4 Pseudophakic Monovision

Boerner and Thrasher1 were the first authors to describe a pseudophakic monovision study in the literature. They retrospectively analyzed the postoperative rate of glasses use in 100 patients with bilateral posterior chamber IOL (PCIOL) monovision. The number of patients needing to wear bifocals postoperatively declined by half with this technique.

There are numerous studies in the literature that demonstrate the impressive success of pseudophakic monovision. Patient satisfaction/spectacle freedom has been approximately 80% or more,^2,3,4,5,6,7 while overwhelmingly most postoperative cataract patients will need to wear reading glasses (160 out of 169 patients) if both eyes were targeted at plano to –0.50 D spherical equivalent.⁸

For the sake of discussion without confusion, we would recommend the following classification for pseudophakic monovision in terms of focal length separation between the two eyes:

• Mini (sometimes referred to as micro or nano), –0.50 to –0.75 D.

• Modest (sometimes referred to as medium), –1.00 to –1.5 D.

• Full (sometimes referred to as traditional or classical), –1.75 to –2.5 D.

4.2 Preoperative Tests for Pseudophakic Monovision

The first eye dominance test recorded was probably by Porta in the sixteenth century, but it did not seem to get much attention until the 1920s.^9,10,11 In the mid-1920s, Sheard^10,11 proposed not to weaken the dominant sighting eye in order to maintain balance. He believed that the object looked at is sighted by the dominant eye in its own line of vision, while the nondominant eye then has to converge until binocular single vision occurs. Thus, there is dominance of one member in each pair, whether we refer to the hands, the feet, or the eyes. He proposed that if the patient has a strongly dominant eye, the clinician should leave the acuity slightly better in this eye than in the fellow eye. Otherwise, the patient may experience some discomfort. If there is no significant eye dominance, then we should try to achieve equal function in both eyes. If prism is needed, it should mostly be placed in front of the weaker eye. These refraction rules proposed by early clinicians such as Sheard were designed to enhance binocular function and lessen asthenopia. They laid out the importance of the role of dominance in monovision. Despite these rules, crossed IOL monovision can still work well within a certain range.4,12

Preoperative dominant eye tests for monovision can be divided basically into two groups. One is a motor sighting dominance test and one is a sensory dominance test. The patient is forced to choose one eye over the other in the motor sighting dominance test, while the patient is asked about balance and perceptual preference in the sensory ocular dominance test. The motor sighting dominance test contains stereo-objects with a disparity beyond Panum’s area, while sensory ocular dominance, also referred to as ocular prevalence, is determined using stereo-targets imaged within Panum’s area.¹³

4.2.1 Commonly Used Sighting Dominance Tests

Hole-in-Card (5 × 9 Inch Card with a Central 1.25-Inch Hole, NearVision CK)

• Done with distance glasses or without glasses, whichever provides better distance vision for the patient.

• Choose a small target at the end of the room, about 10 to 20 feet away.

• The patient extends both arms and holds the card with the thumb and the index finger in both hands.

• The patient is asked to keep both eyes open and to align the hole to the target.

• The examiner covers each eye in turn. The eye aligned with the hole and the target is the sighting dominant eye.

Finger Pointing

• Done with distance glasses or without glasses, whichever provides better distance vision for the patient.

• The patient extends the dominant arm.

• Ask the patient to choose a small target at the end of the room, about 10 to 20 feet away. With both eyes open, have the index finger point at the target.

• Close one eye and then the other. The eye that aligns the index finger and the target is the dominant eye.

• Given the fact that only one arm is used to point at the target, the concern for this test is the potential influence of the hand sidedness. A better way¹⁴ might be to use both extended arms holding a pencil. The patient is asked to align the tip of the pencil with the distance target and the observer occludes each eye alternatively to find out which eye is aligned with the target. The eye that remains in line with the target is the sighting dominant eye.

Finger Hole

• Done with distance glasses or without glasses, whichever provides better distance vision.

• The patient extends both arms and brings both hands together, forming a small hole by crossing the thumbs and index fingers.

• Choose a small target at the end of the room, about 10 to 20 feet away. With both eyes open, view the target through the small hole.

• The examiner covers each eye in turn. The eye aligned with the hole and the target is the sighting dominant eye.

Camera or Kaleidoscope

• Most people automatically use their dominant eye when looking through a camera or a kaleidoscope eyepiece.

• The patient is asked to hold the camera with both hands and bring the camera to the face.

• The eye using the camera is the dominant eye.

Mirror Test

• The patient looks into a small mirror, held at 20 cm, containing a 1-inch diameter circle.

• This results in an optical target distance of 40 cm. The patient sights his or her nose within the mirror.

• Ocular dominance is determined by identifying the sighting eye by alternating eye closure.

There are several ways to do this simulating preoperative monovision mimic test. It gives the patient a chance to experience the mimicked monovision status.

4.2.4 Tolerance Test15 of William Maloney

This is a very meticulous step-by-step test and can yield very detailed information. It works very well but it takes much more time than other tests for the same purpose.

4.2.5 Drs. Finkelman and Barrett Use a More Straightforward Test⁶

Place the distance correction in a trial frame. A + 1.00 D lens is used to demonstrate the predicted outcome of monovision. In their study, the first eye was already operated on with good distance vision and the second eye waiting to have surgery was to be the near eye.

4.2.6 Siepser Blur Tolerance Test

A beautiful demonstration can be found on YouTube by searching “Siepser Blur Tolerance Test.” The value of this test is that it enables a dynamic but still easy-to-perform test for both the patient and the examiner.

Fig. 4.1 Plus lens 1.50 D with a magnetic clip.

Each patient receives a plus lens test, which has two different meanings: the first is the mimic test where the plus lens with a magnetic clip ( Fig. 4.1, Fig. 4.2, Fig. 4.3, Fig. 4.4) is placed in front of the nondominant eye (decided by hole-in-card). By doing this, the patient will have a chance to understand that the distance vision of the near eye is not very clear although the reading vision is better. The patient will need to tell the examiner if he or she finds this condition acceptable.

The second part of the plus lens test is the sensory dominance test with a magnetic clip-on plus lens ( Fig. 4.1, Fig. 4.2, Fig. 4.3, Fig. 4.4) placed in front of each eye monocularly when the patient wears their regular distance glasses, if there was no significantly improved vision with a new manifest refraction ( Fig. 4.3), or in front of the new manifest lenses placed in a trial frame for distance if there was improvement with the new manifest ( Fig. 4.4).

The sensory dominance test with the plus lens is tested for distance where the patient is asked to binocularly look at the distance visual acuity chart at 20 feet while the plus lens is placed in front of each eye monocularly. The patient is asked to report in front of which eye the plus lens is placed when he or she has overall better binocular vision and feels more natural and comfortable. If the patient feels better when the plus lens is in front of the right eye, then the left eye is the sensory dominant eye since the patient feels more uncomfortable when the dominant left eye is blurred due to the extra plus lens.

Fig. 4.2 Plus lens is clipped on the regular glasses.

Fig. 4.3 Plus lens tested with her regular glasses.

The results from different sighting dominance tests for the same person can vary. For example, the hole-in-card shows OD as dominant, but the camera shows OS. It would be interesting to find out if these patients have relatively weak dominance or no clear preference for either the right or left eye. It is possible that these patients have a better prognosis for success with IOL monovision, regardless of whether it is conventional or crossed monovision, than those patients with strong dominance for one eye. Patients with alternating dominance (no sighting preference) have better interocular blur suppression, which is another predictive factor for monovision success, while those with a strong sighting preference have reduced blur suppression, decreased binocular depth of focus, and higher monovision failure rates.16,17,18,19,20,21,22,23 For alternating sighting dominance patients, the binocular depth of focus is almost equal to the sum of monocular depths of focus, but in those with strong sighting dominance, the binocular image becomes blurred as the object moves from the dominant eye’s clear range to the nondominant eye’s clear range, and thus the binocular depth of focus in those patients is much less than the sum of the monocular depths of focus.²¹ Extremely strong dominance examples can be seen in amblyopia and monofixation syndromes, where the suppression for the stronger eye is very difficult, and that is the reason why those patients do not do well with monovision.

Fig. 4.4 Plus lens tested with trial frame when the new refraction offers better vision.

Sighting dominance is not fixed, but rather plastic and fluid, even with the same testing method. More than half a century ago, Charnwood24 had already noted that sighting dominance changes when the refractive correction changes. That is probably the reason why, in most situations, crossed monovision works as well as conventional monovision. Based on 10 ocularly healthy young patients, Khan and Crawford25 were able to demonstrate that in a reach-grasp task for targets within the binocular visual field, patients switched between left and right eye dominance depending on horizontal gaze angle. The sighting dominance can even change as the person looks at different places in the visual field and even at different times.¹⁶

In the aging population, the vision and the refractive status can change significantly as a consequence of cataract formation and refractive index change. It is not rare to notice that sighting dominance changes due to cataract density change and due to refractive status change or vision change. A recent study by Schwartz and Yatziv²⁶ noted eye dominance changed after cataract surgery due to improved vision: of the 33 patients included, 7 patients (21.2%) had a change in ocular dominance from nondominance to dominance due to improved vision.

The hole-in-card test is the most popular motor sighting dominance test, while the plus lens is the most popular sensory dominance test in cataract surgery IOL monovision clinical settings. Between the two types of tests, there is no simple conclusion in terms of correlation. That means the motor sighting dominant eye is not always the sensory dominant eye and is not always the better vision eye.^{13,14,16,27,28,29,30,31,32}

A prospective study with 51 emmetropic patients, aged 18 to 60 years, done by Seijas et al¹⁴ compared multiple tests of motor sighting dominance and sensory ocular dominance. Patients were divided into two groups: 18 to 35 years old (26 patients) and 35 to 60 years old (25 patients). In the motor sighting test group, hole-in-card, kaleidoscope, point-a-finger, and convergence near point tests were evaluated. Hole-in-card and kaleidoscope tests had the most certain results, while the rest had more variation. There was no good agreement among these tests either. In the sensory test group, + 1.00 D lens, W4D test, polarized test, Haidinger’s test, and distance stereo test were evaluated. Plus 1.00 D lens was found to have the most certain results, while the rest had more variation. Similar to the motor sighting tests, very poor correlation existed among the different tests in the sensory group. The agreement of motor sighting dominant eye with hole-in-card with + 1.00 D lens sensory ocular dominance tests was only 58% in young patients (15 of 26) and 40% (10 of 25) in older patients. The agreement among other motor and sensory tests was much lower. It was apparent that most of the studied patients did not show clear ocular dominance, because the results were quite variable and little correlation was found among the tests. Most people have a constantly alternating balance between both eyes. This study result suggests that most patients may do well with IOL monovision. Those who do not tolerate monovision might be those who have strong and clear ocular dominance or those with contraindications for monovision. It is true that the main emphasis of preoperative monovision tests is not to find out which eye is dominant, right versus left, but it is to find out who might not be able to tolerate surgically created monovision. For example, those who have the same eye dominant for both distance and near may have clear and strong ocular dominance and therefore may have a high chance of failure, because monovision forces them to change the viewing distance.

A study³¹ by Suttle et al was in agreement with the study by Seijas et al.¹⁴ Generally speaking, ocular dominance shows poor agreement between methods within individuals. Those experimental studies correlate well with our clinical practice. They appear to explain why overwhelmingly most crossed IOL monovision patients do as well as conventional IOL monovision patients, as long as contraindications are avoided and the anisometropia level is kept at a mini or modest level.

An experimental study using a stereoscope in 60 new contact lens–wearing monovision patients by Collins and Goode²² demonstrated that when the sensory rivalry dominant eye matched the sighting dominant eye, blur suppression and monovision were better. Our own study of 10-year follow-up IOL monovision patients supports that phenomenon: among 45 IOL monovision patients who had both tests, 24 out of 45 were matched between hole-in-card and plus lens test at distance; 18 out of 45 were unmatched and 3 out of 45 were uncertain. The matched group had better results, especially in satisfaction and nighttime driving evaluation with statistically significant p-values.

We do not have an easy way to quantitatively measure rivalry sensory dominance in our busy clinic. The most commonly used test seems to be the plus lens. If the same eye is dominant in the rivalry sensory test for both far and near, that patient probably has a strong rivalry sensory dominance and is likely to have more difficulty in blur suppression and may have a high chance of monovision failure. Full monovision should be avoided in those patients. It would be valuable to have rivalry sensory tests for all prospective IOL monovision patients, to objectively and quantitatively measure the strength of rivalry dominance, although we still do not know how much impact dense cataracts have in terms of reliability and accuracy.

Handa et al18 studied 20 IOL monovision patients; 16 of them were successful and 4 of them were unsuccessful with a follow-up of more than 6 months after cataract surgery. The study was able to quantitatively measure sensory dominance and suggested that strong sensory ocular dominance has a direct negative impact on the success of IOL monovision. The study was done when the patients already had IOLs in their eyes, so it would likely be quite different if measured preoperatively with the cloudy cataract. Strong ocular dominance may have a high chance of monovision failure since it is harder for those patients to have blur suppression. When the nondominant eye is fixing, the dominant eye will not easily suppress the blur, which will produce visual stress on binocular balance.

Ooi and He did a well-designed experiment with 32 patients who had equal vision of at least 20/20 in each eye, good stereoacuity of at least 40 seconds of arc, and normal color vision with no previous history of ocular disease or surgery.³⁰ The hypothesis was if the inferiority of the nondominant eye is solely due to a weakness in its monocular channel before the convergence of binocular information, its other visual functions, such as contrast sensitivity and suprathreshold brightness judgment, when measured monocularly, should also be inferior relative to those functions in the dominant eye. However, if interocular imbalance caused by sensory eye dominance is largely attributable to processes at the binocular visual stage, at which the information from the two eyes has combined, the monocular information retained in the monocular channels should be equally preserved. Hence, the contrast sensitivity and suprathreshold brightness judgment functions measured in each eye should be the same. By adjusting the strength of the stimulus to find the neutral point between the two eyes, rather than adding neutral density filters, the study was able to quantitatively measure sensory dominance of each eye. That study had quite a few interesting findings:

1. There was no correlation between the sighting dominant eye and sensory dominant eye.

2. The monocular contrast sensitivity test did not directly correlate with the motor sighting dominant eye test result or sensory dominance test.

3. Monocular brightness perception did have a nearly significant relation with the sensory dominance test (p = 0.0512), suggesting that there was a statistically meaningful relationship between interocular imbalance (sensory dominance) and monocular brightness perception.

4. Monocular brightness perception did not have direct correlation with the sighting dominance test.

5. Sensory dominance could not be predicted from refractive status.

6. Based on the above findings, their main conclusion was that the sensory dominance is largely attributable to processes in binocular, not monocular channels, since monocular input of either contrast or brightness did not correlate with sensory dominance.

4.2.9 Is a Preoperative Monovision Mimic Test Necessary?

Our own study reviewed all cataract surgery charts from January 2005 to December 2014 at F. Z.’s office. Based on preoperative plus lens tests, patients were divided into tested and untested. Parameters at their last office visit were analyzed including visual acuity, satisfaction, spectacle independence, real-life gross depth perception issues, and nighttime driving difficulties when they did not wear glasses. In total, 359 cases were qualified (30 passed away, 87 lost contact during 10 years) and 194 patients were enrolled with de-identified surveys, 100 with the plus lens test, and 94 without the test. Average postoperative follow-up was 34 months in the tested group and 36 months in the untested group. Mean anisometropia was 1.38 D in the tested group and 1.21 D in the untested group (p = 0.006). Both groups had very high satisfaction of 95% or above. That study suggested that a preoperative mimic test may not be necessary to achieve high patient satisfaction in IOL monovision as long as a good preoperative monovision consultation was provided.

The preoperative monovision mimic test has two meanings: one being a sensory dominance test for the clinician, and one being a mimic test for the patient. First, we know sighting and sensory dominance do not always match. Second, we usually choose the sighting dominant eye for the distance eye and the nondominant eye for near, and this has been the convention. The test has unique merit in giving the patient a mimicked experience of one eye for far and one eye for near; plus, the test is brief and can be done by a technician. This test indirectly tells each prospective monovision patient the fact that monovision is not perfect, but it is a compromise. Therefore, our recommendation is to do this test for every prospective IOL monovision patient. If the patient does not like the sensation when sitting or walking with the presence of a plus lens, then it is not a good idea to offer IOL monovision to this patient, even if there are no other contraindications. The plus lens test may not be absolutely necessary and critical,5,33 but it is advisable as part of the preoperative consultation.

4.2.10 Is a Contact Lens Trial Test Necessary for IOL Monovision?

Many surgeons do a contact lens trial, mainly in patients being evaluated for monovision laser refractive surgery,^{34,35,36,37,38} with a very understandable rationale: to be cautious since surgically induced monovision is hard to reverse if a patient is not a good candidate.

Reilly and colleagues³⁷ reviewed 82 patients who elected to undergo surgical monovision with LASIK between 2000 and 2003. Thirty of the 82 underwent a contact lens trial of monovision before the surgery and none of them elected monovision reversal. There were 52 patients who did not undergo a contact lens trial prior to the surgery; 2 of them underwent monovision reversal. These findings were similar to a study done by Goldberg,³⁶ who found that 5 of 114 monovision patients who elected to have their monovision reversed had not had a trial of contact lens monovision or a history of contact lens monovision correction. Although it may seem intuitive to offer patients contemplating monovision a trial of contact lens monovision correction, some might argue that the reversal rate is so low that a trial is not warranted.

IOL monovision is different from laser vision–induced monovision. Without a contact lens trial, the IOL success rate can still be very high. A study by Greenbaum⁵ with 140 IOL monovision patients demonstrated a 92% (129 patients) success rate. There was no contact lens trial performed prior to the decision for monovision. Of note, for the patients in that study, the near vision eye was aimed at –2.75 D and the distance vision eye was aimed at emmetropia, although there was no average anisometropic level between the two eyes listed in the paper. Many other pseudophakic monovision studies, also with high patient satisfaction and spectacle independence rates, but lower levels of anisometropic level than 2.75 D, did not have contact lens trials prior to cataract surgery.^{2,3,4,6,7,33,39,40}

In individuals with myopia or hyperopia without significant astigmatism, a contact lens trial should be relatively easy. However, contact lens trials can be difficult or not realistic if there is severe astigmatism, or if the patient is unable to use a lens due to a systemic or ocular condition or advanced age, if the patient does not want to go through the process, or if the cataract is dense with very poor vision. The process of a contact lens trial itself can be difficult and it can be discouraging for most elderly patients if they never used contact lenses before.

For many years, when I (F. Z.) received questions from surgeons who had just started to do pseudophakic monovision, a frequent question was “is it necessary to have a contact lens trial prior to the surgery?” My answer was typically “It is reasonable, but not necessary.” It is probably reasonable or even advisable to conduct a contact lens trial for those patients with demanding personalities, and those patients who are not willing to consider glasses after surgery if they are not able to tolerate postoperative IOL monovision. With dense cataracts and poor vision, the accuracy is questionable. The pseudophakic monovision patient population is clearly different from the laser vision correction monovision group. Among several thousand IOL monovision cases over two decades, I (F. Z.) have had only two patients who had a contact lens trial prior to decision-making. The first case was a psychiatrist who had a history of high myopia, retinal detachment, monofocal contact lens monovision 15 years earlier, and then multifocal contact lenses prior to cataract surgery. She was a candidate for either monofocal IOL monovision or multifocal IOLs since she had monofocal contact lens monovision and multifocal contact lenses before and was happy in both situations. My first choice was monofocal IOL monovision rather than multifocal IOLs due to her relatively young age, imperfect macula on optical coherence tomography (OCT), high level of corneal astigmatism (there was no combined multifocal IOL with toric IOL on the US market at that time), and her frequent need for nighttime driving. In order to make sure that IOL monovision would be her first option rather than a multifocal IOL, I had her go through a 2-week monofocal contact lens trial set by my optometrist colleague. She ended up as a happy IOL monovision patient. The second patient who had a contact lens trial already had a modest level of anisometropia IOL monovision, but she wanted to have full monovision with her near eye about –3.00 D to cover her reading. We typically discourage separating two eyes by more than 2.50 D, but that patient insisted on having 3.00 D anisometropia with a piggyback IOL, so a contact lens trial was ordered for her before the piggyback IOL decision. (See the case report “How Much Anisometropia Works Best?” later in this chapter.) Otherwise, without contact lens trials in two decades of practice, I only had to take two patients back to the operating room for a piggyback IOL, one to reverse monovision because I made a mistake in choosing the dominant eye for near (see the case report “Which Eye for Distance and Which Eye for Near?” later in this chapter) and one to enhance her from modest monovision to full monovision (see the case report “How Much Anisometropia Works Best?” later in this chapter). A contact lens trial for the first case would have been helpful to avoid the piggyback IOL, but not for the second case.

Most commonly used is the hole-in-card test.14,31 One can also use a camera (a disposable camera costs only a few dollars and it is very handy to keep one in each room).

Without dominant eye tests and detailed past ocular history, monovision will have a higher rate of failure or, even worse, risk of postoperative deterioration of already compromised extraocular muscle (EOM) balance or even fixation switch diplopia (see details in the section “Fixation Switch Diplopia” later in the chapter).

Plus Lens Mimic Test

This test is also very simple and should be part of the preoperative consultation. A trained assistant can do this test well. As mentioned earlier, this test has two functions, one as a sensory dominance test for the clinician and one as a mimic tolerance test for the patient. With this mimic test, the patient will have an idea of what to expect as the postoperative status and it helps the patient to have realistic expectations. The patient will know that the uncorrected distance vision of the near eye will not be as good as the uncorrected distance vision of the distance eye. This test will also give you an idea if the patient has any issues in terms of anisometropic imbalance when sitting and walking.

Cover and Uncover Test

This test should not be missed. It tells you the basic EOM condition. It is also advisable to check all of the main gaze positions, including head tilt to right and left, since some congenital fourth nerve palsies can be asymptomatic. This assessment may be difficult or impossible if the cataract is very dense in one eye or both eyes. Sometimes, if the patient’s vision is not good enough to allow fixation on the 20/400 chart, one can use a penlight as the target. Most IOL monovision candidates are able to perform this test. In those with very dense cataracts, either most do not have any desire to be spectacle independent (so no need to offer monovision) or they are not good candidates for IOL monovision, such as those with longstanding unilateral traumatic cataracts.

The above-mentioned three tests are highly recommended for every prospective IOL monovision candidate. Some tests are very helpful and become absolutely necessary in certain specific conditions. If you suspect a patient may have mild amblyopia or monofixation syndrome, or if cover and uncover testing reveals some phoria, then a stereo test, W4D test, and 4-diopter prism base-out test will be very helpful. (See further discussion in the sections “Potential Contraindications and Concerns for Pseudophakic Monovision” and “How to Detect Subtle Contraindications” later in the chapter.) If a patient with a history of successful multifocal contact lens use now wants to have IOL monovision, a monofocal contact lens trial is logical.

4.2.12 Summary of Preoperative Tests

The hole-in-card is the most commonly used sighting dominance test. The plus lens is the most commonly used sensory dominance and monovision mimic test for pseudophakic monovision. Sighting dominance test results are not fixed but can be plastic; they can change as vision or refractive status changes. A sighting dominance test prior to surgery is highly recommended since it can provide guidance in terms of conventional versus crossed IOL monovision. Plus lens sensory dominance and monovision mimic tests are valuable and advisable as part of preoperative consultation. The cover and uncover test should never be missed if it is possible to perform for IOL monovision candidates. It is reasonable to conduct a contact lens trial in specific cases, but it is not necessary for most routine cases.

If a patient absolutely wants to be 100% glasses free and is not willing to consider backup glasses, then do not offer monovision or any premium IOL. No one can offer a 100% guarantee. In my early career, I avoided offering IOL monovision to those marked as “perfectionist” on our survey sheet, but I have felt very comfortable for the last few years since I started using a “priority” strategy. (See details in the section “Which Eye to Operate upon First?” later in the chapter.) Basically, when this strategy is used, we have two chances to achieve the priority goal. By far, we do not need to change the preoperative plan if we have accurate biometry and deliver seamless surgery in the overwhelmingly majority of patients, but it is always a good idea to have a backup second chance. I do not recall any unhappy patient with a demanding personality since I have used this priority strategy for the last few years.

Generally speaking, astigmatism has a negative impact on monovision performance and spectacle independence.41,42,43 Astigmatism of 0.50 D or more in the dominant eye is less likely to yield very successful IOL monovision. The nondominant near vision eye, however, allows greater astigmatic tolerance. We have sometimes noted that patients still have good spectacle independence when the dominant eye has good distance vision and the nondominant near eye has residual astigmatism at the 1.00 to 1.50 D level. A study with added astigmatism in the spectacle plane in pseudophakic patients suggested that the allowable limit for astigmatism was in the range of 1.0 to 1.5 D for functional distance and near vision in both multifocal and monofocal IOL patients.44 Some studies suggested an acceptable amount of astigmatism of 1 D or less for IOL monovision.^2,3,33 A study by Greenbaum⁵ reviewed 140 IOL monovision patients with a 92% (129 patients) success rate. Two diopters of preoperative astigmatism was listed as an exclusion criterion from enrollment for that study, although no postoperative residual astigmatism information was given in the paper.

A study documented the fact that distance acuity significantly deteriorated with uncorrected astigmatism, but near acuity improved with uncorrected myopic astigmatism and deteriorated with uncorrected hyperopic astigmatism.⁴⁵ How do we interpret this? Well, if you are presbyopic, put your correction for distance in a trial frame, and then test your reading. You will appreciate if you have an extra –1.00 D cylinder, but not + 1.00 D cylinder. The same study also suggested that together with the uncorrected myopic astigmatism, higher-order aberration improved near vision function in pseudophakic eyes.

If a patient wants to have IOL monovision but does not want to correct corneal astigmatism (for whatever reasons, usually due to the cost) and you find that the dominant eye has significant astigmatism which is not at or near your main incision location, you might better talk him or her out of monovision simply due to the potential effect of astigmatism on the final result. Although study⁸ at Moorfields in 2009 suggested that the impact of astigmatism on reading-spectacle independence was smaller than the impact from hyperopia, meaning hyperopia causes more difficulty than astigmatism in terms of reading glasses, astigmatism is still a significant factor for glasses wear. As part of the Gutenberg Health study, a population-based cross-sectional study was conducted in the general population of Germany.⁴⁶ That study⁴⁶ had a total of 13,558 participants (49% female) with a mean age of 54.0 years (range, 35–74 years). The prevalence of refractive astigmatism (> 1.0 D) was 13.0% in right eyes and 12.0% in left eyes, and 85% of these participants wore distance spectacles.

Besides blurring the image, the effect induced by astigmatism may be due to meridional interocular suppression. This is more obvious for the dominant distance eye. The impact of astigmatism on the visual and optical system can also be associated with aniseikonia. Tolerance for aniseikonia varies from patient to patient. It is when the difference in image size or meridional distortions approaches the patient’s tolerance that symptoms of aniseikonia become manifest and troublesome. Meridional distortions are more poorly tolerated, especially when they are oblique (not horizontal or vertical).⁴⁷

Once surgery is done on both eyes, it is wise to observe the patient’s function for a few months before considering surgical correction if there is significant residual astigmatism, especially if the residual astigmatism is in the near eye. (Typically, postoperative corneal topography is not billable, but we have found that it is important to do it on every patient who had a limbal relaxing incision [LRI] to find out the residual astigmatism, although we do not file a charge for it. It is also helpful in personalizing one’s own LRI nomogram.) These patients often do well without further management while still maintaining reasonably good vision and satisfactory glasses independence. Simple LRI or laser vision correction can be used when the residual astigmatism needs correction. For toric implant patients, the intervention should be within the first few weeks to the first few months after the original surgery.

4.4.1 Summary of IOL Monovision and Astigmatism

Generally speaking, the less residual astigmatism the better, although it is quite reasonable to consider leaving a small amount with the rule cylinder due to aging change in postoperative years, especially in younger patients. Correction of astigmatism is critical for the dominant distance eye; residual astigmatism < 0.50 D seems to be essential to get good patient satisfaction and spectacle independence. The reading eye has greater astigmatism tolerance.

4.5 How Much Anisometropia Works Best?

We would recommend the following classification for Pseudophakic monovision in terms of focal length separation between the two eyes:

• Mini (sometimes referred to as micro or nano), –0.50 to –0.75 D.

• Modest (sometimes referred to as medium), –1.00 to –1.5 D.

• Full (sometimes referred to as traditional or classical), –1.75 to –2.5 D.

Spectacle independence after cataract surgery targeting bilateral emmetropia with monofocal IOLs is known to be low, in the range of 1 to 11%.48 Another study noted that overwhelmingly most postoperative cataract patients needed to wear reading glasses (160 out of 169 patients) if both eyes were targeted for a plano to –0.50 D spherical equivalent.⁸

IOL monovision aims to create intentional anisometropia that increases depth of focus. How much focal length separation between the two eyes will be the best? There is no consensus regarding this concern. It varies depending on the clinician’s experience and preferences as well as the patient’s needs. Generally speaking, –1.25 to –1.50 D defocus is the preferred arrangement.^{4,16,49,50,51,52,53,54} It is worth keeping in mind that pseudophakic accommodation can typically add 1 D to help near vision. More than 1.50 D can have a more noticeable impact on stereovision and contrast sensitivity.

The majority of mini-monovision patients (anisometropia less than 1.00 D) do well for far and intermediate focal distances, but usually need some help for reading, especially for prolonged periods. Labiris et al did a randomized study³⁹ with 38 mini-monovision patients and 37 multifocal IOL patients in 2014. For the monovision group, the dominant eye was aimed at –0.50 D and the nondominant eye was aimed at –1.25 D, with an average anisometropia level of 0.80 D. The spectacle-free rate for the monovision group was 31.4%. A study by Wilkins et al³³ found a 25.8% spectacle-free rate when the average power of the near eye was at –0.92 D.

High spectacle independence and patient satisfaction are achievable for most patients in both the conventional and crossed pseudophakic groups, which was demonstrated in a study by Zhang et al.⁴ It is a common assumption that low anisometropia between the two eyes has a higher chance of needing glasses, but that study suggests that a modest anisometropia level (average about 1.15 D) can still achieve a high patient satisfaction rate (more than 95% were “happy” or “very happy” among 60 participants in that study) with both conventional and crossed IOL patterns; the spectacle independence rate was also impressive: only 1 patient among the 30 conventional IOL monovision group needed reading glasses “all the time” and 3 patients among the 30 crossed IOL monovision group needed reading glasses “all the time.” For distance-related activities, 86.6% in the conventional group “never or only occasionally need glasses” and 90% in the crossed IOL monovision group “never or only occasionally need glasses.”

A study by Fawcett et al⁵⁵ with 32 adults who had post-LASIK/PRK monovision after more than 6 months of follow-up demonstrated a difference of stereoacuity in the two groups: low anisometropia (< 1.50 D) versus moderate anisometropia (1.50 D or more). Median stereoacuity values were 100 seconds of arc for patients with low anisometropia and 150 seconds of arc for patients with moderate anisometropia.

Our own 10-year IOL monovision review found complete spectacle freedom in 36.2% of a low anisometropic group (0.75–1.0 D, mean: 0.88 ± 0.12 D), 46.9% in a modest anisometropic group (1.01–1.50 D, mean: 1.31 ± 0.13D), and 40.9% in a high anisometropic group (> 1.50 D, mean: 1.93 ± 0.25 D), although there was no statistically significant difference among these results. Modest level anisometropia (mean, 1.31 D) had a higher rate of complete spectacle independence than either the low (mean, 0.88 D) or the high anisometropia group (mean, 1.93 D). That result was not expected because we previously assumed that a higher anisometropia level would have a higher rate of glasses freedom. In that study, the > 1.50 group had a higher rate of needing glasses for nighttime driving; this was probably the main reason why complete spectacle freedom was lower in the high anisometropia group (mean, 1.93 D) than the modest group (mean, 1.31 D). The rate of needing glasses all the time for nighttime driving in that study was 21.9% in the high group, 15.3% in the modest group, and 14.8% in the low group. There was no statistically significant difference in the trend p-value (0.438).

Monovision has some inevitable limitations. Once the near eye is –1.50 D or more, the intermediate vision, stereovision, and contrast will show a more noticeable negative impact. Ambati et al²¹ noted that stereovision decreased greatly when the blur was more than 1.75 D. A study by Loshin et al⁵³ demonstrated that, up to a moderate anisometropic level of 1.50 D, monovision blur appeared to have a limited negative impact on high-frequency contrast sensitivity and binocular summation, but as add power increased, the whole frequency range was involved and binocular summation significantly decreased.

An experimental study was conducted by Schor et al56 with different anisometropic levels at 1.25, 1.75, and 2.50 D in different luminance testing environments to mimic nighttime driving. The study revealed that interocular suppression of blur was greater with lower level anisometropia, which means better monovision function, which was less likely to produce asthenopia. Five binocularly normal participants in an experimental study by Simpson⁵⁷ suggested that anisometropic blur induces a central suppression scotoma in the defocused eye. The suppression scotoma was shown to increase in size as the degree of anisometropia increased.

Pardhan and Gilchrist⁵² studied 10 young healthy participants aged 17 to 28 years with a + 0.50, + 1.00, + 1.50, + 2.00, + 2.50, + 3.00, or + 3.50 D defocusing lens placed over one eye to compare contrast sensitivity. That study demonstrated that the maximum contrast sensitivity was in the absence of any monocular defocusing, with full binocular summation, about 42% higher than the monocular value. Monovision up to 1.50 D would still show binocular summation, but the binocular summation disappeared when the monocular defocusing power reached 1.50 D. The study also showed a phenomenon called binocular inhibition, defined as a condition where binocular is lower than monocular sensitivity. Binocular inhibition did not seem to occur until the defocusing add reached 1.50 D. As the power approached + 2.50 D, the binocular inhibition reached its maximum level. As the power of the defocusing lens increased, the binocular sensitivity reverted back to the monocular level, indicating suppression of the defocused eye. Binocular inhibition was also noted objectively in a study done by Trick et al⁵⁸ in which 10 normal young participants were tested with visual evoked responses (VERs) with different levels of neutral density filters. In a typical binocular situation, both eyes view the same stimulus pattern and the amplitude of the binocular VER is approximately 1.4 times larger than the amplitude of either monocular response, but when the luminance difference between the two eyes was large enough, the amplitude of VER of both eyes became smaller than the amplitude of either monocular VER. These interesting studies remind us to consider the threshold or zone for our IOL monovision practice: which level of anisometropia would be most likely to give our patients the best balanced visual function?

Stereopsis and both photopic and mesopic sensitivity decreased as the add increased from 0.75 to 1.75 D in an experimental study of 82 simulated pseudophakic monovision patients by Hayashi et al.⁵¹ The same study suggested 1.50 D anisometropia as the optimal refraction for IOL monovision based on the balance of vision for distance, intermediate and near, stereopsis, and contrast sensitivity.

With optical analysis, we can demonstrate the difference in the not-sharply-focused-zone (NSFZ) between modest monovision and full monovision. If OD is plano with 20/20 vision and OS has –1.50 D IOL monovision in a 60-year-old bilateral pseudophake, assuming a pseudophakic accommodating power average of 1.00 D, the length of the NSFZ is 33.3 cm, from 100 to 66.7 cm. However, if we give OS –2.50 D IOL monovision for the same patient, the NSFZ will be 60 cm, between 100 and 40 cm. (See the section “Optics and Neurophysiology of Pseudophakic Monovision” in Chapter 2 for details.)

Duke-Elder and Abrams claimed that each 0.25 D difference between the refraction of the two eyes causes 0.5% difference in size between the two retinal images and a difference of 5% is the limit which can usually be tolerated with ease.⁵⁹ They also mentioned that patients might experience discomfort when they have more than 2.00 D anisometropia due to an artificial heterophoria.

A Case Report

A female patient of F. Z. was 50 years old when she came to have a cataract evaluation in 2002. She had a posterior subcapsular cataract (PSC) in her left eye. Her left eye was dominant. Left eye cataract surgery was performed in January 2003 with a 20/20 uncorrected distance vision result. Her right eye, also with a PSC cataract, was operated on in May 2007. It was aimed at –2.15 D but ended up at –2.75 D. Manifest refraction did not show significant astigmatism. Vision of her right eye was 20/20 with –2.75 D. Overall, she has been happy without glasses or contact lens for most daily activities, especially for reading and her job as a dental hygienist. The only time she needs backup aid is for nighttime driving. She uses both glasses and contact lenses as single vision for distance as a backup, but she much prefers to wear a contact lens for her right eye for nighttime driving rather than use her backup glasses. It is likely that it is more comfortable for her to wear a contact lens for her right eye than to wear a pair of glasses for her right eye correction of –2.75 and left eye correction of plano due to the difference of aniseikonia. At 2.75 D, aniseikonia would be 6% with spectacles but only 0.5% with contact lenses. At her visit in February 2017, a prescription for backup glasses was given: plano on top OU with + 1.50 bifocal OU because of her complaint of not being able to clearly see the dashboard when she was driving at nighttime with her contact lens in her right eye. She does not need to wear any glasses or contact lens during the daytime. Retrospectively, I wish I had aimed her right eye at –1.50 D only, since arm’s length vision was the main need for her job. I predominantly used traditional full monovision at a level around 2.00 D in my early career years.

A study in 2002 by Greenbaum5 consisting of 140 IOL monovision patients (120 with visually significant cataract and 20 with clear lens exchange for high ametropia) demonstrated a 92% (129 patients) success rate. The subjective criterion for success was patient acceptance and satisfaction with IOL monovision at a 6-month or 1-year final interview. The objective criteria were uncorrected distance vision of 20/30 or better and uncorrected near vision J1 or better. In that study, the dominant eye was aimed to be emmetropic and the nondominant eye was targeted at –2.75. The spectacle dependence rate for distance was 5.8% and for near was 8.4%. There were no stereovision or contrast tests reported in the study.

In a prospective comparison study² in 2011 between IOL monovision and multifocal IOLs, we found that most near-full monovision patients did very well: 77% (17/22) never needed reading glasses; 81.8% (18/22) were “very happy” and “absolutely would recommend it to family members.” In terms of overall patient satisfaction and significant visual complaints, the monofocal IOL monovision group did better than the multifocal IOL group. The average refractive power in the near vision eye in the monovision group was –1.92 D, with a median of 1.88 D.

A Case Report

A 65-year-old well-educated woman whose occupation involved doing payrolls presented in 2014 for cataract evaluation. She complained of decreased vision in both eyes, especially when driving. Her uncorrected distance vision was OD 20/200 and OS 20/100; uncorrected near vision was OD J1 and OS J1 +. Her corrected distance vision was OD 20/40 and OS 20/30. Preoperative refraction was OD –2.75D and OS –2.25D. Hole-in-card and camera tests showed that the right eye was dominant. Past ocular history and the cover and uncover test revealed no contraindication for IOL monovision. She had been wearing a contact lens for OD only for 15 years and her OS was her near eye without a contact lens. She wanted to keep her monovision. Her three main hobbies were cooking, biking, and shopping. The decision for her refractive goal was made based on these main hobbies: OD plano and OS –1.25 D. The right eye was done in June 2014 and left eye in July 2014. She came back 2 years later, with OD 20/20 plano and OS 20/20 –1.00 D sphere. Near vision without correction OD was J16 and OS J7. She wore + 1.50 D readers. She was not happy with the reading ability of her left eye. She wanted to know if she could get rid of the readers. “Make my left eye stronger so I do not have to wear readers.” To make sure that full permanent monovision was what she really liked, a trial contact lens was offered. She preferred + 1.75 D over her left eye rather than + 1.50 D. Popular piggyback IOLs with 0.50 D increments, such as the Staar AQ5010, was not available anymore, so we had to choose either a + 2.00 D or + 3.00 D Alcon MA60MA 3 piece sulcus lens. A + 2.00 D lens would give a refractive outcome of around –2.25D and a + 3.00D lens would give an outcome between –2.75 D and – 3.00D. She strongly preferred + 3.00 D. “With the contact lens of + 1.75 and my –1.00 myopia, I can read very well, so near –3.00 will be just fine for me.” (She was amazingly familiar with the optics.) Surgery was done in November 2016 (F. Z.’s second monovision-related piggyback IOL in 20 years). Postoperative uncorrected distance vision was OD 20/20, OS 20/400; corrected distance vision was OD plano 20/20, OS 20/20 with –2.75. Uncorrected near vision were OD J16 and OS J1 +. Stereopsis and contrast sensitivity tests showed significant decreases comparing full monovision with the piggyback with modest monovision prior to piggyback ( Table 4.1 and Table 4.2).

Table 4.1 Preoperative and postoperative stereopsis comparison

Despite the downside from full monovision of 2.75 D, she is very happy with her overall condition and does not have any complaints. However, when she was asked if she noted any other differences between prepiggyback and postpiggyback, she acknowledged that she has to use backup glasses more often for nighttime driving and also has to get closer to her computer. That situation fits what we discussed in the section “The Not-Sharply-Focused-Zone in Full Monovision” in Chapter 2.

4.5.3 A Customized Approach Is Preferred

Generally speaking, “one-size-fits-all” is not the best approach for IOL monovision refractive cataract surgery. The best approach is a customized plan to match each patient’s daily life needs. It is more important to make the patient happy than to have a higher spectacle independence rate. If a retired patient loves outdoor activities and computer work, but does not do much small print reading and does not mind backup readers, then a modest monovision plan with the dominant eye at plano to –0.25 D and nondominant fellow eye at around –1.00 D to –1.25 D will be excellent for that patient. IOL mini-monovision did not seem to cause any problems in our practices as long as the patient understood the possible need and was willing to use backup readers for small print. If a patient requests to be glasses free for both far and near with reading as the priority goal, then aiming the nondominant eye at –1.50 to –2.00 D will be very appropriate. Full monovision will offer a chance of being completely glasses free for almost all daily activities except nighttime driving. These patients can be very happy with their visual function. The downside is that, as the anisometropia level increases, stereovision decreases, especially if the nondominant eye ends up more myopic due to biometry errors. Then the patient will be more likely to need glasses or contact lenses more than originally hoped to achieve comfortable bilateral vision.

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