Extended wear 21 Chapter

which, when combined with lens adhesion, could precipitate an acute red eye response8 (see Section 29.1.1).

21.1.4 Physical and chemical compatibility of lens materials

The physical aspects of materials must be considered to avoid mechanical trauma from friction or surface hardness. Weight distribution is similarly important. Materials should be chemically inert to avoid surface deposits which could irritate the ocular tissues.

21.2 Approaches to extended wear

On a practical level, lenses for overnight wear can be divided into three possible groups:

Flexible wear (daily plus)

Many patients requesting extended wear are prepared to accept lenses that they can use safely on an occasional overnight basis, remaining happy to remove them most evenings. This type of lens wear gives maximum flexibility and safety. On a day-to-day basis, they are taking advantage of the excellent physiological properties of either silicone hydrogel or high Dk rigid lenses, while at the same time incurring very little risk of adverse ocular response to occasional overnight use.

Extended wear (up to 1 month)

Silicone hydrogels have proven relatively safe for extended wear on a monthly disposable basis, although some patients can be encouraged to remove them more frequently. A schedule of 6 nights’ wear followed by 1 night’s rest gives a realistic routine for many patients; some prefer 5 nights during the working week with daily wear at the weekends. Proper lens cleaning and disinfection are maintained on removal, and a consistent regimen is developed. Disposable lenses, fit well into such a routine.

Continuous wear (over 1 month)

Over 1 month may be regarded as continuous wear or long-term extended wear. It is suitable or even essential for many medical cases, poor lens handlers or for some vocational uses. For most normal eyes, wearing schedules beyond 1 week put additional stress on the cornea and must be used with great caution. Many patients in fact seem to develop a ‘sixth sense’ of when lenses need removal. In the past, a minority using conventional lenses encountered fewer problems with handling, breakage, solutions and infections by leaving them in the eyes for several months.

243

Section three Hydrogel and silicone hydrogel fitting

21.3 Patient selection

21.3.1 Indications and contraindications

Indications

The majority of extended wear lenses are fitted at the patient’s request for purely cosmetic reasons, although they may be required for a particular project such as sailing or mountaineering. With proper supervision, extended wear works well but there is always the possibility of serious problems such as microbial keratitis, oedema or vascularization (see Section 21.7). Even the most permeable silicone hydrogels require great care not only by the practitioner but also by the patient who must be made aware of potential hazards. The clinical needs must therefore be carefully balanced against the possible risks.

There are, however, many patients for whom extended wear is the most suitable, if not the only possible, form of visual correction:

•Aphakics, where handling and visual difficulties preclude daily wear.

•Other poor lens handlers.

•Therapeutic cases (see Chapter 32).

•Young children where daily handling is not feasible (see Chapter 31).

•Vocations where good vision is required immediately on waking.

•Where all soft lens disinfection systems create problems and daily disposables are not feasible.

•Where patients have no facility for lens disinfection.

•As the rear component of a low vision aid system and handling is impossible.

Contraindications

•Patients with only one fully funtioning eye.

•Patients known to be predisposed to corneal oedema.

•Where there has been a previous instance of microbial keratitis.

•Existing corneal vascularization.

•CLIPC.

•Patients unlikely to follow practitioner advice.

•Patients unable to remove lenses.

•Diabetics, where the corneal epithelium is likely to be more fragile.

•Smokers.

•Where financial constraints may preclude aftercare and regular lens replacement.

•Where patients are known to be a long way from optometric or medical help in the event of a severe ocular emergency.

244

Extended wear 21 Chapter

21.3.2 Advantages of soft and rigid lenses in extended wear

Advantages of soft lenses

•More comfortable.

•Easier adaptation.

•Easier to fit.

•High Dks with silicone hydrogels.

•No problems with foreign bodies.

•No 3 and 9 o’clock staining.

•Less risk of loss.

Advantages of rigid lenses

•Very high Dks possible.

•Tear pump on blinking.

•Avoidance of trapped debris.

•Reduced risk of infection.

•Better vision.

•Fewer corneal changes.

•Lenses do not cover the entire cornea.

•No lens dehydration.

•Fewer deposit problems.

•Easier lens maintenance.

•No solutions sensitivity.

•Longer lifespan.

Despite the numerous advantages of rigid gas-permeable lenses, they are still fitted in only a minority of cases for extended wear because they are perceived by the patient as more difficult to wear, they are more difficult to fit and give greater problems with foreign bodies.

21.4 Soft lens fitting and problems

21.4.1 Fitting

The main concerns are to provide the maximum possible oxygen supply and to avoid corneal insult. Fitting considerations therefore include the following:

•Patients where possible should be fitted with silicone hydrogels.

•Large refractive errors (both plus and minus) beyond the range of silicone hydrogels are fitted with high water content hydrogel lenses.

•Lenses should be as flat as possible consistent with stability of fitting and vision.

•Lenses become tighter with wear, particularly overnight because of dehydration.

245

Section three Hydrogel and silicone hydrogel fitting

•Lenses become tighter with age and deposits (see Section 29.4).

•Examination during fitting and aftercare must be carried out with the slit lamp, otherwise small degrees of lens movement cannot be seen.

•Some degree of movement is essential for proper tears exchange to remove debris from beneath the lens on blinking.

•Some thicker lenses or those with a high modulus can cause dimpling.

PRACTICAL ADVICE

It is generally correct to refit silicone hydrogels to existing patients with extended wear. Some, however, will be unable to tolerate the higher modulus of the new material. If lenses have not settled within about 3 days, return to the original lens.

21.4.2 Problems

Lens dehydration and corneal desiccation

Lens dehydration and corneal desiccation can occur with silicone hydrogels but they are usually better than with hydrogel lenses. The typical result is punctate staining in the mid-periphery of the cornea in the exposed area of the palpebral aperture. With the higher modulus materials, it can be associated with superior arcuate staining (see SEALs, Chapter 19.5). Where there is no obvious fault with the fitting, it is usually necessary to change to another make of lens with different dehydration characteristics.

Contact lens-induced acute red eye (CLARE)

The sudden, acute red eye is a serious and unpredictable complication of soft extended wear giving gross conjunctival hyperaemia. It normally occurs on waking and is usually unilateral, associated with varying degrees of pain, photophobia, lacrimation and small limbal infiltrates. The likeliest causes are an inflammatory response to trapped debris and toxins beneath the lens8,9 and to deposits on the lens surface. With conventional hydrogel lenses the incidence was higher with heat disinfection and was reduced with peroxide systems.

If an acute red eye reaction should occur, the courses of action are:

•Remove lenses.

•Ensure patient attends for immediate examination.

•Consider referral.

•Wait several days for the condition to resolve.

•Change to new lenses.

•Consider refitting a different type of lens.

•Consider a peroxide disinfection system.

•Possibly change to daily wear – essential if the red eye recurs.

246

Extended wear 21 Chapter

Infections

Infections with extended wear lenses tend to be more severe and longer lasting than with daily wear lenses. It was originally thought that, with the introduction of silicone hydrogels and their vastly improved oxygen transmission, the incidence of infection would be reduced. This has not been the case, although the results are generally less severe. Where corneal infiltrates are present and the condition is thought to be microbial keratitis, immediate removal of lenses and referral for medical treatment is absolutely essential to minimize the risk of permanent visual loss (see Section 29.1). Other actions are as above for red eye.

The risks of infection are reduced with:

•Frequent lens replacement.

•Maximum oxygen supply to the cornea.

•Proper lens mobility.

•Proper lens cleaning and maintenance by the patient.

•Hygienic lens handling by the patient.

•Frequent replacement of lens cases.

Inflammatory response and corneal infiltrates

Corneal infiltrates as an inflammatory response are seen fairly commonly in extended wear, even with silicone hydrogels (see Section 29.1). They may be either symptomatic (CLPU, IK) or asymptomatic infiltrates (AIK, AI) and discovered only at a routine aftercare examination. Where there are other signs or the patient experiences symptoms, the practitioner must be absolutely certain that a sterile infiltrate is not actually microbial keratitis. If in doubt, the patient should be referred for a medical opinion. Further actions are as above for an acute red eye.

Contact lens-induced papillary conjunctivitis (CLIPC)

The main causes of CLIPC with extended wear are lens deposits and allergic reactions, worse during the hay fever season. It occurs with silicone hydrogels as a result of mechanical irritation with high modulus lenses or an allergic response to the silicone in the material. The courses of action are given in Section 28.3.5.

Oedema

Stromal oedema is the cornea’s response to insufficient oxygen under c!osed eye conditions (see Section 28.3.2). It is almost never seen with silicone hydrogels but with less permeable lenses may show in a variety of ways:

•Single stria, indicating 5–6% swelling.

•Several striae, indicating 7–10% swelling.

•Stromal folds, indicating 10–12% swelling.

•Epithelial microcysts.

•Subjective symptoms of cloudy vision.

247