The cornea has the most refractive power of any structure in the eye. Preoperatively, the surgeon evaluates the cornea for abnormalities that might impair vision and diminish the expected improvement in vision:
tear film abnormalities (these should be addressed as described in the previous section Blepharitis and Acne Rosacea)
epithelial basement membrane dystrophy
irregular astigmatism (this can be assessed by corneal topography and masked by a gas-permeable trial contact lens to determine the effect on vision impairment)
Corneal irregularities affect the accuracy of keratometry and lead to erroneous calculation of lens power. In an eye with epithelial basement membrane dystrophy, epithelial debridement may help produce a smoother corneal surface (Fig 12-2). After debridement, it is necessary to wait a few weeks before repeating keratometry so the corneal surface can become smooth and stable. Mild corneal stromal opacities are unlikely to reduce vision in the presence of a pristine anterior refractive surface.
Figure 12-2 Irregular corneal astigmatism occurs in patients with epithelial basement membrane dystrophy. This corneal condition can further decrease vision in a patient with cataract-related vision impairment, in which case the vision improvement after cataract surgery might be less than expected.
(Courtesy of Christopher J. Rapuano, MD.)
In patients with a history of herpes simplex virus (HSV) infection, epithelial or stromal keratitis may be exacerbated after cataract surgery. Although the Herpetic Eye Disease Study (HEDS) did not specifically address HSV following surgery, the results did show that prophylactic treatment with oral acyclovir (400 mg, twice daily) reduces the incidence of recurrent HSV keratitis. Because recurrent stromal keratitis may result in loss of visual acuity, many ophthalmologists use oral acyclovir, famciclovir, or valacyclovir perioperatively and observe the patient closely for recurrent keratitis postoperatively.
Cataract surgery in patients with keratoconus poses a challenge in IOL calculation. These patients have increased potential for unexpected refractive outcomes, given the irregular astigmatism and potentially abnormal keratometry values. Preoperatively, it is important to inform patients with significant keratoconus that they likely will need contact lens correction of the irregular astigmatism after cataract surgery. The surgeon can attempt to stabilize the keratoconus, if possible (eg, with collagen crosslinking) to predict the refractive outcome more accurately.
Excerpted from BCSC 2020-2021 series: Section 11 - Lens and Cataract. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.