Article

Aspheric Multifocal Gas Permeable CLs: Fitting, Evaluation, and Problem-Solving

Aspheric multifocal gas permeable contact lenses are an excellent design choice for correcting presbyopia with a contact lens. When properly fitted, these lenses can provide great vision at all distances. Thus, they’re the first choice for patients who have vocations and avocations that require clarity at a variety of distances. This article will discuss the various aspheric multifocal gas permeable contact lens designs that are available, the process for selecting the proper lens for each patient, and ways to evaluate the on-eye performance of the lens. Lastly, we will review problem-solving approaches to improve visual outcomes.

Designs

Aspheric multifocal gas permeable contact lenses are available in a variety of configurations. All of these designs fall under the classification of simultaneous image designs. This means there are multiple images being presented to the visual system at one time. The majority of these simultaneous image designs are center-distance, with a progression of plus power as the lens periphery is approached. This power change is accomplished by a flattening of the back surface as you move from center to edge. The rate of flattening is referred to as eccentricity, or simply the e-value. To work properly, the aspheric multifocal does need to translate up on downgaze, so that the power for near can be more easily accessed.

Early designs for the aspheric gas permeable multifocal contact lens were generally back-surface designs that were often fit 4 to 5 diopters steeper than the flat keratometry reading. This created a high eccentricity. While this approach provided good centration and vision, it frequently resulted in corneal molding. In these cases, patients could experience difficulty seeing clearly with their spectacles when they removed the contact lenses at the end of the day.

Modern lathing equipment has made it possible to design aspheric multifocal gas permeable contact lenses that have much lower back surface e-values, thus avoiding the unwanted spectacle blur associated with corneal molding. It is becoming increasingly popular to place the asphericity on the front surface of the contact lens. These manufacturing changes have allowed many different variations of the gas permeable multifocal to be generated. For example, it is possible to create bi-aspheric and multi-aspheric gas permeable lenses by placing aspheric curves on both the back and front of the lens. Also, an aspheric front surface multifocal can be put on just about any back-surface configuration of gas permeable lens. These advances in technology have made it feasible to correct high degrees of corneal toricity, while maintaining a stable visual result. It is even possible to correct any number of corneal irregularities, thus improving vision and quality of life for these patients, while at the same time providing a multifocal correction.

Patient Evaluation

To select the appropriate gas permeable multifocal design, it is essential to evaluate the motivation for wearing contact lenses, as well as the patient’s refractive and ocular health status.

Regarding motivation, some patients want freedom from eyeglasses for certain activities, while others want to have the vision they had in their youth. In either case, it is important to discuss what a gas permeable, aspheric multifocal lens can and cannot do. Patient education is a key factor. Patients must understand that presbyopia is a compromise. No contact lens will be perfect. However, if they’re realistic in their expectations, and they’re willing to work through the process, most of their needs will be met most of the time.

The patient examination should include a comprehensive history as both ocular and general health are important. Has the patient worn or attempted to wear contact lenses in the past? If so, why did they stop? Because of comfort or unsatisfactory vision? Were they soft or gas permeable contact lenses? A refraction must be undertaken to determine the proper prescription for optimal vision at distance and near. Keratometry and/or corneal topography should also be performed.

Next, an examination of the external eye is needed. The lids must be evaluated for tonicity. Good apposition to the globe for the upper lids is required to obtain good translation up on downgaze. Also, look at where the lids are positioned relative to the corneal limbus, both superior and inferior (Figure 1). Observe the lids for signs of blepharitis, and evaluate the meibomian glands. If the meibomian glands are clogged, some form of warming therapy would be prudent to try to increase tear retention. Lids scrubs would be good to help alleviate the blepharitis. This can be performed with mild shampoo and a facecloth, or with any one of the numerous lid hygiene products on the market.

Figure 1. Evaluation of the lid-to-limbus position is important in aspheric lens fitting.

An evaluation of tear quality and quantity is needed. Quantity can be gauged by looking at the tear meniscus on the lower eyelid. If you have a meniscus height of a millimeter or more, tear volume should not be a problem. If the meniscus is scant, it may be wise to perform a Schirmer’s test to assess production, and an osmolarity test to look for inflammatory markers. If issues are found, they should be addressed before beginning lens wear. There are two prescription products available to treat aqueous deficiency. It may also be helpful to perform a tear film break-up time test to evaluate the retention of the tears being produced. (This goes back to the condition of the meibomian glands). As a rule, a tear break-up time of 10 seconds or greater is normal. If the break-up time is between 5 and 10 seconds, contact lens wear can be more challenging. A tear film break-up time of less than 5 seconds could mean that eyeglasses are a better correction option, unless remediation can be achieved.

Closely associated with the tear film is the lid wiper located at the superior lid margin-tarsal conjunctiva interface. This can be assessed utilizing lissamine green dye and lid eversion. If the lid wiper area isn’t smooth, contact lens comfort, as well as general eye comfort, could be compromised. While the lid is everted, evaluate the superior tarsal plate. Look for signs of inflammation such as injection, with or without a papillary or follicular reaction. A certain amount of non-specific conjunctival reaction is to be expected, but if there are giant papillary changes, treatment should be undertaken before beginning contact lens wear. It is also helpful to note other conjunctival observations, such as pinguecula on the bulbar conjunctiva or concretions in the tarsal plates. Pinguecula are generally not an issue with corneal gas permeable lenses, but desiccation sometimes can occur at the corneal limbus adjacent to the raised area.

Also of importance is the presence of pterygia, which encroach on the cornea from the conjunctiva. Depending on the size of the pterygium, it could influence the fit and thus the visual performance of the contact lens. The cornea should also be evaluated for superficial punctate keratopathy (SPK), corneal dystrophies and degenerations, and any other irregularities. SPK could signal issues with dryness and/or lid closure abnormalities. There could be epithelial basement membrane dystrophy present, which may suggest recurrent corneal erosion difficulties. There may be scars from prior trauma or infection. Corneal vascularization, at any level of the cornea, should be noted. There may be clinical signs of keratoconus, such as thinning, a Fleischer ring, or Vogt’s striae. Lastly, the endothelium could exhibit evidence of corneal guttata, which need to be monitored for signs of corneal decompensation. Luckily, the high oxygen transmissibility of modern gas permeable contact lens materials generally minimizes hypoxic stress.

Next, look at the iris to assess the horizontal visible iris diameter (HVID) and the size of the pupils. Pupils should be evaluated in average room light and dim light. This information is important in selecting the overall lens diameter and the optic zone diameter of the contact lens. Pupils of 5 mm or less in average lighting conditions lend themselves well to aspheric multifocal gas permeable designs. In this situation, the pupil will be small enough to access the near power zones of the lens as the lens translates up in the reading position. Additionally, pupils of this size should not lead to flare and glare in dim light conditions, such as night driving. Very large pupils could create visual difficulties. In this instance, a translating, or alternating image design, would be a better option. Conversely, very small pupils could make accessing the power zones of an aspheric multifocal lens more difficult, and a translating design would again be preferable.

While less significant to the fit of the contact lens, the crystalline lens should be evaluated for any cataracts which could influence vision. Also, a dilated fundus examination should be performed to avoid missing potentially vision threatening conditions, along with signs of systemic maladies.

Contact Lens-Fitting Conundrum

There is much debate about which approach to fitting gas permeable contact lenses is better. On the one hand, there are the diagnostic fit proponents. These practitioners prefer to place a lens of known parameters on the eye and evaluate the physiologic fit and vision at the first visit. The thought is that the practitioner gets a better idea of the visual capabilities of the design in question, along with the ability to assess the lens-to-cornea fitting relationship. After over-refraction to determine the best vision, and a fluorescein evaluation to determine any parameter changes necessary, the final lens can be ordered. The problem with this approach is that the patient may not fully appreciate the visual benefit or the comfort that the final contact lens will provide. Additionally, with this scenario, the practitioner must have two to three different diagnostic sets to cover the various patient types encountered. Those in the empirical fit camp would rather use their favorite laboratory’s fitting nomogram to design the first lens based on the patient’s refraction and other physical characteristics. While this necessitates an additional visit, patients are able to experience a full correction of their vision, and the comfort of the best-fitted contact lens, with the first lens placed on their eye. Also, there’s no need for diagnostic lens sets. In truth, it probably doesn’t matter which approach is used. Modern gas permeable contact lens lathing technology produces great lenses, so practitioners and patients appreciate better vision and comfort than ever before.

Problem Solving

Even the seasoned practitioner can’t be successful 100% of the time. With aspheric multifocal gas permeable contact lenses, there can be issues with the fitting characteristics of the lens design chosen, as well as vision problems. As was stated at the beginning of this article, proper lens positioning and translation are key to achieving the best vision with a multifocal design (Figure 2). Improper lens positioning can lead to decreased comfort. So, if the patients are having issues with their lenses, listen to what they’re telling you. This will guide your approach to troubleshooting.

Figure 2. Shown is an optimally fitting GP aspheric multifocal.

If the complaint pertains to poor distance vision, it could be that the lens-to-cornea fitting relationship is not optimal, or that the lens power is incorrect. If the contact lens is riding high on the cornea in primary gaze, the intermediate, or even the reading power can be positioned into the visual axis. An over-refraction will reveal the need for more minus power at distance (however, the near vision will suffer). If the lens power is truly correct, when the lid is pulled away from the contact lens and the lens drops into its proper position, vision will be normal at distance. These actions confirm that the issue is with the fit of the lens. Then it is a matter of steepening the base curve, thinning the lens edge, or both, to achieve a better fit and improved vision. The lens diameter could also be decreased, but this might compromise the optic zone, thus affecting vision in a different way. On rare occasions, the lens may ride lower than expected. Again, if repositioning the lens makes distance vision better, flattening the base curve, enlarging the overall lens diameter, or adding a minus carrier lenticular should result in a more superiorly positioned lens and an improvement in vision.

If the complaint pertains to poor near vision, you again must discover if the power is off or if the lens is not positioning properly. Over-refraction may show the need for more plus power for near, but don’t be fooled by vision improvement alone. Look at the physical fit of the lens. If it fails to translate up on downgaze, measures to improve translation are in order, such as changing to a flatter base curve, increasing the overall lens diameter, adding a minus carrier, or a combination of these corrections.

Lastly, if the complaint is of poor comfort, evaluate the lens-to-cornea fitting relationship again. Fluorescein evaluation will show you a too steep lens or an overly flat lens. Both can compromise comfort, and the patient usually states that the center of their eye is irritated. If the lens edge is irregular, or if it is an edge lift problem, the complaint will be toward the periphery, and/or involve lid sensation. This is another area where your fluorescein evaluation will guide you. Changes to the base curve, the eccentricity value, and/or the periphery of the lens should eliminate the problem.

With any of these issues, laboratory consultants can help. They are experts with their design, and they have encountered every problem you can imagine. It is highly recommended to discuss any possible changes with them before re-ordering a lens.

Parting Thoughts

It’s safe to say that today’s aspheric multifocal gas permeable contact lens offerings are varied and very sophisticated. There are dozens of independent GP laboratories in the United States and Canada, manufacturing top-notch designs using state-of-the-art materials. For many patients, vision can be far superior with a gas permeable multifocal contact lens. All of us should embrace the opportunity to meet the growing need for presbyopic contact lens correction and be proactive with our patients. Challenge yourself and have some fun! ■