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•Some PMMA wearers find the gelatinous feel of a soft lens quite unpleasant on the eye.

•Many PMMA wearers have solutions difficulties, both with allergic response and the additional complexities of soft lens disinfection.

PRACTICAL ADVICE

•Try to avoid refitting PMMA directly with soft.

•Where it is essential to provide PMMA wearers with soft lenses, it is generally better to refit first with rigid gas-permeable lenses as an intermediate step. After about 6–8 weeks the cornea and refraction may have stabilized sufficiently to make soft lens fitting feasible.

•Daily disposable lenses are the preferred method of coping with continuously altering corneal curvature and refraction.

•PMMA wearers have frequently had poor compliance for many years and require strong advice on hygienic procedures with soft lenses.

30.2 Prescribing spectacles for contact lens wearers

The factors to consider include:

•Establishing the correct Rx.

•Providing a correction that is visually comfortable.

•Time of examination.

•When the spectacles are likely to be worn.

•Type of contact lens worn.

Non-tolerances are a frequent problem because, irrespective of contact  lens type, many patients find great difficulty in adapting to the different nature of a spectacle correction worn in front of rather than on the eye. Typical problems are:

•Visual distortion (e.g. sloping floors and bowed door frames).

•Different spatial perspective.

•Different image size.

•Restricted field of view.

•Reflections, especially at night.

•Intolerance to full correction.

•Intolerance to cylinders. Typical causes are:

•No spectacle refraction for several years and a correction showing a very marked increase in myopia.

•Patients may literally have worn no spectacles for decades because with PMMA it was not feasible to obtain a satisfactory result.

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•Astigmatism which is very different in axis and power compared with any previous correction.

•Contact lens monovision (deliberate or accidental) may give difficulty with either bifocals or separate distance and near spectacles.

30.2.1 Rigid gas-permeable lenses

Most modern lenses cause far fewer problems than those previously encountered with PMMA. Some degree of corneal moulding may still be encountered, particularly with astigmatic eyes, where aspheric designs or back surface simultaneous vision multifocals are used. Pronounced distortion occurs in cases of lens adhesion and refraction should be postponed.

Satisfactory refraction is usually achieved immediately on removal of lenses. If the result does not correlate with the contact lenses, keratometry and any previous spectacles, it should be repeated as a morning refraction.

Where PMMA has been refitted with rigid gas-permeable lenses, wait about 2 months before refracting for spectacles, by which time most corneal changes will have resolved. This is confirmed by monitoring ‘K’ readings.

PRACTICAL ADVICE

The once common advice of removing lenses for 2 or 3 days before refraction now rarely has any merit. It is extremely inconvenient for patients, especially if they are highly myopic with no current spectacles and does not necessarily give a more accurate result.

There are two realistic times for examination, the choice of which should be discussed with the patient according to when the correction is more likely to be used:

1.Afternoon, immediately on removal of lenses, gives the closest approximation to a correction for use in the evenings when lenses have been removed for the day.

2.Morning, prior to insertion of lenses, simulates the occasion when contact lenses will not be worn for a day.

If there is a difference, the morning result is usually less myopic. Undercorrecting the evening refraction can often give a satisfactory compromise.

30.2.2 Soft lenses

Corneal curvature and refractive changes are far less common with soft lenses, although some steepening of the vertical meridian may occur. There is little mechanical moulding and, where oedema is present, it generally extends from limbus to limbus without localized changes in curvature or physical distortion. Refraction on removal of soft lenses generally gives a good result but where there is any doubt it should be repeated in the morning.

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GENERAL ADVICE

•Explain the potential problems to the patient.

•Take notice of any pre-contact lens spectacles, however old.

•Give maximum possible binocular addition.

•Consider undercorrecting spheres by 0.25–0.50 D.

•Consider undercorrecting or omitting cylinders, especially if oblique.

•Be careful not to change eye dominance.

•Repeat refraction on another occasion if it does not correlate with the contact lenses, keratometry and previous correction.

30.3 Rigid lens modification

Modifications may be found necessary either during initial adaptation or at annual aftercare examinations. Adjustments are usually better left to the laboratory, especially with modern rigid lenses; technically these procedures should not be carried out by practitioners unless they are CE accredited. There are some occasions, however, when it is beneficial for lenses to be modified on the spot for which the following minimum of equipment is required:

•Drum with motorized spindle.

•Velveteen pad.

•Reversible suction holder or chuck.

•Selection of convex radius tools with polishing tape.

•Polishing medium.

30.3.1 Possible modifications

It is possible to make the following modifications or have them made by a laboratory:

•Blending and flattening peripheral radii.

•Reducing TD.

•Minor changes to BVP (up to –0.75 D and +0.50 D).

•Repolishing or reshaping edges.

•Repolishing front surfaces.

•Fenestration.

•Truncation.

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PRACTICAL ADVICE

•All modifications tend to make the fit looser.

•It is not feasible to tighten the fit by modification.

•Do not attempt to modify surface-treated lenses.

•Modern rigid lens materials require care to avoid distortion.

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