Topography has become an indispensable tool in cataract surgery. Originally developed for keratorefractive procedures, it is now used to determine candidacy for corneal as well as cataract procedures. Preoperatively, it was used to determine if the keratometry readings are appropriate for excimer ablation of the patient’s refractive error, and to rule out ectatic changes prior to surgery. As refractive surgery expanded to include lenticular procedures and premium lenses, the technology grew. Topography is now used to assess corneal shape, regular astigmatism for toric IOL implantation, corneal spherical aberration for aspheric IOL implantation, and alignment.
Ocular alignment should also be addressed preoperatively to ensure success using a premium lens. Clinical assessment of angle Kappa and angle Alpha should be performed prior to discussion of IOL options. (Figure 1) Angle Kappa is the difference between the visual axis and center of the pupil. This is particularly important in keratorefractive surgery for hyperopes, or in presbyopic treatments. Angle kappa distance of 0.5 is considered large, and KRS may be problematic.
Angle alpha is the angle between the visual axis and the center of the limbus. The center of the limbus is thought to represent the center of the lens capsule and is used to predict where the IOL will be positioned after implantation. Current IOL technology employs haptics which center the IOL in the capsular bag, which is correlated with the corneal limbus. If the IOL within the bag is not aligned with the visual axis, the patient will not look through the center of the IOL. This will induce higher-order aberrations and negatively affect visual function. Toric IOLs also require proper alignment. Decentration of toric lenses may induce astigmatism or reduce the power of the cylinder resulting in residual refractive error. An angle alpha distance of 0.5 is considered high for Symfony and toric IOLS, and 0.3 is the common limit for diffractive IOLs which are less forgiving.
Tomography may be combined with topography to identify those with ectasia, form-fruste keratoconus, or pellucid marginal degeneration. Tomography yields three-dimensional images using 2-dimensional sections and is capable of measuring pachymetry as well as curvature and elevation.
Topography is a powerful tool for dry eye, as well. Ocular surface disease may manifest as irregular astigmatism. Demonstration of the effect dry eye has on the corneal surface pretreatment, and again post-treatment is an effective educational tool. It helps them understand how the dry eye changes the vision and they are more likely to follow instructions when they know they will be measured again in one month to assess the treatment plan. Figure 2 demonstrates the effect of dry eye on the central cornea in a patient with a KAMRA inlay. Slit lamp exam found significant punctate keratitis over the central cornea. Repeatedly saying, “You have dry eye” only frustrated the patient when punctal plugs and artificial tears failed to significantly improve the problem. Note the corneal and refractive cylinder in the central 1 mm, which explains why no loose lens would improve the vision at near. Lifitegrast ophthalmic solution twice daily with omega-3 supplementation improved the keratitis, and continued use is improving her vision at near.
Topography is also helpful for assessment of patients with surgical complications. This is particularly true in case of optical alignment and irregular astigmatism not visible upon slit lamp examination.
A 67-year-old male presented complaining of inability to see at all distances, photopsia, and distortion of lights. He had undergone RK years ago, followed by cataract surgery with a Symfony IOL (Johnston and Johnston, New Brunswick, NJ) 4 months prior. Refraction revealed +0.25-2.00 x 130 (20/60) with significant shadowing of letters. Dilated fundus exam revealed the implant was decentered superiorly relative to the pupil. Topography revealed a small, inferiorly decentered optical zone, and increased corneal coma. Angle alpha was 0.667. (Figure 3)
Evaluation of the patient’s topography illuminated several issues that result in poor vision.
- The I-S value is extremely high. (-10.95D) due to the decentered optical zone. These indices are often used to indicate keratoconus, where the steep area is inferior. In this case, the steep area is superior to the visual axis, and the flatter area is inferior due to excimer treatment decentration.
- Coma in the visual system results from inferior displacement of the corneal optic zone and may manifest as vertical monocular diplopia or smearing of the image.
- There is excessive positive corneal spherical aberration (+0.895µm). The average spherical aberration of the cornea is +0.27 μm because the cornea is steeper centrally. Excessive positive or negative spherical aberration causes halos.
Topography can be a beneficial addition for investigation into a patients functional vision loss and is not just for LASIK anymore.
The Dysfunctional Lens Index (DLI) suggests the visual quality is reduced due to posterior astigmatism or capsular haze. Internal aberrations are much less than corneal. If the cornea is not corrected the patient will most likely be disappointed by their outcome.
What options exist to help this patient?
- Spectacles may correct residual refractive error but not higher order aberrations.
- Gas perm contact lenses would address both lower and higher order aberrations, but the corneal curvature makes this a challenging fit. Patients who paid for multifocals are generally unhappy with paying for contact lenses, particular speciality lenses that would be required in this case.
- Topography-guided surface ablation would directly address irregular astigmatism and result in a more natural corneal shape. This is not widely available in the US, however.
- Lens exchange using a monofocal IOL, which is more forgiving of the alignment issues.
The lens was exchanged for a monofocal IOL. The patient reported vision appeared brighter and less blurred the day after surgery. At one month, the patient was correctable to 20/30 with +0.75-1.50 x 135 and was happy with vision which was subjectively better than preoperatively.
- Park CY, Oh SY, Chuck RS. Measurement of angle kappa and centration in refractive surgery. Curr Opin Opthalmol. 2012;23(4):269-275.
- George H.H. Beiko. The Fundamentals of Spherical Aberration. CRST Europe. Jul 2012. https://crstodayeurope.com/articles/2012-jul/the-fundamentals-of-spherical-aberration/ accessed 8/30/2017.