Are Silicone Hydrogel Contact Lenses Overrated?
I have no doubt that within the next five years, each new soft disposable contact lens will be a silicone hydrogel lens. The substantial oxygen transmissibility of silicone hydrogel contact lenses improves ocular physiology.
However, oxygen transmission isn't the only element that determines successful contact lens wear. Other factors have prompted me to continue prescribing a hydrogel contact lens for most of my patients.
Current Silicone Hydrogel Pros and Cons
Like many of my colleagues, when CIBA Vision first launched its Night & Day contact lens, I prescribed it for a large number of patients for continuous wear. Indeed, many patients successfully wear this lens. But an unexpected number of my patients came back, complaining of symptoms such as light sensitivity, redness, burning, itching and foreign body sensation. While I haven't had any cases of microbial keratitis among my silicone hydrogel patients, I've seen many other lens-related findings that are now documented in the literature: Solution incompatibility to PHMB, inflammatory infiltrates, mucin balls, giant papillary conjunctivitis (GPC) and superior epithelial arcuate lesions (SEALs).
Researchers believe that the fairly high modulus of the Night & Day lens underlies the mucin balls, GPC and SEALs. The newer silicone hydrogel lenses such as Acuvue Advance with Hydraclear (Vistakon) and O2Optix (CIBA) have a lower modulus, so I hope to observe reductions in these particular findings.
I also expect the contact lens/solution incompatibility issue to also work itself out as manufacturers are already formulating solutions without PHMB. This is an important industry step with the increasing use of silicone hydrogel lenses, because at present, patient adherence to doctor-recommended care systems is questionable.
I suspect that continuous wear patients are even less adherent and are more likely to use private label solutions. Last year, private label solutions held a 25 percent market share (AC Nielsen Data in November 2004), so for the sake of silicone hydrogel patients, I hope manufacturers also eliminate PHMB from these solutions.
Despite industry adjustments to the early flaws of silicone hydrogel lenses, I'm guarded about whether the industry will have as much success eliminating acute episodes of superficial ocular inflammation that are related to continuous wear.
Continuous Wear Misconceptions
When a lens remains on the eye for a long, continuous period, metabolic debris, including CO2 and lactate, forms under the lens. With insufficient tear exchange, especially without daily lens removal, these and other waste products accumulate. Exotoxins produced by normal eyelid flora may also accumulate and are effective in inciting inflammatory reactions.
So, how do higher Dk materials designed for continuous wear improve removal of post-lens ocular waste? They don't! I see this as a problem with how some manufacturers position silicone hydrogel lenses to practitioners, as well as to consumers. Although oxygen transmission is crucial for successful lens wear, it's not the only factor. Good physiology, comfort, handling, vision and convenience are also important, if not more so. Unfortunately, high oxygen transmission doesn't guarantee that the lens adequately meets all these criteria.
Nevertheless, certain practitioners have already professed silicone hydrogels as the new standard of care even for daily wear. I believe such a claim is unfounded and mistaken. The analogy that silicone hydrogels are to existing hydrogels as GPs are to PMMA is just too severe and distorting. For the vast majority of patients, it's still perfectly reasonable, if not compassionate, to prescribe advanced hydrogel lenses. I would argue that it's not prudent to prescribe silicone hydrogels in instances in which you have reason to believe that a patient will abuse his prescribed wearing schedule. This is a real possibility with patients who wear silicone hydrogels for daily wear because such patients might disregard their doctors' direction and begin unsupervised continuous wear after assuming that the package's label, indicating FDA approval for continual use, applies to anyone.
A Hydrogel Alternative
While it may be true that the incidence of microbial keratitis is no different between silicone hydrogel continuous wear and hydrogel daily wear, I don't hear much from silicone hydrogel promoters warning practitioners and patients about superficial ocular inflammation related to silicone hydrogel wear. Until the industry further characterizes silicone hydrogel technology when any other "surprises" come out in addition to PHMB incompatibility, GPC, SEALs and mucin balls my lens of choice remains the CooperVision Proclear Compatibles. I'm not yet convinced that the available silicone hydrogels offer substantial clinical improvements compared to advanced hydrogel lenses like Proclear, at least for daily wear. In some cases, because of the previously mentioned lens-related findings, silicone hydrogels have caused more eye health-related concerns than hydrogel lenses have.
My hope is that contact lens practitioners will carefully evaluate how silicone hydrogel contact lenses benefit our patients. �New� doesn�t always mean �better.� Meanwhile, I�m looking forward to ensuing generations of silicone hydrogel lenses, which promise to offer more fail-safe clinical performance.
Dr. Chou is in group practice in San Diego. He is also a consultant for the California Board of Optometry and is a clinical investigator and consultant to CooperVision, SynergEyes and Ophthonix.