Empirical Bitoric: Neil Pence, OD, FAAO
LS, a 62-year-old female, was referred to possibly be fit with corneal GP contact lenses. She has Fuch’s dystrophy. Four years ago she had a cataract removed OS, and at the same time a corneal transplant was performed. The patient reports being fit with a corneal GP lens 4-6 months after surgery, and “seeing better, but the lens repeatedly blinked out of the eye,” and after 4-5 lost lenses, she was prescribed an aphakic soft contact lens with spectacles worn over it. She stated that she only wore this lens “a few months,” before being informed that the induced corneal hypoxia had caused neovascularization into and very nearly throughout the entire graft. She was instructed by her eye doctor to discontinue contact lens wear, and the resultant corneal irregularity from the extensive neovascularization caused very poor best corrected spectacle acuity. Therefore, she had been uncorrected in the left eye for over three years.
Entering spectacles: Entering VA with spectacles:
|OD:||+1.25 – .75 x 105||20/40-|
|OS:||Balance lens||Hand Motion|
OD: +1.50 -1.00 x 105; 20/40-
OS: +12.00 -6.00 x 7; 20/200-
OS: 39.75 @ 165 / 46.75 @ 75 (distorted mires)
Patient Consultation and Education
LS was advised that a corneal GP lens would provide a more uniform surface to the left eye, with the tears filling in some of the corneal irregularity behind the contact lens. Because of the large amount of corneal astigmatism, a toric back surface would be required. Due to the ghost vessels throughout the cornea, the patient was advised that the expected acuity level was unknown but in fact may not be very good. A GP lens was offered as her only viable corrective option for the left eye and she desired to proceed.
Because there wasn’t an appropriate trial lens, a bitoric lens was designed empirically utilizing the Mandell-Moore Bitoric Lens Guide (available on the GPLI website).
Mandell-Moore Calculations Follow:
|Flattest K||Steepest K|
|3. Enter K||39.75||46.75|
|4. Spectacle Power||+12.00||+6.00|
|5. Vertex Adjusted line||+14.00||+6.50|
|6. Insert Fit Factor||-0.25 +0.25||-0.75 +0.75|
|Add all lines|
|7. Final CL Rx||39.50 +14.25||46.00 +7.25|
The lens ordered was thus 39.50D / 46.00D, powers +14.25D / +7.25D, 9.2mm overall diameter. The only way the lens can meet these specifications is to add cylinder power on the front surface, which then makes it a bitoric (both surfaces are toric). The difference between the base curves (6.50 diopters) and the difference between the power meridians (7.00 diopters) are not the same. Therefore, this particular bitoric lens will have the effect of a cylinder lens on the eye (i.e. it is not a “spherical-power effect” bitoric).
Follow-Up Care/Final Outcome
Upon dispensing of the left corneal GP lens, it was noted to position a little inferior, move and fit reasonably well, and the acuity was 20/25. The lens was dispensed, and when LS returned in one week, she was very pleased with her vision. An over-refraction of +1.00 sphere brought her vision to 20/20-. A second lens of the same base curves was ordered, but with powers of +15.25D / +8.25D (i.e. +1.00 was added to each meridian).
When LS reported to be dispensed the new lens, she was in fact 20/20-. What had been her “useless eye” was now her good eye, and she was very appreciative. As an interesting post-script, one month later LS won the largest prize in the Indiana lottery. She felt it was indeed a lucky time for her: first, getting improved vision; then second, winning the lottery. She promptly retired from her job and moved to Florida, but not before ordering a back-up corneal GP bitoric contact lens.
Discussion/Alternative Management Options
LS is a dramatic case of the great benefit GP lenses can be in cases of corneal irregularity. She had poor vision with any spectacle lens for her left eye, due to the extensive irregularity caused by the neovascularization. The network of ghost vessels was so extensive, there was apprehension whether fitting a lens would be worth the effort.
When she was previously fit a GP lens, it was probably a spherical base curve. With her 7D of corneal astigmatism, this lens had a difficult time staying on the eye. Utilizing a back-surface toric or bitoric design allows the lens to much better contour the cornea. The design of the lens was very simple and straightforward using the Mandell-Moore Guide.
LS’s spectacle acuities and vision were poor, and the refraction outcome was not satisfactory. Keratometry was also less precise than normal. These combined to cause the power to be off a little in the calculated lens, but this was easily adjusted using the OR of the initial lens to order the second lens.
While certainly a specialty lens, there really were no special skills or knowledge needed to be able to manage this patient. By simply being able to fill in the blanks on the Mandell-Moore Guide, the life of this patient was significantly improved.
There are three easy to use guides for designing bitoric corneal GP contact lenses available on the GPLI website: The Mandell-Moore Bitoric Guide, The Quinn Bitoric Guide, and The Newman Bitoric Guide.
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