Prescribing for Astigmatism
When Astigmatism Doesn’t Follow the “Rules”
BY TIFFANY GATES, OD; HEIDI G. MILLER, OD; & TIMOTHY B. EDRINGTON, OD, MS, FAAO
Irregular astigmatism is associated with ectatic disorders, ocular surgery, trauma, pterygia, dry eye, corneal scars, or any other condition that alters the structure of the cornea (Wang, 2008). Designing a lens that will contour irregular corneal peaks and valleys may pose a challenge. This article will discuss two possible GP lens “add-ons” that can help create an ideal contact lens fit.
Keratoconic corneas are asymmetrical, with a steeper inferior quadrant compared to the superior quadrant, often causing inferior contact lens edge lift (Wang, 2010). One solution for excessive inferior edge lift is to steepen the lens quadrant at 6 o’clock. Some lens designs alter only the inferior edge of the lens, leaving the base curve unchanged. Others allow the base curve and edge of any of the four quadrants to be independently steepened or flattened.
With any design, it is important to identify which curves of the lens are being altered: base, peripheral, or edge?
Case Example A 17-year-old male presented with an irregular cornea OD secondary to penetrating keratoplasty. Note the steep curvature in the inferior-temporal quadrant on the corneal topography axial map (Figure 1). This area does not represent ectasia, but rather a sector in which the corneal curvature steepens posteriorly. Spectacle-corrected visual acuity OD was 20/80, which improved to 20/20 with a spherical large-diameter corneal GP lens. In assessing the fit, the inferior-temporal quadrant of the GP lens showed excessive edge lift (Figure 2), causing discomfort and frequent lens ejection.
Figure 1. OD corneal topography.
Figure 2. Initial spherical GP lens with inferior-temporal edge lift.
Solution We prescribed 1D of prism at 6 o’clock, specifying the steep quadrant (1D steeper than the base curve) to be 20º left of the prism (or at about 7 o’clock). Based on these modifications, the overall lens fit and comfort improved, allowing the patient to achieve full-day wear of his lens.
Prism is often utilized to help center a lid-attached or superiorly positioned GP lens, stabilize a toric or quadrant-specific design (as demonstrated in the above case), and rarely for the correction of small vertical phorias. Values usually range from 0.50 to 4 prism diopters depending on the manufacturer and lens design.
When astigmatism presents in an irregular manner, investigate which GP lens “add-ons” may help improve the lens comfort and fit. CLS
For references, please visit www.clspectrum.com/references and click on document #229.
Dr. Gates and Dr. Miller are the cornea and contact lens residents at the Southern California College of Optometry (SCCO) at Marshall B. Ketchum University (MBKU). Dr. Edrington is the cornea and contact lens residency coordinator at SCCO. He is also a Fellow of the American Academy of Optometry and a Diplomate in their Cornea, Contact Lenses, and Refractive Technologies Section.