Introduction
Presbyopia refers to the decrease in accommodative capacity that accompanies aging. It occurs when an individual’s accommodative power is no longer able to allow for sustained and comfortable near-vision work. Most clinical efforts have been directed toward optical arrangements that promote simultaneous near and distance acuity for pre-presbyopic and presbyopic patients.
Monovision is a method of presbyopic correction whereby one eye is optically corrected for distance vision and the other eye for near vision. The near-vision eye may be placed in focus at a reading distance or at an intermediate distance. Currently, monovision may be achieved in practice by means of contact lenses; intraocular lenses; conductive keratoplasty (CK); or refractive techniques, such as photorefractive keratectomy (PRK), laser in situ keratomileusis (LASIK) and corneal inlays.
Achieving Monovision
One procedure to achieve monovision in the presbyopic population is NearVision CK, or conductive keratoplasty, a nonincisional and nonlamellar surgical procedure approved by the US Food and Drug Administration (FDA).
The procedure uses controlled release of high-frequency, low-energy electric current to generate thermal energy in the cornea and cause corneal steepening by stromal collagen shrinkage ( Figs. 35.1 and 35.2 ).
Researchers estimate that 90 million people in the United States either have presbyopia or will develop it in the next 10 years.
The clinical technique of monovision is widely accepted and has a long history of use. The monovision approach has been used clinically with contact lenses for years, showing a near monovision success rate of 76%. Patient satisfaction after conventional monovision refractive surgery ranges from 72% to 86%.
Monovision may be associated with compromises of binocular visual function, decreased contrast sensitivity, and reduced stereopsis; some people are not able or willing to accept these compromises. It has also been widely reported that patients can experience glare and other night-vision difficulties. These monovision-related issues emphasize the need to balance good near visual acuity with maintenance of comfortable binocular vision providing some intraocular blur suppression.
A number of factors contribute to a successful monovision patient. Careful prescreening of patients is important, along with a contact lens monovision trial or a history of successful monovision contact lens wear.
Finally, patient education is critical. Patients need to understand that monovision is a compromise between distance vision and near vision. There may be a need for continued spectacle use, even though, in a successful monovision patient, that dependence on spectacles would be substantially reduced.
We attempt here to clarify the factors predictive of clinical success and visual performance in monovision patients.
Ideal Monovision Result
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 depths of focus without interference from the blurred image of the other eye. Inherent in the monovision concept is the fact that, at any given distance, the image in one eye will be blurred and the image in the other eye will be in focus. At any given 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 other eye (interocular blur suppression). Blur suppression aids distance binocular vision so that only a slight reduction in distance acuity occurs.
Monovision is considered successful in a given individual if it is satisfactory 85% of the time and if spectacles over monovision are needed only 15% of the time.
Visual Performance in Monovision
Binocular Visual Acuity
Jain and colleagues reviewed six articles addressing the effect of monovision on binocular visual acuity and found the effect to be mild. High-contrast and low-contrast visual acuities at standard room illumination were reduced by 0.04 to 0.08 logMAR unit and 0.04 to 0.09 logMAR unit, respectively. This reduction was slightly higher (0.10 logMAR unit) under low-illumination conditions. The effect on visual acuity was particularly pronounced when the dominant, distance-corrected eye had a residual astigmatic error at an oblique axis.
Interocular Blur Suppression
Monovision success depends on interocular blur suppression. In successful wearers of monovision lenses, the interocular suppression of blur was approximately two orders of magnitude greater than in unsuccessful wearers of monovision lenses. Of note, interocular blur suppression becomes less effective under dim illumination conditions, which accounts for the well-known poorer visual performance of monovision patients under night driving circumstances.
Two tests used to measure the ability to suppress interocular blur are the anisometropic blur suppression test and the American Optical (AO) vectographic test. The first measures the suppression of monocular blur of focused contours that is essential for clear binocular vision under monovision conditions. The test target is a back-illuminated circular aperture in a white card with rheostat-controlled front illumination. At zero front illumination, a clear image from the distance-corrected eye and a blurred image from the near-corrected eye can be perceived. The front illumination is increased until only the clear image is perceived. The anisometropic blur suppression is proportional to the luminance contrast threshold (log% contrast) at which the clear image, not the blurred image, is perceived. Log numbers 1 to 5 correspond to background luminance of 563, 320, 56, 6, and 0.6 cd/m 2 , respectively. Anisometropic blur suppression increased by approximately an order of magnitude for small spot sizes following short-term (1 day) adaptation to anisometropia. In marked contrast to the preadapted stage, blur suppression was greater when the blurring lens (near correction) was placed over the nondominant eye. In contrast to the anisometropic blur suppression test, the AO vectographic test measures ocular dominance characteristics with nonfused diplopic-like images. It reveals that suppression of the blurred eye in monovision was enhanced by increasing the amount of anisometropia and that a 0.50 diopter (D) to 1 D greater add was required to induce 100% suppression at near than at distance.
Stereoacuity
Reduced stereoacuity is considered to be the major disadvantage associated with monovision. The average normal value for stereopsis is 20 arc sec and, for persons older than 40 years, 58 arc sec. A report by Kirschen and coworkers found that near stereoacuity decreased from a median of 50 arc sec with bifocal contact lenses to 200 arc sec with monovision. Patients in whom monovision is successful exhibit a lower reduction in stereoacuity than do unsuccessful monovision patients. Of note, Harris and colleagues found a significant increase in stereoacuity in monovision contact lens wearers after an initial adaptation period: initial monovision levels averaged 151 arc sec and, after 3 weeks of adaptation, 90 arc sec.
Contrast Sensitivity
The contrast sensitivity function increases when the stimulus is viewed binocularly rather than monocularly (binocular summation). Thus in the absence of monocular defocus, the binocular contrast sensitivity is approximately 42% greater 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. If the defocus is further increased (beyond +2.5 D defocus), the binocular contrast sensitivity reverts to the monocular level, indicating suppression of the defocused eye. In successful monovision patients, ocular blur reduces binocular summation of middle to high spatial frequencies (> 4 cycles per degree). With increasing ocular blur, the binocular summation at lower frequencies is also reduced; in addition, the peak contrast sensitivity shifts toward higher spatial frequencies. When ocular blur is greater than +2 D, binocular summation is essentially lost. Because binocular summation is affected at higher spatial frequencies, monovision is not suited for occupations requiring fine, detailed work. The contrast sensitivity function at low photopic levels (10 cd/m 2 ) shows no significant differences between monovision and other forms of presbyopic corrections. At this low luminance, the suppression of interocular blur in monovision is poor.
Peripheral Vision and Visual Fields
Monovision causes no significant effect on binocular peripheral visual acuity. Peripheral visual field width is marginally better (1 degree to 3 degrees) in the nondominant eye (corrected for near) than in the dominant eye. The average decreases in size of the visual field through the far-point lens and the near-point lens are within the variation in measurements expected when taking fields. Static visual fields are not adversely affected by monovision correction.
Binocular Depth of Focus
The binocular depth of focus is the range in which an image may move without noticeable blur under binocular viewing conditions (without changing accommodation). In patients in whom neither eye is clearly dominant (i.e., in whom there is no sighting preference), the binocular depth of focus is approximately equal to the sum of the monocular depths of focus. However, in patients with a strong sighting preference, the image becomes blurred as the object moves from the monocular clear range of the dominant eye to the monocular clear range of the nondominant eye. Therefore in patients with a strong sighting preference, the depth of focus under monovision conditions is considerably less than the sum of the monocular depths of focus.
Phorias
Patients using monovision tend to exhibit a small-angle esophoric shift. At distance, this manifests as an esophoria. At near, the effect is offset by the fact that presbyopes generally exhibit a moderate to large exophoria at near. The magnitude of the esophoric shift is believed to correlate with the degree of binocular stress created by monovision. The esophoric shift at distance in a successful monovision contact lens (0–0.6 prism diopters) was found to be less than the shift in unsuccessful monovision wearers (2.1–2.2 prism diopters). Interestingly, the magnitude of esophoric shift is less when the dominant eye is corrected for distance, thus lending support to the generally accepted custom of correcting the dominant eye for distance.
Task Performance
Monovision appears to be associated with adverse effects on stereoacuity and contrast sensitivity. The question is whether these effects have clinical significance. The effect of monovision on the performance of various visually oriented near tasks can be assessed by comparing an individual’s performance of these tasks under monovision conditions, under monocular viewing conditions (i.e., with one eye covered), and under binocular viewing conditions (i.e., with full near correction for both eyes). Use of this method revealed that monovision reduced performance of near tasks by 2% to 6% when compared to performance of the tasks under binocular viewing conditions. However, this reduction was minimal when compared with the 30% reduction seen under monocular viewing conditions with near tasks requiring high stereopsis.
Binocular Visual Acuity
Jain and colleagues reviewed six articles addressing the effect of monovision on binocular visual acuity and found the effect to be mild. High-contrast and low-contrast visual acuities at standard room illumination were reduced by 0.04 to 0.08 logMAR unit and 0.04 to 0.09 logMAR unit, respectively. This reduction was slightly higher (0.10 logMAR unit) under low-illumination conditions. The effect on visual acuity was particularly pronounced when the dominant, distance-corrected eye had a residual astigmatic error at an oblique axis.
Interocular Blur Suppression
Monovision success depends on interocular blur suppression. In successful wearers of monovision lenses, the interocular suppression of blur was approximately two orders of magnitude greater than in unsuccessful wearers of monovision lenses. Of note, interocular blur suppression becomes less effective under dim illumination conditions, which accounts for the well-known poorer visual performance of monovision patients under night driving circumstances.
Two tests used to measure the ability to suppress interocular blur are the anisometropic blur suppression test and the American Optical (AO) vectographic test. The first measures the suppression of monocular blur of focused contours that is essential for clear binocular vision under monovision conditions. The test target is a back-illuminated circular aperture in a white card with rheostat-controlled front illumination. At zero front illumination, a clear image from the distance-corrected eye and a blurred image from the near-corrected eye can be perceived. The front illumination is increased until only the clear image is perceived. The anisometropic blur suppression is proportional to the luminance contrast threshold (log% contrast) at which the clear image, not the blurred image, is perceived. Log numbers 1 to 5 correspond to background luminance of 563, 320, 56, 6, and 0.6 cd/m 2 , respectively. Anisometropic blur suppression increased by approximately an order of magnitude for small spot sizes following short-term (1 day) adaptation to anisometropia. In marked contrast to the preadapted stage, blur suppression was greater when the blurring lens (near correction) was placed over the nondominant eye. In contrast to the anisometropic blur suppression test, the AO vectographic test measures ocular dominance characteristics with nonfused diplopic-like images. It reveals that suppression of the blurred eye in monovision was enhanced by increasing the amount of anisometropia and that a 0.50 diopter (D) to 1 D greater add was required to induce 100% suppression at near than at distance.
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