Ординатура / Офтальмология / Английские материалы / Jaypee Gold Standard Mini Atlas Series CORNEALTOPOGRAPHY_Agarwal, Jacob_2009

MINI ATLAS SERIES: CORNEAL TOPOGRAPHY

TOPOGRAPHIC DECENTRATION

To evaluate decentration on corneal topography, both axial curvature and elevation maps are useful. The axial map provides the refractive result of ablation, i.e. the optical zone. A large corneal curvature gradient between treated and untreated cornea, such as that resulting from a highly myopic correction, creates a smaller optical zone, increasing the refractive effect of the decentration. Curvature maps indicate surface shape using the axial radius of curvature, or the distance along the normal from the surface to the optic axis. Once a radius is determined, it is converted to a dioptric value using a paraxial keratometry formula, resulting in error for more peripheral points.

In contrast, elevation maps using an appropriate reference surface describe subtle variations in surface geometry and are valuable when true topography is required. Therefore, elevation maps are far more valuable in both diagnosing and treating corneal decentration, and in monitoring surface changes.

Wavefront Aberrations

The growing application of wavefront aberrometry demonstrates increased higher order aberrations in patients following keratorefractive surgery, specifically those with decentered ablations. Mrochen et al found that subclinical

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decentrations less than 1 mm significantly increase wavefront aberrations, deteriorating the optical quality of the retinal image. All Zernike coefficients increased postoperatively, with coma being the predominant higher order aberration. Decentrations as small as 0.2 mm increased wavefront aberrations; however, those less than 0.5 mm are considered clinically insignificant.

Wavefront aberrometers offer a variety of displays describing aberrations, including Point Spread Functions (PSF) and Snellen letter appearance simulations. Examples of a wavefront map, simulated Snellen letters, and point spread function with increased coma of a patient with a decentration are illustrated in Figures 8.1 to 8.3.

MANAGEMENT OF DECENTERED ABLATION

While the diagnosis of decentration is fairly straightforward, reducing or eliminating symptoms associated with decentration can be difficult. The most frequently used nonsurgical method involves gas permeable lenses, which restore visual quality by optically reshaping the anterior cornea surface. Comparing BSCVA to the visual acuity obtained with a gas perm (GP) lens may provide the prognosis of visual success. Unfortunately, achieving fitting success can be difficult, often requiring novel lens types such as reverse geometry or specialty aspheric designs.

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MINI ATLAS SERIES: CORNEAL TOPOGRAPHY

FIGURE 8.1: Wavefront map in a patient with increased coma

FIGURE 8.2: Simulated 20/40 Snellen letter “E” in a patient with increased coma

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FIGURE 8.3: Point spread function in a patient with increased coma

An example of such a gas perm fitting is shown in Figure 8.4. Even in cases when the vision significantly improves, failures result due to poor comfort and limited wearing time. Most patients are not receptive to returning to contact lenses after investing time and money in refractive surgery.

We typically address any tear film instability or deficiency by aggressive ocular surface lubrication. Punctal plugs and topical 0.05% cyclosporin (Allergan, Irvine, CA) both work well to smooth the irregular surface and decrease visual symptoms. Treatment of meibomian gland disease using oral doxycycline or topical Azacite (Inspire Pharmaceuticals, Durham, NC) may be used to stabilize

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MINI ATLAS SERIES: CORNEAL TOPOGRAPHY

FIGURE 8.4: Rose K Irregular Corneal (Blanchard Contact Lens INC, Manchester, New Hampshire) design on a patient with irregular astigmatism resulting from a decentered ablation. Note the asymmetry in the fluorescien pattern. The patient was 20/100 best corrected in spectacles, and 20/20 with this lens

the tear film, possibly increasing the Snellen acuity and decreasing the aberrations such that the patient appreciates visual improvement without further surgery.

Surgical Correction of Decentered Ablation

Several options for surgical correction of decentered ablations exist with variable results. PTK and purposeful decentered ablation in the opposite direction of the initial

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treatment have been proposed but such techniques are not widely used. Correction of decentration using wavefrontdriven treatment, Custom Corneal Ablation Pattern treatment, and topography-guided treatment are more likely to be used in patients suffering vision loss following keratorefractive surgery.

Wavefront-guided Treatment

Custom wavefront treatments have been used to address the visual effects of a previous refractive surgery, including decentered ablation Topographical abnormalities translate into wavefront aberrations, which can be addressed with wavefront-guided treatment. While severely irregular corneas may not be adequately measured using wavefront aberrometry, when data can be captured, it has been found to improve vision.

In case I, a patient presented after undergoing LASIK followed by an enhancement and epithelial cleaning in her hometown. Uncorrected vision was 20/70, which corrected to 20/20 with a refraction of –1.75 + 0.50 × 128. Topography and aberrometry are shown in Figures 8.5 and 8.6. Gas permeable over-refraction yielded 20/20 vision with significant improvement in subjective visual quality. WaveScan-guided (AMO, Santa Ana, CA ) PRK using ablation was performed, and the patient reported improved

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MINI ATLAS SERIES: CORNEAL TOPOGRAPHY

FIGURE 8.5: Preoperative topographical map showing superior decentration

visual quality the following day. Later, she reported although the vision was better than prior to custom treatment, shadows persisted. Postoperative aberrometry is shown in Figure 8.7. Note that while the refractive error was greatly reduced, the RMS value did not significantly improve.

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FIGURE 8.6: Preoperative WaveScan aberrometry maps showing coma, as expected with a decentered ablation

Topography-guided Treatment

Topography-guided options are limited to Custom-CAP (Carl Zeiss Humphrey, Jeno, Germany) treatments in the United States. Use of Custom-Cap has been shown to be effective in correcting decentered ablation. It enables the surgeon to create a customized excimer laser ablation pattern to reduce areas of topographic irregularity on the anterior corneal surface via computerized simulations.

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MINI ATLAS SERIES: CORNEAL TOPOGRAPHY

FIGURE 8.7: Postoperative WaveScan aberrometry maps. Note the RMS values for higher order aberrations did not significantly improve

Creation of plans directing the excimer laser beam size, shape, depth, and location to match the corresponding irregularity on the computerized corneal topography elevation map results in a more regular corneal shape. The laser beam varies in diameter from 0.6 mm to 6.5 mm, and can be centered or offset in any direction from the visual axis. The required VisionPro software (Carl Zeiss Humphrey, Jeno, Germany) allows the programming of up to 20 different sequential ablations.

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FIGURE 8.8: Preoperative topographic map in a patient with a decentered ablation

Case II utilized this technology to correct a decentered ablation in a patient with a history of previous LASIK to correct myopia. She was best corrected to 20/40 and complained of shadows and distorted vision. Her preoperative topography is shown in Figure 8.8. The VisionPro software ablation plan is shown in Figure 8.9. Following treatment, her best corrected vision improved to 20/25 and the postoperative topography showed improved regularity (Fig. 8.10).

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