proposal that the lens remains pliable with age and that presbyopia is due solely to lens growth and crowding that prevents optimum ciliary muscle action.
Glasser A, Kaufman PL. The mechanism of accommodation in primates. Ophthalmology. 1999; 106(5):863–872.
Schachar RA. Cause and treatment of presbyopia with a method for increasing the amplitude of accommodation. Ann Ophthalmol. 1992;24(12):445–447, 452.
Strenk SA, Strenk LM, Koretz JF. The mechanism of presbyopia. Prog Retin Eye Res. 2005; 24(3):379–393. Epub 2004 Dec 19.
Nonaccommodative Treatment of Presbyopia
Monovision
Currently in the United States, monovision is the technique used most frequently for modifying presbyopia in individuals with phakic eyes. In this approach, the refractive power of 1 eye is adjusted to improve near vision. Monovision may be achieved with contact lenses, laser in situ keratomileusis (LASIK), surface ablation, conductive keratoplasty, or even lens surgery. The process involves intentionally undercorrecting a patient with myopia, overcorrecting a patient with hyperopia, or inducing mild myopia in an individual with emmetropic vision. Historically, the term monovision typically referred to the use of a distance contact lens in 1 eye and a near contact lens in the other. A power difference between the 2 eyes of 1.25–2.50 D was targeted on the basis of near acuity demands. Currently, many refractive surgeons target mild myopia (–0.50 to –1.50 D) for the near-vision eye in the presbyopic and prepresbyopic population. The term modified, or mini-, monovision is more appropriate for this lower level of myopia for the near-vision eye. Mini-monovision is associated with only a mild decrease in distance vision, retention of good stereopsis, and a significant increase in the intermediate zone of functional vision. The intermediate zone is where many visual functions used for activities of daily life occur (eg, looking at a computer screen, store shelves, or a car dashboard). For many patients, this compromise is an attractive alternative to constantly reaching for reading glasses. Selected patients who want better near vision may prefer higher amounts of monovision correction (–1.50 to –2.50 D) despite the accompanying decrease in distance vision and stereopsis. Future directions in monovision may involve modification of corneal asphericity to improve depth of focus.
Patient selection
Appropriate patient selection and education are fundamental to the overall success of monovision treatment. Although monovision can be demonstrated with trial lenses in the examination room, a contact lens trial period at home is often more useful. Patients whose vision is neither presbyopic nor approaching presbyopia are typically not good candidates for monovision, as they are usually seeking optimal bilateral distance acuity. However, patients in their midto late 30s should be counseled about impending presbyopia and the option of monovision.
The best candidates for monovision are patients with myopia who are over the age of 40 years and who, because of their current refractive error, retain some useful uncorrected near vision. These patients have always experienced adequate near vision simply by removing their glasses and therefore understand the importance of near vision. Patients who do not have useful uncorrected near vision (myopia worse than –4.50 D, high astigmatism, hyperopia, or contact lens wearers) may be more accepting of the need for reading glasses after refractive surgery. For most patients, refractive surgeons routinely aim for mild myopia (–0.50 to –0.75 D, occasionally up to –1.50 D) in the nondominant eye. It is prudent to give the patient a trial with contact lenses to ascertain patient acceptance and the exact degree of near vision desired. Patients should understand that loss of
accommodation is progressive, so that monovision may not be permanent, and corrective glasses may eventually be required.
Reinstein DZ, Carp GI, Archer TJ, Gobbe M. LASIK for presbyopia correction in emmetropic patients using aspheric ablation profiles and a micromonovision protocol with the Carl Zeiss Meditec MEL 80 and VisuMax. J Refract Surg. 2012;28(8):531–541.
Rocha KM, Vabre L, Chateau N, Krueger RR. Expanding depth of focus by modifying higher-order aberrations induced by an adaptive optics visual simulator. J Cataract Refract Surg. 2009;35:1885–1892.
Conductive Keratoplasty
As discussed in Chapter 7, conductive keratoplasty (CK) is a nonablative, collagen-shrinking procedure approved for the correction of low levels of hyperopia (+0.75 to +3.25 D). The procedure is approved by the US Food and Drug Administration (FDA) for the treatment of presbyopia in individuals with hyperopic or emmetropic vision.
Multifocal IOL Implants
The IOL options for patients undergoing cataract surgery have increased in recent years. Patients may select a traditional monofocal IOL with a refractive target of emmetropia, mild myopia, or monovision; or they may opt for a multifocal or an accommodating IOL for greater range of focus.
The first multifocal IOL to be granted FDA approval in the United States has since been replaced by other lens designs originally including zonal refractive and apodized diffractive IOLs. The zonal refractive lens design utilizes refractive power changes from the center of the lens to the periphery to provide distance and near correction. In contrast, diffractive lens designs employ a series of concentric rings to form a diffraction grating (see BCSC Section 3, Clinical Optics) to create 2 separate focal points for distance and near vision (Fig 9-4). Some diffractive lenses are apodized, meaning that the diffractive step heights are gradually tapered to allow a more even distribution of light, which theoretically makes for a smoother transition among images from distance, intermediate, and near targets. Currently there are no zonal refractive IOLs available in the United States. Zonal refractive lenses, however, are available in a variety of styles in Europe. Examples of this type of lens include the Rayner M-flex T (Rayner Intraocular Lenses Ltd, East Sussex, United Kingdom) and the Lentis Mplus intraocular lens (Oculentis GmbH, Berlin, Germany) (Fig 9-5). In addition, IOLs with trifocal optics are available in Europe; examples are the FineVision (PhysIOL, Liège, Belgium) and the AT Lisa tri (Carl Zeiss Meditec, Jena, Germany).
Figure 9-4 Example of a diffractive multifocal IOL. Left, schematic of the frontal view. Right, schematic of the side view. (Left
image courtesy of Abbott Medical Optics Inc.)
