contact lens case reports
Correcting Astigmatism with GP Lenses
BY PATRICK J. CAROLINE, FAAO, & MARK P. ANDRÉ, FAAO
Figure 1. Patient KR's slit lamp appearance and corneal mapping OU.
|Figure 2. A more characteristic corneal topography of a patient with PMD.|
|Figure 3. Fluorescein patterns of the patient's 17.00D. toric GP lenses.|
Throughout the years, we've reported a number of cases involving pellucid marginal degeneration (PMD), which commonly manifests as a bilateral thinning of the inferior peripheral cornea. The thinning begins approximately 1.0mm above the inferior limbus, resulting in high degrees of against-the-rule (ATR) corneal astigmatism. The optical management of PMD involves a wide range of modalities including spectacles and GP, toric soft, hybrid, piggyback, semi-scleral and scleral contact lenses.
Patient KR is a 34-year-old male with a 10-year history of PMD. Two years ago, he presented to our clinic with a three-month history of progressive lens intolerance with his current spherical GP lenses. Slit lamp examination revealed bilateral inferior thinning of the cornea approximately 1mm from the lower limbus. He had some deep stromal scarring that was slightly greater in the left eye than the right (Figure 1). Keratometric readings were OD 41.00 @ 94/54.50 @ 004, OS 40.00 @ 94/54.00 @ 004 with 13.50D of corneal astigmatism OD and 14.50D OS. Topography revealed an uncharacteristic symmetry of the corneal astigmatism along the horizontal meridian, resembling a simple case of high ATR corneal astigmatism. More commonly, the corneal astigmatism in PMD takes on a horizontal pattern that resembles two kissing pigeons (Figure 2).
Designing a Bitoric GP Lens
Traditional toric GP manufacturing techniques crimp or bend the plastic into the appropriate astigmatic shape, then a spherical radius is cut. This technique has generated toric lenses with up to 7.00D or 8.00D of toricity. More recently, computer-controlled toric lathing systems that can manufacture significantly higher toric corrections have replaced the crimping technique.
Because of the symmetry of KR's astigmatism, we designed the following custom toric GP lenses: OD BCs 40.00/54.00, powers +0.75/17.25, diameter 9.0mm, OS BCs 40.00/54.00, powers +0.25/17.00, diameter 9.0mm (Figure 3). He has successfully worn his lenses for the past two years with a wearing time of 16 hours each day and visual acuities of 20/25.
Patrick Caroline is an associate professor of optometry at Pacific University and is an assistant professor of ophthalmology at the Oregon Health Sciences University. Mark André is director of contact lens services at the Oregon Health Sciences University and serves as an assistant professor of optometry at Pacific University.