3 Non-Pseudophakic Monovision

A study of 411 children by Weakley noted spherical myopic anisometropia of > 2.00 D or spherical hyperopic anisometropia of > 1.00 D resulted in a statistically significant increase in amblyopia and decrease in bifixation when compared to nonanisometropic patients.1 It was also noted that if there is anisometropia of > 1.50 D of hyperopia and > 3.00 D of myopia, it will be more likely to make a young child amblyopic.² In terms of monofixation syndrome, anisometropia > 1.50 D will put a child at a 50% risk of becoming a monofixator; > 2.00 D will increase the risk to almost 100%.²

Thus, those patients presenting as “natural monovision” in the office at cataract age with good foveal fusion and stereopsis without monofixation syndrome might not necessarily have been born with that same level of anisometropia. These patients are lucky from a glasses independence point of view for most of their lifespan. Evans³ also noted that those patients could do well without any glasses or contact lenses for both far and near. The dominant eye typically has good distance vision but the near vision eye may need some myopic lens correction if good distance vision is required. As they age, their crystalline lenses will change, which may cause their refractive status to change. The stereovision for these natural monovision individuals can be expected to be perfect or nearly perfect with 40 seconds of arc at near without glasses or contact lenses.

The clinical importance for these patients is twofold: the clinician should be cautious to make sure that no monofixation and amblyopia exists; and these may be the best candidates for intraocular lens (IOL) monovision when they need cataract surgery should they want to maintain a high level of spectacle independence.

As an example, I (F. Z.) have always had good vision. My uncorrected distance vision was 20/20 with my right eye and better than 20/15 (1.33 in decimal) with my left eye without glasses on my college entrance physical examination. My right eye probably had mild myopia but I never had a refraction before I became an ophthalmologist since I did not have any eye problem. In my 40 s, I noticed more myopic change in my right eye, while my left eye, the dominant eye, remained plano until my mid-50 s. Up to my mid-50 s, my right eye was –1.50 D sphere and my left eye was plano, and I was doing well with natural monovision without any readers until my mid-50 s. I did not have to wear reading glasses until age 57 when I noticed mild difficulty with small print and my refraction at that time was: right eye –1.00 D and left eye + 0.50 D, which I believe was due to my early cataract formation. My unaided Titmus stereovision was 40 seconds of arc at age 57. My unaided Worth Four-Dot test has always been normal at near as well at 20 feet. A cover and uncover test showed 4 PD exophoria at distance in primary gaze. I have worn single vision + 1.50 D readers for most of my regular size print reading since age 57. Over-the-counter + 1.50 D works just fine. I did wear a pair of single vision glasses for distance during my residency training at the Kellogg Eye Center of the University of Michigan in Ann Arbor in my late 30 s, mainly to bring the right eye to plano. I do have distance glasses when I am driving at night but I do not have to use them. For most nighttime driving, I do not use them, although I typically use them in bad weather conditions. I have never worn bifocals, since most of the time I do not need help from glasses for distance activities.

Contact lens monovision was practiced much earlier than IOL monovision, and many literature citations in this book are from contact lens monovision. Monovision was first proposed by Richard Westsmith, MD, of California in 1958 when he fit a + 1.50 D contact lens for his own left eye for reading, while his right was the dominant eye for distance without a contact lens. His unaided distance vision was 20/20 in each eye.4 The first reported clinical study of contact lens monovision was by Fonda⁵ in 1966, who also used different powers of add in his bifocals.

The advantages of contact lenses over spectacles are improved cosmetics, decreased peripheral distortion, less aniseikonia, and increased field of view. In spite of the compromise of slightly decreased stereovision, some studies^6,7 suggested that monovision contact lens correction was the most successful and most popular presbyopic management modality when compared with other presbyopic contact lens management, such as bifocal contact lenses. Evans³ reviewed more than 100 publications to evaluate the literature on the use of monovision for correction of presbyopia. The overall success rate with contact lenses was 59 to 67%. This is obviously lower than the rate from corneal laser vision correction monovision of 72 to 97.6%.^8,9,10 Contact lens wear itself is a dynamic process and contact lens handling can certainly have an impact on the success rate. Contact lens monovision has a higher rate of failure if we include the factor of contact lens intolerance. A literature review by Jain et al¹⁰ noted that the success rate increased from 69 to 81% if the contact lens intolerance factor was excluded. The variable success rates reported might be due to differing criteria used to define success and variations in study duration, ambiguous definitions of the base population, and lack of criteria for minimum contact lens–wearing times.

Contact lens–induced monovision was also demonstrated to be successful for management of symptomatic diplopia.^11,12,13

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