Are Silicone Hydrogel Contact Lenses
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
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
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
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
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
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
Contact Lens Spectrum, Issue: July 2005