PSEUDOPHAKIC MONOVISION

Monovision with monofocal IOLs offers a viable modality for presbyopia correction in pseudophakic patients. However, what constitutes the desired refractions for pseudophakic monovision is a matter of controversy. According to Kristian Naser MD, surgeons should consider targeting -0.25 D pure sphere for the distance eye and -1.25 D pure sphere for the near eye as that strategy optimises the range of functional vision without introducing any significant downsides.

His recommendation was based on the findings from a mathematical model used to determine the binocular distribution of refractions yielding the least defocus over the most extended fixation interval and the subsequent confirmation of the theoretical prediction in a small clinical study (Næser K, Hjortdal JØ, Harris WF. Pseudophakic monovision:optimal distribution of refractions. Acta Ophthalmol 2013; doi: 10.1111/aos.12148. Electronic publication ahead of print).

“Our aim in conducting this research was to determine the distribution of refractions that yields not just mediocre visual acuity, but the best uncorrected visual acuity over the most extended fixation interval in patients with binocularly implanted monofocal IOLs,” said Dr Naser, Dept of Ophthalmology, Randers Regional Hospital, Randers, Denmark.

“According to our data, monovision with spherical refractions of approximately -0.25 D and -1.25 D may yield spectacle independence for distance and intermediate vision along with some near vision, although patients may need an add for close reading,” he said. Dr Naeser addressed the potential compromises with monovision. “Binocular problems, such as insufficient suppression of the defocused eye leading to haloes and glare at night, reduced stereoacuity and binocular inhibition should be minimal with this monovision strategy that creates just 1.0 D of anisometropia, and so we expect patients will not be seeking IOL exchange for optical reasons. And, if this moderate monovision is not tolerated, it can be fully reversed with spectacle correction because the induced aniseikonia is minimal.”

The mathematically-derived set of binocular refractions was determined through application of a formula for describing monocular defocus at any fixation distance as a function of refractive sphere and cylinder [Næser K, Hjortdal J. Acta Ophthalmol. 2011;89:111-115]. Applying the model to the binocular situation involved solving simultaneous equations to identify the combination of refractions that gave the smallest area under the individual defocus curves for the chosen fixation interval. Considering the fixation interval of 0.33m to 6.0m, -0.27 D pure sphere for the distance eye and -1.17 D pure sphere for the near eye were identified as the optimal refractions. “Astigmatism must be avoided at all costs, since any astigmatism will cause blur throughout the fixation interval,” Dr Naser said.

The clinical evaluation included 22 patients bilaterally implanted with an aspheric monofocal IOL who were overrefracted to have a spherical refractive error of -0.25 D in both eyes and then the desired monovision targets of -0.25 D and -1.25 D (rounded from the model for practical purposes). The results showed that with monovision, patients enjoyed good visual acuity (<0.2 logMAR) over a distance range between 7.0m and about 55cm and had no loss in distance visual acuity for the fixation interval from 6.0m to 33cm.