Before you send another patient home with silicone hydrogel contact lenses, make sure he's using an appropriate lens care regimen.
By Lyndon Jones, Ph.D., F.C.Optom., F.A.A.O., Waterloo, Canada
No doubt, you're familiar with the different adverse reactions that can occur between contact lens materials and lens care solutions. Most complications, including documented reactions between FDA Group IV lenses and
OptiSoft, and CSI and Silsoft lenses with Hydrocare,1 probably occur when contact lenses selectively adsorb and release preservatives from lens care solutions. Once released, low molecular weight agents such as chlorhexidine,25 benzalkonium chloride,4 sorbic acid6 and thimerosal4 can damage the corneal epithelium, resulting in the formation of infiltrates,710 superior limbic keratoconjunctivitis,11 pseudodendrites12,13 and corneal staining.7,10,14
Excessive corneal staining has become relatively rare as manufacturers replace old-type preservatives and disinfectants with newer high-molecular weight preservatives. Worldwide, millions of patients successfully use modern, one-step, no-rub, multipurpose products that contain one of three antibacterial agents: Hydrogen peroxide, polyhexamethylene biguanide
(PHMB) or polyquaternium-1 (Polyquad).35
Recent studies suggest that not all solutions are compatible with all contact lens materials in all patients, particularly patients wearing silicone hydrogel lenses. Recent work indicates that certain care regimens may cause asymptomatic corneal staining in some patients when used with daily wear silicone hydrogel lenses.1518
Keeping your patients' eyes healthy can simply be a matter of understanding the interaction between specific lens care solutions and modern lens materials. This article investigates what we know about combinations that may result in corneal staining.
COLLECTING OBJECTIVE EVIDENCE
Studies have reported that using certain
PHMB-based care regimens with specific polyHEMA-based contact lens materials -- notably high water neutral materials (FDA Group II) containing N-vinyl pyrrolidone -- may result in corneal staining.1921 Until recently, evidence of corneal staining with silicone hydrogel contact lenses was anecdotal, but several well-controlled, randomized studies now have examined the relationship between lens care regimens and silicone-based lens materials.
► Polyquaternium-1-based vs.
PHMB-based. In the first published controlled study of corneal staining with
PHMB-based solutions, researchers fit patients adapted
to soft contact lenses with daily wear silicone hydrogel lenses (PureVision).15 Fifty patients wore the lenses during two consecutive 1-month periods, using either a polyquaternium-1-based system
(Opti-Free Express) or a PHMB-based system (ReNu MultiPlus). Investigators observed significant asymptomatic corneal staining in subjects using the
PHMB-based system, with 37% of patients demonstrating a level of staining consistent with a classic solution-based toxicity reaction. Examples of diffuse staining patterns observed in this study are shown in Figures 1 and 2.
Only 2% of subjects using the polyquaternium-1-based system developed similar staining. Researchers noted that degree of staining didn't correlate with any significant symptoms and patients using both cleaning regimens reported no difference in lens comfort or solution preference.
► Hydrogen peroxide-based vs. polyquaternium-1-based. In a study by Amos,2225 patients wearing Focus Night & Day lenses used a peroxide-based regimen
(AOSept Plus) and a polyquaternium-1-based system (Opti-Free Express). He found that patients using the hydrogen peroxide-based system developed marginally less overall staining than those using the polyquaternium-1 regimen. Furthermore, no AOSept patients showed atypical staining; and two of the 25 patients (8%) using
Opti-Free exhibited abnormally high staining.
► Hydrogen peroxide-based vs.
PHMB-based. The Centre for Contact Lens Research in Waterloo18 refitted 20 myopic patients experiencing soft contact lens-related dry eye symptoms with daily wear PureVision and Focus Night & Day lenses. In this crossover study, patients wore one of each lens type on their contralateral eyes for 2 months, during which they were randomized to a
PHMB-based system (ReNu MultiPlus) or a hydrogen peroxide system (AOSept). Each group switched to the other cleaning system halfway through the study. We found that:
- Patients using the hydrogen peroxide care system showed no significant difference from baseline corneal staining and no evidence of high staining.
- Patients using the PHMB-based system had
significantly more corneal staining at both follow-up visits than at baseline, with more staining observed with the PureVision lenses than with the Focus Night & Day lenses.
- Forty-seven percent of the PHMB-PureVision eyes and 21% of the
PHMB-Focus Night & Day eyes showed staining.
- Staining was generally mild (< 25 on a 0100 scale) and frequently appeared as a stain-free center surrounded by an unusual annular pattern (See Figures 3 and 4) as previously reported.15,17
► PHMB-based solutions of varying compositions. Amos23 also studied the incidence and severity of corneal staining in patients wearing Focus Night & Day contact lenses, but he tested
regimens exclusively. After 1 month, patients using the ReNu system exhibited 24% atypical staining, compared with no accentuated staining among patients using the Focus Aqua
(AQuify MPS) regimen.
These studies suggest that:
1. Unacceptable staining is more likely when PureVision lenses are used with certain
PHMB-based sytems than when Focus Night & Day lenses are used with the identical systems.
2. Staining is relatively rare with peroxide- and polyquaternium-1-based regimens.
3. Regimens with identical concentrations of PHMB can behave differently, depending on solution formulation.24
THE LIPID FACTOR
Although we don't completely understand the mechanism underlying corneal staining seen with certain combinations of silicone hydrogel lenses and care regimens, we may gain some insight by studying how
PHMB-based solutions interact with other contact lens materials. Researchers have noted corneal staining in patients who use certain
PHMB-based products with lipid-attracting, neutral, high water content materials (FDA group II).19,20,2527
One theory is that one or more components in certain products first bind to and then are released from lipid deposits on the silicone hydrogel lens surface, mimicking a drug delivery device. The lack of lipids in in vitro assays may explain why this phenomenon occurs only in in vivo studies, which show that the pattern of lipid uptake is usually identical to the pattern of observed corneal staining.
Whatever the underlying mechanism, recent controlled studies have shown that the role of PHMB in silicone hydrogel daily wear staining has been over-simplified. Silicone-based lenses can interact with certain care regimens, producing relatively asymptomatic corneal staining patterns that are distinctly different from those observed with
HEMA-based lenses. But not all PHMB-based systems are equally likely to cause staining. Some
PHMB-based products create as little staining as hydrogen peroxide or polyquaternium-1 products,23 and many patients can use any disinfecting solution with their silicone hydrogel lenses without developing clinical problems.
When switching daily wear patients from conventional hydrogel contact lenses to silicone hydrogel lenses, you should consider whether the patient's current lens care system is entirely compatible with the new lens material. If a patient continues using the same care regimen and presents with symptoms of solution sensitivity or toxicity, we tend to blame his discomfort on the new lens material instead of an interaction between the material and care regimen. This same patient may not exhibit excessive corneal staining with a different silicone
hydrogel-compatible system, so switching solutions followed by repeat fluorescein testing may be necessary.
Compared with other soft contact lenses, silicone hydrogel lenses have a unique chemistry. Features that make this material more sensitive to solution-based nuances than other commercially available hydrogel materials include:
- Very high lens oxygen permeability28,29
- Low levels of protein uptake27,30
- High levels of lipid uptake27,30
- Reduced in vitro wettability31,32
- Reduced levels of dehydration33,34
- Material stiffness.28
Based on these differences, I believe silicone hydrogel lenses are unique enough to warrant a new FDA soft contact lens classification group (Group V).
KNOW THE DIFFERENCES
Acknowledging the singular properties of silicone hydrogel materials and learning how they interact with different lens care regimens can help us avoid corneal staining in certain silicone hydrogel contact lens patients.
For a complete listing of contact lens care systems and their ingredients, go to
http://www.clspectrum.com/class/ to view "Contact Lenses and Solutions Summary."
Dr. Jones is the associate director of the Centre for Contact Lens Research at the University of Waterloo, Ontario, Canada.
Several studies have shown that the severity of contact lens-related corneal staining typically increases over a 4-week period,15,18 is remarkably asymptomatic14,15,17,22,23 and appears most prominently about 2 hours after patients apply their contact lenses.21 The severity of corneal staining then abates as patients continue wearing their contact lenses, so that fluorescein testing reveals reduced levels of staining 6 hours after lens application.21
Time-sensitive fluctuations in corneal staining patterns may explain why some practitioners fail to document excessive corneal staining. We typically see patients when corneal staining is less evident -- after they've been wearing their lenses 6 to 8 hours. And because solution-related corneal staining is
asymptomatic, we're less likely to perform fluorescein evaluations on patients who don't report discomfort.
The best strategy for detecting solution-related corneal staining is to incorporate fluorescein evaluation with a yellow barrier filter into every silicone hydrogel follow-up exam, ideally testing patients 2 hours after they apply their lenses.
Percentage of Abnormal Staining Reported
in Daily Wear Silicone Hydrogel Studies
||Focus Aqua (Aquify MPS)
| Pure Vision
| Focus Night & Day
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Contact Lens Spectrum, Issue: July 2004