Section three Hydrogel and silicone hydrogel fitting

Epithelial microcysts

Microcysts contain fluid and cellular debris, appearing as small vesicles in the corneal epithelium with diameters of 15–20  m. They are seen with high mag-

nification with the slit lamp and exhibit reverse illumination. Microcysts are an indication of hypoxic corneal stress, although small numbers may be considered sufficiently normal to ignore in the absence of other signs. They are generally seen after 2–3 months of extended wear, but can occur sooner.10 They can also take several weeks to resolve.

Mucin balls

Mucin balls (pre-corneal deposits) are usually associated with first generation silicone hydrogels but can be seen with other lenses (see Chapter 19). They are translucent grey or opalescent in appearance and consist of cell debris and other

products of corneal metabolism. There is no reverse illumination and diameters are 10–50  m. Mucin balls are benign, with most patients completely asymp-

tomatic. They should, however, be distinguished from microcysts and simple dimpling with air bubbles.

Vascularization

Vascularization, practically unknown with silicone hydrogels, is a longer term response of both daily and extended soft lens wear. Vessels may be either superficial, extending from the limbal arcades, or deeper, stromal neovascularization where there is a much greater risk that they may extend into the pupil area. The first action to consider is refitting other types of lens with silicone hydrogels but it is often necessary to change from extended to daily wear; from soft to rigid; or, in severe cases, to cease contact lens wear altogether.11

Deposits

Because lenses are removed infrequently for cleaning, there is a general predisposition to deposits (see Section 29.4). These are largely avoided with disposables but are kept to a minimum with peroxide systems.

Breakage and loss

Breakage occasionally occurs with the lower modulus silicone hydrogels where they are worn on a flexible basis. Loss is not a great problem, although some of the smaller, thinner lenses are occasionally rubbed out of the eye during sleep.

21.5 Rigid gas-permeable fitting and problems

There are now several high Dk gas-permeable materials available for extended wear. These include the Bausch & Lomb Quantum 2, Menicon Z-alpha and Fluoroperm 151. Their main use is with orthokeratology for overnight wear after which they are removed during the day (see Chapter 14).

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21.5.1 Fitting

The main considerations are:

•Sufficient oxygen with a high Dk material.

•Adequate edge lift to avoid lens adhesion.

•Avoiding excessive edge lift and 3 and 9 o’clock staining.

•Good centration, particularly avoiding low riding lenses because of the risk of adhesion.

•Sufficient lens movement to provide a good tear pump.

•Avoiding lenses that ride onto or over the limbus.

21.5.2 Problems

Overall, there are fewer problems with rigid lenses for extended wear. They do not cover the entire cornea, give a regular tear pump and are easier to remove and clean. By their nature, they avoid absorption, solutions reaction, red eye and vascularization. However, other difficulties can occur:

3 and 9 o’clock staining

3 and 9 o’clock staining is the rigid lens equivalent of soft lens dehydration. It rarely causes discomfort but, because patients continue to wear lenses, its effect may become cumulative with unacceptable conjunctival injection (see Section 28.3.2 for possible remedies).

Infections

Infection with rigid lenses is infrequent. Care, however, is required where lens adhesion occurs because of the possibility of corneal ulceration.9

Contact lens-induced papillary conjunctivitis

CLIPC can occur with rigid gas-permeable lenses because of deposits, allergic response or mechanical irritation.

Oedema

Microcysts and striae occur where materials with insufficient Dk have been used.

Vascularization

Vascularization occasionally occurs in the horizontal meridian associated with prolonged 3 and 9 o’clock staining.

Breakage and loss

Breakage occurs with some of the more brittle high Dk materials and where lenses have crazed or otherwise deteriorated with time. Loss is no worse than with daily wear.

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Lens adhesion (binding)

Most studies report lens adhesion to the cornea in a minimum of 10% of patients immediately on waking.12 Adhesion relates to lens design, material, lid pressure and changes in tear constituents (see Section 28.3.6).

21.6 Other lenses for extended wear

21.6.1 Silicon lenses

Silicon elastomer lenses (see Section 7.6) have extremely high Dks and are occasionally used for extended wear. One of their main applications is with babies to avoid the loss and damage that occur with soft lenses (see Section 31.5.1).

21.7 Long-term consequences of extended wear

Metabolic activity

Reduced oxygen supply to the epithelium reduces metabolic rate and therefore cell mitosis. Since the epithelium would not remain intact with decreased cell production and a constant rate of cell death and removal at the anterior epithelial surface, some compensation must occur.

Infection and inflammatory response

The risks of infection, including microbial keratitis and contact lens peripheral ulcers (sterile infiltrates) are significantly greater if the cells at the anterior epithelial surface become functionally less resistant as a result of long-term extended wear13 (see Chapter 29).

Corneal thinning

Epithelial thinning occurs as cell production and wastage rates reach a new equilibrium. Stromal thinning results from long-term chronic oedema.5

Endothelial polymegathism

Polymegathism is an irregularity in the cell size of the corneal endothelium caused by a lowering of pH14 due to increased lactic acid (resulting from lensinduced hypoxia) and increased carbonic acid (resulting from lens-induced hypercapnia). There does not seem to be a change in cell density, but a size increase of 27% has been reported.5 The chances of complications with surgery or trauma are reported to be significantly greater after extended wear.15

Vascularization

The extension of limbal blood vessels into the cornea is a serious indication of chronic hypoxia. The vessels may extend from any position around the limbus,

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but tend to be further advanced beneath the top lid. Any increase should be monitored and, if the vessels grow more than 2 mm into the cornea, extended wear should be discontinued.

Neovascularization

New vessel growth at a deep stromal level is an unacceptable response to corneal stress caused by hypoxia, limbal compression, tissue damage or an acute infection. If allowed to continue and encroach within the pupil area, vision may be reduced as the surrounding tissue is changed by a lipid keratopathy.

PRACTICAL ADVICE

To minimize the risk of potential problems, both shortand long-term, patients should:

•Wear lenses with maximum oxygen transmissibility.

•Return for regular aftercare visits, not only during the fitting stage but for as long as extended wear continues.

•Replace lenses on a regular basis, preferably with disposables.

•Use peroxide and enzyme systems with conventional lenses.

•Receive written instructions.

•Remove lenses in case of illness, especially with upper respiratory tract infections.

•Avoid wearing lenses while flying.

•Receive advice that they must remove lenses if they suffer red eyes, discomfort, reduced vision, persistent lacrimation or other abnormal symptoms.

•Use goggles when swimming.

•Know where 24-hour emergency help is available in case of severe infection.

References

1.Holden BA, Mertz GW. Critical oxygen levels to avoid corneal oedema for daily and extended wear contact lenses. Investigative Ophthalmology and Visual Science

1984;25:1161.

2.Holden BA, Sweeney DF. Ocular requirements for extended wear. Contax 1987;May:13–18.

3.Harvitt DM, Bonasnno JA. Re-evaluation of the oxygen diffusion model for predicting minimum contact lens Dk/t values needed to avoid corneal anoxia.

Optometry and Vision Science 1999;76:712–19.

4.Holden BA, Sweeney DF, Sanderson G. The minimum precorneal oxygen tension to avoid corneal oedema. Investigative Ophthalmology and Visual Science

1984;25:476.

5.Holden BA, Sweeney DF, Vannas A, Nilsson KT, Efron N. Effects of long-term extended contact lens wear on the human cornea. Investigative Ophthalmology and Visual Science 1985;26:1489.

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6.Hamano H, Hori M, Hamano T, Cawabe H, Mikami M, Mitsunaga S. Effects of contact lens wear on mitosis of corneal epithelium and lactate content of aqueous humor of rabbit. Japan Journal of Ophthalmology 1983;27:451.

7.Madigan MC, Holden BA, Kwok LS. Extended wear of hydrogel contact lenses can compromise the corneal epithelium. Investigative Ophthalmology and Visual Science

1986;27(suppl.):140.

8.Millodot M. Effect of the length of wear of contact lenses on corneal sensitivity. Acta Ophthalmologica 1976;54:721.

9.Zabkewicz K, Swarbrick H, Holden BA. Clinical experience with low to moderate Dk hard gas permeable lenses for extended wear. Transactions

of the British Contact Lens Association Annual Clinical Conference 1986;3: 101–2.

10. Efron N. Epithelial microcysts. In: Contact lens Complications. 2nd ed. Oxford: Butterworth-Heinemann; 2004. p. 116–21.

11. Efron N. Corneal neovascularisation. In: Contact lens Complications. 2nd ed. Oxford: Butterworth-Heinemann; 2004. p. 153–61.

12. Swarbrick HA, Holden BA. Rigid gas-permeable lens binding: significance and contributing factors. American Journal of Optometry and Physiological Optics

1989;64:815–23.

13. Sweeney DF, Grant T, Chong MS, Fleming C, Wong R, Holden BA. Recurrence of acute inflammatory conditions with hydrogel extended wear. Investigative Ophthalmology and Visual Science 1993;34:S1008.

14. Bonanno JA, Polse KA. Measurement of in vivo human corneal stromal pH: open and closed eyes. Investigative Ophthalmology and Visual Science 1987;28:522–30.

15. Rao GN, Aquavella JV, Goldberg SH, Berk SL. Pseudo aphakic bullous keratopathy

– relationship to preoperative corneal endothelial status. Ophthalmology 1984;91:1135.

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