One of the most important factors in attaining an on-target refractive outcome with cataract surgery is a clear and regular corneal surface. Our patients are expecting better and better outcomes—especially with the array of premium IOL options. So the stakes are high.

Garbage In ...

The cornea accounts for about two-thirds of the eye’s refractive power. While we have many different formulas and algorithms to calculate IOL power, they can be useless if corneal measurements are inaccurate or irregular. When there is incorrect or poor quality input, it inevitably produces faulty output—and the potential for a refractive surprise. As the saying goes: garbage in, garbage out.

Watch Out for These

Several corneal conditions can contribute to imprecise pre-op corneal measurements. The most common we see that need to be addressed before cataract surgery are:

• Ocular Surface Disease (OSD)
• Salzmann’s nodules
• Epithelial basement membrane dystrophy (EBMD)
• Pterygium

Benefits of Treatment

Treating these conditions with something as simple as artificial tears for a few weeks can make a profound difference in the quality of pre-op measurements. But sometimes, a more invasive surgical intervention is needed to ensure that the cornea is ready to proceed. The referring optometric physician plays an invaluable role in managing this process.

Another benefit of treating corneal pathology before cataract surgery is to help ascertain how much the cataract (vs. any corneal issue) truly impacts a patient’s vision. When asked, many will be symptomatic for an ocular surface-related condition. Occasionally, we have patients with such an improvement in visual symptoms after treating their corneas that they can postpone cataract surgery altogether.

Finally, cataract patients desiring premium lens implants are more likely to achieve maximum satisfaction when pre-existing OSD is identified and treated prior to surgery.

Ocular Surface Disease

By far, the most common corneal condition encountered before cataract surgery is OSD. According to some studies, it affects the majority of patients referred for the procedure. Changes to the cornea due to OSD can profoundly affect keratometry measurements—sometimes by as much 2D—and be the source of unwanted post-op refractive surprises.

Helpful Algorithm

Because of the increasing demands for optimal refractive outcomes, the Cornea Clinical Committee of the American Society of Cataract and Refractive Surgery has created a handy pre-op OSD algorithm. The flow-chart is meant to help eye doctors incorporate the evolving knowledge about OSD and advanced diagnostic technologies and therapies into practice. Click here to download and print copies for use in your office.

The OSD algorithm is divided into essential tests and optional tests. Essential tests include:

• a questionnaire
• tear osmolarity
• matrix metalloproteinase-9 (MMP-9)
• doctor-performed LLPP (look, lift, pull, push) slit lamp exam

A key highlight to the algorithm is that it differentiates between non-visually significant OSD (NVS-OSD) and visually significant OSD (VS-OSD).

• NVS-OSD – Prophylactic treatment should be started, but surgery can proceed.
• VS-OSD – Patient must be treated and followed until converted to NVS-OSD.

In an ideal comanagement setting, the algorithm would be implemented before the patient is referred for surgery, underscoring optometry’s primary care role and expertise.

Salzmann’s Nodular Dystrophy

Salzmann’s is characterized by gray-white to bluish superficial nodules generally located in the mid-periphery or near the limbus. The diagnosis is usually made at the slit lamp. If the lid is not lifted during the slit lamp exam, they can go undetected. The nodules consist of extracellular accumulation that is subepithelial in the presence of a thinned or absent Bowman’s layer.

The nodules may or may not stain with fluorescein. On topography, they can induce a large amount of irregular astigmatism with subsequent increased astigmatism on manifest refraction. When symptomatic to irritation or a reduction in vision secondary to irregular astigmatism, surgical management with a superficial keratectomy can be done. This involves mechanically removing the epithelium, finding the lamellar plane at the nodule’s depth, and peeling the nodule away. Re-epithelialization occurs six to eight weeks later, and the cornea is topographically stable enough to proceed with repeat IOL measurements.

Epithelial Basement Membrane Disease

EBMD, with its characteristic maps, dots, and fingerprints, is a common dystrophy affecting the anterior cornea. These changes represent thickened corneal epithelial basement membrane which has migrated superficially into the substance of the epithelium. When occurring in the visual axis, they can cause reduced vision from irregular astigmatism as well as imprecise keratometry values and subsequent IOL calculations. EBMD can be treated surgically with superficial keratectomy and diamond burr polishing or phototherapeutic keratectomy. Biometry measurements can be taken six to eight weeks later.

Pterygium

Pterygium are wing-like masses of fibrovascular tissue that extend from the conjunctiva to the cornea in the palpebral fissure along the horizontal meridian. They can present in various forms on a continuum from asymptomatic and quiescent to highly vascular types that progress rapidly. Pterygium can affect vision by either encroaching on the visual axis itself or by inducing corneal astigmatism—especially if they are 3mm or larger.

As a rule of thumb, pterygium 3mm or larger should be excised before cataract surgery as the induced astigmatism is often reversible. Typically, surgical excision is followed by a conjunctival autograft glued over the bare sclera. The cornea should stabilize in six to eight weeks, after which biometry measurements can be taken and IOL calculations performed.

Conclusion

Delaying cataract surgery to address any ocular surface problem can seem unnecessary to patients anxious for visual rehabilitation. But it is well worth the extra time to ensure they achieve as good a refractive outcome as possible. The role comanaging doctors play in this process cannot be underestimated.