contact lenses
Silicone Hydrogels:STOP and Consider
Consider a different perspective when prescribing contact lenses.
BRIAN CHOU, O.D., F.A.A.O.

I spent eight years at the University of California at Berkeley: four as an undergraduate and four as an optometry student. Of course, Berkeley has a reputation for student protests and radicalism. While I was never interested in those sorts of activities, my time at Berkeley taught me to be more critical about popular opinion and the information that comes from the media. So it is in that spirit that I take the contrarian viewpoint in presenting an argument for hydrogel lens technology amid the push for silicone hydrogels.

For the past few years, there's been a repeated message — a mantra if you will — within our contact lens industry. It goes something like this: "Your eyes need oxygen," "Your eyes need to breathe," "Enjoy the freedom of continuous vision correction," "Your eyes need silicone hydrogels for healthy wear." These messages have run through continuing education programs and in the trade publications.

I could take the easy way out by saying the same thing and providing corresponding citations; doing so would be comfortable and convenient. However, I argue that it's still reasonable and compassionate for the prudent practitioner to prescribe advanced hydrogel lenses for daily wear.

When silicone hydrogels became commercially available, I, like many other practitioners, prescribed these lenses for my patients, predominantly for extended wear. After all, the hallmark of silicone hydrogels is high oxygen permeability. The concept back then was that high oxygen transmission would solve a host of contact lens problems, from wearing discomfort all the way to minimizing serious complications such as microbial keratitis. Have we reached the oxygen "promised land?"

Evidence to the contrary?

We were rudely awakened when the first few reports of microbial keratitis with silicone hydrogel extended wear came forth, appearing in journals in 2002 and 2004.

As it turns out, there is mounting evidence that high oxygen transmissibility does not surmount the risks of extended wear. Perhaps it is best surmised in a recent article by Stapleton, et al, that, while the ocular health benefits of silicone hydrogel lenses have increased the period for which patients can wear lenses overnight, the risk of infection is still similar to that for soft lens made from other materials. So overnight wear still carries a higher risk factor for infection than daily wear, regardless of lens material. This assessment echoes the recent findings by Schein, et al, in a large multi-center, post-FDA approval study of silicone hydrogel extended wear in 2005. The researchers' data showed that the incidence of microbial keratitis in patients wearing lotrafilcon A lenses for up to one month of extended wear was not significantly different from what has been previously reported for patients in hydrogel lens extended wear up to one week.

As further evidence that high oxygen transmissibility does not surmount the risks of extended wear, researchers from the United Kingdom last year reported their findings of patients who presented with contact lens-associated corneal infiltrative events (CIEs). They found that patients in extended wear, whether with hydrogel or silicone hydrogel lenses, had an eight times greater incidence of CIEs compared with those in daily wear. The absence of a statistically significant difference in the incidence of CIEs between the two lens materials (when worn on an extended wear basis), they said, suggests that hypoxia may not play a significant role in causing CIEs when soft lenses are worn overnight. The researchers did find, however, that silicone hydrogel wearers had less severe CIEs than hydrogel wearers in extended wear. For this reason, silicone hydrogels are appropriate for any patient who insists on wearing contact lenses for extended wear.

An oxygen hypothesis

Why doesn't high oxygen transmissibility significantly reduce the incidence of microbial keratitis? One hypothesis is that when a contact lens is on the eye for an extended duration of time, even if the eye receives adequate oxygen, the accumulation of waste products in the post-lens tear film may compromise the barrier function of the eye, permitting inflammation and infection. Even if the eye receives a lot of oxygen, waste products such as lactate, carbon dioxide, shed epithelial cells and bacterial byproducts can stagnate and create a cesspool behind the contact lens, as studies have shown.

Tear exchange is key

What might it take to give extended wear a shot at success without incurring a significantly higher incidence of microbial keratitis than daily wear lenses? There are industry efforts underway to incorporate antimicrobial substances into contact lenses and this may help. However, the real key may involve increasing tear exchange behind the lens. One such technology is called "microchannel." An article in last year's Optometry & Vision Science evaluated these prototypical soft lenses; the posterior periphery of the lenses contain radial microchannels that improve tear exchange between the post-lens tear film and the eye's tear reservoir. The authors found that these microchannel contact lenses did not appear to affect lens comfort or vision. However, they did significantly reduce conjunctival staining versus standard contact lenses, suggesting that increased tear exchange can have physiological benefits. Whether these microchannel lenses will become a viable technology remains uncertain.

A false sense of security?

Do laypeople know that silicone hydrogel lens wear does not eliminate the health risks associated with extended wear? As eye care professionals, we cannot assume they do.

Our responsibility is to disclose to our patients the significantly increased health risks of extended wear. Let me be clear that this is not an indictment on the technology behind silicone hydrogel lenses: Silicone hydrogel lenses are an amazing advancement in material science. But my concern is that silicone hydrogel lenses may give practitioners a false sense of security that we no longer need to provide our patients with informed consent about the increased risks of extended wear. Worse, since most of the current silicone hydrogel lenses are direct-to-consumer advertised, some patients may buy into a manufacturer's promotional message of extended wear even when the practitioner warns against doing so. Put into perspective, critics correctly point out that extended wear with silicone hydrogels appears to hold less risk of visual loss than laser vision correction. Yet patients undergoing laser vision correction learn about the inherent surgical risks through informed consent. Our patients may not know that extended wear carries more risk of serious infection than daily wear. Arguably, too few patients in extended wear lenses receive appropriate informed consent. 

Patient realities

So what about the surveys that indicate the majority of people want to sleep in their contacts overnight? I don't dispute the likelihood that most patients like the idea of being able to sleep in their contact lenses, at least occasionally. Whether they actually do is another issue, especially when informed about the associated health risks.

What about the argument that practitioners should always prescribe silicone hydrogels because they offer greater safety if a patient sleeps with a contact lens? True, some patients will never listen to their doctor and sleep in their lenses anyway. However, we are not absolved from the responsibility of advising patients about the increased risks of extended wear, along with proper contact lens care and hygiene. The reality is that as doctors, we influence our patients' behaviors.

Daily-wear and safety

Daily-wear is still the safest contact lens wearing modality, and it is still the dominant category of soft contact lenses. Roughly 92% of new soft contact lenses sales were for daily wear, according to Health Products Research (HPR) data from the fourth quarter of 2005. With silicone hydrogel lenses, HPR cumulative data shows there's a notable shift from the extended wear emphasis toward daily wear, punctuated with the introduction of lenses such as Acuvue Advance (Vistakon), O2Optix (CibaVision) and Acuvue Oasys (Vistakon).

Behind the buzz

Yet to what extent does it make sense to prescribe silicone hydrogel lenses for daily wear? After all, there are advanced hydrogel lenses that already exceed the minimum Dk/t requirement to avoid corneal swelling in the open eye state. Still, we all know that we doctors are prescribing silicone hydrogel lenses much more frequently. Could the growing prescription rates reflect the practitioners' belief that if they are not prescribing silicone hydrogel lenses, they are second-class practitioners exposed to undue medicolegal risk? Could it be that eye care professionals feel compelled to join the bandwagon? Ideally, one would hope that the increased prescribing rates of silicone hydrogels is a sole consequence of improved clinical performance relative to existing contact lenses.

Oxygen and comfort

Daily wear clinical performance is multifaceted, comprised of elements like vision, comfort, handling and ocular health. Certainly, silicone hydrogel lenses have superbly addressed the issue of ocular hypoxia, which is one important subset of ocular health. But the other aspects of ocular health, including minimizing inflammation and infection, may not be achieved with silicone hydrogels. Furthermore, there is still no definitive link between high oxygen transmissibility and improved vision, comfort and handling.

As clinicians, we know the clinical signs of ocular hypoxia. Corneal edema manifests with stromal folds and striae, epithelial microcysts and endothelial polymegathism. Limbal neovascularization and ocular redness are also potential signs of oxygen insufficiency. Of course, we must not forget that there are also non-hypoxic causes of neovascularization and ocular redness. For example, allergy and blepharitis can cause unusual redness. Interestingly, all the afore- mentioned signs of ocular hypoxia, except for ocular redness, are usually asymptomatic. That's to say that it is not unusual for patients to exhibit neovascularization 3.0mm into the cornea with a large number of microcysts, without any complaints.

The type of corneal edema that causes visual symptoms is epithelial edema, observed in patients with acute angle closure glaucoma and pseudophakic bullous keratopathy. Still, there were patients in whom the magnitude of ocular hypoxia — even if completely asymptomatic — was such that, before silicone hydrogels, we would have taken them out of soft contact lenses in favor of glasses or GP contact lenses. Silicone hydrogel lenses have rescued these patients.

With more than 30 million contact lens wearers in the United States, an estimated 10% discontinue lens wear each year, according to recent data from Alcon. Before silicone hydrogel lenses, a portion of these discontinuations were practitioner-decided because the patient had unacceptable amounts of ocular hypoxic signs. Yet the overwhelming majority of contact lens discontinuations originate from patients who stopped wear themselves. In one study, Young, Veys and Pritchard reported that 51% of previous wearers cited discomfort as the principal reason for giving up contact lenses.

Oxygen isn't the only factor

Could the increased oxygen supply of silicone hydrogels solve the discomfort issue? If there is a direct link between oxygen transmission and comfort, then all practitioners should prescribe hyper-Dk gas permeable lens materials in greater volumes since many of these lenses have oxygen permeability that exceeds what current silicone hydrogels offer. Alas, not everyone can tolerate the comfort of GP lenses due to lid-lens edge sensitivity. What influences comfort, more so than oxygen, are characteristics such as edge design, modulus, surface wetting and dehydration and deposit resistance.

In our pursuit to create high-Dk silicone hydrogel lenses, have we neglected other important attributes for contact lens clinical performance, including comfort? Perhaps. The early modulus-related surprises such as mucin balls, contact-lens papillary conjunctivitis (CLPC), superior epithelial arcuate lesions (SEALs) and conjunctival flaps (CEFs) tell us there's more to contact lens wear than oxygen. The reported incompatibilities between the preservatives in some solutions and silicone hydrogels is another reminder of this. Lens and solution manufacturers are correcting these early shortcomings.

Consider the "upgrade"

Often, practitioners hear of silicone hydrogels in the context of an "upgrade" for hydrogel lens wearers. However, silicone hydrogels are not an upgrade for patients if they do not offer improved clinical performance and only increase the lens cost. In the same way, you aren't really upgrading if you are used to driving a high-performance sports car then switch to an early hybrid electric vehicle (HEV). Sure, the HEV may provide you with exceptional gas mileage, but at the expense of acceleration, cornering and braking. New-generation HEVs bring substantially improved driving performance, although not yet to the level of their gasoline-only counterparts.

The big question

So, are hydrogel or silicone hydrogel lenses "better?" This is not a fair question to ask. With- in each of these categories, there are diverse material options, so there's no "average hydrogel" or "average silicone hydrogel." Hydrogel lenses have polymacon at one end with omafilcon on the other end. Among silicone hydrogel lenses, you have early generation materials like lotrafilcon A at one end, and newer generation materials such as senofilcon A and comfilcon A on the other end of the spectrum. My point is that as practitioners, we must think before prescribing rather than robotically reaching for silicone hydrogel lenses.

With increasing competitive pressures in contact lens sales due to the Fairness to Contact Lens Consumers Act and the emergence of alternative distributorship, optometrists have shifted to emphasize professional services, and rightly so. Our refractive decisions should be guided by which contact lens serves the best interests for the individual patient. If we don't exercise our expertise in evaluating both hydrogel and silicone hydrogel lenses for each patient on a case-by-case basis, then there's little value in our contact lens services beyond what the patient could decide on his or her own. At present, practitioners are still well justified to prescribe advanced hydrogel lenses, for silicone hydrogel lenses have not definitively shown head-and-shoulders improvement in overall clinical performance for daily wear.