the contact lens exam

Irregular Astigmatism is Not Limited to Keratoconus

BY GREGORY J. NIXON, OD, FAAO

With the advent of advanced technology, practitioners have the benefit of being able to correct almost any refractive error with contact lenses. I recently experienced a scenario highlighting benefits and challenges of correcting complex refractive errors.

Unfortunate Circumstances

A 46-year-old female who had just undergone cataract surgery in both eyes secondary to early onset posterior subcapsular cataracts was referred to me for contact lens fitting. After nearly two years of poor vision, she was excited at the thought of seeing clearly with her first pair of glasses after surgery. Unfortunately, her spectacle prescription of OD –0.75 –1.75 ×145 and OS +0.75 –1.00 ×029 caused problems. While the degree of anisometro-pia may not look excessive at first, an optical cross evaluation of the prescription shows a 3.25D difference along one of the oblique meridians between the eyes. This resulted in blur, ghosting, eyestrain and visual distortions consistent with aniseikonia.

Figure 1. Topography image of right eye.

Opportunity for Contact Lenses

Just like all forms of anisometro-pia, this patient was ideal for contact lens correction. She readily adapted to the fit and comfort of soft toric lenses. Despite 20/20– acuity, she complained of a mild but chronic ghosting/monocular diplopia in the right eye.

At first, I was convinced that her soft toric fit must not be optimally stable to provide consistent astigmatism correction. However, we tried multiple prism-ballasted and thin-zone-stabilization lenses with cross cylinder corrections to account for the over-refraction, yet never corrected the problem. The persistence of symptoms caused concern about post-operative cataract complications such as posterior capsular opacification (PCO) or cystoid macular edema (CME), but we ruled these out.

Back to Basics

Knowing that her symptoms were not caused by PCO or CME, I further investigated her cornea. A topography image of her right eye (Figure 1) shows an irregular bowtie pattern in the axial view consistent with irregular astigmatism. The map also shows a corneal irregularity measurement of 0.88 microns, falling outside the normal range of 0.03 to 0.68 microns. A diagnostic GP evaluation provided crisp 20/20 acuity with no diplopia or ghosting. As a result, she proceeded with spherical GP correction full time in both eyes with reading glasses for near work and now enjoys the clearest vision of her life.

This case highlights that irregular astigmatism isn't limited to keratoconus or pellucid degeneration. Also, mild degrees of irregular astigmatism can cause significant visual image degradation. Lastly, this lens fit was initiated using keratometric readings from her presurgical cataract work-up. This highlights the value in obtaining baseline topography during the pre-fit assessment to best determine the appropriate lens for each patient. CLS

Dr. Nixon is an associate professor of clinical optometry and the extern coordinator at The Ohio University College of Optometry. He is also in a group private practice in Westerville, Ohio.