Extended and Continuous Wear Lenses

Oxygen and corneal swelling

When the eye is open, oxygen is supplied to the cornea directly from the atmosphere ( , ). At sea level, the concentration of oxygen in the atmosphere is approximately 21%, corresponding to a partial pressure (PO ₂ ) of 155 mm Hg or 21 kPa. *

* kPa = kiloPascals, where 1 kPa = 7.5 mm Hg.

When the eye is closed, the cornea receives oxygen almost exclusively from the capillary plexus of the palpebral conjunctiva, but at much reduced levels (PO ₂ = 55–60 mm Hg or ~8%) ( , , ). The consequence of this, or any other significant reduction in available oxygen supply, is that the cornea swells due to alteration of its metabolic balance.

described comprehensively how the metabolism of glucose (glycolysis) in the cornea can occur either aerobically through the Krebs (tricarboxylic acid) cycle, or anaerobically by the Embden-Meyerhof pathway. The main differences between these reactions are that the aerobic route is about 18 times more efficient in energy terms ( ), i.e. generating more adenosine triphosphate (ATP) and results in the production of carbon dioxide and water, whereas anaerobic metabolism creates hydrogen ions and lactic acid. If these latter two entities are allowed to accumulate, an osmotic gradient is created that tends to draw water into the cornea. However, under normally oxygenated circumstances, no swelling occurs because the endothelial fluid pump can match the inward and outward flow rates. This is the case, even though the majority of glycolysis (85%) actually occurs anaerobically at normal oxygen levels. Should available oxygen become scarce, the minority of cells that were formerly behaving aerobically, start to shift onto the anaerobic pathway. As more and more hydrogen ions and lactic acid are created by this reaction, the osmotic gradient steepens until it can no longer be opposed by the endothelial pump. The influx of fluid that follows causes the familiar oedema and corneal swelling responses ( ).

Amount of Oxygen Required

The question of how much oxygen is needed to prevent corneal swelling has drawn considerable attention since the original finding that an atmospheric level of only 1.5–2.5% oxygen (11–19 mm Hg or 1.5–2.5 kPa) was required to avoid corneal oedema ( ). This estimate was later increased to 3.5–5.5% oxygen ( ), and provided values as high as 15%, together with the realisation that considerable individual variation occurs in the population. found that individual needs can range from 7.5–21% and suggested that a level of 10% oxygen must be available to the cornea to prevent oedema ( Fig. 12.1 ).

A high value is to be expected given the relative scarcity of aerobic cells in the cornea, therefore oxygen requirements at these levels are only achievable under open eye conditions. During eye closure, the partial pressure of oxygen from the conjunctival vessels is much less than the 21% (155 mm Hg) available from the atmosphere, and so mild corneal oedema during sleep is a regular and normal phenomenon. The average amount of swelling produced is 3–4% ( , , , , , , ), again with considerable individual variation. An extended-wear contact lens will further impede the already reduced oxygen supply to the anterior corneal surface, resulting in greater than normal amounts of oedema.

Preventing this from occurring requires that adequate oxygen is continually delivered into the tear film beneath the contact lens, a process that can follow two possible routes:

1) Percolation of tears around the lens edge into the post-lens space, a process aided by blinking and lens movement. By its nature, this mechanism is active only when the eye is open. Although the possibility of pumping during rapid eye movement (REM) sleep has been suggested ( ), it is unrealistic to expect significant addition to oxygen availability via this means.

2) By diffusion through the lens material which is the main avenue for oxygen delivery in both soft and RGP wear, so it is important to maximise lens transmissibility.

Corneal oxygenation models (see Section 9 , Addendum, available at: https://expertconsult.inkling.com/ )

During the 1980s, attempts were made to establish the minimum oxygen transmissibility Dk/t (Dk = oxygen permeability, t = lens thickness) required to prevent lens-induced oedema during both daily and extended wear ( , , ), with the Holden-Mertz criterion being the best-known model.

As this is roughly the amount of swelling that occurs in the absence of a lens, the Holden-Mertz criterion became the de-facto standard for extended-wear lens oxygen transmissibility.

).

More sophisticated models of corneal behaviour during sleep have since been developed that offer a deeper insight into transport through the cornea of oxygen and other metabolites, such as lactate, bicarbonate and glucose, as well as how these are modified by lens wear. These models are two-dimensional in nature. This means that, unlike earlier one-dimensional attempts, which only considered the central cornea, they take into account the whole corneal profile from centre to periphery. In addition, wear of a lens with any geometry and oxygen permeability can be modelled at the same time ( , , ). The results of these methods show clearly that behaviour differs depending on both depth from the epithelial surface and distance from the corneal centre. As Fig. 12.2a shows for a closed eye with no lens, the partial pressure of oxygen at the corneal centre drops steadily as we move posteriorly from the epithelium until it is almost zero in the mid-stroma. From there it increases towards the endothelium because significant oxygen levels are present in the anterior chamber. The situation at the corneal periphery is even more extreme (see Fig. 12.2b ), with total anoxia prevailing over a large region of the mid-stroma.

Probably because it is relatively difficult to observe and measure, the phenomenon of peripheral corneal swelling during sleep has received little attention. Early measurements using hydrogel lenses of varying negative back vertex power showed overnight peripheral swelling to be in a tight range of about 3–5%, ( ), a value that is supported by . The relationship between lens Dk/t and peripheral swelling has been measured only under open eye conditions ( ). Thus, from an extended-wear perspective, while lenses with transmissibilities above the Holden-Mertz or similar criteria are known to be able to successfully replicate the no-lens, closed-eye, swelling behaviour of the central cornea, no similar threshold yet exists for the corneal periphery. It is likely that any such benchmark will be relatively high and probably above 100 × 10 ^–9 (cm.ml O ₂ )(s.ml.mm Hg) ^–1 because lenses of this approximate transmissibility have previously failed to eliminate peripheral swelling ( ).

Carbon dioxide

Carbon dioxide (CO ₂ ) is not a direct end product of anaerobic glycolysis, but it does tend to accumulate in hypoxic corneas as a product of the bicarbonate buffering of hydrogen ions ( ). If this reaction did not occur, the resulting acidosis would be rapidly catastrophic and so the occurrence of hypercapnia (increased carbon dioxide concentration) is a relatively mild consequence. Nevertheless, hypercapnia does contribute significantly to corneal acidification ( , ) in the closed eye because the eyelid prevents carbon dioxide from diffusing away into the atmosphere. The situation is rapidly normalised on eye opening, provided no contact lens is present.

Because the permeability of hydrogel materials to carbon dioxide is approximately 20 times that of oxygen ( , ), it was originally thought that they did not restrict carbon dioxide efflux from the cornea. However, demonstrated that even thin (0.035 mm) hydrogel lenses provide some barrier to carbon dioxide efflux. Hence, during hydrogel extended wear, the cornea may experience chronic hypercapnia, with little opportunity for recovery to normal open-eye levels between periods of sleep. Silicone hydrogels are unlikely to have the same problem, however, as physical chemistry considerations indicate that this will be at least six times higher than their already relatively high permeability to oxygen (Winterton 2016, personal communication).

Hydrogels

The main component of a contact lens hydrogel material is poly 2-hydroxyethyl methacrylate (HEMA) although other monomers are often added to improve characteristics such as wettability, mechanical strength and so on, or to adjust water content. The Dk of a hydrogel material is dependent on its water content because the hydrophilic monomers attract and bind water into the polymer. Oxygen is transported through the lens by diffusion through the water, and therefore the higher the water content, the more oxygen can be carried. (For early research on lenses trialled for extended wear, see Section 8 , History, available at: https://expertconsult.inkling.com/ ).

Hydrogel lenses continue to be used for extended wear, and those currently available are typically of medium water content (45–60%) with moderate centre thicknesses, in the range 0.05–0.12 mm, giving oxygen transmissibilities between about 20 and 40 barrers. On average, around 10–14% overnight corneal swelling can be expected from such lenses ( ), and this may be accompanied by other hypoxic sequelae (see p. 242 ).

Silicone elastomers

Silicone-based lenses have been available since the 1970s, but a number of unique and frustrating drawbacks have limited these lenses to paediatric aphakia and therapeutic applications. Silicone elastomer lenses became available for aphakic and cosmetic extended wear in Japan and Europe in the mid-1970s and in the USA received FDA approval in 1985 for paediatric and aphakic cases (see Chapter 24 ).

Compared with hydrogel lenses, silicone elastomer lenses are more durable, easier to handle and have higher oxygen transmissibility. They contain negligible amounts of water, with their oxygen transport properties deriving from the silicone content. During overnight wear, the high level of oxygen provided to the eye has actually been found to induce significantly less oedema than occurs during sleep without lenses ( ), and other complications such as endothelial polymegethism were found to be less acute than in hydrogel extended wear ( ). Silicone elastomer lenses were also found to promote wound healing ( ).

Unfortunately, several disadvantages have been associated with their use, including manufacturing problems leading to poor edge shape and fitting difficulties, excessive levels of lipid deposition, corneal adherence, poor wettability and discomfort ( , , , , , , ).

Silicone hydrogels

Silicone hydrogel lenses combine the high Dk of silicone with the benefits of hydrogel materials. The main difference is that Dk is largely determined not by the water content but by the level of silicone in the material. Silicone is intrinsically nonwetting, so various strategies are required to render the lenses adequately wettable for clinical use. This aspect of their chemistry also means they have a different deposition profile to hydrogels, being generally less likely to attract protein and in some cases more prone to lipid deposition.

Although hypoxia is reduced ( , , ), silicone hydrogels remain affected by other problems, in particular, they have not reduced the overall prevalence of microbial keratitis from that found with hydrogels, although recovery times may be shorter ( ).

Silicone hydrogels also have therapeutic applications ( , ) and made piggyback contact lens systems a viable correction system for highly ametropic and keratoconic RGP lens wearers ( ) (see Chapters 20 and 26 ).

Wearer selection (see Chapter 6 )

Selection of suitable candidates for contact lens wear is important for achieving success with any lens type or wear modality and particularly so for extended wear. It involves avoiding those with potential for adverse events and discouraging individuals whose expectations are unrealistic for the product.

Convenience is the primary reason for wanting extended wear. It offers advantages for all, in particular those with specific occupations such as military personnel or those on call, and for certain leisure pursuits such as camping or mountaineering where daily lens removal is awkward or inconvenient.

Overall, wearers find both hydrogels and silicone hydrogel lenses comfortable ( , , ) and increased oxygen does not necessarily enhance comfort ( ).

Continuous wear with silicone hydrogel lenses is well tolerated by those who require contact lenses for therapeutic purposes, and also in piggyback systems where the combination of high Dk/t with a soft lens material promotes corneal healing and the alleviation of ocular discomfort (see Chapter 26 ).

Contraindications for extended- or continuous-wear lenses include:

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  compromised immunity

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  severe allergies

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  patients on systemic medication such as steroids

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  repeated episodes of mechanical or inflammatory events

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  difficulties with daily wear

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  inability to maintain an extended-wear schedule

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  water-sport hobbies.

Younger wearers, i.e. those younger than 25 years of age, should be given particular attention as they may be more prone to infectious and inflammatory events ( ).

Wearers must:

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  understand that contact lens care solutions are still a necessary adjunct to safe lens wear

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  be confident with lens handling and care and maintenance regimens

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  be prepared to remove their lenses at any time should the need arise, which includes keeping a pair of up-to-date spectacles and ensuring that solutions have not passed their expiry date.

Lens fitting

There are no major differences in the approach to fitting hydrogel lenses compared with silicone hydrogel lenses or any other soft lens type (see Chapter 10 ). The main aim is to optimise lens movement and centration, maximise tear exchange and avoid discomfort and lens awareness. Trial lens fitting should be carried out before extended or continuous wear commences and alternative products tried if difficulties or abnormalities are encountered. The lens should not fit tightly or tighten with wear, and should move across the cornea with ease when pushed up by the lower eyelid. Lenses should be slightly loose, with 45–50% tightness using the push-up test ( , ), 0.2–0.3 mm lens movement on blinking and good limbal coverage in all gaze positions.

The numbers who can be fitted successfully with hydrogel and silicone hydrogel lenses are similar, though the reasons for failure may differ. Unsuccessful lens fitting may be caused by insufficient limbal coverage and decentration in both cases, but for some stiffer silicone hydrogels, lens fluting – an intermittent buckling at the lens edge ( Fig. 12.3 ) – can be a cause of failure ( ).

Individuals who experience discomfort during the trial fit may not adapt to this sensation over time, although fitting a steeper BOZR may solve the problem ( ). If this does not resolve the issue, an alternative lens type is necessary.

Aftercare and wearer management

New lens wearers should be adapted to their lenses with a short period of daily wear (minimum of 1 week) which can establish that lens handling and hygiene are adequate before extended wear commences. If no problems are encountered, they can proceed to extended wear over several nights using a new pair of lenses and, if successfully using silicone hydrogels, move on to 30-night continuous wear once they have demonstrated success with extended wear.

Wearers should be seen early in the morning after the first month, then every 3–6 months thereafter. Practitioners who are new to extended wear may wish to see those in their care more frequently in the initial stages of lens wear. They should bring their lens solutions and case to each aftercare visit to discuss and reinforce lens-handling and hygiene techniques. Wearers should be advised to consult their practitioner if problems arise between appointments and need to emergency contact details 24 hours a day.

Current data suggest the risk of infection during 30 nights of continuous wear with silicone hydrogels is similar to the risk of infection during 6 nights of hydrogel extended-wear lenses ( ). Whatever the agreed removal schedule, flexibility should be encouraged and lenses removed as often as necessary to reduce the potential risk of adverse events. For example, lens wear should be discouraged:

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  during upper respiratory tract infections ( )

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  during hospitalisation

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  for water sports (or to wear watertight swimming goggles).

For details of aftercare assessments, see Chapter 10, Chapter 15, Chapter 16 .

Lens care and maintenance

If lenses are removed overnight or for long periods, they should ideally be discarded and a new pair inserted when next required. As a minimum requirement, lenses that are reused should be thoroughly cleaned and disinfected following manufacturers' instructions before they are reinserted.

Lens surface performance

The lens surface is assessed by examining a range of variables that contribute to the biocompatibility of contact lenses.

Wettability (see Chapter 2 , Chapter 5 , Chapter 16 )

Wettability is a subjective measure of tear film quality during lens wear that takes into account the following aspects:

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  pattern in which tears break over a lens

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  speed of tear breakup

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  stability of the tear film

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  appearance of the lipid layer.

The Brien Holden Vision Institute (BHVI) scale (see Appendix B and further information available at: https://expertconsult.inkling.com/ ) for assessing wettability ranges from 0 to 5, where:

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  0 corresponds to a nonwettable surface

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  1 is a surface showing nonwetting patches immediately after blinking

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  2 is the appearance equivalent to a HEMA surface

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  3 is more wettable than HEMA

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  4 is an appearance approaching that of a normal healthy cornea

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  5 corresponds to the wettability of a normal healthy cornea.

Depending on ethnicity, a normal healthy cornea has a NIBUT of between 15 and about 30 seconds and a stable, even, lipid layer. The BHVI scale also uses the wettability of HEMA lenses as a benchmark midway in the scale. HEMA lenses usually have a NIBUT from 5 to 7 seconds ( ) and correspond to grade 2. The surface wettability of silicone hydrogel lenses during extended and continuous wear remains relatively constant over time, irrespective of whether they are worn on a 6- or 30-night lens wear schedule, and is similar to the levels seen with extended wear of hydrogels (grade 2).

Front and back surface deposit accumulation is low during extended wear of soft lenses for both 6- and 30-night wear ( ). However, the deposits that do accumulate vary in the ratio of protein to lipid, with less protein but more lipid usually being found on silicone hydrogel lenses compared with conventional hydrogels ( ). Some soft contact lens wearers develop ‘haze’ and ‘globular’ lens deposits over several days, which can interfere with vision ( ) ( Fig. 12.4 ). These appear to be wearer specific and are likely to be lipid ( ). Occasionally, these deposits will accumulate several hours after lenses are inserted. They can be removed easily by cleaning with a surfactant but make extended wear impractical.

Advice to wearers (see Chapter 6 , Chapter 31 )

The primary responsibility of the contact lens practitioner is to educate wearers about the risks, benefits and realities of their chosen wear modality. Advice for extended or continuous-wear lenses differs in some respects from that for daily wear lenses. Potential acute complications and long-term effects should be clearly explained. Wearers at greater risk of adverse events should be advised accordingly. These include people younger than 25 years of age ( ) and those who may be noncompliant with lens care and wear schedules ( ). Wearers with a history of noninfectious corneal infiltration should be advised on the risk of developing recurrent events ( , ).

Emphasis should be placed on the need for regular overnight lens removal and replacement, and meticulous lens hygiene and disinfection procedures. If there is any unusual redness, discomfort or blurred vision at any stage, lenses should be removed and wearers should consult their practitioner.

The preliminary discussion should be reinforced with a written/online summary detailing the wearer's responsibilities and the risks involved, and these responsibilities should be reiterated at each aftercare visit. This should include the following ( , ):

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  an information leaflet/website address

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  practitioner-wearer agreement

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  instruction sheet

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  frequently asked questions

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  informed consent

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  documentation for what to do in an emergency.

This information also serves to formalise the necessity for compliance, particularly with continuous wear (see Chapter 31 ).

Corneal staining

Extended wear lenses do not usually require the use of solutions so corneal staining usually results from the lens fit (see Chapters 9 and 10 ).

Hypoxia