Article Date: 10/1/2002

CL Solutions
Are All Multipurpose Contact Lens Solutions Created Equal?
Studies show that slight differences in formulations make big differences among patients.
BY KENNETH A. LEBOW, O.D., F.A.A.O., Virginia Beach, Va.

Contemporary multipurpose solutions (MPSs) for contact lenses have changed dramatically over the years. The original versions of these products primarily contained thimerasol and chlorhexidine -- both of which are extremely strong disinfecting and sensitizing agents -- and resulted in many red eyes and uncomfortable patients.

New generation solutions contain different preservatives such as Polyquad (polyquaternium-1), Aldox (myristamidopropyl dimethylamine) and PHMB (polyhexamethylene biguanide), which are kinder to the corneal physiology. But are all contemporary MPSs created equal?

ILLUSTRATION BY MARK HEINE

Knowing the difference

All too often I hear practitioners reason that all contact lens solution are the same, so it's good enough for patients to use whichever one they want. Yet patients often return to our offices with particular solution preferences even after we've specifically recommended a given brand.

For example, in one clinical study comparing ReNu Multi-Purpose solution with Complete Multi-Purpose solution, investigators showed that differences in the formulations of the two solutions (slight as they may be) have significant effects on the performance of the solution and on patient preferences for one solution over the other.

Patients know what they like and they can tell the difference between products -- even if the only perceptible difference is the price! Obviously, clinical signs and symptoms may vary depending on the lens brand, care products or a particular combination of the two that the patient uses.

Understanding staining

Any time a solution containing a disinfectant comes in contact with the ocular surface the potential for cytotoxicity develops, which we can measure clinically by observing corneal staining with fluorescein. Unfortunately, many of us don't always take the time to remove a soft lens and stain the eye. However, we'd be surprised at what we found if we did.

Studies show that corneal staining is preservative- and concentration-related. Moreover, certain hydrogel lens categories (especially Group II lenses) demonstrate more staining than others, possibly as a result of the absorption of preservatives into the lens matrix overnight.

Even the new silicone hydrogel lenses aren't immune from this situation. It's been reported that not all MPS preservatives are compatible with these materials. While preservatives are generally the first solution component implicated when we have a problem, Nichols, et al. recently evaluated 500 lens wearers and found that the severity of staining depended on how often the subjects replaced their lenses, whether they wore conventional or disposable lenses and how meticulous they were about cleaning their lenses.

MPS musts

Today's MPSs must function on three critically different levels:

1. Contact lens disinfection

2. Lens cleaning

3. Lens conditioning.

All of these functions are necessary to produce safe, comfortable contact lens wear, but more importantly, we must maintain a balance between bacterial disinfection, ocular toxicity and patient compliance.

In terms of disinfection efficacy, the two leading MPSs (Alcon's Opti-Free Express and Bausch & Lomb's ReNu MultiPlus) demonstrate extremely high anti-bacterial activity against the stock FDA test organisms. The cumulative log kill rates for these two products show little difference in their total disinfection efficacy on these stock strains of bacteria, yeast and fungi. In fact, this high level of disinfection has enabled these two brands to achieve stand-alone approval by the FDA. Because more than 50% of the contact lens wearing population doesn't rub their lenses, it's comforting to know that these solutions disinfect lenses without the need for rubbing.

It's interesting to note however, that "stand-alone" FDA testing criteria don't involve an assessment with the lens present in the solution. Rather, the required 3-log reduction for bacteria and 1-log reduction for yeast and fungi are performed without a contact lens in the test inoculum. The actual issue for the real world use of these solutions is what happens when we add a contact lens to the test media.

Because some MPSs actually absorb or bind the disinfectant within the contact lens matrix, we can alter the antimicrobial efficacy of the solution in the presence of a contact lens. This may also explain why some disinfecting solutions (especially those containing high levels of PHMB) are more toxic to the corneal epithelium than others. Over time, PHMB bound to lipid deposits from the contact lens matrix is released to the corneal epithelium resulting in staining.

Pointing out the differences

Other real-world testing situations help show the differences in MPSs.

Anti-microbial activity. It's critical that MPSs maintain their disinfection efficacy because they're often exposed to high temperatures, virulent strains and higher concentrations of organisms and organic contamination.

In all of these situations, various contemporary MPSs react differently from the reported results in FDA testing situations. Some solutions (particularly biguanide-preserved solutions) merely have reduced anti-fungal and anti-yeast effectiveness when exposed to high temperatures, while others are almost completely inactivated by it. Because patients are often non-compliant in their care of contact lenses, it behooves us to use the most effective and aggressive MPS that's still safe.

Anti-Acanthamoeba activity. Acanthamoeba keratitis is a real problem that was originally thought to be prevented only by heat or hydrogen peroxide disinfection. All of the major MPSs demonstrate efficacy against the troph forms of Acanthamoeba.

Alcon's Opti-Free Express with Aldox has demonstrated hydrogen peroxide-like efficacy kill rates against the cystic form of these ubiquitous and potentially sight-threatening organisms. Moreover, for hydrogen peroxide solutions to be effective against this amoeba, they must maintain an adequate peroxide concentration (3% or greater) for at least 4 hours. Hence, one-step peroxide-based care systems are ineffective against Acanthamoeba. This is also why hydrogen peroxide-based disinfect-
ing solutions aren't acceptable for storing contact lenses long term. Once the peroxide is reduced to water and oxygen, any remaining micro-organisms can re-infect the solution.

Removing lipids and proteins. MPS cleaning agents must address both lipid and protein removal to be truly effective. Some solutions use Pluronic surfactants (SOLOcare and Complete) while others use various forms of Tetronic surfactants (Opti-Free Express and ReNu MultiPlus). These surfactant cleaners emulsify lipid-based soilants on lenses preventing re-attachment of proteins or other materials to the lens surface.

However, specific formulations of Tetronic also work as surface-acting agents facilitating hydrogel lens wetting at a molecular level. While this concept may seem strange for hydrogel materials, it isn't, because much like their rigid gas permeable counterparts, hydrogel lens polymers have hydrophilic and hydrophobic sites on the lens surface.

It has been demonstrated that a specific formulation of Tetronic (1304) causes an adsorption of a balanced hydrophilic/hydrophobic polymer to the hydrophobic centers on the lens allowing the creation of enhanced hydrophilic centers that result in increased lens wettability. This is much the same action we achieve by soaking gas permeable lenses in a conditioning solution overnight.

Simply put, this formulation of Tetronic cleans the lens surface (which improves lens wetting) and holds more water to the lens surface, keeping it wetter longer. As a result of this dual-surface action, subjective evaluations of all day, end of day and long-lasting ocular comfort, especially among self-diagnosed dry eye patients, are also significantly improved.

Basic cleaning facts. Passive cleaners also play an important role in lens cleanliness. While contact lens surfaces may appear macroscopically clean, they can be microscopically dirty and antigenicly reactive. In this situation, the main contaminant is lysozyme, which represents 90% of the total protein dissolved in the tears. Lysozyme has been found to loosely bind to the lens surface as well as absorb into the lens matrix. Hence, a more challenging cleaning activity is to reduce the total lysozyme load from a contact lens.

Passive cleaners such as citrate-based buffering solutions actually facilitate the cleaning action of the water molecule to bind lysozyme and remove it from a contact lens. In studies comparing borate-based buffers with citrate-based buffers, citrate-formulated solutions remove greater amounts of lysozyme during overnight lens soaking.

While Tetronic 1304 has been demonstrated to be a comfort-enhancing ingredient, some MPSs incorporate macromolecules (for example, hydroxypropyl methylcellulose -- HPMC in Allergan's Complete MPS) in their chemical formulation to improve lens wetting and comfort by conditioning the lens surface. The use of this traditional ocular lubricant in a MPS is thought to produce a thicker and longer-lasting layer of fluid on the lens surface that absorbs into both Groups I and IV lens materials, gradually releasing from the lens over time.

Consider the differences

The next time you consider saying that all solutions are created equal, stop and think about the alternatives that are currently available to you and your patients. Many studies of various MPSs have demonstrated that significant differences among these products exist.

Consider the different types of disinfectants, various concentrations of similar disinfectants, cleaning activity and wetting properties of these solutions as a possible remedy to your patient's complaints.

Dr. Lebow is in private practice specializing in contact lenses, clinical research and consultation to industry. He's a member of the American Optometric Association, a past president of the Virginia Optometric Association and a Fellow of the American Academy of Optometry.

 


Optometric Management, Issue: October 2002