Silicone Hydrogel Lenses and The Presbyope
BY CRAIG W. NORMAN, FCLSA
The contact lens field is once again at a defining moment, a moment similar to that time in the mid-1980s when disposable soft lenses entered the scene.
Today, silicone hydrogel contact lenses promise improved corneal health through better oxygen exchange. What's more, continual research has brought us materials that are quite compatible with the ocular surface and the tear film.
With this combination, we can now offer patients safe, comfortable
options for daily and continuous wear. But what does this mean for our presbyopic
Age alone brings potential difficulties for contact lens wear among presbyopic patients. Dry eye is not uncommon in patients over 40, particularly postmenopausal women. In addition, poor tear quantity and quality due to aging may be exacerbated by medical conditions and the accompanying use of systemic medications. Near tasks, such as computer use, also contribute to dryness problems.
Tear instability has less to do with reduced tear volume and quality of the tear layer. It is more related to the strength of tears binding to the ocular surface and the efficiency of the blinking that re-forms the tear film. Surface-treated silicone hydrogel lenses provide good wetting characteristics and tear deposit resistance and, along with their lower water content as compared to conventional disposable soft lens materials, help to provide a "wetter" contact lens. This is even more important if overnight or continuous wear is chosen as a wearing schedule for the presbyope.
Another factor to consider is that many patients in this age group are long-term hydrogel lens wearers and may be suffering from corneal oxygen deficiency, resulting in limbal redness and neovascularization. Silicone hydrogel lenses have been proven to decrease these signs through improved corneal oxygenation.
Silicone hydrogels also provide a level of wearing convenience that many presbyopes are seeking the ability to safely nap or sleep in their contact lenses.
Extended or continuous wear can be particularly useful for patients such as hyperopes who have difficulty seeing up close during the lens application and removal process and for patients who have difficulty handling contact lenses.
Latent Hyperopes Appear
Many presbyopes interested in contact lenses for the first time are latent hyperopes manifesting the need for vision correction for the first time both at distance and at near. It's not uncommon to need monovision lens powers ranging from +1.00D to +4.00D in these cases. Since there's less average oxygen transmissibility across lenses with higher plus powers, there's a great change of corneal hypoxia among these patients. The greater oxygen exchange provided by silicone hydrogels is ideal for these patients.
Monovision for Now
Right now, monovision is the only silicone hydrogel option for correcting presbyopia in the United States. This may seem a step backwards for those of us who have embraced multifocal/ bifocal soft contact lenses (although a temporary one as will be discussed later), but for many practitioners, monovision is "business as usual." With reported success rates of 60% to 80%, monovision remains a successful correction option for presbyopes.
As recently as early 2005, Harris and colleagues1 reported that eyecare practitioners preferred to fit monovision rather than bifocal contact lenses. Studies such as this, combined with clinical experience, show monovision is still a viable alternative, particularly for emerging presbyopes.
In addition, the recently introduced silicone hydrogel toric designs will improve monovision success by correcting low amounts of astigmatism that we previously were forced to mask with spherical lenses.
Given the improved oxygen exchange, the lower water content and the surface wettability of silicone hydrogel contact lenses, practitioners definitely should consider them for patients who are successful monovision wearers.
Multifocals in the Future
Soon, we'll have more options for the presbyope. First, we'll see proven designs many of them familiar multifocal/bifocal designs switched into silicone hydrogel materials.
More intriguing is the possibility of new designs and the recasting of older styles. For instance, translating bifocal designs, which provided excellent vision but lacked adequate corneal oxygenation when made of conventional soft lens materials, are back on the drawing board for many researchers working with silicone hydrogels.
Fitting Pearl: Hydrogel to Silicone Hydrogel
|Some hydrogel lens wearers may experience subclinical hypoxia, resulting in "myopic creep." After being refitted into a silicone hydrogel lens of the same power, these patients may be overcorrected at distance, thus increasing their presbyopic symptoms. Make sure you follow up with these patients within a few days of refitting to evaluate the need for a power adjustment.|
Translating bifocal soft lenses, like their GP counterparts, are quite thick because of the ballasting needed to keep the lenses in proper position for a specific visual task. This thickness won't be as much of an issue for corneal health with silicone hydrogel lenses due to their significantly higher Dk/t. This should give us more options in "true" add powers and may even allow for trifocal styles that will be helpful to computer users.
Silicone hydrogel lenses are here to stay. As with any early generation technology, design choices may be limited right now, but we can easily add these lenses to our presbyopic offerings for monovision candidates.
Craig Norman is director of the Contact Lens Section at the South Bend Clinic in South Bend, Indiana. He is a fellow of the Contact Lens Society of America and is an advisor to the GP Lens Institute.
1. Harris MG, Kuntz S, Morris C, Zardo DF. Use of contact lens corrections in optometric practices. CL Spectrum 2005.4:42-46.