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Stand alone test

•Primary standard: indicates a reduction by 99.9% (3 log units) of a challenge of the test bacteria and 90% (1 log unit) reduction for fungi at the minimum recommended disinfection time (MRDT).

•Secondary standard: for products that can be used as part of a care regimen if they meet the criteria of a combined log reduction against the three bacteria of at least 5 log units with a reduction of at least 1 log unit for any single bacterium and that there is no growth of the test fungi at the MRDT.

Regimen test

Following the full recommended care regimen, no more than 10 viable organisms per lens should remain.

Acanthamoeba is not included in this standard at the moment as there are no specifically identified species type, no standard method of recovery nor quantification of survivors and no standard test methods.

26.2 Solution for soft lenses

The general principles for soft lens solutions are similar to those used for rigid lenses, but there are potentially more difficulties because of the possibility of interaction with the material. Viscosity agents are not generally employed and the pattern of use is often different, since many solutions must be partnered to ensure their complete antimicrobial efficacy.

The ideal care system should have the following core requirements:

•Effective – reduce or eliminate ocular pathogens.

•Compatible – non-toxic to the ocular surface.

•Maximize comfort – reduce deposits and condition surface.

•Simple to use.

•Affordable.

26.3 Disinfection

Disinfection may be by means of either chemicals (cold) or heat.

26.3.1 Chemical disinfection

Cold disinfection uses either preserved chemicals or unpreserved oxidative systems.

Multipurpose solutions

Combine disinfection and cleaning. The soaking solution can be used for rinsing. Ideal for disposable lenses (e.g. Renu MultiPurpose, Quattro, Focus Aqua, Complete EasyRub, Optifree Express); easy to use, economical, but there is a possibility of an allergic reaction to some of the preservatives. The product can be kept up to three to six months after opening.

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Table 26.2  Polyquad systems

Table 26.3  Polyhexanide systems

Formulation: high molecular weight preservative; sequestering agents, lubricating agents (Tables 26.2 and 26.3).

Oxidative systems

Oxidative systems are generally unpreserved and use hydrogen peroxide or chlorine-based compounds as the disinfecting agent.

Hydrogen peroxide (H2O2)

Most peroxide systems use a 3% concentration. They include a sodium or phosphate stabilizer to prevent the rapid decompensation of the otherwise unstable H2O2.1 The systems need neutralizing agents to convert the peroxide into a safe neutral solution. The two-step systems were seen as the ‘Gold Standard’ but they are now in the main discontinued. Three percent hydrogen peroxide has a broad antimicrobial efficacy. Although 10 minutes is effective, longer soaking times of between 1 and 4 hours are recommended to ensure adequate antifungal and antiprotozoan activity, with 6 hours required to kill all Acanthamoeba cysts. This usually means that an overnight soak is desirable but this is not possible with a one-step system unless the neutralization stage is omitted for overnight storage.

A tablet of enzyme cleaner can be included with the peroxide solution (e.g. Ultrazyme).

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Advantages of hydrogen peroxide

•No reaction to preservatives.

•Efficient method of disinfection especially when used overnight.

•Enhances cleaning.

•Prolongs lens life.

Disadvantages of hydrogen peroxide

•The peroxide must be neutralized.

•The neutralizing time is sometimes lengthy.

•Complicated to use and understand.

•One-Step systems offer no on-going disinfection for intermittent wearers

•One-Step systems give no protection against Acanthamoeba due to fast neutralization times.

•Possibility of patient error. Three options are available:

1.Catalytic neutralization

(a)Using catalase, a naturally occurring bovine catalyst which is highly specific for the speedy decomposition of hydrogen peroxide, either in solution (e.g. OxySept two step, which has now been withdrawn) or with a time release tablet coated in hydroxypropylmethylcellulose (e.g. OxySept One Step). Time release tablets delay neutralization for the first 20 minutes. The concentration of peroxide does not fall below 1% and there is then almost complete neutralization by 2 hours. A pink

colourant (vitamin B12) is used to aid compliance in tablet use and safety of lens wear.

(b)Using a platinum coated disc (e.g. AOsept, EasySept). There is a rapid neutralization in the first 2 minutes (from 3% to 0.9%) followed by a much slower phase than catalase because the molecules have to migrate through the buffered saline to the disc surface instead of being evenly distributed throughout the solution. This slower phase may take up to 6 hours. The disc is replaced with each new bottle since a case is included. If the neutralizing solution is buffered to a high pH (e.g. AOSept), the kill time is slower. The neutralization does not depend on the concentration of the catalyst and no by-products are formed except water and oxygen. The recent inclusion of cleaning agents aids lens cleaning during the early neutralization phase when bubbling at the disc is at its peak (e.g. Sauflon Multi, AOSept Plus)

2.Low concentration

Preservative free solutions are available which contain hydrogen peroxide but do not behave in the usual manner. Hydrogen peroxide is present at extremely low concentration (0.01%) and, although forming a synergy with the chlorite ion, also performs a stabilizing role (e.g Regard (Advanced Eye Reseach) and Synergi (Sauflon)).

Both products contain surfactants and wetting agents to improve lens wetting and comfort.

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3.Dilution

By rinsing and soaking in saline (e.g. the now discontinued Quik-Sept).

Chlorine systems

Chlorine-based systems contain active ingredients such as sodium dichloroisocyanurate (e.g. Softab) called a low chlorine system or para-dichlorosulphamoyl benzoic acid (e.g. Aerotab) called a high chlorine system. They are now no longer available in the UK.

26.3.2 Heat disinfection

Heat disinfection is generally carried out using unpreserved normal saline at a pasteurization temperature of 80°C for 10 minutes. The addition of sodium edetate gives calcium-removing properties. Lenses must always be allowed to cool before insertion. This method is now rarely used.

In emergency, patients could disinfect lenses stored in 0.9% saline by boiling in a saucepan of water. Care is required that the case can withstand high temperature.

Microwave

Non-pressurized microwave disinfection can be carried out and past techniques included the Microclens. The system consisted of the disinfector unit containing Microclens saline into which the contact lens case, filled with the same solution, was placed. The unit was microwaved for 1–2 minutes.

Saline

Saline was extensively used for heat disinfection but is now mainly used for rinsing.

A solution of 0.9% saline is referred to as isotonic, having an overall sodium chloride concentration equivalent to that of human tears. A solution with higher salt concentration is hypertonic and one with a lower concentration hypotonic.

Normal (0.9%) saline may be either buffered or unbuffered and is available in the following formats:

•Preserved in multidose bottles (e.g. CIBAVision).

•Unpreserved in unit dose form (e.g. Amidose).

•Unpreserved in aerosol form – but no longer readily available.

•Drip-feed bags with one-way valve (not obtainable in the UK except in some hospital departments).

WARNING

Home-made saline, using purified water, ceased in the UK in 1988 with the withdrawal of salt tablets. Acanthamoeba infection in the USA has been linked to home-made saline.2

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Tap water

Tap water must not be used for soft or silicone hydrogel lens storage and insertion, or for cleaning the lens case, because of:

•Serious risk of microbial contamination (e.g. Acanthamoeba).

•Trace metals and salts.

•Lens adhesion as a result of hypotonicity.

Ideally, any use of tap water should be avoided in the hygiene regimen, except for hand washing. This fact is now mentioned on solution leaflets: ‘Do not rinse and store your lenses or lens case with tap water’ (e.g. Complete Multi-Purpose Solution).

26.3.3 Cleaning solutions

Formulation: surface-active agents; preservatives.

In addition to cleaners specifically formulated for soft lenses (e.g. Soflens Sensitive Eyes Cleaner), rigid gas-permeable lens cleaners which do not contain benzalkonium chloride can be used. Alcohol-based cleaners are effective against lipids. For denatured protein, in the past a surfactant-polymeric bead cleaner combined a cleaning agent with microscopic, polystyrene-like beads.3

Daily manual surface cleaning is important to:

•Remove lipids, inorganic deposits, some proteins and insoluble contaminants by manual action.

•Overcome the hydrophobicity of oily deposits with surface-active   agents.

•Assist chemical disinfection by removing deposits that could interfere with antibacterial activity.

•Remove contaminants that supply nutrients to bacteria.

•Disperse mucus film to minimize the potential for binding of any large molecular size antimicrobial agent.

Second generation multipurpose solutions claim to achieve physical cleaning as they often contain a cleaning agent in their formulation. Although soaking in the solution gives a potential cleaning action (e.g. Optifree Express), the manual ‘rub’ is important to reduce the bacterial load and accumulated deposition.

The No-Rub concept is deprecated as it is generally considered essential  that a ‘rub n rinse’ step be performed not only to reduce bioburden and so  maximize disinfection but also to disperse mucus film, minimizing the potential for binding. In the case of silicone hydrogel lenses, the rub and rinse stage is  even more important to reduce the level of lipids which can be a problem  with some lenses. Deposits are exacerbated in patients with meibomian gland dysfunction.

Rinsing is indeed an integral part of the cleaning and disinfection process. Cleaning and rinsing together have been shown to remove 99.9% of microorganisms from the lens.

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Other cleaning methods

•Ultrasonic units.

•Ultraviolet units.

•Spinning devices.

26.3.4 Case hygiene

Case hygiene is one of the most frequently overlooked aspects of lens care hygiene. Certain microorganisms secrete a protective capsular layer of slime around their cell membrane called a glycocalyx (biofilm). Often, the contact lens case becomes a reservoir of contamination of biofilm from lenses and microorganisms, along with other associated debris. For this reason, proper care of the lens case is an important factor in keeping the lenses clean and reducing the chance of infection. Various studies have shown that more than 50% of contact lens cases are found to be contaminated with bacteria and 4% with amoebae.

After the lenses have been inserted, the case should be rinsed with disinfecting solution or sterile saline and left open to air dry. A dry case is important, as microbes cannot multiply in a dry environment. Lens cases should be replaced at least 3-monthly but, ideally, every month.

26.3.5 Rewetting solutions

Moisture retaining lubricants with or without preservative (e.g Refresh Contacts). Multidose solutions are available containing Purite as the preservative.

For reducing surface protein with extended wear silicone hydrogels, multidose solutions are used containing Tyloxapol for lipids and debris, and Tromathamine to emulsify and displace protein (e.g. Complete Blink-N-Clean).

26.3.6 Periodic cleaners

Enzyme tablets

Enzyme tablets are used for the removal of protein from the lens surface. Weekly cleaning is suggested, but patients with peroxide are able to use them less frequently. Tablets are dissolved in saline or used in the actual disinfecting solution (e.g. Oxysept One Step). Some second generation multipurpose solutions incorporate a sequestering agent and so do not need a separate system (e.g. RenuMultipurpose, OptiFree Express). Sequestering or chelating agents, such as EDTA, citrate and certain buffering agents (phosphate) found in the other multipurpose solutions, all lay claim to passive protein removal.

Liquid enzyme is also available, e.g. as a pancreatin based product. One drop is placed in the lens case with the disinfecting solution on a nightly basis (e.g. SupraClens).

Formulation: papain (Hydrocare or Bausch & Lomb); subtilisin A (Ultrazyme); pancreatin (Clenzyme); lipase, pronase, protease, sodium edetate (Amiclair).

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