Scleral lenses vault the cornea to create a regular optical surface. In many cases, a spherical scleral lens can provide satisfactory vision. However, in some cases, residual astigmatism (RA) may remain that can cause blur or a “ghosting” effect. Common sources of RA include lenticular astigmatism from the natural lens and flexure from bending of the scleral lens on the eye. To aid in diagnosing a patient’s visual symptoms, a sphero-cylindrical refraction can be performed to determine whether a power change improves the vision.
Determining the Cause of the RA
If RA is present, taking topography or keratometry of the lens while on the patient’s eye can reveal the source of the residual power. If the topography or the keratometry shows a spherical lens surface, the RA is due to a posterior structure in the eye and most often is lenticular.
If the topography or keratometry shows a toric lens surface, the lens is flexing on the eye. In this scenario, the RA may be due to either lens flexure alone or to both flexure and lenticular astigmatism. In the latter case, lens flexure must be addressed first before determining any remaining lenticular astigmatism.
Lens Adjustments to Manage RA
When RA is due to flexure, the lens parameters can be adjusted to prevent the lens from changing shape while on the eye. Increasing the center thickness is one method to reduce flexure. In patients who have compromised endothelium or corneal transplants, be cognizant of the resultant reduction in oxygen transmissibility when the lens is made thicker. In these cases, choose a material with the highest-available Dk to lessen the chances of hypoxia-related complications.
Modifying the landing zone to better align with the sclera is another method to reduce flexure. A poorly aligned or decentered lens may result in flexure when pressure is applied from anatomical features such as tight lids.
When RA is due to the anatomical lens, a toric correction can be added to the front surface of the scleral lens. When the amount of RA present is low, a spherical equivalent correction can often provide satisfactory vision. However, when the RA is too great for a patient to achieve acceptable vision, adding the astigmatism correction to the front of the scleral lens is the best option.
If a front-toric scleral lens rotates on the eye throughout the day, the lens needs to be stabilized to prevent poor or fluctuating vision. Methods to stabilize front-toric scleral lenses include back-toric or quadrant-specific peripheral curves and prism. DeNaeyer et al (2016) found that less than 6% of scleras are truly spherical. For non-spherical scleras, most lenses can be stabilized using back-toric or quadrant-specific landing zones, whereas spherical scleras may require prism.
Newer technologies for fitting scleral lenses have been developed to improve both stability and landing zone alignment, including corneoscleral tomography- or topography-guided designs or impression-based scleral designs. These techniques have the potential to create a lens with a more accurate contour to match the ocular surface and to yield better vision and comfort outcomes.
Check for Aberrations
Finally, if lens adjustments cannot achieve the expected visual potential and all other ocular health and lens fitting issues can be ruled out, the reduced visual potential may be a result of higher-order aberrations. Some scleral lens manufacturers can incorporate information from aberrometers or use decentered optics in their lens designing process to reduce symptoms; but, these options are limited, and it remains difficult to fully correct these aberrations. Educating patients about the source of their visual symptoms and utilizing available technologies to minimize them as much as possible can improve quality of life and vision. CLS
For references, please visit www.clspectrum.com/references and click on document #313.