Hyperguides

Ophthalmology / Refractive surgery

Monovision

Neda Shamie, MD Peter J. McDonnell, MD
Article


Introduction

The form of optical correction chosen by an individual reflects his or her lifestyle including occupational and recreational needs. As such, a patient who is active in sports and outdoor activities that require excellent distance acuity may desire optimal distance correction, necessitating reading glasses or bifocals for near vision if the patient is presbyopic. Presbyopia is believed to reflect a loss of accommodation due to stiffening of the lens with age, a weakening of the ciliary muscle/lens zonule apparatus, or a combination of mechanisms1-3; symptoms usually begin early in the fifth decade of life.4 People in the presbyopic age range who read extensively for work and pleasure may value good uncorrected near vision above all else and may choose to be undercorrected for distance in the case of myopia or overcorrected in the case of hyperopia with their glasses, contact lenses, or surgery. Monovision is designed for presbyopic patients who desire vision at both distance and near without requiring reading glasses.5 Monovision as part of the surgical correction of refractive error has been recommended for patients who seek to minimize their future need for corrective lenses in any situation.6,7

Indications

Monovision is an option offered to presbyopic patients when one eye (the "reading eye") is left slightly nearsighted while the other eye is corrected for the full distance correction. Monovision can be created with contact lenses, refractive surgery, and intraocular lenses. Not all patients are good candidates for monovision, however, and not all patients will successfully adapt to it.5,8,9 In one literature review of published results on monovision with contact lenses,5 the mean success rate was 76% (434 successful monovision patients out of 573); exclusion of contact lens-related intolerant patients resulted in improvement in the success rate to 81%. It is possible that the literature contains a bias toward reporting success, but it is clear that a considerable percentage of patients do not tolerate monovision. For this reason, a preoperative trial of monovision is often recommended.

Blur Suppression

The mechanism by which patients adapt to monovision is interocular blur suppression, with the blurred image from one eye being suppressed so that it does not interfere with the image from the in-focus eye. According to Schor and colleagues,10 patients with successful monovision correction have, on average, two orders of magnitude greater interocular blur suppression than patients with unsuccessful monovision. Also, they report that the suppression mechanism may be less effective in dim illumination.

Results

Although many patients are successfully corrected with monovision, concerns exist regarding adverse consequences of monovision, including difficulty performing tasks that require depth perception and good eye-hand coordination (e.g., ball sports, surgical procedures, handling machinery, and driving). Monovision has been found to produce either no change in binocular acuity or a small to moderate change for both contact lens-induced5,11 and surgically created7 monovision. Contact lens-induced monovision reduces stereopsis; the magnitude of the reduction decreases with time.12 Monovision is reported to cause no measurable change in peripheral visual acuity or visual field width.13 The contrast sensitivity function increases by the square root of 2 when the stimulus is viewed binocularly rather than monocularly. Thus, in the absence of monocular defocus, the binocular contrast sensitivity is approximately 42% higher than monocular contrast sensitivity. With increasing monocular defocus, the binocular contrast sensitivity decreases steadily and then falls below the monocular contrast sensitivity, showing binocular inhibition. The majority of clinicians aim for an interocular difference of 1.5 D in postoperative refractive error. If the defocus is further increased (beyond 2.5 D defocus), the binocular contrast sensitivity reverts back to the monocular level, indicating suppression of the defocused eye.14

Studies of near task performance (such as card filing and letter editing) demonstrate a 2% to 6% reduction in efficiency with contact lens-induced monovision.15 The success and safety of monovision correction in pilots remain a matter of controversy. Some authors claim success with monovision in pilots6 and one study reports excellent performance of military pilots rendered monocular with patching.16 Conversely, the National Transportation Safety Board's investigation of an October 19, 1996, Delta Airlines Flight 554 non-fatal crash at La Guardia Airport concluded that "the probable cause of this accident was the inability of the captain, because of his use of monovision contact lenses, to overcome his misperception of the airplane's position relative to the runway during the visual portion of the approach."17

Three series report on outcomes of surgical induction of monovision. Wright and colleagues7 describe results in 21 patients; 20 of 21 (95.2%) patients were measured to have 20/25 or better binocular uncorrected visual acuity postoperatively. None of these patients used reading glasses, and 19 of the 21 patients were reportedly pleased with the monovision. Jain and colleagues18 reported on 42 patients with surgically induced monovision; 82% of their patients were satisfied, and the investigators reported success with "crossed monovision," in which the nondominant eye was corrected for distance vision. Lee and Steinert (unpublished observations) reported on 432 patients and noted that women were twice as likely as men to choose monovision. Measurement of satisfaction in this series revealed that 96% of monovision patients were satisfied, but overall satisfaction was higher in patients with bilateral distance correction.

Summary

Surgical creation of monovision can be successful in a high percentage of appropriately selected patients, reducing dependence on corrective lenses. As with all refractive surgical procedures, preoperative discussion will help patients make intelligent choices between monovision and bilateral distance correction. The typical reduction of stereoacuity should be described to patients, and a preoperative trial of monovision (ideally with contact lenses) with the reading eye typically left 1.5 D nearsighted is helpful. We strongly encourage offering monovision as an option for patients considering refractive surgery.

References

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