Small-Aperture Strategies for the Correction of Presbyopia

H. Burkhard Dick


Curr Opin Ophthalmol. 2019;30(4):236-242. 

In This Article

Part II: Small Aperture Optics Topics of Interest

Effects of Reduced Light Transmission

It has been proposed that reducing the amount of light reaching the retina through a small aperture could reduce retinal image quality or cause an unwanted reduction in contrast sensitivity.

Trindade et al. for example, theorized that patients implanted with the Morcher add on small-aperture device would struggle in low-light conditions. However, only one of the 21 patients in their prospective case series complained of reduced acuity under low-light conditions.[7] The authors surmised that the Stiles-Crawford effect, in which pupil luminance is not proportional to the pupillary area because of differing degrees of brightness from central and peripheral rays, was responsible for the unexpected tolerance.

Using an adaptive optics simulator, Artal and Manzanera evaluated perceived brightness when small apertures (3.0 mm and 1.6 mm in diameter) were presented monocularly to the participant.[17] They found perceived brightness to be 1.24 to 1.51 times greater than what one would expect from theoretical calculations based on the aperture size. About 1/3 to 1/5 of the greater-than-expected perceived brightness effect could be explained by the Stiles-Crawford effect, but most of the effect was unexplained. Additionally, the authors theorized, neuroadaptation and binocular effects could further increase perceived brightness in real visual settings.[17]

In a later publication, these authors evaluated perceived brightness in the two eyes of patients implanted monocularly with Kamra small aperture inlays.[18] Patients implanted with the inlay exhibited an enhanced brightness perception compared to their untreated fellow eye -- again, larger than what could be expected due the Stiles-Crawford effect. Neural adaptation could be responsible.

A related issue is whether the discrepancy in light transmission between pupils of two different sizes matters when a small aperture is implanted or induced monocularly. The Pulfrich effect is an optical phenomenon characterized by a distorted perception of object motion induced by an interocular marked difference in retinal luminance. When this optical phenomenon is present, the path of a pendulum appears as an elliptical rather than a lateral movement. Reports suggest that any Pulfrich effect with a small aperture inlay is very small; patients seem to neuro-adapt to reduced illuminance in the treated eye. This would be expected to be the case with a small-aperture IOL, as well, although it has not been studied.

Ultimately, of course, researchers and clinicians want to know whether small-aperture optics affect contrast sensitivity, contrast acuity, or visual performance. Numerous studies have now shown some reduction in monocular contrast sensitivity under some lighting conditions or at some spatial frequencies, but minimal change to binocular contrast sensitivity. Elling et al. for example, reported discrete reduced contrast sensitivity under binocular mesopic conditions with glare in eyes with a small-aperture inlay, compared to the control group, but the difference was not statistically significant.[14] Small aperture inlays have previously been shown to provide better contrast sensitivity than multifocal IOLs. And, in the European multicenter trial of the IC-8 IOL, contrast acuity was equivalent between the IC-8 and monofocal IOL eyes.[1]

Monocular Versus Binocular

Contemporary applications of small-aperture optics principles have most often taken a monocular approach, with a device implanted in one eye to improve binocular near vision while maintaining binocular distance vision. However, given that small-aperture technologies have a minimal effect on monocular distance, does it make sense to use them bilaterally for ideal summation and clear focus at all distances?

Stereoacuity is a concern with any monocular solution. One author, testing young patients in dim light, found a deterioration in stereopsis at near and intermediate distances, as well as a loss of binocular summation at some distances in patients wearing a small-aperture contact lens, especially when combined with monovision.[19] These findings may or may not apply to real-world conditions in presbyopes with an implanted monocular small-aperture in better lighting conditions.

We know that monocular small-aperture surgery with a myopic target produces results that are quite different from monovision. Even small amounts of monovision (0.75 D) with a contact lens can reduce stereopsis, with the effect increasing with a greater degree of difference between the eyes. But, in a prospective study in 60 patients, there was no significant change in stereoacuity six months after monocular Kamra inlay implantation.[20]

My colleagues and I compared six cases in which patients undergoing cataract surgery were implanted bilaterally with the IC-8 IOL to 11 cases in which patients received the IC-8 in one eye and an aspheric monofocal in the fellow eye: Bilateral implantation of the IC-8 IOL resulted in an extended range of focus, with better intermediate and near vision than monocular implantation. However, satisfaction was higher in the monocular group.[21] Larger studies are needed to confirm which approach is superior.