Ординатура / Офтальмология / Английские материалы / Ophthalmic Drugs Diagnostic and Therapeutic Uses 5th edition_Hopkins, Pearson_2007

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References

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Chapter 11

Contact lens solutions

Contact lens solutions are as old as contact lenses. The first description of a contact lens was given by Fick, in 1888, who experimented with different solutions to fill the space between the cornea and the back surface of the glass scleral shell. He considered that a 2% solution of ‘grape sugar’ (i.e. dextrose) was suitable for this purpose. In 1892, Dor reported that having first tried a glucose solution and then an artificial serum, ‘physiological saline’ was satisfactory, and this formed the basis of contact lens solutions for many years thereafter.

In the late 1940s, the advent of rigid corneal lenses manufactured from the oxygen-impermeable material polymethylmethacrylate (PMMA) encouraged the popular use of contact lenses as an alternative to spectacles. Commercially produced contact lens solutions date from this period. Initially, two types of solution were introduced, one for ‘wetting’, which was intended to promote tear flow over the PMMA surfaces, and another for ‘soaking’, with which the storage case was filled to achieve disinfection of the contact lenses. A later development was the introduction of daily surfactant cleaners and subsequently dual-, tripleand multipurpose solutions were introduced in an endeavour to simplify the lens-care regimen to enhance compliance with its proper use. Gaspermeable rigid materials, which evolved rapidly from the 1970s, tend to be more susceptible to deposits than PMMA but less so than hydrogels.

Soft hydrogel contact lenses were introduced in the 1960s and became widely available a decade later. Their arrival necessitated the development of new systems of lens care. For example, benzalkonium chloride, an effective preservative that had been used extensively in rigid contact lens solutions, was found to bind to the hydrogel polymer.

Hydrogel lenses have a greater affinity for surface deposits than rigid lenses and this problem was addressed by means of the following, alternative strategies:

•The introduction of enzymatic cleaners intended for use about once a week.

•The development of various ‘deposit resistant’ hydrogels.

•Limitation of the ‘lifetime’ of the lenses: originally, this option was achieved by the planned replacement of lenses after 6 or 12 months. The development of the moulding method of manufacture permitted the low-cost production of disposable lenses that are discarded after 1 month’s or after 1 day’s use.

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Ideally, contact lens solutions should:

•be sterile

•not discolour the lens (Wardlow & Sarver 1986) or change its properties or parameters

•not be toxic or irritative to the eye

•have a mode of use that encourages patient compliance.

THE DAILY ROUTINE FOR CONTACT LENS CARE

•Wash hands thoroughly and dry with a clean towel.

•Remove the contact lenses from the storage case (or blister pack in the case of daily disposable hydrogel lenses).

•Insert the lenses.

•At the end of the wearing period, wash the hands and dry thoroughly.

•Remove the lenses and, if they are of the daily disposable type, discard them.

•If the lenses are not to be discarded, apply a surfactant cleaner or multipurpose solution that performs this function, and rub the lens between the fingers to remove all debris. Multipurpose solutions described as ‘no rub’ claim to make this action redundant.

•Rinse the lenses with sterile saline or multipurpose solution and inspect them. If they do not appear to be clean and clear, repeat the previous step. Place the lenses in a storage case filled with a solution that can disinfect them.

•All planned or unscheduled replacement hydrogel lenses, and some rigid gas-permeable lenses, require the use of enzymatic cleaners, usually on a weekly basis.

WETTING SOLUTIONS

Although dedicated, ‘stand-alone’ wetting solutions are no longer available they are mentioned here briefly to establish the importance of the wetting function of multipurpose solutions for rigid contact lenses.

Wetting solutions were developed in the 1950s with the specific aim of rendering the surface of a rigid contact lens relatively hydrophilic. They were applied to the lens following its removal from the storage case (i.e. to a disinfected lens). They contained a surface active agent that reduced the contact angle of tears on the contact lens surface and provided a viscous ‘coating’ that encouraged the lens to adhere to the finger during insertion. The solution also acted as a lubricant between the contact lens and cornea, thereby enhancing comfort.

Before the advent of wetting solutions, patients were sometimes encouraged to use saliva for this purpose. Although being available at no cost and having good wetting and viscoelastic properties, the use of saliva involves an unacceptable microbiological hazard. Today, it remains necessary to warn patients not to lick a lens to remove foreign matter from its surfaces.

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(Penley et al 1989) and to be effective against the cyst of Acanthamoeba culbertsoni (Connor et al 1991).

4.Improves the efficacy of subsequent disinfection by exposing clean surfaces to this process.

5.Removes possible nutrients on which organisms can grow.

6.Can reduce toxic effects from disinfection solutions: daily cleaning removes contaminants to which preservatives might bind.

7.Reduces the adherence of organisms to the lens: it has been shown that tear components absorbed onto the lens surface enhance the adherence of microorganisms to lenses (Butrus & Klotz 1986) but some might adhere even to clean lenses (Duran et al 1987).

These considerations demonstrate that daily cleaning is not just a prelude to the disinfection process but is an integral part of it. Failure to remove tear proteins before disinfection with hydrogen peroxide (or heat) allows the protein to become denatured and act as an antigen.

Daily cleaners contain a surface active agent, or detergent, which reduces the surface tension of fats and other lipophilic substances. The surfactants can be non-ionic, with hydrogel lenses, to minimize interaction with the material. As multiuse solutions, they will also contain a preservative.

The formulations commonly include a chelating agent such as EDTA, phosphate or borate buffers, and sodium or potassium chloride, which determines the tonicity of the solution.

There is no concern about toxicity of standalone cleaning solutions because they should be rinsed thoroughly from the lens surface before it is disinfected. This avoids the possibility of contaminating the soaking solution.

The mode of use of daily cleaners requires that after their application to the contact lens, its surface is rubbed between the fingers for some moments. An efficient rubbing action plays a role in decreasing contamination by microorganisms such as Acanthamoeba polyphaga (Liedel & Begley 1996). The rubbing with cleaner is followed by rinsing with a suitable solution such as sterile saline.

Current practice is dominated by multipurpose solutions which, it is claimed, eliminate the need for a separate daily surfactant cleaner. However, a study that compared the use of a dedicated cleaner as part of a care system with a multipurpose product showed that use of the former resulted in more comfortable and cleaner lenses (Lebow & Christenson 1996).

DISINFECTING SOLUTIONS

With the exception of daily disposable hydrogel lenses, all contact lenses need to be disinfected after their removal and cleaning. Disinfecting solutions have also been described as soaking solutions because they act as a storage medium when the contact lenses are not being worn. If

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rigid lenses were stored dry, any organic contaminants on them would support the growth of microorganisms. The surface wetting properties of some rigid gas permeable materials would be impaired by dry storage.

Wet storage of hydrogel lenses is essential not only to ensure disinfection but also to maintain their hydration because they have a water content that can range from 38 to 80%. Although silicone hydrogel lenses have a much lower water content (25–47%), wet storage is necessary to maintain wetting of their surfaces.

Two types of solution are currently used to achieve disinfection of contact lenses:

•A stable solution containing a preservative that has been formulated for use either with rigid contact lenses or with hydrogel and silicone hydrogel lenses.

•A transient solution containing an antimicrobial agent that is inherently unstable and is broken down by a neutralizer until none of the active ingredient remains. Such solutions were originally introduced for use with hydrogel lenses but can also be used with rigid and silicone hydrogel lenses.

Any system of disinfection should be effective not only against commonly used challenge bacteria such as Staph. aureus, Ps. aeruginosa and

Escherichia coli, but also on bacteria such as Serratia marcescens (Ahearn et al 1986, Parment et al 1986), Haemophilus influenzae (Armstrong et al 1984) and other organisms such as fungi (Brooks et al 1984, Churner & Cunningham 1983) and acanthamoebae. Fungi are a particular problem, as organisms can actually penetrate hydrogel lenses (Bernstein 1973, Filppi et al 1973).

STABLE DISINFECTING SOLUTIONS

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hydrogel materials. If this preservative came into contact with a hydrogel lens, initially it would cause irritation and later become toxic as its concentration increased. Rosenthal et al (1986) have claimed that benzalkonium chloride also binds to rigid gas-permeable materials in a manner different to that of chlorhexidine binding, which is limited to a monomolecular layer. Benzalkonium chloride builds a self-propagating multilayer that reduces surface wettability. In a study of in vivo and in vitro absorption of benzalkonium chloride, Wong et al (1986) demonstrated that rigid gas-permeable lenses did not accumulate significant levels of the preservative. A study by Chapman et al (1990) showed that the level of benzalkonium chloride that can be released from either hydrogel or rigid lenses is of sufficient concentration to be at, or near, the upper limit of safety.

Multipurpose solutions Solutions intended for both cleaning and disinfection of hydrogel lenses were introduced in 1994 and, by 2005, represented the care system used by 91% of patients in the UK (Morgan & Efron 2005). The popularity of multipurpose solutions is due to the fact that they simplify contact lens care for the patient and this, it is hoped, enhances compliance. The most recent development in the quest for simplification is the ‘no rub’ multipurpose solution, which eliminates the rub-and-rinse step.

Nevertheless, one study indicated that the greater the number of steps in a regimen, the greater its disinfecting efficacy. Rinsing the lenses prior to disinfection was considered to assist this process (Rosenthal et al 2004).

These solutions incorporate preservatives regarded as ‘novel’ in the sense that they have not been in general use in other ophthalmic products and are of high molecular weight. Examples are the biguanide preservative polyhexanide [polyaminopropyl biguanide or polyhexamethylene biguanide (PHMB)], which has the proprietary name of DYMED and a molecular weight of 1300. It is used in a concentration of 0.00005 to 0.0001% and provides efficient antimicrobial action even in high dilution.

Polymeric biguanides have found use as general disinfecting agents in the food industry and for the disinfection of swimming pools. PHMB is a membrane-active agent that also impairs the integrity of the outer membrane of Gram-negative bacteria, although the membrane can also act as a permeability barrier. Activity of PHMB increases on a weight basis with increasing levels of polymerization, which has been linked to enhanced inner membrane perturbation. Unlike chlorhexidine but like alexidine, PHMB causes domain formation of the acidic phospholipids of the cytoplasmic membrane. Permeability changes ensue and there is believed to be an altered function of some membrane-associated enzymes.

Significant levels of relatively asymptomatic corneal staining were observed when patients wearing silicone hydrogel lenses used a PHMBbased solution (Jones et al 2002). Polyquad (polidronium chloride) is a cationic detergent with a molecular weight of 5000. It is used in a

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but a non-electrolyte tonicity agent has been used for this purpose. It had been assumed that a sodium chloride equivalent of 0.9% would provide the best solution but Kempster (1984) studied the effect of soaking lenses in solutions of different tonicities and found that those between 1.0 and 1.1% w/v produced minimal changes in corneal thickness. A randomized, double blind study by Fletcher & Brennan (1993) demonstrated that both hypotonic and hypertonic solutions can produce ocular discomfort. Discomfort was minimal with a saline solution of 1.3% (w/v) concentration.

One disturbing experimental study has suggested that contact lens solutions might have the potential to contribute inadvertently to an increased risk for lens-related microbial keratitis, as their topical application increases binding of Ps. aeruginosa and reduces corneal surface cell exfoliation (Li et al 2003).

Multipurpose solutions for use with rigid lenses have similarly adopted preservatives such as polyhexanide and also polixetonium chloride and also include some form of cellulose viscosity agent.

Attention has previously been drawn to the importance of good personal hygiene, such as thorough hand washing before the lens is handled. This precaution can reduce the burden placed upon the solution used to achieve disinfection.

TRANSIENT OR OXIDATIVE DISINFECTING SOLUTIONS

Hydrogen peroxide Hydrogen peroxide was the first chemical means of disinfection of hydrogel lenses to be introduced as an alternative to heat disinfection and the extent of its usage has varied over the years. In 2005, it was used by only 10% of patients in the UK (Morgan & Efron 2005). The principal advantages of hydrogen peroxide are:

•It is free of preservatives, to which some patients exhibit an allergic or toxic response.

•Its broad spectrum of antimicrobial action against bacteria, viruses and yeast. A 3% solution can kill trophozoites of Acanthamoeba castellanii after 3 min and cysts after 9 hours.

The original procedure involved soaking the lenses in 3% peroxide for 5 min, they were then exposed to sodium bicarbonate (0.5% w/v) to accomplish neutralization (Isen 1972). Following two changes of saline, the lenses were stored overnight in fresh saline. The time-consuming nature of this multistep process prevented the popular use of hydrogen peroxide until simpler means of neutralization were developed.

Hydrogen peroxide has been used for many years as a surface disinfectant and for its bleaching effect. Its strength can be described in terms of volumes (10 volume, 20 volume, etc.). A 10 volume solution possesses 10 times its volume of oxygen when it breaks down. In terms of percentage, a 10 volume solution is about 3% w/v. Tragakis et al (1973) found that hydrogen peroxide was very effective against all the organisms used in their tests and similar results were obtained by Penley et al (1985).