In 2009, silicone hydrogel lenses accounted for about two thirds of contact lens sales in the U.S.1
And, when you consider that more than 34 million Americans wear contacts, that’s a substantial number.2 SiHy lenses are becoming the “go to” lens choice for daily wear. Why? High-Dk silicone hydrogel lenses give patients a convenient, comfortable, healthy option for contact lens wear.
Speaking of comfort, one of the most challenging and complex diagnostic dilemmas in all of eye care is dry eye—especially for contact lens-wearing patients. A multifactorial disorder, the term “dry eye” represents myriad contributory elements, pathophysiologic processes and clinical presentations. But, the advent of silicone hydrogel lenses may help these patients continue lens wear comfortably.
Here is a brief rundown of the basics about silicone hydrogel lenses.
What Goes Into the Lens
The silicone in the SiHy lens has an impact on its rigidity and flexibility. The hydrogel component facilitates wettability and fluid transport, which aids in lens movement. Silicone hydrogel lenses generally have a higher modulus, and are therefore more rigid than standard hydroxyethylmethacrylate lenses.
Because silicone is a hydrophobic material, manufacturers are developing methods to make silicone lenses more hydrophilic. Some companies treat the lens surface with plasma to accomplish this, while others incorporate wetting agents that negate the hydrophobia of the silicone. Surface-based wetting agents increase lens comfort as well, and polysaccharide-based agents, such as hydroxypropyl methylcellulose and hyaluronic acid, can be added to rewetting drops to enhance the mucin layer and help keep the cornea wet.
Traditional soft contacts are made from hydrogel polymers—soft, water-containing plastics. Hydroxyethyl methacrylate (HEMA) is used to make the hydrophilic polymer. These lenses rely on the amount of water in the polymer to regulate how much oxygen can pass through the lens. In conventional contact lenses, increasing the water content increases the Dk/t and amount of oxygen that transmits to the cornea through the lens. With SiHy lenses, however, the Dk climbs as the percentage of silicone—not the water content—increases.
Oxygen transmissibility is just one benefit of silicone hydrogel lenses. Due to the increased transmissibility, longer wear times are often possible. SiHy lenses are also more resistant to protein deposits, less prone to drying and easier to handle (due to increased rigidity of the material). Corneal neovascularization, corneal edema, and limbal hyperemia are less likely with SiHy lenses vs. low-Dk lenses. In several studies, patients reported significantly improved comfort while wearing SiHy lenses, especially in adverse environmental conditions.2
Also, high rates of oxygen permeability allow SiHy lenses to play a therapeutic role—for problems such as recurrent epithelial erosions, abrasions, bullous keratopathy, etc.
Why are SiHy lenses a preferred mode of therapeutic contact lens wear? These lenses are usually worn on a continuous basis for a longer period of time. Conventional lenses do not meet the criteria for transmission of oxygen through the lens for overnight wear. SiHy lenses also promote corneal re-epithelialization and tear film quality thanks to their high oxygen transmissibility rates. The use of silicone hydrogels for management of ocular surface disorders has been proven to be safe and effective.3
Although most SiHy lenses are similar in their chemical composition, material variations can cause differences in their performance—so, be sure to match lenses specifically to patients according to their needs and wear habits.
When it comes to wear regimen, several options are available: 30-day continuous wear, two-week daily wear, monthly wear and daily disposable wear, just to name a few. And, some daily-wear lenses are also approved for continuous wear, albeit for shorter durations.
After being worn, SiHy lenses have shown more capacity to retain their initial equilibrium water content—they’re less susceptible to degeneration over time, which prolongs their performance.4
Another study compared how much ultraviolet light various SiHy lenses absorb. Researchers found only a small amount of variability within each brand.5
As with any product, SiHy lenses do have some drawbacks. Subepithelial infiltrates, limbal redness, conjunctivitis, keratitis and corneal ulcers can occur with these lenses, just like any other lens.
Their high modulus of elasticity can cause epithelial compromise, such as corneal staining and topographical changes—however, this will vary by lens, based on material and design. But, in a small study that compared first- and second-generation SiHy lenses’ relation to corneal shape change vs. that of monthly disposable hydrogel lenses (control), wearers of first and second generation SiHy lenses showed greater corneal stability than the control group.6 Some of the patients wearing a first-generation SiHy lens, however, did demonstrate complications related to the mechanical properties of the lens. There were no complications in the group wearing second-generation lenses. This lens technology is an improvement over older lens designs, and it improves as it evolves.
Another drawback: hydrophobia. As previously mentioned, the incorporation of silicone into the hydrogel polymer results in hydrophobia and a possible increase in bacterial adhesion. For example, one study demonstrated that the smallest amount of bacteria binds to conventional hydrogel lenses, whereas lotrafilcon B, a SiHy material, attracted the most.7 Hypothetically, bacterial adhesion to a biomaterial depends on the hydrophobia of the material and on the polymer type used to make the contact lens.8
But, bacterial adhesion is also strongly related to biofilm production, which thrives in an oxygen-rich environment. A biofilm is an aggregate of microorganisms in which cells adhere to each other and other surfaces, such as the lens or lens case. These can develop rapidly, and they contribute to cases of keratitis. Lens-induced hypoxia makes the epithelium more susceptible to bacterial adherence and keratitis.
Also, proteins are absorbed more readily by the less hydrophilic surfaces of SiHy lenses, and lipid deposition likewise increases.
Last but not least, SiHy lenses also tend to be more expensive than HEMA lenses.
Many patients report dryness and discomfort with contact lenses, but SiHy contact lenses have minimized these effects for many patients. The low wetting angle and mid-level modulus affect the comfort of some SiHy lenses.
Key risk factors for MK associated with soft lenses are lack of lens cleaning, poor disinfection and extended wear time.9 Pseudomonas aeruginosa, Staphylococcus, Streptococcus and Moraxella are common causes of MK during contact lens wear. Clinically, an epithelial defect with underlying stromal inflammation and infiltrates is typically visible. Common symptoms are red eye, photophobia, mild to severe ocular pain and irritation while wearing contact lenses.
However, one study revealed that rates of MK were less than 0.18% after the first year of SiHy use, and less than 0.04% of cases resulted in loss of visual acuity.10 These rates are higher than those of daily disposable lenses, but still low.8 A 2005 study in the U.K. found that people who slept in traditional hydrogel lenses were five times more likely to develop MK than those sleeping in SiHy lenses.10 No difference was found for risk of infection when used for normal daily wear.
Another study revealed that continuous SiHy wear has the same risk for MK as low-Dk extended wear.11 The overall rate of MK with a 30-night wear schedule was similar to that for conventional extended-wear soft lenses worn for fewer consecutive nights.11 The clinical severity, however, was lower in the SiHy group.
Infiltrates are equally likely when wearing SiHy lenses. These coin-shaped inflammatory opacities occur in the subepithelial layer and usually result from contact lens overwear. One study concluded that SiHy lenses have the same overall risk for corneal infiltrates as low-Dk hydrogels.12 But, there is a greater risk for contact lens-induced papillary conjunctivitis (CLPC) with SiHy lenses.12 Risk factors for corneal infiltrative events resulting from continuous wear of SiHy lenses include age, blepharitis, bulbar redness, corneal staining and history of previous inflammatory events.
Lessons Put Into Practice
The following two case studies demonstrate some possible complications resulting from SiHy lens wear. Remember that both noncompliant and compliant contact lens patients can experience problems during lens wear. Don’t assume that a patient’s compliance will preclude him or her from lens-related complications (though it might lessen the likelihood of such occurrences).
|1. Note the subepithelial
infiltrate located superiorly in the patient’s left eye. The area did
not stain upon fluorescein instillation.|
• Lens overwear. A 37-year-old white female presented with redness, tearing and photophobia in her left eye that had persisted for two days. She had worn SiHy contact lenses since May 2009, but she’d been wearing contact lenses since 2000, when she first came to our practice. The patient said she’d slept in her contact lenses two nights ago and that the irritation began the following morning. This was the patient’s first experience of irritation related to lens wear.
At her last examination one month earlier, her best-corrected visual acuity was 20/20 O.U. Her pupils were equal, round and reactive to light with no afferent defect. Extraocular muscle motility was full and smooth, and confrontation fields were full to finger counting O.U. Her anterior and posterior segments were completely unremarkable.
At her current examination, she presented with a corrected VA of 20/20 O.U. Her contacts were removed, and upon anterior segment examination, a subepithelial infiltrate was observed on the superior portion of the cornea of her left eye (figure 1). The area did not stain upon fluorescein instillation. Her IOP measured 15mm Hg O.U.
At this time, the patient admitted to sleeping in her contacts for more than six consecutive nights, but that this was the first time she had done it. I placed her on Lotemax (loteprednol etabonate 0.5%, Bausch + Lomb) q.i.d. for seven days and instructed her not to wear her contact lenses.
When she returned for follow-up the next week, her symptoms had greatly improved. She demonstrated no ocular irritation or photophobia at this visit. There was no IOP increase O.S., and the infiltrate was barely visible. I tapered the steroid, and she was able to return to contact lens wear. The patient stated that she no longer wanted to sleep in her contacts and was re-educated on the negative effects of noncompliance.
|2. Note the marginal keratitis in
the superotemporal portion of this patient’s right cornea. The patient
also demonstrated peripheral corneal infiltrates and some ulceration.|
• Bacterial infection. A 39-year-old white female presented complaining of redness, foreign body sensation and tearing in her right eye that had persisted for about two weeks. She stated that the irritation was a “seven on a scale of one to 10.” She had worn the same SiHy lens style since 2004. She denied sleeping in or overwearing her lenses, but when she took her contacts out, her comfort improved.
At her last full eye exam, four months earlier, all findings were normal, and her best-corrected VA was 20/20 O.U. At her current examination, her best-corrected acuity still measured 20/20 O.U. But, once her contacts were removed, anterior segment examination with fluorescein revealed what appeared to be bacterial marginal keratitis superotemporally O.D. (figure 2). This area did stain positively. Peripheral corneal infiltrates and some ulceration were also present. There was no sign of blepharitis, inflamed eyelids or telangiectasia on the lid margins.
I prescribed Vigamox (moxifloxacin, Alcon) q2h and scheduled the patient to return for follow-up in two days. There were no cells or flare present in the anterior chamber. And, because the lesion was in the periphery, there was a low risk of vision loss.
When she returned for follow-up, her best-corrected VA was 20/20 O.U. The redness had greatly improved, but she still experienced irritation “at a level of four out of 10.” Slit lamp examination with fluorescein verified that the staining had drastically decreased, but the infiltrative area still remained.
I decreased the patient’s Vigamox regimen to t.i.d. and added Lotemax q.i.d. O.D. Three days later, at her final diagnostic examination, the keratitis was completely resolved, and there were no ocular symptoms remaining. But, because this was the second instance of similar irritation due to her contact lenses, I recommended that she switch to a different brand with a higher DK to allow more oxygen to the eye. She agreed and successfully resumed lens wear.
Overall, silicone hydrogel lenses offer vast ocular physiological improvements thanks to their highly oxygen-transmissible materials. This increased oxygen permeability may reduce—though not eliminate—the risk of contact lens-related eye problems, as demonstrated in the cases above.
But, their track record and evolving technology could mean that SiHy lenses are the lens type of choice for both extended wear and daily wear.
Dr. Schorner currently practices at University Eye Associates in Charlotte, N.C.
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