Cataract surgery is one of the most common surgical procedures performed in the United States, with approximately 3.6 million cataract surgeries completed each year (Lindstrom, 2018). Despite how routine cataract surgery has become, postoperative challenges may still occur, particularly when pre-existing corneal pathology is involved.
To complicate matters further, today’s surgical cataract patients have high expectations because they recognize that cataract surgery is associated with excellent postoperative visual outcomes. Therefore, it is of particular importance to be thorough in terms of the preoperative evaluation of all patients.
On occasion, patients can present with complicated and misleading clinical signs and symptoms. It is necessary to identify other—less apparent—causes of reduced vision or visual disturbances before recommending cataract surgery.
Comorbidities can influence the surgical outcome by leading to patient dissatisfaction with the results or potentially causing disastrous complications. These non-cataract sources of vision loss or conditions need to be discussed with patients and may need to be treated or referred prior to making a recommendation for cataract removal. Counseling patients about these comorbidities will appropriately set their expectations.
What Changed?
The cornea is remarkable because it is the only transparent tissue in the entire body. A clear cornea contributes to good vision regardless of the state of the crystalline lens. Any ocular surface abnormality can affect post-surgical comfort, physical healing, and, most importantly, visual outcomes. Today, the medical community increasingly appreciates the importance of a clear, regular, smooth cornea as it relates to post-cataract-surgery visual outcomes. Clinicians agree that meticulous assessment, including review of medical and surgical history as well as a thorough preoperative slit-lamp examination, is essential to identifying corneal irregularities (Hermanson, 2016).
Previously, however, the health of the cornea was somewhat ignored prior to cataract surgery. Most cataract surgeons felt that if the cornea was clear enough to see through, the surface was generally adequate (Hermanson, 2016).
Concern with respect to the ocular surface did not grow until 2007 when the first International Dry Eye WorkShop (DEWS) report was published by the Tear Film & Ocular Surface Society (TFOS). This marked one of the first major group efforts to organize and interpret a tremendous amount of research on the subject (TFOS, 2007).
Since then, appreciation for the complexity and importance of the ocular surface as it relates to all aspects of eye health has grown. The highly anticipated, updated TFOS DEWS II report was released in 2017 (Craig et al, 2017).
Also, impressive diagnostic tools now allow practitioners to quantify the amount of blur that each problem causes. For example, one diagnostic device can assess the degree of optical scatter to evaluate visual function and diagnose the source of visual complaints, whether it is the tear film, the cornea, or the lens (Fox, 2015). In addition, corneal topography assessments can quantify existing irregularity in the cornea, while tomography can measure the index of the irregularity. Corneal higher-order aberrations can also be measured, and lens density can be quantified with a tomographer, wavefront aberrometer, or topographer to assess whether the lens or the cornea is contributing to the blur (Kent, 2017).
Challenging Conditions of the Cornea
It is important to look for a range of corneal conditions that can affect the cornea when evaluating cataract surgery candidates. Those conditions generally fall into two categories.
First, certain corneal disorders, such as Fuchs’ endothelial dystrophy, represent a risk of significant visual acuity loss following cataract surgery. Second, conditions such as dry eye, epithelial basement membrane dystrophy, and Salzmann’s nodular degeneration can potentially lessen patient satisfaction after surgery (Hovanesian, 2011).
Challenge: Fuchs’ Endothelial Dystrophy Fuchs’ endothelial dystrophy is a genetic disorder in which the corneal endothelium becomes progressively weak, leading to corneal decompensation and decreased vision. It is the most common endothelial dystrophy affecting human corneas and is a progressive disease in which patients develop corneal guttae, endothelial cell loss, and stromal edema (Alder and Kim, 2012).
Patients who have Fuchs’ are at risk for developing cataracts as their corneal disease worsens. The intersection of these two problems in the same eye is not uncommon. In many cases, it is not clear how much each condition contributes to patients’ decline in vision. Clinical judgment and experience are necessary to distinguish between the contributions (Alder and Kim, 2012).
It is an important consideration for every cataract surgery candidate and requires a thorough assessment of the corneal endothelium. A careful slit lamp exam prior to cataract surgery should always include seeking endothelial abnormalities. When in doubt, pachymetry and specular endothelial cell counts can help estimate risk. Less than 1,000 cells per square millimeter and central pachymetry greater than 600 microns are warning signs of increased risk of decompensation (Hovanesian, 2011).
Challenge: Dry Eye It is essential to ensure that the ocular surface is as healthy as possible before surgery for two reasons. First, cataract surgery has been shown to induce or exacerbate pre-existing dry eye syndrome or other surface-damaging disorders. It can also increase the risk of chronic, postoperative irritation and photophobia (Roach, 2014).
Second, the goal of taking measurements before cataract surgery is to allow the best, most accurate measurements possible, which will result in a very precise surgery. A dysfunctional, unstable tear film will affect the visual outcome by introducing error into the measurements on which intraocular lens (IOL) power calculations depend. Even mild-to-moderate dry eye can disrupt the tear film enough to affect the optics of the cornea (Roach, 2014).
Although dry eye is not an absolute contraindication to cataract surgery, the condition should be aggressively treated before the surgical plan and IOL selection are finalized. The treatment of dry eye disease may delay cataract surgery in some cases, but it is often a worthwhile investment. The treatment plan should be tailored to the severity of the disease state. It is also important for physicians to monitor and treat the condition during the postoperative period as well.
Challenge: Epithelial Basement Membrane Disease Although common, epithelial basement membrane dystrophy (EBMD) remains an often overlooked and frustrating cause of postsurgical disappointment. EBMD (also known as map-dot-fingerprint dystrophy) is an inherited congenital disorder that affects the corneal epithelium and basement membrane. Patients who have EBMD will generate a new basement membrane much more rapidly than is normal, and it encroaches on the corneal epithelium and disrupts the stroma-to-epithelial junction (Schechter, 2016).
Secondary corneal effects include epithelial microcystic edema, recurrent corneal erosion (RCE), and visual axis involvement. Symptoms or visual disturbances range from negligible to pain and significant visual obscurations.
In the perioperative period, EBMD is a potential concern, although it is not an absolute contraindication to proceeding with cataract surgery. If EBMD is suspected, surgery should be delayed until the condition is stable. Due to a propensity for change in the shape of the corneal surface, EBMD is a risk factor for inaccurate biometry and IOL power calculations (Schechter, 2016).
Challenge: Salzmann’s Nodular Degeneration Salzmann’s nodular degeneration (SND) is a disorder in which fibrotic plaques develop on the surface of Bowman’s membrane, most typically in the peripheral cornea but occasionally more centrally. The nodules, which may be multiple, frequently occur in the presence of EBMD, dry eye, or blepharitis.
Even though this is usually a peripheral corneal disorder, the nodules frequently affect the central corneal curvature and keratometric readings because the tear film is irregular over and around them. False keratometry readings can lead to imprecise IOL power selection. Removing these nodules is fairly straightforward and requires only simple instrumentation (Chan, 2014).
Conclusion
Corneal sources of reduced vision are frequently encountered. In addition, even when subtle, they need to be considered when the vision loss is not consistent with the level of cataract to make sure that the right problem is being treated. At minimum, practitioners should inform patients that some vision loss may persist postoperatively.
Fortunately, in most instances, patients who undergo cataract surgery have satisfactory results. While it is true that eyecare practitioners need to be mindful of corneal conditions that may complicate cataract surgery, it is also worth noting the solid benefits that most achieve. CLS
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