Article

WHEN ASTIGMATS BECOME PRESBYOPIC

What you need to know to successfully fit astigmatic patients who have reached presbyopia.

Today’s presbyopes do not want to give up their youthful appearance or compromise their active lifestyle by wearing spectacles. Instead, they generally insist that their eyecare practitioner satisfy their visual needs with the latest technology and advancements. Members of this population are near the peak of their earning power and are typically willing to pay extra for contact lenses that fulfill this demand.

In general, astigmatism can be challenging to correct with contact lenses. Adding the visual demands of presbyopia to the complexity of astigmatic contact lens fitting increases this challenge. Successfully fitting contact lenses that address both of these issues is professionally satisfying and potentially profitable.

THE PRESBYOPIC EYE

Understanding the normal physiological changes that the eye undergoes with age is required to successfully correct the vision of presbyopic patients. First, the pupil size becomes smaller with age. To maximize visual performance, certain multifocal or bifocal contact lens designs take pupil size into account (Bradley et al, 2014; Guillon et al, 2016).

Second, this demographic is susceptible to dry eye. Aqueous tear production and tear stability decrease with age. Meibomian gland dropout and eyelid irregularity (Figure 1) also increase with age, further contributing to tear film instability (Tomlinson et al, 2011). Additionally, common age-related conjunctival changes, such as pterygia or conjunctival redundancy, could further disrupt the tear film and interfere with the fit of contact lenses (Yam and Kwok, 2014). Proactively treating dry eye enables patients to wear their lenses comfortably.

Figure 1. Dry eye is a challenge when fitting presbyopes. Meibomian gland dropout and eyelid irregularity increase with age, contributing to tear film instability.

Age-related endothelial cell loss increases susceptibility to corneal edema (Duman et al, 2016). Thus, it is important to choose a contact lens material and thickness profile with oxygen permeability in mind. Other age-related factors may include decreased corneal transparency, crystalline lens changes, decreased retinal sensitivity, and reduced contrast sensitivity (Grossniklaus et al, 2013). Finally, lid tonicity decreases (van den Bosch et al, 1999), which can affect successful translation with certain GP contact lens designs.

PRESBYOPIC CORRECTION MODALITIES

Multifocal contact lenses offer presbyopic patients the most natural vision possible. They provide correction for two or more working distances and, if fitted successfully, will enable presbyopes to maintain their binocular vision. Multifocal lenses are available in two basic types: rotational and non-rotational designs (Figure 2). The key to successfully fitting multifocal lenses is understanding the differences in the designs and knowing when to employ which type for a particular eye.

Figure 2. Representations of both non-rotational (left) and rotational (right) multifocal designs with distance (D) and near (N) correction.

Rotational multifocal lenses consist of a central annular zone, which provides either distance or near power, surrounded by a peripheral annulus, which corrects either near or distance vision, respectively. The annular curves are spherical or aspheric in design. An aspheric design employs a more gradual flattening of curvature along the annulus that creates increasingly more plus power toward the lens periphery. A rotational multifocal lens can be manufactured with front, back, or front and back aspheric surfaces (Pérez-Prados et al, 2016). Because an aspheric design greatly flattens the peripheral curves, back-surface aspheric multifocals often need to be fit steeper than K to maintain good centration.

Non-rotational multifocal lenses resemble multifocal spectacles, with a distance optic at the top and a near optic at the bottom (Figure 3). These lenses are meant to translate to allow patients to view through the lower reading portion. Non-rotational multifocal lenses are offered almost exclusively as corneal GP lenses that are stabilized with prism inferiorly. Translation is aided by either truncation and/or extra prism. Non-rotational GP multifocal lenses are more popular in segmented designs; however, there are a few concentric annular designs in which the distance optic is displaced slightly superior to the geometric center. There is only one translating soft bifocal contact lens available on the market.

Figure 3. Non-rotational multifocal lenses resemble multifocal spectacles, with a distance optic at the top and a near optic at the bottom.

Translating and non-translating lenses deliver vision differently. Translating lenses offer alternating vision in which lens movement allows patients to view through either the distance or the near portion of the lens (Pérez-Prados et al, 2016). The lower lid pushes the lens up on the eye when patients look down. This movement aligns the visual axis with the reading power of the lens.

In contrast, non-translating multifocal lenses provide simultaneous vision (also known also simultaneous images), in which patients view both distance and near prescriptions at approximately the same time (Pérez-Prados et al, 2016). Although patients report satisfactory vision with such designs, it is compromised due to degraded retinal image quality from focused and non-focused images being placed simultaneously on the retina (Gispets et al, 2002; Pujol et al, 2003). Center-near multifocal lenses are typically available in designs that do not translate on the eye, such as soft or scleral GP lenses. Center-distance multifocal lenses are available in both non-translating and translating designs.

Enhancing distance or near vision in either the dominant or the nondominant eye, respectively, is beneficial to some presbyopes and can be accomplished in a variety ways. One technique is to use different multifocal add powers and/or different designs in each eye.

Modified monovision is an option for presbyopes who need to maintain optimal distance vision. This is achieved using a single-vision distance lens on the dominant eye and a multifocal lens on the nondominant eye. In modified monovision, the multifocal lens placed on the nondominant eye will provide good near vision and acceptable distance vision.

Finally, a true monovision fit is also an option, in which the dominant eye is fitted with a single-vision lens that optimizes distance vision, and the nondominant eye is fitted with a single-vision lens that optimizes near or intermediate vision. Generally, however, as it counters the goal of providing balanced binocular vision, a monovision fit is not ideal (Josephson et al, 1990).

ASTIGMATIC CORRECTION

Understanding the different types of astigmatism and how astigmatic correction can be incorporated into the various multifocal lens designs will help streamline the contact lens fitting process. The following sections discuss contact lens correction in presbyopes who have residual, high, and irregular astigmatism.

Residual Astigmatism Residual astigmatism is the astigmatic refractive error that remains after a contact lens is placed on the eye to correct existing ametropia. When a spherical lens is placed on the eye, the residual astigmatism equals the difference in corneal astigmatism (the keratometry values from the anterior surface corneal topography) and the total refractive astigmatism of the eye (the manifest refraction).

Physiologic residual astigmatism commonly results from curvature refractive index changes from the posterior cornea and the crystalline lens. Characteristic cataract formation in maturing presbyopic patients contributes to lenticular astigmatism. Cortical cataract progression typically creates residual astigmatism; however, nuclear sclerotic lenses also create astigmatism (Tatham and Prydal, 2008). Additionally, the posterior corneal shape changes with age, resulting in residual astigmatism (Ho et al, 2010).

Uncorrected residual astigmatism contributes to unacceptable vision. This depends on the amount of residual astigmatism, patients’ refractive error, and their expectations for vision quality. Highly ametropic patients tolerate greater residual astigmatism compared to lower ametropes. Nonetheless, patients who experience greater than 0.75D of uncorrected residual astigmatism often complain of decreased vision (Bennett et al, 2014).

Types of lenses that correct presbyopic patients who have residual astigmatism include spherical multifocal soft and GP (both corneal and scleral) lenses, soft toric multifocals, front-surface-toric multifocal GP (both corneal and scleral) lenses, back-surface-toric multifocal corneal GPs, and hybrid (central corneal GP fused with a soft peripheral skirt) multifocals.

Spherical corneal GP multifocal, spherical soft multifocal, and hybrid multifocal lenses may be acceptable for astigmatic presbyopes if the level of uncorrected residual astigmatism is low and they can tolerate some amount of blurry vision. Many corneal GP and hybrid lenses are available in designs with front- and/or back-surface aspheric curves that create the add power needed for reading. Also, front-surface aspheric curves are commonly used to create add power in soft multifocal lenses. Of note, contact lenses with aspheric front surfaces improve subjective vision in some astigmatic patients; however, there is no evidence demonstrating any masking or correction of cylinder (Bennett et al, 2014). Figure 4 shows an aspheric corneal GP on an astigmatic and presbyopic eye.

Figure 4. An aspheric corneal GP on an astigmatic and presbyopic eye. Contact lenses with aspheric front surfaces improve subjective vision in some astigmatic patients.

Soft toric multifocal lenses correct residual astigmatism in presbyopic patients. A common design for this lens type is a front-surface center-distance or center-near multifocal lens with the astigmatism correction on the back surface. Although it is possible to correct residual astigmatism with soft toric multifocal lenses, rotation of the lens will reduce visual acuity.

Presbyopes who have residual astigmatism can also be corrected with front-surface GP multifocal lenses. In rotational scleral or corneal GP multifocal lenses, the reading add power is typically placed on the back surface while the astigmatic correction is stabilized on the front surface by either a prism ballast, peri-ballast, or back-toric peripheral curves. Front-surface torics are also available on non-rotational segmented GP and soft lenses, as they already utilize prism ballast for stabilization.

Rotational and non-rotational corneal GP multifocal lenses designed with a back-surface toric will correct residual astigmatism. Due to differences in index of refraction of contact lens plastics and the pre-corneal tear film, back-surface-toric designs will induce residual astigmatism. The ideal situation for a back-surface toric is when toric base curves improve the fit, the amount of astigmatism in the manifest refraction is about 1.5 times the amount of corneal astigmatism, and both the corneal and refractive astigmatism are at the same axis.

High Astigmatism For the purposes of this article, high astigmatism is defined as corneal astigmatism ≥ 2.50D. As GP lenses are able to correct corneal astigmatism, they are a great option for fitting these patients. Contact lens options for correcting presbyopes who have high astigmatism include spherical GP multifocal (both corneal and scleral) lenses, bitoric corneal GP multifocals, soft toric multifocals, and hybrid multifocal lenses.

Both rotational and non-rotational corneal GP multifocal lenses, depending on the design, will work optimally when well-centered or slightly lid attached. It is possible to successfully fit back-surface spherical or aspheric corneal GP lenses on borderline high-astigmatic corrections, such as those with less than 3.00D of corneal astigmatism or those with centrally localized astigmatism (not limbus-to-limbus astigmatism).

Nonetheless, in highly astigmatic corneas, a spherical-back-surface corneal GP will most likely result in a suboptimal fit. A spherical lens on a highly astigmatic cornea will cause problems such as lens decentration and induced flare from misaligned optics; corneal desiccation and staining from excessive peripheral clearance and lens rocking; and flexure-induced fluctuations in vision (Bennett et al, 2014). In most cases of high astigmatism, a bitoric design is ideal when prescribing corneal GP lenses. A bitoric corneal GP on a highly astigmatic eye offers better centration in comparison to a spherical back surface. Both rotational and non-rotational multifocal lenses are available in bitoric designs.

Hybrid multifocal lenses are also an option for correcting highly astigmatic presbyopes. As these lenses have a soft skirt, centration is not typically an issue. Nevertheless, the GP portion of the lens is susceptible to flexure on highly astigmatic corneas with limbus-to-limbus astigmatism. Induced flexure in hybrids, however, can sometimes be remedied by increasing the center thickness of the corneal GP section of the lens. Multifocal hybrid lenses are commonly available in a rotational multifocal design with a front-surface center-near optic.

Some scleral lens designs are manufactured with rotational multifocal optics. These large-diameter lenses center well, which optimally places the multifocal optics in front of the pupil. As scleral lenses vault over the cornea, they correct high amounts of corneal astigmatism (Figure 5).

Figure 5. Some scleral lens designs are manufactured with rotational multifocal optics. Scleral lenses vault over the cornea and correct high amounts of regular and irregular corneal astigmatism.

It is possible for custom-made soft toric lenses to correct high astigmatism and presbyopia (Figure 6). In this lens design, a near-center or distance-center multifocal optic is placed on the front surface of the lens, while the back surface of the lens has the astigmatism correction. Again, with high astigmatism, small amounts of rotation in a soft toric lens will reduce vision.

Figure 6. Custom-made soft toric multifocal lenses correct residual and high astigmatism in presbyopes. The toric markers (arrows at 3 o’clock and 9 o’clock) in the periphery of the lens designate the stability of astigmatism correction.

Irregular Astigmatism Contact lenses are available to correct presbyopia in patients who have keratoconus or post-surgically altered corneas. Both corneal and scleral GP lenses are available with multifocal optics. The multifocal optic is commonly placed on the front surface of GP lenses for irregular corneas. The back surface of a corneal GP incorporates steeper base curves, spherical or aspheric peripheral curves, or reverse geometry peripheral curves to correct irregular astigmatism. Scleral lenses are also a great option for correcting irregular astigmatism because of their ability to vault over the cornea. Multifocal optics are placed either on the front or the back surface of a scleral contact lens.

SUMMARY

Presbyopic astigmats are uniquely challenging to correct with contact lenses. Recognizing the different types of astigmatism and how to add astigmatic correction to the variety of multifocal designs available is key to fitting this special population. CLS

For references, please visit www.clspectrum.com/references and click on document #256.