Visual performance in the correction of astigmatism with contact lenses: Spherical RGPs versus tonic hydrogels

Clinical Article

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Astigmatism with Contact Lenses: Spherical RGPs versus Toric Hydrogels Christopher

Snyder, OD, MS, Nancy I’. Wiggins, OD, and Kent M. Daum, OD, PhD

The purpose of this study was to evaluate visual performance with both spherical rigid gas permeable (RGP) and toric hydrogel contact lenses. We hypothesized that both visual ucuity and performance would be inferior with hydrogel lenses because of dynamic crossed-cylinder effects with eye movement-induced lens rotation. Eighteen subjects (2345 years) having 0.75 2.50 D of astigmatism in each eye and matching corneal toricity were fitted with both types of lenses (providing at least 20/20 SnelIen VA). Seventeen subjects successfully wore each lens type on a full-time basis for 3 weeks prior to testing. Our randomized, crossover study included logMAR threshold visual acuity, contraSt sensitivity, and a VDT text-editing task interrupted by a short reading task in upper lateral gaze. Visual performance was assessed by counting errors identified, the error detection rate, and the total number of lines edited. Subjects rated the comfort, quality, and stability of vision just before and after the VDT text-editing task. Subjective ratings were also obtained at the end of the overall wearing period of each lens type in regard to lenswear comfort, visual clarity, and visual stability. Patients chose which pair of lenses to keep after the study. Our analysis of these data indicated no significant differences between threshold visual acuity or pre-post VDT task-related ratings of visual comfort, quality, or stability between the two types of lenses. P__trnrc C,,C;t;“,;t?.,,,,..I*, I,,“+-” “I,, nnt /l;(Coront hotnrloon lone L,“,IL,cuLJrrrJILLVrLJ lLJYl6L.I WClCL&W” ,l”C U6,,C’ C’LC “CL.WCCI~ KllJ types. The overall ratings at the end of the wearing weeks of each

type of lens indicated that toric hydrogels were better than were RGP lenses for wearing comfort, dryness, and handling, whereas the RGP lenses were rated better for clarity of vision. Stability of vision was rated the same between the two lens types. Seventy-four percent of the subjects chose to continue wearing the toric hydrogels after the study, whereus 24% chose the RGP lenses. These results suggest that toric hydrogels can provide equivalent Although

visual performance

to spherical RGP contact

lenses by the subjects,

there was a strong final preference for

hydrogel lenses, largely due comfort.

to

better overall lens wearing

Keywords: Astigmatism; contact lenses; vtsual ject preference; video display terminal (VDT)

Accepted

for publication

@I 1994 Butterworth-Heinemann

June

1994.

performance;

suh-

Introduction The correction tant clinical

of astigmatic

consideration

general

population

practice

have an astigmatism

refractive

error is an impor-

since one of four subjects

of subjects

presenting

of more than

in the

to an eye care 1 .OO diopter.

’ In

many cases, spherical back surface rigid lenses and toric hydrogel contact lenses are well developed options for eye ,-QVPnrartit;nn~r~ nntiral of CUIL pLULL’L’.“‘L’” tn L\I cnn~irl~ LVL.YL..LLfnr L”I the. L&ILC’ YL’LY’rnmwtinn ~V..~C._IVLLV. astigmatic

refractive

error.

Evaluations of the visual performance, and subject preference between spherical Address reprint requests to Dr. Christopher Snyder at the Department of Optometry, University of Alabama at Birmingham, 1716 University Blvd., Birmingham, AL 35294.

Lenses.

overall clarity of vision was rated as better with RGP

ocular response, RGP and spher-

ical hydrogel contact lenses have been conducted in comparison with the wear of both lens types at the same time, one on each eye, for overnight wear”“3 and in a crossover design.4 To our knowledge, no crossover comparison study

ICE,

Vol.

21, July/August

1994

127

ClinicalArticles has evaluated visual performance and subject preferences between spherical RGP and toric hydrogel contact lenses for subjects having moderate amounts of cornea1 toricityinduced astigmatism. Toric hydrogel contact lenses must remain in a consistent position on the eye so that the lens cylinder will remain aligned with the axis for the subject’s astigmatic error. This avoids the resultant “residual” spherocylindrical power effect from crossed cylinders.’ The dynamic movement of the lens on the eye will lead to a varying crossed-cylinder effect that can manifest itself as an irregular fluctuation between good and poor vision.5 Spherical RGP lenses have an advantage over toric hydrogel lenses for correcting astigmatism in that they do not need to maintain rotational orientation to provide stable, clear vision. We hypothesized that visual performance with toric hydrogel lenses would be inferior to that with RGP lenses because of hydrogel-related dynamic cross-cylinder effects occurring with eye movements. The purpose of this study was to compare the visual performance of astigmatic subjects corrected with spherical rigid gas permeable and toric hydrogel lenses.

Methods Eighteen subjects over the age of 18 years who were free .- -_ of systemic and ocuiar disease, had an astigmatism of 0.752.50 D in each eye with a matching cornea1 toricity within 0.50 D (as measured by keratometry), and were correctable to at least 20/20 Snellen visual acuity in each eye with toric hydrogel and spherical rigid lenses were entered into the study. Each subject was required to wear each type of lens for at least 3 weeks prior to testing in order to be included in the final analysis. One subject was ultimately unwilling to wear her rigid lenses for the required wearing time and was excluded from the final analysis. Patient data are presented in Tubk 1. The initial lens type worn by each subject was randomly assigned and each subject served as their own control in a crossover design-wearing protocol. The subjects were fitted into the toric hydrogel lenses that provided optimal fitting performance and vision (Table 2). Each subject was fitted into rigid lenses made of Fluoroperm 60 material (Paragon Optical), which provided optimal fitting performance and vision in a standard tricurve design.

Table 1. Patient Data 17

Cylinder (D) Comeal toricity (D)

128

/C/X,

period

subjects wore their lenses regularly for at least 3 to ensure that adequate adaptation to each lens type occur. Vision, visual performance, clinical evaluaand subject evaluations occurred after the 3-week and the second

pair of lenses were dispensed

at this

with the second pair of lenses occurred after the next 3-week period.

crossover

visit. The final evaluation

Visual Acuity Testing Visual acuity was measured monocularly with a B-VAT visual acuity tester (Mentor, Inc., Cat. #22-4710) with the subjects attempting to read progressively smaller letters, encouraging guessing, to establish a visual acuity at their threshold for seeing. Final visual acuity was marked as the log of the minimum angle of resolution (logMAR) of the final completed acuity line minus 0.02 log unit/letter for correct letters below the last all-correct line plus 0.02 log unit/letter for misses above the last all-correct line.“’ Contrast Sensitivity Testing Contrast sensitivity was measured with an MCT 8000 Multivision Contrast Tester (Vistech Consultants, Inc.) set for far test distance. The testing was performed monocularly at five spatial frequencies of 1.5, 3, 6, 12, and 18 cycles per degree. Contrast sensitivity was measured at h~EPl;nP ..,;th “LLUC&LI&L **,LLL h,,t “LOL c~nr~~rln OpLLcaL’L

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3 -week visits for each lens type. Visual Performance Testing Each trial consisted of a 24-minute period performing an editing task on a VDT (AT&T 6300) while wearing the lens type that had been worn over the previous 3 weeks.’ (Patients over age 40 years wore an appropriate reading prescription in spectacles, as necessary.) The text was green lettering on a black background; the font was Times, 12 point. Other details regarding the VDT task station are described elsewhere.“,” The editing task was interrupted at 4-minute intervals by a short reading task in upper lateral gaze. This was included to assure an ample and controlled opportunity to stimulate eye movements to affect the on-eye stability of the lens. The goal of the editing task was to identify spelling and grammatical errors in a short story. Visual performance was assessed by counting the number of errors identified, the error detection rate (correct identifications/total number possible), and the total number of lines edited. Subjective Ratings of Vision with the VDT Task

Number Gender distribution Age (years) Refraction Sphere (D)

All weeks could tions,

4 male; 13 female Mean = 34; range: 23-45 Mean: -2.92; SD: 1.87; range: + 1.75 to -6.25 Mean: 1.36; SD: 0.45; range: 0.75-2.50 Mean: 1.51; SD: 0.45; range: 0.75-2.38

Vol. 21, July/August 1994

All subjects evaluated their quality of vision, stability of vision, and comfort of vision before and after the text editing task. The lo-step rating that they marked raneed o--~ from ~~-~~~ 0 (poor) to 10 (excellent). Overall Subjective Ratings All subjects evaluated their lens-wearing comfort, clarity of vision, stability of vision, dryness sensation, and lens-

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Clinical Articles

The present study was designed to fully correct the subTable 5. Comparison of RGP and Toric Hydrogels by Subjective Ratings (N = 34 Eyes)

ject’s astigmatic

Degrees of Freedom

X2 Probability

Significance (Preferred)

Wearing comfort

7

p < 0.001

Clarity of vision

6

p < 0.05

Stability of vision Dryness

8 7

p < 0.4 p < 0.05

Handling

7

p < 0.05

Significant (Hydrogels) Significant (RGPs) ns. Significant (Hydrogels) Significant (Hydrogels)

Table 6. Lenswear History vs. Final Lens-Type Choice Prestudy Lens-wearing History

Lens Type Selected after Study

Toric SCL Sphere SCL RGP No lenswear

5 3

Soft

Rigid

s (4)’

1 0 0 3

Sample size = 17 completed all data collection (18 started). Number in ( ) included a final choice by the 18th subject. ??

chosen

by approximately

previous

lens wearing

half of the subjects history

who had no

(Table 6).

Discussion This study was designed tion within inder matism

powers,

the range i.e.,

correction

to evaluate

astigmatism

correc-

hydrogel

lens cyl-

DC. Low amounts

of astig-

of standard

0.75-2.50 may rotate

toric

many degrees

off-axis

before

In fact, some authors have suggested that the correcting cylinder axis only needs to be “close” in order to achieve clinically acceptable results in the fitting of toric hydrogel contact lenses. l1 This may be the rationale for why significant differences were not found in this study in objective measures of visual performance or in subjective assessment of stability of vision between the rigid and hydrogel lenses, since the amount of cylinder correction needed in the study was relatively low (mean = 1.36 DC). Previous studies with spectacles” and contact lenses’ have suggested that optimal visual comfort and performance in the use of a video display terminal (VDT) is enhanced by the full refractive correction since a small amount of uncorrected astigmatism was shown to be a factor related to subject symptoms with each mode of correction. 9*1o Those studies were designed to compare subject symptoms between full refractive correction and a low amount of induced or uncorrected astigmatism. causing

130

a crossed-cylinder

effect

ICLC, Vol. 21, July/August

of significance.5

1994

refractive

error

using

two methods:

toric

RGP lenses. The data gleaned from the text-editing task results with each type of lens showed no significant differences in visual performance or efficiency between the lens types. This may be credited to the precision of the optical correction of the subjects with each lens type, to the consistency in lens positioning on the eye (especially for the hydrogel toric lenses), or to the small amounts of astigmatism being corrected by the toric hydrogels, leading to a minimal variability of vision with hydrogel lens dynamic malalignment5 during the textediting task. The significant differences between lens types in subjective assessment indicated that the RGP lenses were equivalent to the hydrogels in stability of vision and superior in clarity of vision. Because objective assessment of visual acuity (and the related contrast sensitivity) were equal, we are unable to determine the etiology of the subjects’ assessment. Not surprisingly, the hydrogel lenses were rated as being better than the RGP lenses for wearing comfort. Many subjects who ultimately chose to continue wearing the hydrogel lenses at the conclusion of the study indicated that the wearing comfort was a primary factor in their decision to choose the hydrogels over the RGP lenses. It was interesting that hydrogels were rated better in the area of dryness sensation during wear in that such lenses are often associated with reduced wearing times because of dry-eye symptoms. l2 It is not possible for us to know how the effects of previous lenswear experiences may have influenced the results of ultimate lens choice; however, it was interesting that when examining the subgroup of seven subjects with no previous lenswear four chose toric soft lenses and three chose rigid lenses (Table 6). The wearing order of lens type in the study may have had an effect since three of four who first wore the toric soft lenses chose toric soft lenses and two of three first wearing the rigid lenses chose rigid lenses. The final split on ultimate lens choice showed that, given the opportunity to experience the wearing of both lens types, 24% of the subjects completing the study preferred the RGP lenses. This, along with the fact that about half of the subjects with no lens-wearing experience prior to their participation in this study chose RGP lenses, indicates that RGP lenses are an important fitting option that contact lens clinicians should consider. hydrogel

contact

lenses and spherical

Conclusion The results of this study suggest that well-fitted hydrogel toric lenses can provide visual acuity and visual performance equivalent to that with spherical RGP lenses. Further studies with subjects having higher amounts of astigmatism, in the range requiring custom hydrogel lens powers and bitoric base curve RGP lenses, may provide results that demonstrate significant differences in visual performance.

Correction of astigmatism: Snyder et al.

Acknowledgments We gratefully

acknowledge

Paragon

grant that made this study possible Vote

for her assistance

Optical,

and we thank

in the execution

Inc.,

for a

Ms. Maria

of the study.

References 1. Holden BA. The principles and practice of correcting astigmatism with soft contact lenses. Aust J Optom 1975;58:279299. 2. Fonn D, Holden BA. Rigid gas permeable vs. hydrogel contact lenses for extended wear. Am J Optom Physiol Opt 1988;65(7):53&544. 3. Weiss L. Clinical study of extended wear lenses: Hydrogel vs. gas permeable. Contact Lens Forum 1987;12(9):41-46. 4. Johnson TJ, Schnider CM. Clinical performance and patient preferences for hydrogel vs. RGP lenses: A crossover study. ICLC 1991;18(7,8):13C-134.

5. Snyder C. A review and discussion of crossed cylinder effects and overrefractions with toric soft contact lenses. ICLC 1989;16(4):113-17. 6. Bailey IL, Lovie JE. New design principles for visual acuity letter charts. Am J Optom Physiol Opt 1976;53:740-745. 7. Kitchin JE, Bailey I. Task complexity and visual acuity in senile macular degeneration. AustJ Opton 1981;64:235-242. 8. Ferris FL, Kassoff A, Bresnick GH, Bailey IL. New visual acuity charts for clinical research. Am J Ophthalmol 1982;94: 91-96. 9. Wiggins NP, Daum KM, Snyder C. Effects of residual astigmatism in contact lens wear on visual discomfort in VDT use. J Am Optom Assoc 1992;63(3):177-81. 10. Wiggins NP, Daum KM. Visual discomfort and the astigmatic errors in VDT use. J Am Optom Assoc 1991;62:68C684. 11. Myers RI. Off-axis fitting of soft toric contact lenses. Int Eyecare 1985;1(7):48&488. 12. Robboy M, Orsbom G. The responses of marginal dry eye lens wearers to a dry eye survey. Contact Lens J 1989;17:8-9.

Dr. Christopher Snyder is a Professor of Optometry and serves as the Chief of Contact Lens Services at the School of Optometry of the University of Alabama at Birmingham. He conducts research, teaches contact lens clinical practice, and practices in the University Optometric Group. Dr. Snyder is well published, is co-author of the textbook Contact Lenses-Procedures and Techniques, is a Diplomate in the Cornea and Contact Lens Section of the American Academy of Optometry and has lectured extensively. He serves on the Editorial Review Board of the Journal of the American Optometric Association, is the Contact Lens Editor for Oprometry-Current Literature in Perspective, is the immediate-past Chairman of the Association of Optometric Contact Lens Educators, and is an active member of many professional organizations. Nancy P. Wiggins, OD, practices in a private group optometric practice in Aliquippa, PA. She received her degree from the University of Alabama at Birmingham School of Optometry. With a specialty in contact lenses, she has conducted research and published articles on the subject. She is a member of the American Optometric Association, the American Academy of Optometry, and the Association for Research in Vision and Ophthalmology.

Kent Daum received his OD (1976), MS and PhD (1979) degrees from The Ohio State University. He is currently teaching at the School of Optometry at the University of Alabama at Birmingham. Dr. Daum is a member of the American Optometric Association and a fellow of the American Academy of Optometry.

/UC,

Vol. 21, July/August

1994

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