Soft Lens Care Systems

The Rationale for Disinfecting Contact Lenses

Contact lens practitioners are acutely aware that an eye wearing a contact lens is more likely to become infected than an eye not wearing a contact lens. estimated the risk of contact lens-associated infection as being 60 times greater in a contact lens wearer than in a nonlens wearer. The reasons for this increase in risk are multifactorial, and it is worth considering these factors in the first instance as they essentially form the rationale for contact lens disinfection.

The eye has a number of inherent protective mechanisms to resist infection. These are generally successful, as can be seen in the light of work by , who estimated that potential pathogens are present in the tear film of 5% of a population at any time, yet the prevalence of ocular-surface infection falls far short of this value. The tear film and the blinking process play an important role in the resistance of infection. Basal tear production is of the order of 1–2 min and the overall tear volume is about 7 L, which confirms the rapid turnover of tears at the ocular surface with the consequent removal of microorganisms.

Bacteria in the tear film must also breach the defence provided by proteins in the tear film such as lysozyme, lactoferrin and surfactant protein D. Furthermore, immunoglobulins such as secretory IgA, IgG, IgE and IgM can act to resist infection.

A microorganism that is able to defeat all the above systems is still hampered in its quest to invade and infect the cornea because of the various defences of the corneal epithelium. These include: tight junctions, which prevent the migration of microorganisms between epithelial cells; sloughing of cells from the epithelial surface to remove infected cells before any further harm is caused to the rest of the cornea; the active release of antibacterial factors from the corneal epithelium; and the ‘filter-like’ barrier provided by the basal lamina, which prevents bacteria reaching the underlying stroma ( ).

Contact lens wear adversely affects a number of these defence mechanisms. Perhaps the most significant effect is the prevention of clearance of debris and microorganisms from the ocular surface by the blinking mechanism owing to the protection offered by the contact lens. It has also been postulated that the level of fibronectin is reduced during contact lens wear, thereby increasing the likelihood of bacterial attachment to the epithelium ( ).

A key reason for the increase in ocular infections amongst contact lens wearers is the bioburden of microorganisms introduced to the ocular surface when lenses are applied. Indirect evidence for this is provided by the work of , who analysed the risk factors associated with contact lens-related infections. They found that the risk of infection was significantly increased in those wearers undertaking only irregular disinfection with their contact lenses. The association of inappropriate use of the contact lens storage case ( ) and a lack of handwashing ( ) with an increased risk of contact lens-related infections also supports the notion that contact lens wear presents an increased microbial challenge to the ocular-surface defence systems. As such, it seems clear that whilst contact lens wear renders the eye at greater risk of infection, there is good evidence that the appropriate use of suitable contact lens disinfection systems will reduce the magnitude of this increased risk.

The Rationale for Cleaning Contact Lenses

There are two key reasons why a contact lens should be cleaned prior to disinfection. First, a wide variety of intrinsic and extrinsic debris can adhere to the surface of a contact lens. This can lead to lens distortion, discomfort, an unsightly cosmetic appearance (as soiled lenses can show marked discolouration clearly visible to an onlooker), ocular-surface and eyelid pathology and vision loss ( ). Lens cleaning can mitigate against these problems. found rinsing to be ineffective in removing loosely bound deposits on lenses compared with rubbing. Lens deposits are discussed in Chapter 20 . Second, cleaning acts to enhance the disinfection process by reducing the levels of microorganisms on the contact lens. demonstrated this by contaminating contact lenses with an organic load plus live cells of Pseudomonas aeruginosa or Staphylococcus epidermidis . When lenses were rinsed for 10 seconds, contamination was reduced from 1 million colony-forming units (cfu) per lens to less than 3000 cfu per lens. When the lenses were rubbed with the index finger in the palm of the hand for 10 seconds with three drops of cleaner on each side before the rinsing process, there was a reduction to less than 300 cfu per lens ( Fig. 10.1 ). The importance of contact lens cleaning is supported by the epidemiological work of , who demonstrated that the risk of infection was about three times greater in patients who cleaned their lenses less than twice per week compared with those who achieved at least this frequency of cleaning. have demonstrated that ‘rubbing and rinsing’ with a contact lens solution is more effective in removing pathogenic microbes from a lens surface than rinsing alone or indeed when no treatment is employed. Accordingly, it would seem prudent to recommend that all contact lens care systems include a rub and rinse step as part of the hygiene regimen.

The Evolution of Soft Lens Care Systems

Historically, the care of a soft contact lens was a complex and time-consuming activity for the wearer. The various steps of the lens care process were divided into separate activities with a different bottle or tablet for each. This has changed over time for two reasons: first, the introduction of multipurpose products has meant that more than one component of the lens care process could be undertaken with a single solution. Second, the commercial success of disposable and planned-replacement soft lenses in the 1990s – with the associated emphasis on patient convenience – acted as a powerful incentive for the lens care industry to reduce the complexity of the lens care process as much as possible. In some respects, such as a reduced requirement for regular enzymatic cleaning of soft lenses, this was justified. In others, such as the omission of any form of lens surface cleaning, there appeared to be a related increase in the incidence of microbial keratitis ( ).

The evolution of soft lens care systems in the United Kingdom between 1997 and 2020 is displayed graphically in Fig. 10.2 ( ). It can be seen that, since 2001, multipurpose and hydrogel peroxide solutions have been essentially the only disinfecting systems prescribed for soft lens wearers. Also, evident in this graph is a steady rise is the use of multipurpose disinfecting systems and a decrease in the use of hydrogen peroxide disinfecting systems. A slight reduction in the use of multipurpose disinfecting systems and commensurate increase hydrogel peroxide disinfecting systems in 2006–08, and again in 2016, coincided with reported outbreaks of acanthamoeba keratitis around these times [ ; ], whereby practitioners apparently preferred to temporarily prescribe more hydrogel peroxide systems, which have greater antimicrobial efficacy against acanthamoeba.

Similar trends have been observed worldwide. A survey in 2020 of over 13,000 contact lens fits in 24 countries demonstrated that 88% of soft lens wearers were prescribed a multipurpose product, with the remaining care systems mainly peroxide based ( ).

Physical Methods

The various physical methods of soft lens disinfection rely on energy being imparted to microorganisms to cause lethal cell changes. Heating was the first soft lens disinfection method approved by the US Food and Drug Administration (FDA) in 1972. Disinfection using this approach requires a temperature of 80°C to be maintained for at least 10 minutes. A representative example of one of the heating units available at this time was the Bausch & Lomb system ( Fig. 10.3 ), which reached 96°C for a period of about 20 minutes ( ). In terms of lens disinfection, the heating systems were recognized as being highly effective ( ), even against the protozoan Acanthamoeba ( ). Furthermore, after the initial purchase of the heat unit, the ongoing costs of operation were minimal.

However, there were a number of disadvantages associated with heat disinfection. In normal circumstances, the protein that spoils the surface of a soft contact lens does not denature or does so slowly; however, heating the lens will immediately denature protein with adverse clinical consequences such as reduced acuity, the potential for ocular-surface reactions such as giant papillary conjunctivitis and altered physical lens parameters. With the popularity of low-water-content, nonionic lenses in the early and mid-1970s, this was not as significant a problem as with the use of these systems with more modern, high-water-content and ionic materials that absorb much greater quantities of proteins ( ). Fig. 10.4 demonstrates how a standard single heat disinfection cycle – performed on an ionic, high-water-content lens that had been worn for 8 hours – causes the lens to turn yellow and become deformed.

The heating process was also inconvenient for many wearers. Not only did this method require a nearby source of electricity, but also the system used unpreserved saline, which did not offer any antimicrobial activity. The opportunity for microbial contamination arose if the lenses remained in the cooled saline for a prolonged period, so this disinfection system required the process to be repeated each day with fresh solution if the lenses were not used.

With the advent of planned-replacement lenses, which were generally of high water content and often manufactured from ionic materials more prone to parameter changes, the popularity of heat disinfection waned, and this technique is rarely used today.

Microwave irradiation has been proposed as a potentially cheap and effective method of soft lens disinfection. demonstrated that, although there are some parameter changes when lenses are repeatedly irradiated with a standard 650 W microwave oven, none of these changes are clinically significant. However, as patients often need to care for their lenses in locations remote from a microwave oven, this approach is often impractical or inconvenient.

Studies on the efficacy of ultraviolet radiation for contact lens disinfection have provided equivocal results. Using radiation at 253.7 nm at an energy of 44.3 W/cm ² , found that pathogenic Acanthamoeba cysts and trophozoites survived irradiation of 22 minutes duration. Similarly, , using an identical system, concluded that, although the numbers of microorganisms were reduced by ultraviolet irradiation, the level of survivors was unacceptably high. On the other hand, demonstrated that disinfection could be achieved at the same wavelength for a panel of bacteria using an ultraviolet lamp with a higher energy output of 950 W/cm ² . concluded that any parameter changes with this method were not clinically important.

The use of ultrasound systems for lens disinfection has also been proposed. Although such devices can be shown to have a limited disinfection effect ( ), the efficacy of ultrasound energy is limited by the physical similarity between lens material and solution, meaning that the relatively large amount of energy required to clean the lens successfully would probably damage its surface ( ).

Chlorhexidine- and Thiomersal-Preserved Systems

After the problems associated with heat systems became known, alternative disinfection systems were required that allowed for the simple storage of contact lenses without affecting the lens or causing irritation to the eye. Suitable early candidates were products that contained chlorhexidine gluconate or thiomersal.

Chlorhexidine is probably the most widely used biocide in antiseptics, especially for handwashing and oral products. Its action has been closely studied, and it is believed that its uptake by both bacteria and yeast is extremely rapid. Chlorhexidine damages cell walls and subsequently attacks the bacterial cytoplasmic or inner membrane, or the yeast plasma membrane ( ).

Thiomersal is considered to be a less effective antimicrobial agent overall, although its action against fungi is better than that of chlorhexidine. Due to this, a combination of chlorhexidine gluconate and thiomersal became common in disinfectants for soft contact lenses. However, due to the absorption of these agents onto soft lenses, toxic and hypersensitivity reactions were reported when they were used clinically ( ). The build-up of these preservatives, and the subsequent leaching onto the ocular surface over time, had the potential to cause discomfort and discontinuation of lens wear ( ). These products were ultimately superseded by others that offered a similar level of convenience and antimicrobial efficacy, and a lower adverse reaction rate.

described a novel approach for a chlorhexidine system, known as OptimEyes (Bausch & Lomb). They developed a tablet that, when dissolved in tap water, provided a solution with a chlorhexidine concentration of 0.004%. The solution was shown to be effective against a panel of challenge microorganisms and also against most of the microorganisms found in tap water. This was a controversial development because this product was specifically designed for use with tap water and practitioners were concerned that contact lens wearers would consider standard tap water as an acceptable component of lens care generally. The product was simple and cheap to use, and it provided action against Acanthamoeba . Its disadvantages included the reliance on a supply of rising mains tap water and, importantly, it was contraindicated for use with FDA group IV lenses because the action of chlorhexidine is reduced with ionic lenses. With the increasing popularity of group IV lenses throughout the 1990s, the OptimEyes product did not become a mainstream soft lens care product.