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Up Front | Oct 2002

Monovision: A Viable Presbyopic Treatment

Consider monovision as an option for correcting presbyopia in carefully selected patients.

Monovision is a method of presbyopic correction that usually corrects the dominant eye for distance vision and the nondominant eye for near. Traditionally, it has used contact lenses for correction, but more recently, ophthalmologists have incorporated monovision into refractive surgery. Prior to the conventional refractive surgical procedure, myopic patients are generally able to read by removing their eyeglasses, but they are no longer able to do so after conventional refractive surgery. Many are dissatisfied at the prospect of needing reading glasses and may prefer monovision, which would enable them to retain their ability to read without spectacles and markedly decrease their dependence on spectacles for both near and distance work.

In myopic presbyopes, the surgeon may fully correct the dominant eye for distance vision and undercorrect the nondominant eye, a technique that produces monocular blur without additional external correction. Success with monovision, defined as adequate adaptation after 3 to 4 weeks, occurs in 81% of patients.1-3 The advantages of inducing monovision after refractive surgery include constant optical correction, binocular adaptation, less aniseikonia than with spectacles, and freedom from contact lenses.3

Ideally, the monovision patient should be able to see clearly at all distances. The depth of focus under binocular viewing conditions should be continuous and equal to the sum of the monocular depth of focus.1,3 At any distance, a patient should be able to suppress the blurred image from one eye so that it does not interfere with the image from the other2,3 (interocular blur suppression). Any compromises in binocular visual function (such as in visual acuity, contrast sensitivity, or stereopsis) as a result of monovision should not hinder the patient's ability to function comfortably at home, while driving, or at work.

I have compiled the following list of items relating to visual performance in monovision and factors affecting its success rates.
1. BINOCULAR VISUAL ACUITY Monovision decreases binocular visual acuity, especially under low-illumination conditions and when the dominant, distance-corrected eye has residual astigmatic error at an oblique axis.2-4
2. CONTRAST SENSITIVITY Monovision significantly reduces contrast sensitivity, especially at higher spatial frequencies (greater than four cycles per degree).1-3
3. PERIPHERAL VISION Monovision does not significantly affect peripheral visual acuity. Researchers have reported a small effect on the size of the temporal fields and an even lesser effect on the nasal fields.2
4. TASK PERFORMANCE Near-task performance with monovision is reduced only by 2% to 6% when compared with performing tasks under binocular viewing conditions. In addition, there is reported improvement in task performance with increased adaptation.2
5. INTEROCULAR BLUR SUPPRESSION Patients with successful monovision see clearly by virtue of interocular blur suppression, which increases with the period of adaptation. Interocular blur suppression is less effective in dim illumination, which explains the poorer visual performance that is documented in monovision patients under night-driving conditions.2,4
6. BINOCULAR DEPTH OF FOCUS In patients with alternating dominance, binocular depth of focus is approximately equal to the sum of monocular depth of focus. However, it is considerably decreased in patients with a strong sighting preference.3
7. STEREOPSIS There is only a small, marginally significant reduction in stereo acuity in patients with successful monovision. Moreover, stereopsis significantly increased between the initial monovision levels and after 3 weeks of adaptation.1-4 Overall, the moderate decrease in stereopsis is an accepted tradeoff for the effective elimination of presbyopic symptoms.
8. HORIZONTAL PHORIAS AND VERGENCE Patients with monovision tend to exhibit a small-angle esophoric shift, although this is less significant in those who have successfully adapted to monovision (0 to 0.6 D) than in patients who are unable to adapt (2.1 to 2.2 D). The clinical significance of esophoric shifts of this magnitude may be negligible.
9. OCULAR DOMINANCE The dominant eye has been shown to be superior for spatial-locomotor tasks.5 Correcting the dominant sighting eye for distance improves vector tasks (such as walking, running, or driving a car).
10. PSYCHOLOGICAL AND PERSONALITY TRAITS Certain psychological and personality factors also appear to play a role in determining patients' success with monovision. Structured, detail-oriented people are not as successful in their adaptation as flexible patients.5,6 Men who scored as introverts and women with difficulty perceiving visual patterns against background noise had difficulty adapting to monovision.

PERSONAL PREFERENCE
Patients' personal preferences also play an important role in their level of success with monovision. Patients who read frequently or perform fine near work may be willing to tolerate mildly decreased binocular distance vision in order to be able to perform near tasks comfortably without glasses.1,2

MONOVISION TRIAL
A trial of monovision contact lenses may be the best method of identifying patients who might adapt well to monovision. The success rate increased from 69% to 81% when the contact lens-related failures were excluded.1,2 A 3-week acclimatization period is necessary before diagnosing monovision failure.

If the patient's contact lens monovision trial is unsuccessful, the physician should rule out two possible culprits before attributing this failure to monovision: contact lens-related problems, and residual astigmatism (especially oblique astigmatism in the dominant eye).

An unsuccessful contact-lens monovision trial does not exclude the possibility of including successful monovision by refractive surgery. Creating a multifocal corneal topography after refractive surgery enables patients to read better than their refractive error would suggest.7 Therefore, a small degree of anisometropia is required for good near visual acuity while maintaining an adequate binocular function, which increases monovision success rates as compared with contact lens monovision patients.

If a patient has a refractive error of approximately -1.0 to -2.0 D, instead of a bilateral procedure, the physician can initially perform the procedure only on the eye that is targeted for distance.2 The monovision effect can be reversed easily by targeting emmetropia in the second eye if the patient is unable to adapt to the monovision situation.

CONCLUSION
Monovision refractive surgery is an option for correcting presbyopia in carefully selected patients who demonstrate a potential for adapting to the visual compromises inherent to monovision. Success with the procedure requires appropriate preoperative clinical screening and properly exercised professional judgment. It is possible to perform an enhancement procedure to return both eyes to binocular vision if the monocular result is unsatisfactory.

Elias Jarade, MD, is a research fellow at the Corneal and Refractive Surgery Services, Massachusetts Eye and Ear Infirmary, and the Schepens Eye Research Institute at Harvard Medical School, in Boston, Massachusetts. Dr. Jarade may be reached at (617) 912-0246; ejarade@vision.eri.harvard.edu.
Sandeep Jain, MD, is an attending surgeon at the Refractive Surgery Service, Massachusetts Eye and Ear Infirmary, and the Schepens Eye Research Institute, Harvard Medical School, in Boston, Massachusetts. Dr. Jain may be reached at (617) 573-3234; sjain@meei.harvard.edu.
Dimitri T. Azar, MD, is Director, Corneal and Refractive Surgery Service, Massachusetts Eye and Ear Infirmary in Boston, Massachusetts. Dr. Azar may be reached at (617) 573-4331; dazar@meei.harvard.edu.
1. Sippel KC, Jain S, Azar DT. Monovision achieved with excimer laser refractive surgery. Int Ophthalmol Clin. 2001;41(2):91-101.
2. Jain S, Arora I, Azar DT. Success of monovision in presbyopes: Review of the literature and potential applications to refractive surgery. Surv Ophthalmol. 1996;40:491-499.
3. Johannsdottir KR, Stelmach LB. Monovision: A review of the scientific literature. Optom Vis Sci. 2001;78(9):646-651. 4. Hom MM. Monovision and LASIK. J Am Optom Assoc. 1999;70(2):117-122.
5. du Toit R, Ferreira JT, Nel ZJ. Visual and nonvisual variables implicated in monovision wear. Optom Vis Sci. 1998;75:119-125.
6. Erickson DB, Erickson P. Psychological factors and sex differences in acceptance of monovision. Percept Mot Skills. 2000;91(3 pt 2):1113-1119.
7. Jain S, Ou R, Azar DT. Monovision outcomes in presbyopic individuals after refractive surgery. Ophthalmology. 2001;108(8):1430-1433.
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