Presbyopia Treatments by Mechanism of Action: A New Classification System Based on a Review of the Literature

Figures & data

Table 1 Overview of Mechanisms for Correcting Presbyopia Subcategorized by Method, The Area of the Eye Where the Mechanism Has Effect (“M”), and Location of Application (“A”)

		Extraocular:
	Method	Non-Contact	Surface	Cornea-Sclera	Intraocular
VARYING THE REFRACTIVE POWER OVER TIME
Impermanent	Natural Accommodation				M^a
	Multiple glasses	A, M
	Pharma that stimulates CB		A		M
	Electrostimulation		A		M
	Pharmaceutical that softens lens		A		M
Permanent	Scleral laser micro excision			A, M
	FS laser to lens				A, M
	Scleral implants			A, M
	Accommodating IOL				A, M
VARYING THE REFRACTIVE POWER ACROSS THE VISUAL FIELD
Impermanent	Multifocal spectacles	A, M
	MF CTLs		A, M
VARYING THE REFRACTIVE POWER BETWEEN EYES
Impermanent	CTL Monovision		A, M
Permanent	Laser vision induced monovision (ie, LASIK, surface ablation, SMILE)			A, M
	IOL monovision				A, M
VARYING THE REFRACTIVE POWER ACROSS A RANGE OF DISTANCES
Impermanent	Pharmacological miotics	A			M
Permanent	Small aperture corneal inlay			A, M
	Small aperture IOL				A, M
	Presby LASIK			A, M
	Laser refractive indexing			A, M	A, M
	Refractive corneal inlays			A, M
	Refractive/HOA EDOF/MF IOLs				A, M
	Phakic IOLs (ICL)				A, M
	Diffractive EDOF/MF/TF IOLs				A, M

Notes: ^aNatural accommodation is a mechanism and is therefore not “applied” in any true sense.

Abbreviations: CB, ciliary body; CTL, contact lens; EDOF, extended depth of focus; FS, femtosecond; HOA, higher order aberration; ICL, implantable collamer lens; IOL, intraocular lens; MF, multifocal; TF, trifocal.

Table 2 Relative Advantages and Disadvantage of Differing Approaches for Varying Refractive Power to Achieve Functional Through Focus

Mechanism	Advantages	Disadvantages
Over time^a	Mimics natural mechanism Could achieve goal with no inherent side effects	Technically challenging Has not been successfully achieved
Across the visual field	Easy to implement Convenient Reversible	Potential visual intolerance/image jump Blurred inferior VF Requires neuroadaptation Potential for neck, eye strain Trip hazard
Between eyes	Easy to implement Convenient Reversible	Loss of visual quality, depth perception, contrast sensitivity, and stereo acuity Potential visual intolerance Requires neuroadaptation Trip hazard, especially with incomplete suppression of interocular blur Inherent limits in binocular summation (may require residual accommodation)
Across range of distance	Most similar to natural accommodation Extends DOF	Potential loss of quality and contrast Potential low-light dysphotopsias Can require surgical precision

Notes: ^aMechanism of natural accommodation.

Abbreviations: DOF, depth of focus; VF, visual field.

Figure 1 Varying the refractive power over time describes the eye’s natural ability to shift focus from far objects (person) at time 1 (t₁) to near (smartphone) at time 2 (t₂).

Figure 2 Varying the refractive power across the field requires optics that utilize two or more focusing zones. In this example, representing a bifocal correction, objects at far (person) are focused by an optic powered for far (orange rectangle), and objects at near (smartphone) are focused by an optic powered for near (grey rectangle). The viewer must either move the optic itself or physically adjust the eye gaze to utilize the appropriate viewing zone.

Figure 3 Varying the refractive power between eyes, or monovision, ultimately depends on binocular summation within the visual cortex of the brain. Objects at far (person) are focused by one eye (Eye 1) that is appropriately powered for viewing (indicated by a solid arrow) and a defocused image (indicated by a dashed arrow) is perceived by the fellow eye (Eye 2). The opposite is true for near objects (smartphone). The resulting image is reorganized via binocular neural summation to add focused and defocused images in the visual cortex.

Figure 4 Varying the refractive power across a range of distances (pseudoaccomodation) achieves simultaneous focus from far (person) to near (smartphone). In this illustration, an iris pinhole optic provides pseudoaccomodative ability.

Figure 5 Representative defocus curves associated with various methods for varying the refractive power across a range of distances.

Figure 6 Varying refractive power across a range of distances may be accomplished by one of three mechanisms. Small Aperture/Pinhole Optics (A) function to block peripheral unfocused light rays, while focused light rays reach the retina. Refractive Optics (B) utilize multiple focusing zones. Diffractive Optics (C) leverage interference patterns to create zones/ranges of focus from far to near.