Other tests:

• Worth’s 4-point test.
• Polarized test (dissociative polarized glasses).
• Far stereopsis test (polarized glasses).
• Haidinger’s test (synophore). This test allows quantifying the magnitude of ocular dominance. According to the studies of Seijas et al16, analyzing each of these types of tests separately, they found that the tests that obtained the highest proportion of dominance were those of the hole in the hand, the +1 D, and that of Haidinger.

In most of the published studies on monovision, only one test is mentioned to determine the ocular dominance (usually a motor test: the “hole-in-card test”)1,9. Sometimes, some studies do not even describe the type of test used to determine which is the reading eye10. In spite of this, and since the satisfaction results are very similar between different studies (the majority of patients with successful monovision are those who do not have a frank ocular dominance), we think that patients who do not adapt to monovision would be those who have a strong dominance (and therefore less able to perform the alternation and the suppression of the blurred image).

The ocular motor dominance in emmetropic presbyopes presents a constant alternation; in myopic presbyopes with anisometropia less than 1.75 D the dominance was 50% in the most myopic eye and 50% in the least myopic one. However, when the anisometropia was higher, in 100% of the cases dominated the more myopic eye (fact that questions the adjustment for far vision of the "motor" dominant eye, in monovision). Hyperopic presbyopes are usually less tolerant of the monovision situation19.

The examination of ocular dominance should allow us to rule out those patients with marked ocular dominance (e.g. strabismus), since they will tolerate the monovision worse, since there is no correct binocular coordination, which prevents the adequate suppression of the blurred image.

BINOCULAR SUMMATION

The binocular summation allows the contrast sensitivity to be greater than the monocular, even though both eyes have different sensitivities. When the anisometropia is greater than 1.5 D, the binocular sum begins to disappear. If the anisometropia becomes greater than 2.5 D, the binocular sum may then completely disappear and the binocular contrast sensitivity equals the average monocular contrast sensitivity16.

The so-called by us "help test" makes it possible to simulate how the patient's binocularity will be, in case it is submitted to monovision24. In the clinic, the patient looking at the optotypes from afar, with its far refractive correction in the phoropter, is placed a spherical + 1 D lens in front of the eye for reading. Then, the reading eye is alternately occluded and de-occluded, keeping the eye for far always uncovered. When the patient reports being more comfortable with the reading eye uncovered, we will consider that the help test is positive. However, if the patient is more comfortable with the reading eye occluded, we will consider that the help test is negative. This test can serve as a prognostic test. When the help test is positive, it indicates that the reading eye, even if it is out of focus, helps in distant vision (the patient is able to enjoy the binocular sum, and suitably suppresses the blurred image). On the contrary, if the help test is negative, the patient is not able to suppress the blurred image (in this case, the distant blurred image of the reading eye), and does not enjoy the binocular sum, indicating a worse prognosis in the adaptation to monovision.

For monovision to work, the patient must have a normal binocular coordination, and present good visual acuity in both eyes9. If the binocular coordination is abnormal (strabismus), the patient will have a marked ocular dominance, and it will be difficult to achieve the interocular alternation necessary to suppress the blurred image. If the patient has a moderate or deep amblyopia in one eye, or a situation of low visual acuity in both eyes, it will be difficult for the monovision to be successful (despite having normal binocular coordination), since the image offered by each eye will not have enough quality to allow a useful near and far vision.

CONTACT LENS TEST

We agree with other authors not to routinely include in the preoperative period the “simulation” of the monovision through contact lenses4,5, among other reasons because the tolerance to surgical monovision is greater than with contact lenses, since a better binocular adaptation is achieved, with constant refractive correction, less aniseikonia, and elimination of intolerance and/or complications secondary to the use of contact lenses1. In our protocol, it is a useful test in some specific cases. However, in our protocol, we consider it a test of utility in the postoperative period, as we will see later on.

SELECTION OF THE PATIENT. INCLUSION AND EXCLUSION CRITERIA

Patient selection is important in refractive surgery and becomes relevant in monovision. Patients whose life style requires excellent vision in extreme distances, especially in low light conditions, are not good candidates for monovision. Certain professions, such as pilots, professional drivers, sports requiring very good visual acuity (golf, tennis, hunting ...), are described in some studies as contraindications5,7. A correct anamnesis is essential to know the way of life of the patient, its occupation, sports that he/she practices, hobbies (photographer, hunter) or some activity that requires monocular approach, driving (type, nocturnal/diurnal, routes, etc.).

The age of the patient and his refractive defect are decisive when making the decision to induce monovision. Some authors offer monovision to patients from 35-45 years of age, others recommend it from 40 years of age and others with age equal to or greater than 45 years3,5,6. We value the option of monovision in patients who already suffer from presbyopia, that is to say, patients who, with their distance correction, are not able to read adequately (usually from 45 years of age, with variations). As a rule, we do not perform monovision in pre-presbyopic patients, although we value this option in certain cases (in particular the myopic presbyopes). The lens surgery with an intraocular lens implant must now be considered to improve the patient's visual acuity and to eliminate or reduce the previous refractive defect, if any, minimizing the dependence on glasses and improving the quality of life of patients. After refractive lens replacement with lensectomy and monofocal intraocular lens implant, we consider that monovision should be applied, in general, in all cases of bilateral lensectomy (as long as there are no exclusion criteria), since this way we will be able to minimize the use of the near and intermediate glasses, providing greater patient satisfaction.

The importance of applying a selection protocol leads us to propose the following inclusion and exclusion criteria for monovision3,9.

Inclusion criteria:

• Emmetropic presbyopic patients, or ametropic (myopia, hyperopia, and/or astigmatism).
• Post-surgical refractive state "target" possible.
• Patients who do not demand excellent visual acuity for extreme distances (very far, very close), especially in low light conditions.
• Motivated patients who wish to improve their near vision without glasses, due to their profession or usual activities.

Exclusion criteria:

• Patients who require excellent visual acuity in low light conditions (professional or nocturnal drivers, hunters).
• Demanding personalities, with high expectations on the outcome of the surgery.
• Ophthalmological exclusion criteria: amblyopia, low vision, strabismus, strong ocular dominance, macular diseases, and all those eye diseases that can cause an important loss of vision in the short or medium term.

As we have already mentioned, in the preoperative visit, it is necessary to assess certain aspects of the patient's daily life before inducing monovision. But, in addition, the patient should receive a wide explanation about his or her case: diagnosis (presbyopia), cause (failure in the accommodation or loss of the lens), alternatives (glasses, contact lenses, other techniques), technique (monovision with laser excimer or pseudophakia), prognosis (visual and refractive), advantages and disadvantages, and complications. Another important aspect is the personalization of the Informed Consent. In our group, we added a document attached to the standard document of the main surgical technique, in which the most relevant peculiarities of monovision are mentioned.

The correct selection of the patient, after an exhaustive anamnesis and exploration, is important, but also the patient's education, so that their expectations are similar to the results obtained, which will imply that they are satisfied with the results. We make special emphasis on the concept of "useful vision" (one that covers daily activities and that can be obtained with surgery) and the "extreme visions" for far/near (for which he/she could require the use of glasses). It is necessary to make the patient understand that he should not expect to have a vision like that of a person of twenty years, who does not have any visual defect and does not need glasses.

In general, most patients with monovision do not request glasses. We agree with other authors, in that patients with monovision requested more frequently the spectacle for close than the spectacle for afar10,24.

SIDE EFFECTS. INABILITY FOR ADAPTATION AND REVERSIBILITY

Side effects, described in the medical literature, that may occur in patients with monovision are decreased contrast sensitivity, and decrease in stereopsis, distant binocular visual acuity, and depth of focus7. In addition, monovision can decompensate a latent strabismus25.

In most of the published studies, a reduction of distant steropsis and contrast sensitivity has been demonstrated in patients with monovision, compared with patients without monovision11. However, Wright did not find statistically significant differences in the stereoscopic acuity measured with the Worth’s 4-point test, and with the titmus stereotest6. In the study, Wright established that binocular distant visual acuity, stereopsis, contrast sensitivity, and amplitude of convergence and fusion were similar between a group of patients without monovision and another with monovision. Specifically, the distant stereopsis was somewhat lower in the group of patients subjected to monovision (compared to the control group, with both eyes focused for far), but the difference was not statistically significant. In this study, contrast sensitivity was very similar in both groups. Logically, the group with monovision presented a better near visual acuity without correction6. On the other hand, patients with successful monovision have a lower reduction in distant stereopsis, when compared with patients with unsatisfactory monovision19.

The dissatisfaction of the patient (or inadaptation) to the combined vision (monovision) is a fact to be taken into account and that may involve the reversal of the monovision situation and adjust the two eyes for far vision. Goldberg asked the reasons for their decision to the patients who chose to reverse their monovision, and focus their reading eye for far, finding that most of them reported that they were not comfortable with the situation of having two different eyes, while one of them made the decision because he had difficulty in sports. The patients described the freedom of glasses for far and near as a monovision advantage, and as disadvantages the possibility of needing glasses for night driving, the lack of perfection in distant visual acuity, the situation of having both eyes focused different, and the period of adjustment to monovision. Miranda found in a study that most of the patients who chose to reverse their monovision could not specify a specific reason, while other possible reasons are the excess of myopia in the reading eye, or the lack of emmetropia in the far-focused eye20.

Monovision reversal rates published in the literature vary between 2.4%7, 4.4%5, 5%10 and 7.5%20. Let us summarize the causes of dissatisfaction:

1. Poor vision for distance (the common cause is usually the existence of a residual defect in the eye for far)
2. Poor near vision (due to insufficient myopia in the eye for near)
3. Torpid neuro adaptation. We will resort to the "help test" to assess whether the cause is due to some of the above or other assumptions (a normal help test will imply a poor refractive result; an abnormal help test confirms the poor neuroadaptation). At this time the utility of the "contact lens test" is recognized, when being able to find two assumptions for its adaptation and subsequent assessment by the patient:
   1. When a correct refractive result is observed, but the patient does not obtain any benefit (he will be shown what the visual situation will be after reversing the monovision)
   2. When induced myopia in the reading eye is excessive for the desired purpose (he will be offered a partial reversal of induced myopia)

MINI-MONOVISION AND MICRO-MONOVISION

The mini-monovision is a relatively new term, which defines that situation of monovision in which a slight myopia (between -0.75 and -1 D) is induced in the reading eye. Obviously, this low degree of myopia will only make possible the focus at a moderate distance, not close, of the objects (computer), depending on the age and the situation of the lens (pseudophakia). It is used in incipient presbyopic patients and even pre-presbyopic myopia, in lens surgery with a bifocal multifocal intraocular lens (MIOL) implant and even in those with extended depth of focus lenses.

Monovision with increased depth of focus (micro-monovision or advanced monovision) is based on the induction of moderate myopia in the non-dominant eye combined with an aspherical treatment with excimer laser (micro-monovision or “laser blended vision” with CRS Master and MEL excimer laser; advanced monovision with Custom Q excimer laser Allegretto Eye Q). Although their results have been published, they are not techniques that have obtained wide dissemination15,16.

ALTERNATIVES TO MONOVISION

As an epilogue to this chapter, in our opinion, monovision induced with surgery, either corneal or intraocular, is a useful technique for the treatment of presbyopia. Its indication and execution require a detailed study of the patient, like the rest of refractive surgery, and the knowledge of a series of concepts, such as ocular dominance, the reading eye or the inclusion and exclusion criteria, in addition to its limitations. But more than talking about alternatives to monovision, we must look into “when is monovision an alternative” to other surgical techniques used today in presbyopia surgery.

There is no doubt that, during these years of the 21st century, refractive lens replacement with multifocal intraocular lens implants has gained ground to other techniques, including monovision, to restore near vision26. However, we consider that laser monovision is an appropriate technique in cases of myopic presbytes without crystalline alteration (lens or cataract dysfunction syndrome) who wish to minimize the use of glasses27,28, as is monovision with pseudophakia in those cases in which patients are not good candidates for MIOL implantation (high refractive defects, such as large myopic or hyperopic)29-31.

We advocate to maintain the knowledge and use of this technique, since its visual and refractive results and patient satisfaction allow us to consider it useful in the “armamentarium” of any refractive surgeon.

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