of complications from challenging capsulorrhexis. There are some studies that showed trypan blue had a deleterious effect on corneal endothelial cell in vitro, however this was only true with concentrations much higher than what are typically used or in cases with prolonged exposure to the dye (greater than 6 hours). To minimize the interaction between corneal endothelial cells and trypan blue, some surgeons advocate the use of an intracameral air bubble to provide a barrier. The use of trypan blue intraoperatively during cataract surgery did not cause a statistically significant difference in endothelial cell loss, size, or shape nor did it cause increase in corneal edema or difference in best corrected visual acuity postoperatively.
Cataract Extraction Method and Nuclear Disassembly
When determining the method for cataract removal, some studies have evaluated different techniques and their impact on endothelial cell loss. The most common techniques for cataract removal include phacoemulsification, manual small incision cataract surgery (SICS), extracapsular cataract extraction (ECCE), and femtosecond laser assisted cataract surgery (FLACS). Although the ultimate decision of surgical technique will likely be determined by the surgeon’s own expertise and familiarity, the choice of technique should be one that attempts to minimize endothelial cell loss and maximize successful cataract removal with intraocular lens implantation.
One study by George et al. evaluated endothelial cell loss in a cohort of patients without FECD who were randomized to receive cataract surgery either by conventional ECCE, SICS, or phacoemulsification. The results showed that there was no significant difference in endothelial cell loss post-operatively (4.72% ECCE, 4.21% SICS, 5.41% phacoemulsification). Another study by Zhu et al. evaluated standard phacoemulsification versus FLACS in 207 eyes with FECD. In this study, although the use of femtosecond laser to aid in nucleus disassembly leads to overall decreased amounts of ultrasound energy and time, there was no difference between rates of corneal decompensation or best corrected distance vision at 6 months follow-up. In another study by Yong et al., FLACS was found to reduce endothelial cell loss in eyes with FECD, as compared to traditional phacoemulsification. This difference was especially significant in moderate to high density cataract surgery. At present, the use of FLACS has not been proven to decrease progression to corneal edema and need for corneal transplant in patients with FECD despite reducing endothelial cell loss.
If the decision is made to proceed with phacoemulsification, there can still be further choices as to the method of nucleus disassembly that can be employed. Some of the more common techniques are known as “divide-and-conquer”, “stop-and-chop”, and “phaco-chop”. One study compared these different techniques and their impact on corneal endothelial cell loss in patients without known FECD. Results showed that in patients with denser nuclear cataracts, phaco-chop had significantly less endothelial cell loss when compared to divide-and-conquer and stop-and-chop techniques (5.2% vs 9.1% and 7.2%, respectively). There was a trend towards less endothelial cell loss in all densities of lenses with phaco-chop, but the numbers did not reach significance. There was, however, no difference in final best corrected distance vision.
Thus, when a decision is made to perform cataract surgery alone on a patient with FECD, the choice of technique should be based on the surgeon’s experience, the density of the cataract, and the health of the cornea. However, if corneal thickness is greater than 640 um or endothelial cell density is less than 1000 cells/mm2, a triple procedure may be considered over cataract surgery alone.
Postoperative care for patients with FECD who undergo cataract surgery is very similar to patients without FECD, but there are some additional considerations. All patients with low corneal endothelial cell counts preoperatively should be counseled on prolonged visual recovery when compared to routine cataract surgery patients. These patients often require more frequent administration and more prolonged use of topical steroids in the postoperative period. Additionally, these patients often have more significant and prolonged corneal edema that can negatively impact visual acuity. Fortunately, many patients recover well and most studies show little difference between final visual outcomes in patients with low endothelial cell counts.
Certain risk factors may predispose patients to postoperative pseudophakic corneal edema and prolonged visual recovery and therefore special care should be taken when operating on these patients. Studies have indicated that shorter axial length, diabetes mellitus, longer phacoemulsification time, higher phacoemulsification intensity, and posterior capsular rupture are risk factors for greater endothelial cell loss and progression to bullous keratopathy in patients with already decreased endothelial cell counts.
Medical therapy for the treatment of pseudophakic corneal edema is multifaceted and directed towards the suspected etiology. General therapy includes the use of hypertonic saline (5%) in either solution or ointment form to desiccate the corneal stroma, with one study showing significant improvement in visual acuity in about one third of patients with pseudophakic bullous keratopathy. If there is presence of low-grade inflammation, topical steroids can be beneficial, as anterior uveitis is often associated with corneal edema. Furthermore, if IOP is elevated, this can also compromise endothelial cell function and lead to edema. Medical therapy should be directed towards lowering IOP with the use of one or more topical antihypertensive medications. If medical therapy cannot lower the IOP sufficiently, surgical intervention may be warranted.
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