Ординатура / Офтальмология / Английские материалы / Hyperopia and Presbyopia_Tsubota, Boxer Wachler, Azar_2003

B. THE 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 (4). 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 in focus. Ideally, 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 (known as interocular blur suppression) (5).

Any compromises in binocular visual function (such as in visual acuity, contrast sensitivity, or stereopsis) as a result of monovision should not interfere with the patient’s ability to function comfortably at home, while driving, or at work. Monovision patients may require spectacle correction in order to obtain optimal visual functioning for certain tasks such as night driving or fine near vision tasks. Monovision is considered successful in a given individual if it is satisfactory 85% of the time and spectacles over monovision are needed only 15% of the time (3).

C. MONOVISION SUCCESS RATES AFTER CONTACT LENSES

Many published reports address monovision success rates in contact lens wearers. The reported monovision success rates vary considerably and in large part because differing definitions of monovision success are applied and study designs vary. Jain and associates reviewed 19 articles that met their definition of monovision success, which was the adequate adaptation to 1.00 to 2.00 D of monocular blur after 3 or more weeks of acclimatization (2). In an attempt to predict success rates after refractive surgery, these authors included only reports that studied contact lens patients above 40 years of age with astigmatism of less than 1.00 D, no previous monovision experience, and no previous contact lens intolerance. The mean monovision success rate was found to be 76%. When failures related to contact lens intolerance were excluded, the success rate increased to 81%. The latter figure is important because of its applicability to monovision success rates after refractive surgery (3).

D. MONOVISION REFRACTIVE SURGICAL OUTCOMES

We have evaluated a group of 97 patients over 45 years of age who satisfied the criteria for strict monovision (n 60) and minimonovision (n 37) after LASIK surgery (Table 1). Best-corrected visual acuity (BCVA) before LASIK ranged from 20/15 to 20/30. The average SE before LASIK was 3.71 2.73 D OD and 3.77 2.93 D OS. The lasers used were both the VISX and the Summit (7).

After LASIK, the average SE for eyes corrected for distance was 0.12 0.29 D and 1.32 0.54 D for near vision. The mean anisometropia was 1.20 0.06 D. BCVA after LASIK ranged from 20/15 to 20/30. The mean Snellen uncorrected visual acuity (UCVA) after LASIK in the distance eye was 20/23 (range: 20/15 to 20/60); and in the near eye, it was 20/53 (range: 20/20 to 20/400). Satisfied and dissatisfied patients had similar distance and near UCVA. The average follow-up time was 6.1 4.7 months

(7).

Table 1 Monovision Subdivision Criteria

A.Based on near spherical equivalent (SE) Strict monovision

Distance SE: 0.5 to 0.5 D Near SE: 1.0 D to 3.75 D Anisometropia 0.75 D

Minimonovision

Distance SE: 0.5 D to 0.5 D Near SE: 0.5 D to 1.0 D

B.Based on ocular dominance

Uncrossed monovision

Dominant eye corrected for distance

Nondominant eye corrected for near

Crossed monovision

Nondominant eye corrected for distance

Dominant eye corrected for near

C. Based on near Spherical Equivalent and ocular dominance

Conventional monovision

Strict monovision

Uncrossed monovision

Crossed minimonovision

Minimonovision

Crossed monovision

E.SATISFACTION WITH MONOVISION AFTER REFRACTIVE SURGERY

Of our 97 LASIK patients, 78 (80.4%) reported satisfaction with visual outcome after LASIK; 19 (19.6%) were not satisfied; 37 (47.4%) of the satisfied patients were happy with visual outcome, and 41 (52.6%) expressed no complaints. Of those who were not satisfied with their outcome, 4 (21.1%) were unhappy with the quality of distance vision, 3 (15.8%) were unhappy with near vision, 2 (10.5%) were unhappy with both distance and near vision, and 6 (31.6%) complained of imbalance. Four patients (21.1%) were unhappy for reasons unrelated to monovision; three complained of dry-eye symptoms and one of floaters. Satisfaction was unrelated to age, gender, dominance, myopia, type of laser, and type of microkeratome ( 2 NS). Of the 78 satisfied patients, 29 (37.2%) fulfilled criteria for minimonovision; of these, 10 (34.5%) had crossed monovision. Of the 78 satisfied patients, 49 (62.8%) fulfilled criteria for strict monovision; 12 of the 49 (24.5%) had crossed monovision. There was no statistically significant difference in satisfaction between strict and minimonovision groups or between uncrossed and crossed monovision groups. The average anisometropia of the unsatisfied patients was 1.05 D and of the satisfied patients 1.23 D; the difference was not significant. There was also no statistically significant relationship between lines of BCVA lost after LASIK and satisfaction with visual outcome.

Patients who were treated with strict monovision versus minimonovision were similar with regard to distribution of age, gender, dominance, myopia, type of laser used, and type of microkeratome used. Minimonovision patients had an average anisometropia of

0.68 D; strict monovision patients, 1.52 D (P .001). Twenty-nine (78.4%) minimonovision patients and 49 (81.7%) strict monovision patients were satisfied with visual outcome after LASIK. The difference in satisfaction between the two groups was not statistically significant. Only three patients in our entire study were monovision failures requiring retreatment; all had strict monovision before retreatment. One was dissatisfied with distance vision, and two complained of imbalance.

Other studies have examined monovision success rates after refractive surgery. In 42 presbyopic myopic patients with monovision induced by refractive surgery, Jain and associates found a monovision success rate of 88% (1). One case report examined an aircraft pilot who underwent PRK with intentional undercorrection of the dominant eye. Postoperatively, the patient noted no deleterious visual effects and was still able to pilot an aircraft (8).

Wright and colleagues studied binocular function and patient satisfaction in 21 presbyopic myopic patients between the ages of 37 and 53 in whom monovision induced by refractive surgery (9). Sixteen emmetropic patients served as a control group. In the monovision group, 20 patients (95.2%) had binocular visual acuity of 20/25 or better. No patient in the monovision group used reading glasses postoperatively, whereas 4 of 16 patients (25%) in the control group used such glasses. Stereoacuity was slightly lower in the monovision group but not statistically significantly so. Patient satisfaction was very high in the monovision group.

Anecdotal evidence indicates that refractive surgery patients are often able to read better than their refractive error would suggest; for example, a 50-year-old myope corrected to plano in one eye and 0.75 D in the other may still able read fine print. This phenomenon has been attributed to the creation of a multifocal corneal topography after refractive surgery (10,11). Therefore, a smaller degree of anisometropia may be required to obtain adequate visual function for near and distance for refractive surgery monovision patients. This would serve to preserve binocular visual function and increase monovision success rates compared to monovision contact lens users.

F. PREOPERATIVE COUNSELING

All patients who opt for monovision should be informed of the adverse effect monovision may have on some visual function parameters (2,3). Specifically, they need to be informed of the risks of reduced binocular visual acuity, stereoacuity, and contrast sensitivity. In addition, they need to be made aware of the risk of distance and near ghosting as a result of incomplete blur suppression. Blur suppression appears to be particularly problematic under night driving conditions because, as mentioned earlier, interocular blur suppression becomes less effective under dim illumination conditions (2,3). Therefore, patients must be advised of the need for distance glasses when driving. Liability is an important consideration when selecting a refractive patient for monovision (3,12). Therefore, discussions of the risks and benefits associated with monovision need to be carefully documented in a patient’s chart.

It is important to ascertain the personal preference of the patient. Some patients (particularly those who are active in sports) wish to have the most optimal distance vision possible and are willing to tolerate difficulties with near vision and associated need for reading glasses in order to achieve this. These patients should be fully corrected for distance vision in both eyes. Other patients (particularly those who do a lot of reading or other 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. These patients may wish to be undercorrected in both eyes (3).

G. MONOVISION TRIAL

Although the best way to demonstrate the effects of monovision preoperatively is with a monovision trial with contact lenses (13,14), this is often impractical. A monovision trial can also be performed with spectacles. However, spectacles may induce magnification and minimization effects and, therefore, a monovision trial is more accurately conducted with contact lenses. If a patient has a refractive error of approximately 1.00 to 2.00 D, a monovision surgical trial can be performed (12). Instead of a bilateral procedure, surgery is initially performed on only one eye, which is targeted for distance. If the patient is unable to adapt to the monovision situation, the other eye is treated and targeted for plano.

It is important to allow for at least a 3-week acclimatization period before concluding whether or not monovision is appropriate for a given individual. If a patient experiences difficulties with a monovision contact lens trial, two problems must be ruled out before one declares that monovision has failed in that patient. First, accurate contact lens fitting must be ascertained. Second, the clinician must ensure that all residual astigmatism has been corrected by performing a spherocylindrical overrefraction. As mentioned previously, even small amounts of uncorrected astigmatism can have a substantial negative effect on binocular distance visual acuity (3).

A major benefit of a contact lens trial is that adjustments to the monovision arrangement can be made before refractive surgery is performed. For example, switching the distance and near eyes can serve to relieve undesirable visual symptoms (16). One common complaint with monovision is blur at an intermediate distance. Slightly reducing the add in the near eye can relieve this symptom, although this change may compromise near vision. Plus power may also be added to the distance eye, although this change may reduce distance vision. Even small (0.25-D) changes can make a large difference in creating an acceptable monovision situation for a patient (3,17).

An unsuccessful contact lens trial does not necessarily mean that surgically induced monovision will be unsucessful (17). A patient may respond to refractive surgery with a gradual transition into monovision as a result of regression of the refractive result, in contrast to a contact lens trial. For example, a patient may initially be 0.75 D in the distance eye and 0.25 D in the near eye but, after several weeks, may have regressed to plano in the distance eye and 1.00 D in the near eye. This gradual transition may allow for an easier adaptation to monovision.

H. DETERMINING THE EYE FOR DISTANCE

Different approaches have been used for selecting the distance-vision eye. Among these are (1) correcting the left eye for distance for increased driving safety (18); (2) using handedness to determine which eye is corrected for distance, (i.e., matching the selected eye to the patient’s handedness) (19); (3) designating as the near-vision eye the eye with the closer near point of convergence; and (4) using the “swinging-plus test” to select the near eye. In the latter test, the patient walks around the examination room with a 1.50 lens first over one eye, then over the other eye, and the eye that is most comfortable with the plus lens then is designated as the near-vision eye (3,4).

The most commonly used approach, however, is determining which eye is the dominant eye and correcting that eye for the most commonly used viewing distance (11), which is generally considered to be the far distance. The dominant eye has been shown to be superior for spatial-locomotor tasks such as walking, running, or driving a car (2,20). Blur suppression appears to be greater when the dominant eye is corrected for the most commonly used distance (i.e., far) (3). Correcting the dominant eye for distance also produces less esophoric shifts (21).

The dominant eye is generally identified by use of sighting dominance tests (22). One of the more common tests is the hole test (8), for which the patient is asked to frame an object that lies at an intermediate distance from him or her with a triangle created by his or her outstretched arms while keeping both eyes open. The eye that is in alignment with the object and the hole is considered the dominant eye.

I. CROSSED MONOVISION

Crossed monovision occurs when the nondominant eye is corrected for distance and the dominant eye for near. This can happen either accidentally or intentionally (1). Crossed monovision may be the intended goal when, for example, a contact lens monovision trial demonstrates better visual function if the nondominant eye is corrected for distance. A patient may also change his or her mind regarding monovision versus full distance correction for both eyes after the nondominant eye has already been treated for distance and the dominant eye has not yet been treated. Patients who wish to have only one eye treated and who are markedly more myopic in the nondominant eye may elect to have the nondominant eye corrected for distance (1,3).

Unintentional crossed monovision can occur when correction in the dominant eye is less than expected in patients requesting full distance correction for both eyes. Conversely, in patients desire equivalent undercorrection in both eyes, an overcorrection in the nondominant eye can produce crossed monovision. Unintentional crossed monovision is a result of the fact that refractive surgery is not a completely predictable procedure (1).

J. UNCROSSED MONOVISION VERSUS CROSSED MONOVISION

Of our 97 LASIK patients, 69 (71.1%) had uncrossed monovision, and 28 (28.9%) had crossed monovision. The average age was 51.7 0.5 years for patients with uncrossed monovision and 49.7 0.7 years for patients with crossed monovision (p 0.04). The two groups were similar with regard to distribution of gender, dominance, myopia, type of laser used, and type of microkeratome used ( 2 NS). The average anisometropia of uncrossed monovision patients was 1.28 D, and of crossed monovision patients 0.98 D (p 0.03).

Of the 69 uncrossed monovision patients 56 (81.2%) were satisfied after LASIK, as were 22 (78.6%) of the 28 crossed monovision patients. Two (33.3%) of the 6 patients dissatisfied with crossed monovision and 2 (15.4%) of the 13 patients dissatisfied with uncrossed monovision were unhappy for reasons unrelated to monovision, such as dryeye symptoms and floaters. One patient each (16.7%) complained of poor distance vision, poor near vision, imbalance, and poor overall quality of vision in the crossed monovision group. Of the 13 dissatisfied uncrossed monovision patients, 3 (23.1%) complained of poor distance vision, 2 (15.4%) were unhappy with near vision, 5 (38.5%) felt imbalanced, and 1 (7.7%) was unhappy with overall quality of vision.

Among minimonovision patients, 19 of the 24 (79.2%) patients with uncrossed monovision were satisfied, as were 10 of the 13 (76.9%) with crossed monovision. Of the monovision patients, 37 of the 45 (82.2%) patients with uncrossed monovision were satisfied, as were 12 of the 15 (80.0%) crossed monovision patients. Of the 3 patients who were monovision failures, 2 had crossed monovision. One patient with crossed monovision was retreated to uncrossed monovision, while the other two patients did not have a change in crossed monovision status. The patient with uncrossed monovision before and after retreatment remained dissatisfied.

K. VISUAL PERFORMANCE IN MONOVISION

1. Monovision Failures

All 13 patients dissatisfied with monovision outcome were offered retreatment, but only 3 (23.1%) elected to undergo a second procedure. Most patients chose to defer retreatment for one of three reasons (1) the patient was unwilling to sacrifice near and intermediate vision for sharper distance vision, (2) symptoms were not bothersome enough to merit risks of additional surgery, and (3) the patient was willing to give additional time to adjust to monovision. Patients were then prescribed glasses for distance vision or reading, depending on the complaint, or were to be re-evaluated for retreatment following some period of adjustment. Many patients were then lost to follow-up. This suggests that the degree of dissatisfaction was relatively mild and that many patients eventually adjust to monovision or wear glasses on occasion for specific activities.

Overall patient satisfaction with monovision after LASIK was 80.4%, compared to 80.6% in contact lens wearers after exclusion of contact lens intolerance. Furthermore, the satisfaction among strict monovision, minimonovision, uncrossed monovision, and crossed monovision groups did not differ significantly from satisfaction in contact lens wearers.

Due to the retrospective nature of most monovision refractive surgical studies, there was no standardized instrument to measure patient satisfaction. Rather, patient dissatisfaction was noted if the patient had any complaints or negative comments about vision at the last follow-up visit. In addition, near vision was not documented in many charts and could not be analyzed as an outcome. It was also difficult to determine from charts whether monovision was intended or whether regression of one or both eyes led to a monovision result. These factors may be better analyzed through a prospective study.

2. Interocular Blur Suppression

Two tests used to measure the ability to suppress interocular blur are the anisometropic blur-suppression test and the American Optical vectographic test. The anisometropic blursuppression test indicates that the interocular suppression of blur is greater for smaller degrees of anisometropia (2). Both testing modalities indicate that blur suppression is greater when the dominant eye is corrected for distance.

Monovision success is dependent on interocular blur suppression. In successful wearers of monovision lenses, the interocular suppression of blur was found to be approximately two orders of magnitude greater than in unsuccessful wearers of monovision lenses (2). Of note, interocular blur suppression becomes less effective under dim illumination conditions (2), which accounts for the well-known poorer visual performance of monovision patients under night driving circumstances.

3. Binocular Visual Acuity

Jain and colleagues (1,2) 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 found to be 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 (23).

4. Stereoacuity

Reduced stereoacuity is considered to be the major disadvantage associated with monovision (24). Jain and coworkers reviewed twelve articles that examined the effect of monovision on stereoacuity (1,2). When near stereoacuity under monovision conditions was compared to stereoacuity under binocular viewing conditions, a mean decrease of 37 arc seconds (from 87 to 124 arc seconds) was found. The average normal value for stereopsis is 20 arc seconds and, for persons over 40 years of age, 58 arc seconds (13,25). A more recent paper by Kirschen and coworkers found that near stereoacuity decreased from a median of 50 arc seconds with bifocal contact lenses to 200 arc seconds with monovision (26).

Patients in whom monovision is successful exhibit a lower reduction in stereoacuity than do unsuccessful monovision patients. Patients in whom monovision was unsuccessful were found to have a 50 to 62 arc seconds greater reduction in stereoacuity as compared to successful monovision patients (1).

5. Contrast Sensitivity

When two eyes are used instead of one, visual performance, and especially contrast sensitivity, greatly improves (binocular summation). Contrast sensitivity increases by a factor of 2 when the stimulus is viewed binocularly; therefore, binocular contrast sensitivity is 42% greater than monocular contrast sensitivity. With increasing monocular defocus, the binocular contrast sensitivity decreases steadily until it is actually worse than monocular contrast sensitivity (binocular inhibition) (27). If the defocus is increased beyond 2.50 D the binocular contrast sensitivity reverts back to the monocular level, indicating suppression of the defocused eye. Because monovision results in loss of binocular summation, or may even result in binocular inhibition, monovision results in a significant reduction in contrast sensitivity, especially at higher spatial frequencies (greater than 4 cycles per degree).

L. PERIPHERAL VISION AND VISUAL FIELDS

Monovision appears to have no significant effect on peripheral visual acuity and only a minimal effect on binocular visual field width (14).

1. 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 (3).

2. 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 successful monovision contact lens (0 to 0.6 prism diopters) was found to be less than the shift in unsuccessful monovision wearers (2.1 to 2.2 prism diopters) (7,28). 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 (21).

3. Task Performance

Monovision appears to be associated with adverse effects on, in particular, stereoacuity and contrast sensitivity in particular. 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 the tasks by 2 to 6% when compared to performance of the tasks under binocular viewing conditions. However, this reduction was quite minimal when compared with the 30% reduction seen under monocular viewing conditions with near tasks requiring high stereopsis (29).

M. FACTORS INFLUENCING MONOVISION SUCCESS

On the basis of the above-mentioned findings, poor candidates for monovision are patients who exhibit minimal interocular suppression of blur, patients with large esophoric shifts with monovision, and patients with a significant reduction in stereoacuity with monovision. Certain psychological and personality factors also appear to play a role in determining the success of monovision (30).

An additional consideration is sighting preference. The inputs from the two eyes are not identical in their relative influence on cortical cells: the dominant eye produces a greater response to a given stimulus than does the input from the other eye. Those individuals who do not have a strong sighting preference (i.e., who have alternating dominance) appear to have constant interocular blur suppression and therefore tend to be more successful with monovision. Furthermore, the choice of eye that is corrected for distance, whether the dominant or the nondominant eye, appears to have an effect on monovision success.

In 16 articles reviewed by Jain and coworkers, the average age of successful monovision users ranged from 48 to 55 years (1,2). No articles were found that compared the success rate in younger versus older presbyopes. Two articles examined the difference

between the average age of successful versus unsuccessful monovision patients but failed to find any statistically significant difference in age between the two groups (5,31).

N. CONCLUSIONS

Monovision has been evaluated extensively in contact lens users, but few studies comment on its success in refractive surgery. Furthermore, the impact of the magnitude of anisometropia created in monovision patients has not been fully characterized. We have introduced a new term, minimonovision, to characterize monovision patients with a lesser degree of near vision correction than full monovision (7). The inclusion criteria for strict monovision and minimonovision are mutually exclusive. We found that strict monovision and minimonovision groups had comparable satisfaction rates. We also found that crossed monovision patients overall were as satisfied as uncrossed monovision patients, and that within the minimonovision and monovision subgroups, crossed monovision did not affect satisfaction. The rate of satisfaction for monovision after LASIK was similar to the rate in contact lens wearers.

Monovision is associated with some compromises of visual function, the extent of which depend on the particular individual and the requirements imposed by different viewing conditions. However, for those refractive surgery patients willing and able to adapt, these compromises constitute reasonable a trade-off for reducing dependence on near-vision correction.

Refractive surgery may be used to take advantage of the monovision option in presbyopic refractive surgery patients. However, this option should be pursued only after careful preoperative screening and counseling of the patient. Creating a monovision situation with refractive surgery constitutes a practical alternative to other surgical treatment modalities for presbyopia, such as scleral expansion/relaxation and multifocal corneal treatment.

REFERENCES

1.Jain S, Ou R, Azar DT. Monovision outcomes in presbyopic individuals after refractive surgery. Ophthalmology 2001; 108:1430–1433.

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.Sippel KC, Jain S, Azar DT. Monovision achieved with excimer laser refractive surgery. Int Ophthalmol Clin 2001; 41:91–101.

4.Schor C, Erickson P. Patterns of binocular suppression and accommodation in monovision. Am J Optom Physiol Opt 1988; 65:853–861.

5.Schor C, Landsman L, Erickson P. Ocular dominance and the interocular suppression of blur in monovision. Am J Optom Physiol Opt 1987; 64:723–730.

6.Fonda G. Presbyopia corrected with single vision spectacles or corneal lenses in preference to bifocal corneal lenses. Trans Ophthalmol Soc Aust 1966; 25:78–80.

7.Chang MA, Kloek CE, Zafar S, Jain S, Azar DT. Analysis of strict monovision and minimonovision LASIK surgery in presbyopes. Arch Ophthalmol 2002. Submitted.

8.Maguen E, Nesburn AB, Salz JJ. Bilateral photorefractive keratectomy with intentional unilateral undercorrection in an aircraft pilot. J Cataract Refract Surg 1997; 23:294–296.

9.Wright KW, Guemes A, Kapadia MS, Wilson SE. Binocular function and patient satisfaction after monovision induced by myopic photorefractive keratectomy. J Cataract Refract Surg 1999; 25:177–182.

10.Wilson SE, Klyce SD, McDonald MB, Liu JC, Kaufman HE. Changes in corneal topography after excimer laser photorefractive keratectomy for myopia. Ophthalmology 1991; 98: 1338–1347.

11.Moreira H, Garbus JJ, Fasano A, Lee M, Clapham TN, McDonnell PJ. Multifocal corneal topographic changes with excimer laser photorefractive keratectomy. Arch Ophthalmol 1992; 110:994–999.

12.Harris MG, Classe JG. Clinicolegal considerations of monovision. J Am Optom Assoc 1988; 5:491–495.

13.Emmes AB. A statistical study of clinical scores obtained in the Wirt steropsis test. Arch Am Acad Optom 1961; 38:398.

14.Collins MJ, Brown B, Verney SJ, Makras M, Bowman KJ. Peripheral visual acuity with monovision and other contact lens corrections for presbyopia. Optom Vis Sci 1989; 66: 370–374.

15.Hom MM. Monovision and LASIK. J Am Optom Assoc 1999; 70:117–122.

16.Bennett ES, Henry VA. Bifocal contact lenses. In: ES Bennett, VA Henry, eds. Clinical Manual of Contact Lenses. Philadelphia: Lippincott, 1994; 362–398.

17.McLendon JH, Burcham JL, Pheiffer CH. Presbyopic patterns and single vision contact lenses II. South J Optom 1968; 10:33–36.

18.Sanchez FJ. Monovision: which eye for near? Contact Lens Forum 1988; 13:57.

19.Lebow K, Goldberg J. Characteristics of binocular vision found for presbyopic patients wearing single vision contact lenses. J Am Optom Assoc 1975; 48:1116–1123.

20.Trevarthan CB. Two mechanisms of vision in primates. Psychol Forsch 1968; 31:299–348.

21.Rigel L. Which modality works best? When monovision makes sense. Rev Optometry 1998; 13:90.

22.Coren S, Kaplan CP. Patterns of ocular dominance. Am j Optom Arch Acad Optom 1973; 50:283–292.

23.Collins M, Goode A, Brown B. Distance visual acuity and monovision. Optom Vis Sci 1993; 70:723–728.

24.Erickson P, Schor C. Visual function with presbyopic contact lens correction. Optom Vis Sci 1990; 67:22–28.

25.Wirt SE. A new near-point stereopsis test. Optom Weekly 1947; 38:647–649.

26.Kirschen DG, Hung CC, Nakano TR. Comparison of suppression, stereoacuity, and interocular differences in visual acuity in monovision and Acuvue bifocal contact lenses. Optom Vis Sci 1999:832–837.

27.Pardhan S, Gilchrist J. The effect of monocular defocus on binocular contrast sensitivity. Ophthal Physiol Opt 1990; 10:33–36.

28.McGill EC, Erickson P. Sighting dominance and monovision distance binocular fusional ranges. J Am Optom Assoc 1991; 62:738–742.

29.Sheedy JE, Harris MG, Busby L, Chan E, Koga I. Monovision contact lens wear and occupational task performance. Am J Optom Physiol Opt 1988; 65:14–18.

30.Du Toit R, Ferreira JT, Nel ZJ. Visual and nonvisual variables implicated in monovision wear. Optom Vis Sci 1998; 75:119–125.

31.Koetting RA. Stereopsis in presbyopes fitted with single vision contact lenses. Am J Optom Arch Am Acad Optom 1970; 47:557–561.