Ординатура / Офтальмология / Английские материалы / Medical Contact Lens Practice_Millis_2005

all year, sometimes with seasonal exacerbations. Both SAC and PAC are type I reactions, but the type of antigen differs in the two diseases – pollens are the more likely cause of SAC, whereas PAC is more often due to the house dust mite, cosmetics or animal dander, which reach the eye from the eyelid margins. Neither SAC nor PAC is sight-threatening.

Management of SAC and PAC

Once the allergen has been identified contact with it should be avoided if possible and certainly reduced to a minimum. Cold compresses are comforting and relieve symptoms temporarily or the eye may be irrigated several times a day with unpreserved saline.

Topical vasoconstrictors and antihistamines may help minimize symptoms, but their use should be limited to short courses because rebound redness due to chronic vasodilation may result. This is sometimes difficult to achieve because these drops are available without prescription.

Topical mast cell stabilizers such as sodium cromoglycate or lodoxamide are useful, particularly as prophylactics before an attack. They have a slow onset and so are less useful in the acute stage. Patients who have annual attacks are advised to commence treatment with a mast cell degranulator about 3 weeks before the anticipated attack. Oral antihistamines may be given, but side-effects such as drowsiness can limit their use. Corticosteroids are effective but have serious side-effects and are not usually necessary. Desensitization may be helpful in cases once the allergen has been identified.

Even when SAC and PAC are not caused by contact lenses they may be exacerbated by them. In mild cases it is often possible to continue with lens wear, and neither the lens design nor the material needs to be changed.

Giant papillary conjunctivitis

Giant papillary conjunctivitis is a noninfectious inflammatory condition involving the upper tarsal conjunctiva, and characterized by the presence of papillae (Fig. 7.3).3 Giant papillary conjunctivitis has been associated with contact lens wear, particularly soft lenses, and has also been reported in

Figure 7.3 Papillary conjunctivitis.

Figure 7.4 Localized papillary conjunctivitis caused by a proud suture.

association with sutures that protrude above the surface (proud sutures) (Fig. 7.4), ocular prostheses and retained contact lenses.

Contact lens-related papillary conjunctivitis (CLRPC) causes itching, excess mucus production (Fig. 7.5), blurred vision due to a coated lens and, as the lens surface becomes sticky with discharge, it decenters with the blink. In the early stages itching may be noted after removal of the lens, but as the disease progresses itching is present when the lens is worn. Friedlander4 noted that itch was the most reliable symptom of allergic conjunctivitis.

Papillae occur in the upper tarsal area and vary in size, depending on the stage of the disease,

Figure 7.5 Increased mucus production associated with CLRPC papillary conjunctivitis.

from micro to macro papillae, which are larger than 0.3 mm. The exact distribution depends on the type of lens worn, and it is important to exclude any papillae at the extreme medial and lateral areas of the eyelid when making the diagnosis because these are normal. The everted eyelid should be examined carefully because symptoms may result from a few localized papillae. The upper tarsal conjunctiva is swollen and edematous, and in some cases the conjunctival reaction is sufficiently severe to cause a mechanical ptosis due to the increased weight of the eyelid.

As the disease progresses symptoms and mucus discharge increase and the lens tolerance gradually falls. The papillae increase in size and number and there is evidence of copious mucus discharge on the surface of the papillae, the apices of which may stain with fluorescein. Eventually it is no longer possible to wear a contact lens because it frequently decenters.

It is believed that CLRPC results from an allergic response to antigens on the lens, and the effect of repeated mechanical trauma. The allergic response may be to the deposits on the lens, or may be associated with the solutions used. The exact etiology is still uncertain.

Histologically eosinophils and basophils are found in the epithelium and substantia propria of the conjunctiva, and mast cells are present in the epithelium (where none are normally found). The mast cells are degranulated.5 There are increased levels of IgE, IgG, IgM and complement factors in the tears, together with normal lysozyme levels

and reduced levels of lactoferrin, and these level return to normal when lens wear ceases, whic supports the histology of an immune mechanism.

Fowler and Allansmith6 examined the surface of contact lenses and found the posterior surfac was smoother than the anterior surface. Deposit start to form immediately on insertion of the len and the layers, formed gradually, become mor complex and cannot be entirely removed, eve with professional cleaning (see Ch. 5). The con tinued use of aging lenses and suboptimal len care, often associated with cases of CLRPC, ar likely to increase deposit formation, and the irregu lar surface they create may cause mild trauma. Trauma due to a badly fitting or damaged lens o with a poor or unsuitable edge will further increas the damage associated with the roughened anterio surface.

Management

The aims of treatment of CLRPC must be to:

●remove the antigen

●reduce deposit formation

●modify the immune reaction.

Removal of antigen Cessation of lens wear wil result in resolution of the CLRPC, but the aim of th clinician and the patient is to enable lens wear to b maintained. Stopping lens wear for a period of 4– weeks will allow the inflammation to become qui escent, but the papillae remain for many months Once symptom free, the patient can be refitted wit new lenses of the same type, and many will achiev successful wear. Greater success may be obtaine by fitting another polymer. The advent of frequen lens replacement has reduced the need to refit th soft lens wearer with RGP lenses, and they are use ful for those patients in whom lens wear is essentia for medical reasons and who cannot carry on wit their normal activities without their contact lenses We have found that changing to 1-day or weekl wear and using topical lodoxamide before insert ing the lens and after the lens has been removed a night allows the symptoms to resolve while main taining wear in many cases.

Reduce the deposit Improved lens hygien increases the chances of patients being able to wea

their lenses without the disease recurring. Use of a surfactant and a rub and rinse step are vital. Allansmith and Ross8 thought it important to use a preservative-free system, and in particular to avoid thimerosal and chlorhexidine. They believed that hydrogen peroxide systems were best tolerated. Daily disposable lenses have the added advantage that they do not require any solutions other than unpreserved sterile saline with which to rinse or rewet the lens.

Weekly use of enzyme tablets is recommended for conventional lenses, but care must be taken to rinse off all traces of the enzyme.

It has been suggested that reducing the overall size of the lens will decrease the amount of deposit, but hypersensitivity reactions are not dependent on the amount of antigen present. Nevertheless smaller RGP lenses are easier to modify and repolishing the lens will remove existing deposit.

Frequent changes of lens reduce the build up of deposits9 and are associated with a lower risk of developing GPC10 and have been found to be advantageous for those with environmental allergies.11

Modification of the immune response

Antihistamines Levocabastine (Livostin® Novartis Ophthalmics) is the latest selective H1 antagonist that has been found to be more effective than the mast cell stabilizer sodium cromoglycate in SAC, and will control itching, redness and eyelid swelling.12 Its use is currently limited to 4 weeks per year and it is normally instilled twice a day. If it is necessary to increase the frequency of the dose disposable lenses will limit the problems of uptake and accumulation. To date most antihistamines have been found to be of limited benefit in CLRPC.

Mast cell stabilizers Sodium cromoglycate is the most commonly used mast cell stabilizer and is now available without prescription in the UK. It is best used in a quiet eye, but has been found to help maintain lens wear in patients who have recurrent disease despite good lens hygiene and attention to lens fit and material.13 The suggested regimen is to cease lens wear when possible and use the drops four times a day, and restart lens wear when the eye is quiet. The frequency of

the dose should be decreased by one drop per day every 3 months; it is increased again if symptoms recur and then tailed off to a maintenance dose of one drop per day or alternate days. Lodoxamide (Alomide® Alcon) is a more recent mast cell stabilizer that is available on prescription only.

Corticosteroids Topical corticosteroids are effective in controlling the symptoms and signs of allergic conjunctivitis, but the potential complications of cataract, raised intraocular pressure and a possible increased risk of infection make their use inadvisable in most cases.14

Vernal keratoconjunctivitis

Vernal keratoconjunctivitis is an uncommon condition that occurs most often in those under 25 years of age, and in males rather than females. It is not related to lens wear. Large cobblestone papillae with flat tops, which may stain with fluorescein, are found on the tarsal or limbal conjunctiva (Fig. 7.6). The tarsal form is more common in Caucasians and the limbal form in Afro-Caribbeans.

Symptoms are similar to other allergic diseases with itching, watering and a white, stringy discharge. The cornea may become involved and a loss of vision may result from pannus formation or lipid deposits. Trantas’ dots, which consist of degenerate epithelial cells and eosinophils, may occur at the superior limbus.

There is an increase in the number of eosinophils and basophils in the epithelium and substantia propria of the conjunctiva, and increased plasma cells in the epithelium. Mast cells are found in the epithelium where normally there are none. There are raised tear and serum IgE levels, but normal IgA.2

Management is with cold compresses, topical antihistamines and vasoconstrictors. Mast cell stabilizers such as sodium cromoglycate, and corticosteroids may be necessary if corneal changes are severe.

Atopic keratoconjunctivitis

Atopic keratoconjunctivitis occurs in atopic patients who frequently have an eczematous condition of the eyelids as well as generalized

Figure 7.6 Vernal conjunctivitis showing cobblestone papillae.

eczema. It is a serious condition associated with chronic inflammation of the eyelid margins, papillae, injection and chemosis of the bulbar conjunctiva, and corneal epithelial defects, which may result in sight-threatening plaque formation and neovascularization. It may give rise to atopic subcapsular cataract formation15,16 and increase the risk of microbial keratitis. Glaucoma may occur secondary to the use of corticosteroids.

Contact dermatoblepharitis

Contact dermatitis may affect the eyelids and is caused by topical medication, cosmetics and environmental allergens. It may occur as a type I reaction with a rapid onset, with swelling of the eyelid, itching, redness and conjunctival chemosis. Rarely patients experience a systemic anaphylactic reaction, with a fall in blood pressure and possible respiratory and circulatory failure. These reactions have been noted with topical anesthetics and some antibiotics.

Figure 7.7 Reactivation of thimerosal sensitivity.

In some cases contact dermatitis appears to be type IV reaction because the onset is delayed by up to 72 hours, and the eyelids show eczematou changes with thickening, scaling and redness, which may cause scarring and distortion of the eyelid.

The most common iatrogenic causes are atropine antibiotics such as neomycin, gentamicin and th preservative thimerosal (Fig. 7.7). Although ther are no longer contact lens solutions preserved with thimerosal, some eyedrops are, and if used frequently the individual may become sensitized.

The symptoms resolve with removal of th offending allergen, antihistamines and mast cel stabilizers. In severe cases a short course of corti costeroids should be considered.

Allergic eye disease is commonly encountered in contact lens practice and causes a great deal o misery for those individuals who have it. A carefu history and assessment will differentiate between those cases in which the contact lens can b incriminated and those in which lens and lens car are not involved.

1.McGill JI, Holgate ST, Church MK, et al. Allergic eye disease mechanisms. Br Ophthalmol 1998;82: 1203–1214.

2.Ehlers WH, Donshik PC. Allergic ocular disorders: a spectrum of diseases. CLAO J 1992;18:117–124.

3.Allansmith MR, Korb DR, Greiner JV, et al. Giant papillary conjunctivitis associated with an ocular prosthesis. Am J Ophthalmol 1977;92: 368–371.

4.Friedlander MH. Ocular allergy. Int Ophthalmol Clin 1985;25:35–41.

5.Henriquez AS, Kenyon KR, Allansmith MR. Mast cell ultrastructure. Comparison in contact lens associated giant papillary conjunctivitis and vernal conjunctivitis. Arch Ophthalmol 1981;99: 1266–1272.

6.Fowler SA, Allansmith MR. Evolution of contact lens coatings. Arch Ophthalmol 1980;98:95–99.

7.Lustine T, Bouchard CS, Cavanagh DH. Continued contact lens wear in patients with giant papillary conjunctivitis. CLAO J 1991;17:104–107.

8.Allansmith MR, Ross RN. Treatment of giant papillary conjunctivitis. Trans BCLA Int Conf 1988;5:38–42.

9.Palmisano PC, Ehlers WH, Donshik PC. Causative factors in unilateral giant papillary conjunctivitis. CLAO J 1993;19:103–107.

10.Porazinski AD, Donshik PC. Giant papillary conjunctivitis in frequent replacement contact lens wearers: A retrospective study. CLAO J 1999; 25(3):142–147.

11.Bucci FA, Tanner JB, Moody KJ. The clinical performance of the Surevue, Focus, and CSI daily wear lenses in allergic and normal contact lens patients. CLAO J 1994;20:159–166.

12.Davies BH, Mullins J. Topical levocabastine is more effective than sodium cromoglycate for the prophylaxis and treatment of seasonal allergic conjunctivitis. Allergy 1993;48:519–524.

13.Kruger CJ, Ehlers W, Luistro AE, Donshik PC. Treatment of giant papillary conjunctivitis with cromolyn sodium. CLAO J 1992;18:46–48.

14.Asbell P, Howes J. A double-masked, placebocontrolled evaluation of the efficacy and safety of loteprednol etabonate in the treatment of giant papillary conjunctivitis. CLAO J 1997;23:31–36.

15.Friessler KA, Lang GE, Lang GK. Allergic diseases of the lids, conjunctiva and cornea. Curr Opin Ophthalmol 1997;8:25–30.

16.Buckley RJ. Allergic eye disease – a clinical challenge. Clin Exp Allergy 1998;28:39–43.

Extended-wear (EW) lenses are soft or RGP lense that are worn continuously during the day an overnight. They are worn by patients who:

●find handling lenses difficult

●have high refractive errors who find that it i very convenient to have fully corrected vision i they wake at night

●use them as therapeutic lenses.

The concept of EW arose as a result of cornea infections occurring with daily-wear soft lenses. I was believed that the source of the pathogeni organisms was the solution in which the lens wa stored, and that by reducing lens handling an eliminating contaminated solutions the risk of infec tion would be greatly reduced. Unfortunately thi was not the case and contact lens-related microbia keratitis (CLRMK) and contact lens acute red ey (CLARE) were both found to be complications o EW, with a higher incidence than for daily-wea lenses.

THE CLOSED EYE

Hypoxia

Oxygen reaches the open eye from the atmosphere which contains 21% oxygen, dissolved in the tea film at a partial pressure of 155 mmHg (20.7 kPa) This is reduced to 8% (at 55 mmHg, 7.3 kPa) whe the eye is closed in sleep, and the cornea relies fo the supply of oxygen on the subtarsal and limba blood vessels.

A contact lens acts as a barrier to the supply o oxygen to the cornea. With an RGP lens and the ey

open the oxygen that reaches the cornea beneath the lens depends on the oxygen permeability (Dk) of the lens polymer, the thickness of the lens, and the tear pump action. However with most soft lenses it is related to the water content and thickness of the lens, and there is virtually no tear pump action.

In the closed eye there is minimal tear pump action, even with RGP lenses, and a contact lens acts as a greater barrier under these conditions, further reducing the oxygen supply to the eye. The temperature of the closed eye is 3–4°C higher than the open eye resulting in a greater need for oxygen.1

Clinically changes due to hypoxia can be seen in all layers of the cornea.

Epithelium

In the epithelium microcysts may be observed on the slit lamp in the midperiphery of the cornea. They appear to consist of apoptotic cells or cell debris and develop after 3 months of EW. Apoptosis is cell death in which a genetic mechanism programs orderly steps for the destruction of the cell. The nucleus fragments, there are changes in the plasma membrane and the formation of small, circular or irregular blebs that are asymptomatic. Microcysts increase over three months and resolve over a similar period when lens wear ceases. The response is less with RGP lenses than with hydrogels.2

Reduced metabolic activity and diminished mitosis result in epithelial thinning, and chronic hypoxia causes a reduction in the density of the hemidesmosomes, which increases the risk of epithelial damage and loss. McNamara et al., in a small study, have shown a reduction in epithelial barrier function associated with EW, with apparent recovery in the day following overnight wear.3

Stroma

The stroma becomes edematous, and swelling is greater centrally than peripherally. The stromal substance thins, but this may not be obvious until several days after lens wear ceases, when any edema has settled. Unlike epithelial thinning it may take months to recover.

Endothelium

Endothelial changes include:

●bleb formation, in which small swellings develop on the posterior endothelial surface as soon as a lens is inserted, reach a peak in 30 minutes, and gradually subside to a low level in 1 hour – they resolve when the lens is removed

●polymegethism, which is a variation in cell size seen with increasing age and when a contact lens is worn

●bedewing, which is thought to be the result of clusters of inflammatory cells on the posterior surface, and may cause discomfort and reduced contact lens wearing time.

These changes are similar to those seen with other types of contact lens wear.

Corneal edema

The hypoxia resulting from overnight lens wear causes a decrease in aerobic and an increase in anaerobic metabolism, which results in an accumulation of lactic acid in the stroma and creates an osmotic gradient. This draws water into the stroma, causing corneal swelling, which is easily measured with a pachymeter and becomes clinically evident as striae in the posterior cornea when edema levels reach 6%; stromal folds occur with 10–15% edema.4

Up to 4% corneal swelling occurs in the normal, non-contact lens wearing eye during sleep, but with a thick, low-water content lens this may rise to 10–20%. Holden et al.5 found 10–15% overnight swelling with hydrogel lenses that was dependent on the water content of the lens and the average lens thickness. Greater amounts of swelling were also found to occur peripherally under thick-edged, high-minus lenses.

On opening the eye the swelling starts to reduce. This occurs more rapidly with RGP lenses than with soft ones because:

●more of the cornea is directly exposed to atmospheric oxygen

●the tear pump is restored with the movement of the lens on the eye

●higher gas transmissibility (Dk/L) is possible with RGP lenses.

The cornea was found to be capable of eliminating an average of 8% edema during the open eye period of EW so that at the end of the day some residual edema may remain. Holden and Mertz6 found a Dk/L of 87 10 9 was necessary to limit edema to the 4% level found in non-lens wearing eyes, and a Dk/L of 34 10 9 would limit overnight swelling to 8%, which would permit the eye to recover during the open eye period. Until very recently no lens has been able to meet these requirements.

As the Dk/L of a lens increases there is a decrease in overnight corneal edema and microcysts, but other responses, which are unrelated to hypoxia, such as lens adherence and three and nine o’clock staining that occurs with RGP lenses worn overnight, are unaltered.7 If ultrathin lenses are fitted to try to minimize hypoxic swelling this may result in desiccation-like corneal staining.8 The environment becomes more acid and, as the cornea swells, the lenses tighten on the eye.

Changes in the tear film

With a contact lens on the eye, the tear film is divided into anterior and posterior tear layers, and it is the posterior layer that is critical for successful EW.

In the open eye the tear film contains bactericidal lysozyme, bacteriostatic lactoferrin and secretory immunoglobulin A (sIgA), and is believed to be under neurological control. In the closed eye this secretion alters, and is replaced by a slower flow in which the total protein is increased, the sIgA may form up to 80% of the tear protein, and serum proteins, such as albumin, increase to 1–2% of their serum concentration.9 The complement system, which is part of the innate defence system involved in inflammatory reactions, has been shown to be active, with the concentration of all components greatly increased under these conditions.10

Carney et al.11 reported that EW does not appear to alter the concentration of the major tear film proteins, suggesting that any adsorption to the lens surface is replenished, but Stapleton et al.12 found the concentration of both total proteins and individual proteins increased significantly. In addition the closed eye tear fluid contains large numbers of polymorphonuclear leukocytes (PMNs), many

of which are actively phagocytic.13 These change together suggest an active, subclinical inflamma tion is taking place.

Tan et al.14 showed that changes in tear film taking place during eye closure do so in a clearly defined sequence. The number of epithelial cell gradually increases with increasing periods of sleep and reflects the slow collection of exfoliated cells in the stagnant tear film. Similarly concentrations o sIgA and albumin increase with longer closed-ey periods. They found an initial rapid increase in plasmin-like activity in the first 2–3 hours, which then plateaued, and then decreased with continued eye closure. Plasmin is an enzyme concerned in th complement cascade. This increase in plasmin-lik activity occurs before PMNs are recruited suggest ing it may be a complement-driven process. PMN increase slowly in the first few hours and rapidly thereafter due to active chemotactic recruitment.

Bacterial adherence and infection

The adherence of Pseudomonas aeruginosa has been shown to be enhanced when lenses are worn on an EW basis.15 In addition cytotoxic strains of thi bacteria can damage epithelia on an uninjured corneal surface providing there is prolonged bacte rial contact,16 as occurs in EW. These factors may contribute to the increased risk of microbial kerati tis seen with EW. No difference has been found between the type of microorganisms present in the openor closed-eye states, but clinically impor tant levels of Gram-positive organisms were pres ent in the closed-eye samples, which suggests tha eyelid closure increased the growth of norma flora.17

PATIENT SELECTION

Patient selection for EW must be undertaken with great care.

Any inflammation, infection, or limbal vascu larization, and any corneal or conjunctival stain ing should be resolved before fitting an EW lens and contraindicate an EW lens if they recur Patients with a dry eye are at greater risk of compli cations, and elderly aphakic individuals often hav poor tear function. Patients with a past history of CLRMK should only be refitted with lense

with extreme caution. Patients wearing lenses for therapeutic reasons may have to be refitted, but must be kept under close supervision.

Systemic disorders, including diabetes mellitus, may be relative contraindications to contact lens wear, but EW lenses have been worn successfully by diabetics.18 The use of medications that can cause dryness must also be taken into consideration. Smoking may enhance the risk of complications with EW, but the mechanism is unclear.

Anesthetic corneas carry an increased risk of infection, but EW bandage contact lenses may be used in the treatment of neurotrophic ulcers, providing there is increased supervision. The advent of silicone hydrogel lenses should decrease the risk, but frequent supervision is still necessary.

Previous problems with lenses and a history of poor compliance are factors that must be taken into consideration when assessing the risks of EW.

It is important that patients who are considered for EW contact lenses are fully informed of the risks of this modality, particularly:

●the increased risk of microbial keratitis

●the greater cost

●the inconvenience of frequent, regular follow up

●the need to live near to the practice and be able to reach help the same day, even on weekends and public holidays.

The practitioner must be able to arrange full-time cover in the event of an emergency. Unless this is possible the patient should be advised against EW.

The need for EW should be discussed with the patient to see if a safer alternative is acceptable. Often patients dislike caring for their lenses and are happy to change from a conventional or frequent replacement lens to a 1-day disposable lens, which does not need to be cleaned.

Extended-wear lenses are useful for myopic and hyperopic patients and for those aphakes who have been deemed unsuitable for an intraocular lens implant (IOL) or who have not been fully corrected by the IOL. They may be prescribed for those who have handling problems (e.g. patients with rheumatoid arthritis). Environmental circumstances may prevent frequent manipulation of lenses and an EW lens is indicated in circumstances such as trekking, camping or long-distance sailing, when weekly EW may be more practical than frequent lens handling.

EXAMINATION AND FITTING

A full ophthalmic examination is carried out to identify any contraindications and any pre-existing conditions should be treated before commencing lens wear. It may be helpful to grade conditions such as redness, microcysts and staining to monitor the condition. Whenever a grading scale is used a note of the type of scale used should be recorded in the notes to aid comparison with subsequent results.

Soft lenses for extended wear

Soft lenses for EW may be lowor medium-water content thin lenses or thicker high-water content lenses, and are available as conventional or as frequent replacement lenses, which are changed weekly, fortnightly or monthly (Table 8.1). Few toric lenses are available, and these only in a limited range, but fitting a more rigid material can be successful with mild degrees of astigmatism or a high Dk RGP lens may be tried.

Fitting guidelines are usually available from manufacturers and it is simplest to follow them. Alternatively, an initial trial lens 0.5 mm flatter than the flattest keratometry reading (K) for a 13.0 mm diameter lens, or up to 1.0 mm flatter for a 14.5 mm lens should be selected, with the nearest power to the patient’s refractive power. The lens should be allowed to settle on the eye for 30 minutes and the fit assessed. There should be 0.5–1.0 mm of movement with the blink. The lens should be fitted looser rather than tighter because of the risk of the lens tightening overnight. A tight fit should always be avoided. If the fit is satisfactory, over-refract and adjust the power accordingly.

A thin myopic lens may create a plus-lens tear film centrally and may appear to need an increase in power, but a flatter fit that negates the tear lens is indicated. A thick plus lens may create a negativelens tear film that requires a change of power. In some cases of poor vision a larger optic zone is helpful.

All patients should be taught to insert and remove their lenses. If insertion proves impossible a relative should be taught to insert the lenses, but the patient must be able to remove the lens in an emergency. The patient should be advised to take care when inserting a lens to avoid touching the

posterior lens surface with fingers or lashes to minimize the risk of infection.

The advent of the disposable lens has removed the need to take the lens out once a week to clean, disinfect, and then reinsert it. It is advisable to leave all types of lens, with the possible exception of silicone hydrogel lenses, out for one night when the lens is changed.

Written instructions must be provided, including information on where to seek advice in an emergency, and to remove the lens if the eye is red, painful or the vision is blurred. Patients should examine the eye each morning for redness, and should not go to sleep with the lens on the eye if they have symptoms.

All EW lens wearers are advised to instill a drop of unpreserved lubricant, for example Vislube® (Chemedica), Refresh® (Allergan) or sterile normal saline in each eye on waking, then to blink 10–20 times to rehydrate and clean the lens and to encourage its movement with the blink. Patients may instill further drops during the day if the eyes are dry.

Unpreserved drops are preferred to avoid toxic reactions to the preservatives.

COMPLICATIONS OF EXTENDED-WEAR LENSES

Fitting problems

As with other lenses, EW lenses may be too flat o too steep. If there is some doubt regarding the fi of a lens it is better to fit looser rather than tighter All EW lenses should be checked 24 hours afte they are initially placed on the eye to assess the fit

Blurred vision

If blurred vision occurs in the first week of wear i is probably a symptom of adaptation, but if it per sists for longer than this:

●the lens should be changed for a flatter one, or

●the material should be changed to increase th Dk/L, by reducing the thickness or increasin the water content.

Halving the thickness of a lens doubles th equivalent oxygen percentage (EOP), and for ever 10% increase in water content the oxygen perform ance is doubled.