Ординатура / Офтальмология / Английские материалы / Ophthalmic Drugs Diagnostic and Therapeutic Uses 5th edition_Hopkins, Pearson_2007
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CONTACT LENS SOLUTIONS 167
Two-step disinfection When used to disinfect contact lenses, it is necessary to neutralize the peroxide after an interval of time and early products were described as ‘two-step’, which means that the patient caries out disinfection and neutralization as separate stages in the procedure. The advantage of a two-step system are:
•The extended duration of antimicrobial action that can, for example, be overnight.
•In hot, humid climates that favour microbial proliferation, overnight storage in hydrogen peroxide is advantageous.
•Suitable when contact lens wear is intermittent rather than daily.
It is important that neutralization of the peroxide is not undertaken too rapidly. Holden (1990) has warned that a short exposure of 10–15 min is inadequate to ensure disinfection of fungi and acanthamoebae and he recommended a disinfection period of at least 2 hours.
In one system, a solution containing the enzyme catalase is added to the storage case after the hydrogen peroxide has been discarded.
Although a two-step system is compatible with non-ionic high water content hydrogel lenses, ionic high-water-content lenses undergo marked hydration changes as a consequence of the lengthy immersion in hydrogen peroxide. Lowe et al (1993) concluded that the latter lens type should not be used with this peroxide method.
A disadvantage of some two-step processes is the need for the patient to remember to initiate neutralization. The pH of hydrogen peroxide is in the range of 3.5–4.5 (McKenney 1990) and the patient who omitted to neutralize it before inserting a lens into the eye would make this mistake only once due to the resultant degree of discomfort! After neutralization, the final pH is between 6.15 and 7.64, and the level of residual peroxide is 0–70 ppm (Harris 1990). Any discomfort experienced by patients using hydrogen peroxide is therefore likely to be due to pH rather than the residual concentration. It would appear to be a wise precaution to advise patients to rinse their lenses liberally with sterile saline prior to insertion.
Other agents that have been used to neutralize hydrogen peroxide include sodium pyruvate, sodium sulphite and sodium thiosulphate.
One-step disinfection One-step systems were introduced to simplify the procedure for the patient and have the advantage that the vital step of neutralization cannot be overlooked. The disadvantages are:
•
•
The duration of the disinfection stage is less than with two-step products.
Unsuitable for long-term storage of lenses.
About 90% of the hydrogen peroxide systems now used are one-step. In one such system, a platinum disc in the storage case is used as a catalyst to break down the peroxide into oxygen and water. Having filled the case with 3% (w/v) peroxide, there is an initial rapid phase of neutralization resulting in a concentration of 0.9% (w/v) after 2 min, which is followed by a slow phase to approximately 15 parts per million (ppm) after 6 hours. Such a level is below the threshold of subjective
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Other transient disinfectant solutions
sensitivity (50–300 ppm) and that at which adverse effects on the mitotic activity and movement of epithelial cells occur (30 ppm) (Tripathi & Tripathi 1989). To maintain efficacy of neutralization, the disc is replaced every time further supplies of peroxide are purchased.
Another one-step system uses a tablet coated with hydroxypropyl methyl cellulose, which contains catalyse, which is slowly released after 30 min with reactive neutralization to a final peroxide level of 1 ppm after about 2 hours.
Microbiological contamination of the contact lens storage case regularly receives attention as a possible source of infection in wearers. Wilson et al (1990) found that cases disinfected with peroxide systems showed a lower level of microbial contamination than those in which other methods of disinfection had been used. Frequent replacement of the storage case remains the best means of avoiding the risk of its contamination (Devonshire et al 1993).
Although hydrogen peroxide was adopted for use with hydrogel lenses, it can also be used with rigid and silicone hydrogel lenses. However, it has been reported that one silicone hydrogel (PureVision) suffered parameter changes when used with a one-step peroxide system (Oxysept 1-Step) (Dumbleton & Jones 2002).
Alternative transient solutions have been introduced from time to time but have fallen into disuse. Oxidizing agents such as iodine and chlorine have antimicrobial effects, especially against anaerobes.
A brown-coloured iodine solution was reduced by a neutralizing agent to the inactive colourless iodide, the reaction producing a mixture of electrolytes, none of which were foreign to the eyes.
Chlorine, which like other halogens acts a disinfectant and mild cleaning agent, was used in two products one of which contained dichloro-sulphamoyl benzoic acid (halozone) and the other sodium dichloro-isocyanurate. Both products liberated their active agents from tablets that were dissolved in unpreserved saline. According to Copley (1989) and Ferreira et al (1991), chlorine provides effective disinfection of clean hydrogel lenses, but one such system tested by Lowe et al (1992) failed to meet the United States Food and Drug Administration (FDA) recommendations for bacterial challenge.
LIMITATIONS OF PRESENT METHODS OF CONTACT LENS DISINFECTION
Two examples can serve to demonstrate the limitations of contact lens disinfection with a multipurpose solution or with hydrogen peroxide. Bacillus cereus is a possible aetiological agent of contact lens-associated keratitis. Heat and many types of contact lens disinfecting solutions appear to be ineffective in eradicating B. cereus from contaminated contact lens cases whereas hydrogen peroxide was sporicidal (Pinna et al 2001).
Adenovirus has been shown to survive chemical and hydrogen peroxide disinfection but not heat sterilization (Kowalski et al 2001).
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As heat sterilization is generally unavailable in a practice, patients with adenoviral keratoconjunctivitis should simply discard contact lenses worn at the onset of the infection. On completion of treatment, new lenses should be used.
SALINE SOLUTIONS
Mention has already been made of the important role played by sterile saline solutions in ‘rinsing’ contact lenses.
Saline is also used as a medium for various enzymatic cleaners, which are mentioned later. Another use of saline is to fill the storage case of hydrogel lenses when they are subjected to thermal disinfection. This method of disinfection requires that the lenses in their storage case are placed in either an electrical heater or in a small vacuum flask, which is filled with boiling water.
Saline solutions should have a pH between 6.8 and 8 and an osmolarity between 280 and 320 milliosmoles per litre to ensure comfort and compatibility with hydrogels.
The first method of disinfection of hydrogel lenses employed a saline solution in which the lens was heated to at least 100°C. Although heat disinfection of hydrogel contact lenses offers the advantages of efficacy, low cost and absence of preservatives, it is now very seldom used. The fact that heating tended to shorten the lifetime of lenses is no longer a disadvantage in view of the common use of disposable or planned replacement types. In the early 1970s, salt tablets were supplied so that they could be dissolved in the correct amount of purified water in order to produce normal saline for use with heat disinfection. This method of producing saline was implicated in the outbreak of Acanthamoeba keratitis in contact lens wearers (Moore et al 1986) and salt tablets for such use are no longer sold in the UK.
Preserved saline Preserved saline has been used to enhance the efficacy of heat disinfection but is generally used today to rinse contact lenses. The function of preservatives in saline solutions is to maintain solution sterility and not to provide a means of contact lens disinfection. Preservatives can pose the risk of allergic and toxic reactions. Shaw (1980) demonstrated the relationship between preservatives and the incidence of solution allergy. In a double masked randomized crossover study, six subjects used an unpreserved saline solution and a preserved saline solution (0.001% thiomersal and 0.1% EDTA). These subjects had a probable history of sensitivity to preserved solutions which was confirmed in this investigation.
Preserved saline solutions containing EDTA as part of the preservative system have a useful deposit resistant effect compared with an unpreserved saline solution (Moore et al 1980).
Polyhexanide and Purite are preservatives that are currently used in saline. The latter is a form of stabilized chlorite, has been adopted as the
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preservative in a saline solution. The chlorite acts as an oxidizing agent and is combined with chlorate and a trace amount of chlorine dioxide. Chlorine dioxide free radicals, which are effective against bacteria, viruses, yeasts and fungi, are generated in the presence of microbial contamination.
There is evidence that preserved saline can become contaminated with Gram-positive bacteria (Sweeney et al 1999).
Pressurized containers Both buffered and unbuffered forms of sterile saline have been made available in aerosol-type containers. This mode of presentation of saline requires no preservatives.
Single-use containers Available both preserved and unpreserved, sachets have the major advantage of remaining sterile until opened. Their disadvantage is the greater cost.
Non-clinical use of When a saline solution is used specifically to undertake standardized saline for contact tests to determine dimensions, physical, chemical and biological lens testing characteristics of contact lenses and contact lens materials, it should be formulated in strict accordance with the relevant International Standard
(ISO 10344:1996).
ENZYMATIC CLEANERS
Tablets containing an enzyme are dissolved in either saline or distilled water to remove denatured protein deposits from the surfaces of contact lenses. They were originally introduced for use about once a week with hydrogel lenses but, if necessary, can also be used with rigid contact lenses. However, fluorine is present in rigid gas permeable lenses to provide good oxygen permeability and has relatively low surface energy, which reduces the adhesion of deposits.
A film of protein, mainly lysozyme (Hosaka et al 1983) but with amounts of albumin and globulin from the tears, can build up on lenses and be difficult to remove with the surfactants in the daily cleaning solutions. Treatment with proteolytic enzymes is necessary to remove this form of contamination. Proteins are macromolecules composed of chains of amino acids, which are linked by peptide bonds. Proteolytic enzymes hydrolyse these bonds either at the end of the molecule, stripping off units of amino acids each time, or by breaking the protein into smaller and smaller multi-amino acid segments. The former are called exopeptidases and the latter endopeptidases. Like all enzymes, the proteolytic enzymes have optimum working conditions, i.e. temperature and pH. Tonicity does not seem to be so important, as some tablets can be used in saline while others can be used in purified water. Another property that enzymes share with all proteins is the possibility of producing allergic reactions. Bernstein et al (1984) reported a case of local anaphylaxis resulting from the use of papain and a similar case was reported by Santucci et al (1985).
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Papain, one of the enzymes used in protein remover tablets, is a thermostable endopeptidase found in the pawpaw fruit. Other enzymatic cleaners utilize pronase, pancreatin or subtilisin. Pronase is comprised of a bacterial protease, a lipase and a pronase, and pancreatin is composed of protease, a lipase and an amylase. Subtilisin is derived from the bacterium Bacillus licheniformis. In a comparison of the efficacy of papain and subtilisin, the former proved to be better with medium and heavy deposits and the latter was better with light deposits (Larcabal et al 1989).
Although hydrogen peroxide cannot be used as a solvent for papain because it is rendered ineffective, it can be used with pronase and subtilisin, resulting in enhanced disinfectant action.
It has been suggested that enzymatic cleaners are unnecessary when hydrogen peroxide is used for disinfection because this agent causes swelling, which is followed by shrinkage upon neutralization and these changes might be thought to dislodge surface contamination. However, when chemical disinfection and enzymatic cleaning was compared with oxidative disinfection alone (Lasswell et al 1986), the lenses treated by the former method were significantly cleaner than those treated by the latter.
The use of protein-removing tablets is not an alternative to daily cleaners because the two agents are complementary. Lenses treated with either type of cleaner were contaminated more than those treated with both (Fowler & Allansmith 1981).
COMFORT DROPS
These drops, which are also referred to as re-wetting, conditioning or cushioning drops, can be instilled into the eyes of contact lens wearers at any time during wear to enhance comfort.
The eye can tolerate adverse atmospheric conditions, and while it can also tolerate the wearing of contact lenses, it sometimes has difficulty in coping with both. In particular, dry, smoky environments can produce changes in the lens–tears system, which turns an acceptable situation into an intolerable one. Certain drugs such as antihistamines, which have an antimuscarinic effect, can reduce contact lens tolerance. Hormonal changes in women can also modify the quantity or quality of tears. In such situations, a comfort drop can be used. These are in reality artificial tears with one extra property, i.e. lens compatibility. They contain a viscosity-increasing substance and, if intended for use with rigid lenses, a wetting agent. If they are in a multidose presentation the drops will contain a suitable preservative.
As comfort drops are for ‘in use’, that is, to be applied to the eye while the lens is being worn, the requirement for preservative concentration is the same as for wetting solutions, i.e. the concentration must be kept as low as possible. For rigid lenses, there are formulations containing polyvinyl alcohol and benzalkonium chloride. The concentration of the latter should not exceed 0.004% w/v for reasons considered in the section on wetting solutions. In the case of hydrogel lenses, the problem
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of preservative binding to the surfaces reduces the available options. For this reason single dose unpreserved saline can be used as comfort drops.
Sibley & Lauck (1974) reviewed the usefulness of comfort drops and concluded that their use reduced patient problems and increased comfort. However, Efron et al (1990) found that any enhancement of the pre-lens tear film is only transient and that the level of hydration of hydrogel lenses is unchanged.
ALTERNATIVE METHODS OF HYDROGEL CONTACT LENS DISINFECTION
Electrical systems employing ultraviolet light and/or ultrasound have been shown to be only partially effective when subjected to a microbiological challenge (Palmer et al 1991, Phillips et al 1989, Scanlon 1991).
However, Dolman & Dobrogowski (1989) found that a 253.7 nm ultraviolet (UV) light with an intensity of 1100 μW/cm2 achieved satisfactory disinfection with short exposure times. A suspension of 104/mL Acanthamoeba polyphaga was sterilized in less than 3 min. Harris et al (1993) similarly concluded that ultraviolet radiation of this wavelength is an effective and rapid method of disinfecting contact lenses.
Rohrer et al (1986) used standard 2450-MHz microwave irradiation to achieve sterilization of hydrogel contact lenses contaminated with a variety of bacterial, fungal and viral corneal pathogens.
According to Harris et al (1989), a standard 600-W microwave oven can provide effective and rapid (90 s) disinfection of hydrogel lenses and up to 40 lenses can be disinfected at the same time.
Hiti et al (2001) concluded that microwave treatment is a very effective, easy and cheap method to keep contact lens cases free of acanthamoebae, thus considerably reducing the risk of an Acanthamoeba keratitis.
The efficacy of standing wave unit and an ultrasound unit was evaluated by Efron et al (1991). The former provided better cleaning but neither unit delivered effective disinfection.
In the Purilens system, hydrogel lenses in a container filled with an unpreserved, buffered sterile saline solution are placed in an electrically powered unit and subjected to subsonic agitation (60 cycles per second) to clean them. The solution is irradiated with ultraviolet light (UV-C at 253.7 nm at one inch for 15 min) to achieve disinfection of the lenses (Choate et al 2000). However, it has been reported that Acanthamoeba cysts can survive this process and it has been suggested that use of a preservative solution may be advisable with this device (Hwang et al 2004).
HYGIENIC CARE OF TRIAL LENSES
In the UK, 96% of new contact lens patients are fitted with soft lenses (Morgan & Efron 2005). The ready availability of disposable soft lenses allows the practitioner to undertake fitting and wearing trials with freshly opened, sterile lenses. After assessment of the fit or a period of wear, the lenses are discarded.
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In the case of rigid lenses, it is usually considered to be necessary to evaluate the fit of several lenses from a non-disposable fitting set to decide the specification to be ordered. The use of trial lenses assumes particular importance when complex designs such as bifocals are to be supplied.
In the light of the guidelines published by the US Centers for Disease Control and Prevention (CDC) and the American Academy of Ophthalmology (AAO), Smith & Pepose (1999) concluded that:
Trial contact lenses that are disinfected with a 2-hour soak in 3% hydrogen peroxide are effectively rid of all pathogens of concern. After disinfection, rigid lenses should be stored dry, and soft lenses should be stored in a sterile, preserved solution. Repeat disinfection should be routinely performed at 1-month intervals to prevent re-growth of organisms.
However, in 1999 concern was expressed in the UK that a contact lens from a trial set could act as a vector for the transmission of variant Creutzfeldt–Jakob disease (vCJD) from one patient to another. The background to prion disease and the risk of its transmission by ophthalmic devices such as contact lenses, applanation tonometer prisms and goniolenses has been the subject of a comprehensive review by Macalister & Buckley (2002).
The procedure currently endorsed by various British optometric organizations is as follows:
•Immediately on removal of the trial lens from the eye, it is cleaned with an appropriate solution (i.e. surfactant cleaner or multipurpose solution). It should be noted that if the lens were allowed to dry, any infective agent would become fixed making it more difficult to break down.
•The lens is immersed in a 2% solution of sodium hypochlorite (20 000 ppm of available chlorine) for 1 hour at room temperature.
•On removal from this solution, any excess is shaken off.
•The lens is rinsed with sterile saline (or freshly boiled water at room temperature). Because a 2% solution of sodium hypochlorite would be extremely toxic to ocular tissue, all traces must be removed from the lens before it is next placed on the eye.
•The lens is disinfected with an appropriate solution.
Two products are commercially available, Menicon Progent and Menicon Meni-LAB, which utilize 0.4 and 0.5% sodium hypochlorite, respectively. An experimental study has demonstrated the efficacy of these products in the disinfection of rigid gas-permeable lenses contaminated with prions (Comoy et al 2003).
PATIENT EDUCATION
It is the practitioner’s responsibility to train patients to ensure that contact lens care products are used correctly and safely. It is important to mention the risks associated with contact lens wear and to review signs and symptoms of ocular infection. Ky et al (1998) found that:
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A sizable proportion of contact lens wearers do not adequately adhere to recommended contact lens care, and many have an inadequate understanding of contact lens care guidelines. Therefore, it is important that practitioners place more emphasis on patient education at the time of initial contact lens fitting and reinforce such instruction during follow-up visits.
Thus, at each aftercare appointment it is essential to review the procedures actually being followed by the patient in an endeavour to maintain compliance with instructions and advice previously given. Nevertheless, the occurrence of contact lens-related corneal ulcers has been reported in patients using conventional and frequent replacement daily wear hydrogel contact lenses who were considered to be fully compliant in their use of multipurpose solutions (Najjar et al 2004).
When contact lens wear is terminated for a period of more than 24 hours, the patient must be advised of the need to disinfect them before further wear. All solutions and the contact lens storage case should be replaced regularly. It is inadvisable to use simultaneously lens care products from different manufacturers due to possible incompatibility of preservatives, pH, etc. Patients should be warned not to use solutions that have passed their expiry date.
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