7 Limitations of Pseudophakic Monovision

Wheatstone1 was believed to be the first to explain stereopsis, in 1838: “… the mind perceives an object of three dimensions by means of the two dissimilar pictures projected by it on the retina.” Because each eye views the visual world from slightly different horizontal positions, each eye’s image differs from the other, forming horizontal disparity, or binocular disparity. Wheatstone showed that this was an effective depth cue by creating the illusion of depth from flat pictures that differed only in horizontal disparity. To display his pictures separately to the two eyes, Wheatstone invented the stereoscope, which is still used today.

A review of contact lens and laser vision correction monovision by Jain et al² noted that stereopsis in monovision ranged from 23 to 73 arc seconds for distance, and 50 to 113 arc seconds for near, with a mean of 87.5 for near. Unsuccessful monovision had 50 to 62 arc seconds reduction in stereopsis compared with successful monovision patients. The presbyopic population was noted to have a significant decrease in stereopsis tested with the Titmus test with near correction when compared with the prepresbyopic population, but no difference was noted between males and females.³ Another study, with 60 normal subjects aged 17 to 83 years, demonstrated that overall stereoacuity measured by all tests, including TNO and Titmus, showed a mild decline with age, and a significant drop off after age 50 with the Titmus test.⁴

Stereovision decreases when anisometropia increases.^5,6,7,8,9 A study by Fawcett et al⁶ of 32 adults with LASIK/PRK-created monovision and 20 age-matched controls, with more than 6 months of follow-up, demonstrated a negative impact on stereovision and sensory fusion due to longstanding induced monovision. The monovision patients were divided into two subgroups based on anisometropic level: < 1.50 D as the low anisometropia group (n = 18) and 1.50 D or greater as the moderate group (n = 14). For the monovision study group patients, a single contact lens was used to correct the near eye for distance vision of 20/20 or greater at 6 m and then reading glasses were used to achieve vision of 20/20 or greater at 33 cm for reading. With this binocular full correction, a Randot stereovision test was performed at near and Worth Four Dot fusion test at 33 cm for extramacular sensory fusion and at far (3 m) for macular sensory fusion. These tests were done within 15 to 20 minutes of contact lens placement. A statistically significant difference was noted when the study groups were compared with the control group: the median stereovision was 100 seconds of arc for the low anisometropic group, 150 seconds of arc for the moderate anisometropic group, and 40 seconds of arc for control subjects. For foveal fusion tested with the Worth Four Dot test at 3 m, all control subjects passed the test. Among patients with moderate anisometropia, 50% failed (p = 0.009). Among the low anisometropia group, 22% failed (p = 0.182). All subjects, both study and control patients, passed the extrafoveal Worth Four Dot test at 33 cm. This study not only suggested the impact of anisometropia level on stereovision and binocularity, but also suggested the susceptibility of fusion function in adults. Compromised binocularity may not recover immediately (within 15–20 minutes) even with full correction of anisometropia. The nature of binocularity can be plastic and the recovery may take longer. It would be very interesting if the same test were done again at 1 week and 3 weeks to compare with the results from the 15- to 20-minute test. Then, we would be able to tell if surgically induced longstanding monovision (at least 6 months in this study) has any permanent impact on stereovision and fusion, as well as how long the readaptation would take. No known study is available on intraocular lens (IOL) monovision in this regard. We routinely assume that backup glasses or contact lenses can fully recover binocularity immediately, but the real impact with accurate laboratory measurement is yet to be determined.

A study by Zhang et al10 noted that average stereopsis was 127 seconds of arc when the average anisometropia was 1.93 D in 22 IOL monovision patients. The impact of IOL monovision on stereopsis was noted to be minimal when mini-monovision was applied.¹¹ A study by Goldberg¹² of 432 consecutive LASIK patients (51% full distance vision correction, 49% monovision correction with anisometropic levels following the age-based anisometropia Goldberg monogram¹²) noted almost identical results from their survey between the two groups in terms of depth perception.

There is no doubt that monovision has a negative impact on fine stereopsis, but when it comes to gross depth perception in normal activities of daily life, the negative effect is minimal. Our own 10-year IOL monovision review with a de-identified survey noted that 94.3% had no problem or pretty much no problem; 5.2% consider “it does not bother me even when I do not wear glasses”; and only 0.5% had to wear glasses all the time to correct their depth perception problem. (See details in the section “What Benefits Can Pseudophakic Monovision Bring to Your Practice?” in Chapter 1.)

7.2 Induced Diplopia

Contact lens–induced monovision can induce some esophoric shift (from exophoria to orthophoria or to less exophoria, from orthophoria to esophoria or from less esophoria to more esophoria), but usually it is very small, averaging 1 to 3 prism diopters.^13,14 No effect was noted on vertical alignment.

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