Section ONE Preliminaries

be made between one and four minutes after instillation. It is available in the form of strips which are wetted by saline and it is used, like rose bengal, for the evaluation of the cornea and conjunctiva. It is not readily available worldwide (see also Section 4.2).

Patient questionnaire on dry eyes

Screening with the aid of a questionnaire prior to contact lens fitting can give valuable information in assessing a marginally dry-eyed patient. Questionnaires have been published by McMonnies4 and, in a modified version, by CooperVision and others.5 The topics covered are:

•Symptoms experienced and extent of problems.

•Atmospheric conditions that make the eyes sensitive.

•Effects of alcohol.

•Medication taken (e.g. hormone-replacement therapy, antihistamines, tranquillizers).

•Presence of systemic conditions with dry eye side effects.

•Reports of dryness in other parts of the body.

•Sleep-induced problems.

5.6 Patient suitability for lens types

The majority of patients are now fitted with either soft or rigid gas-permeable lenses although, given the choice, most will opt for soft on grounds of comfort. Scleral, combination, PMMA or other lens forms are needed only occasionally. It is often immediately obvious from the preliminary examination and discussion which type is likely to be more suitable. Many patients can be successful with either soft or rigid lenses, but ideally, it is necessary to assess lenses of each type in order to evaluate lens performance on the eye.

5.6.1 Soft lenses

Soft lenses are now the likely first choice for most patients because of their superior initial comfort. They are, however, particularly indicated in the following cases:

New patients

•Rigid diagnostic lenses give unsatisfactory comfort because the lids or cornea are obviously too sensitive.

•Rigid diagnostic lenses give poor centration.

•The Rx is spherical and hypermetropic.

•The Rx is spherical with astigmatic ‘K’ readings (see Section 5.4).

•The pupils are very large or decentred.

•Rapid adaptation is required.

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Preliminary considerations and examination 5 Chapter

•An irregular wearing schedule is anticipated.

•Where there is poor or incomplete blinking prior to fitting.

•Older patients.

•There are awkward anatomical features likely to give poor rigid lens positioning (e.g. low lower lid; proptosed eyes; decentred corneal apex).

•Dusty geographic or working environment.

•Patients need the security of a lens which it is almost impossible to dislodge from the eye (e.g. sports or vocational use).

Refits or previous failures

Where rigid lenses have failed because of:

•Poor comfort.

•Poor vision.

•Flare and reflections.

•Poor centration.

•Oedema.

•Poor blinking.

•3 and 9 o’clock staining or vascularization.

•Other persistent corneal staining.

•Persistent conjunctival injection.

•Poor handling or repeated loss.

•Limbal vascularization.

5.6.2 Silicone hydrogel lenses

As silicone hydrogels become available in an increasingly wide range of parameters, they are gradually assuming priority over hydrogel lenses for most new soft lens patients because of their superior physiological properties. They are, however, particularly indicated in the following cases:

•Thick lenses (e.g. with high Rxs) where Dk/t is likely to be inadequate with a hydrogel lens.

•Extended or flexible wear.

•Corneal oedema with previous lenses.

•Persistent conjunctival injection with previous lenses.

5.6.3 Rigid gas-permeable lenses

Rigid gas-permeable lenses should still be considered in the following cases:

New patients

•Soft trial lenses give unsatisfactory vision.

•Significant corneal astigmatism is present (>3.00 D).

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Section ONE Preliminaries

•Corneal irregularity is present (e.g. keratoconus, grafts).

•Complete corneal coverage is inadvisable (e.g. pterygium, old scar).

•Dry eyes have been diagnosed.

•An extremely high Dk is required.

•VDUs are used full-time.

•Dry geographic or working environment.

•There is a history of hay fever, vernal conjunctivitis or giant papillary conjunctivitis prior to fitting.

•The appearance of the limbal vessels prior to fitting suggests that vascularization is a likely consequence with soft lenses.

•Patients are unlikely to comply with soft lens disinfection and daily disposables are not appropriate.

•Handling difficulties are likely with soft lenses (e.g. low myopes with ultrathin lenses; very small palpebral apertures).

•Patients wish to avoid the ongoing costs of disposable lenses.

Refits or previous failures

Where soft lenses have failed because of:

•Poor vision.

•Poor comfort.

•Dry eyes.

•Poor centration or fitting.

•Poor handling or repeated breakage.

•Corneal vascularization.

•CLIPC.

•Repeated infections.

•Unacceptably short lifespan and daily disposables are not possible.

•Frequent deposits.

•Solutions allergies.

•Materials allergy.

References

1.Walker J. The forgotten generation. Optician 2008;1 February.

2.Stone J. Near vision difficulties in non-presbyopic corneal lens wearers.  

Contact Lens Journal 1967;1:14–6.

3.Applegate RA, Massof RW. Changes in the contrast sensitivity function induced by contact lens wear. American Journal of Optometry 1975;52:840–6.

4.McMonnies CW, Ho A. Patient history in screening for dry eye conditions. Journal of American Optometric Association 1987;58:296–301.

5.Guillon M, Allary J-C, Guillon J-P, Osborne G. Clinical management of regular replacement: Part I. Selection of replacement frequency. ICLC 1992;19:104–20.

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Section

Preliminaries ONE

The tear film and CHAPTER6 dry eyes

6.1 The tear film

The tear film is typically considered to be a three-layered structure between 6 and 9  m in thickness1 comprising:

•a mucoidal basal layer

•an aqueous component

•a thin superficial lipid layer

The tear film is now thought to be more complex with a mucin gel attached to the corneal microvilli and a larger aqueous and mucin interface resting on the gel with a much thinner lipid layer as the boundary between the tears  and air.2

6.1.1 Maintenance of the tear film

As the eye closes during a blink the lipid layer is compressed between the lid margins. Mucin is moved to the upper and lower fornices from where it is excreted through the tear ducts. It is replaced by a new layer, created by the lids pushing against the surface of the eye. As the lids open, a new aqueous layer is spread across the now hydrophilic epithelial surface. The lipid layer, made thicker from lid closure, spreads out producing a monolayer across the aqueous to reduce tear evaporation. The new tear film is relatively unstable and tear evaporation reduces the thickness and allows lipids to diffuse towards the mucus. The now contaminated mucin begins to lose its hydrophilicity and the tear film

©2010 Elsevier Ltd, Inc, BV

DOI: 10.1016/B978-0-7506-7590-1.00011-X