High Prescriptions - Clinical Tree

Introduction

Most patients with high prescriptions benefit both optically and cosmetically from contact lenses, more so than patients with low prescriptions. Fitting the lenses is not always straightforward partly due to the different lens thickness but also to the medical aspects of the associated eye condition. Extra vigilance is required at aftercare to ensure that both the health of the eyes and the state of the contact lenses are adequately assessed.

General Points

Patients requiring high prescriptions are at a disadvantage when dealing with their lenses, as their unaided acuity is especially poor.

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Lens handling
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  Insertion – Everything needs to be set up to insert the lenses and the spectacles removed at the last moment. If the patient cannot manage at all without glasses, an empty frame can be glazed in one eye only with the lower part of the other rim removed. The first contact lens can then be inserted through the empty side of the frame.
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  Removal – Spectacles need to be to hand when lenses are being removed.
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  A mislaid lens is harder to find, so it is safer to work over a clean towel on a designated surface.
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  Thick (positive) lenses are easily scratched.
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  Thin (negative) lenses, both soft and rigid gas-permeable (RGP), can distort or crack with handling. The back vertex power (BVP), especially in RGP lenses, may alter with energetic lens cleaning.
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Vision
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  Spectacle aberrations are more obvious when alternating between spectacles and contact lenses, and the pin-cushion effect of aphakic spectacles is especially difficult to cope with.
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  Objects appear larger with hypermetropic or aphakic spectacles and smaller with myopic spectacles (see Chapter 7 ).
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  Distance judgement is likewise affected when alternating between the two forms of correction and patients who drive need to be warned about this.
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  Where acuity is good, lens parameter tolerances are not always adequate and replacement lenses may be unacceptable to the patient, even when the new lens is ordered with exactly the same parameters as the previous lens.
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Low-cost disposable lenses are available in limited powers only (although the range of parameters is improving all the time).
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Where lenses are custom-made, errors are more likely to occur in their manufacture, and replacement lenses may not be identical.
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Before reordering lenses that have been worn for some time, recheck parameters as they are liable to have altered.
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Thick lenses develop deposits more than thin ones. It is advisable that lenses are replaced at regular 6- or 12-monthly intervals, as all high-power lenses are prone to scratching and excessive buildup of deposits.

Helpful hints include the following:

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Order different tints to help differentiate left and right lenses.
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Provide a lens case with different-colour tops, or mark one top with indelible ink or nail varnish.

Ordering High-Power Lenses (see Chapter 7 and also https://expertconsult.inkling.com/ )

Prescriptions up to +/–20.00 D are available as standard disposable soft or silicone hydrogel lenses (Coopervision). For prescriptions over these powers, or if an astigmatic or multifocal prescription is required, a spectacle overcorrection can be ordered to wear with spherical disposable lenses. Some companies (e.g. Mark'Ennovy, Spain) make disposable lenses in most spherical and toric prescription powers, but these can be expensive and the lens fit may not be as good.

If disposable lenses are not suitable, the following must be taken into account when prescribing custom-made lenses:

Lens power

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Refract and measure the back vertex distance (BVD).
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The lens power at the cornea equals the back vertex power (BVP) of the contact lens to be ordered. This is either calculated or read from a chart (see Appendix A and Further Information available at: https://expertconsult.inkling.com/ ).
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For astigmatic corrections, the power of the lens at the cornea must be calculated separately, in the two principal meridians.

Lens material

The ideal material used for high-power lenses should have the following properties:

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high oxygen permeability
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good wettability
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good deposit resistance
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good scratch resistance.

In addition, extra properties of RGP lenses are:

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high refractive index (for thinner lenses)
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low specific gravity (positive lenses less likely to drop).

Extra properties of soft lenses are:

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bound water
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silicone hydrogel.

Unfortunately, the properties required are not all available in a single material.

Manufacture of High Prescriptions

The manufacture of lenses is covered in Chapter 29 , but there are some differences to be noted when manufacturing high prescriptions.

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The lenses are all made in lenticular form ( Fig. 21.1 ).
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Junction thickness needs to be calculated to ensure that it is not too thin in hypermetropic or aphakic prescriptions and not too thick in myopic prescriptions (see below).
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If the lenticular or front optic zone diameter (FOZD) is made smaller, the lens may fit better, but if it is too small, flare will be a problem. With multicurve RGP lenses, where practical, the FOZD is made the same or up to 0.5 mm larger than the back optic zone diameter (BOZD) to avoid the problem of flare.
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Soft lenses are made in the dry state (xerogel), and high powers are difficult to check accurately. The lenses need to be left longer to hydrate after manufacture and inaccuracies are commonly found once the material is fully hydrated.

Aphakia

General points

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The natural crystalline lens is absent, so more ultraviolet light is able to reach the retina.
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The risk of retinal detachment and glaucoma is increased.
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The cornea may be compromised due to previous surgery or trauma.
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Contact lenses can have a dual purpose, for example, to correct the refractive error and as a therapeutic lens for an aphakic patient with mild bullous keratopathy (see Chapter 26 ).

Nowadays, the usual surgical procedure is to have an intraocular lens (IOL) inserted so that neither thick spectacles nor contact lenses are required. However, aphakia is still common in:

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infants and children born with cataracts who are unable to have IOLs at the time of surgery (see Chapter 24 )
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young uveitis patients, when an IOL is risky
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patients with ectopia lentis, when the dislocated lens has been surgically removed or when the lens is dislocated enough to use the aphakic portion (see Fig. 24.10 )
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traumatic aphakia patients, when there is no capsular support for an implant. Traumatic aphakia often occurs in manual labourers who are not very dextrous or diligent when it comes to lens handling and care. It is frequently associated with other ocular trauma, which can affect the appearance of the eye and also makes the eye photophobic ( Fig. 21.2 ). An aphakic lens with a tint or a prosthetic lens may then be required (see Chapter 25 and ‘Unilateral aphakia’ below)
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  patients with explanted eyes, when an IOL had to be removed because of problems
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  those in whom a problem occurred at the time of the initial operation such that it was not possible to insert an IOL.

Elderly patients

Occasionally elderly patients are seen who had cataracts removed before IOLs were the norm. It is frequently necessary for a carer to manage the contact lenses. Alternatively, consider continuous wear in patients who would otherwise be unsuitable, for example, when neovascularisation has progressed, as elderly patients have a shorter life expectancy and quality of life is often more important. If the lens is to be removed more regularly, the patient would be incapacitated. However, if the risk of infection is too high this is not advisable, especially as the elderly are not always keen to ask for help if they have problems ( ).

Issues encountered with elderly patients include:

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handling difficulties
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poor tears
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poor endothelium
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lid problems:
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  epiphora
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  entropion/ectropion
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  loose lids:
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    poor lower lid support
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    ptosis
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    poor lid closure/blink.
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other ocular pathology:
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  glaucoma
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  retinal detachment
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  corneal problems.
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    dystrophy
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    keratitis.

Surgery or trauma

Many aphakic eyes are particularly difficult to fit, for example:

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failed IOLs
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glaucoma surgery, particularly trabeculectomy, which produces a filtration bleb; or glaucoma filtration tubes, which may be irritated or affected by a contact lens
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off-centre or enlarged pupil
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multiple pupils (polycoria)
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decentred corneal apex
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induced astigmatism.

Advantages of contact lenses over spectacles

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better field of vision
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less peripheral aberrations
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cosmetically more acceptable.

Disadvantages of contact lenses over spectacles

Key point

Acuity is not as sharp due to the reduction in image size – this can be a problem in cases of impaired vision.

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Bifocal spectacles may still be necessary to wear with the lenses, as high power multifocal contact lenses are not particularly successful. The thick lenses move too much with blinking, compromising the vision. However, some patients do well with multifocals, so they should not be dismissed altogether (see Chapter 13 ).

Aphakic Lens Fitting

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Choose a material that incorporates an ultraviolet inhibitor as the natural protection from the crystalline lens has been removed.
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Consider extended wear (whether soft or RGP) especially in an elderly patient (see above). In infants and young children, short periods of extended wear may be necessary, but it is always advisable to encourage daily lens removal from the outset as problems such as infections and scarring developed in childhood can lead to amblyopia. These, and any neovascularisation that develops, will affect the patient for life (see Chapter 24 ).
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Because of the lens thickness, aphakic lenses are liable to sit low ( Fig. 21.3 ).
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The lenses move more with blinking. This is especially a problem when only one eye is aphakic.
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The oxygen permeability of the lens is poor compared with a low-power lens of the same material.
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Unwanted long-term effects are more likely. Optimum fitting is even more important than for low prescriptions because patients are more dependent on their contact lenses as alternating with spectacles is not usually practical or convenient (see ‘General Points – Vision’ [CR] ).
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Scratches and deposits are common. Protein may build up, especially at the junction between the lenticular portion of the lens and the carrier ( Fig. 21.4 ).

RGP lenses

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The centre thickness is typically 0.30–0.40 mm.
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As previously mentioned, lenses are manufactured in lenticular form in order to reduce the weight. This helps to shift the centre of gravity back towards the eye, which helps improve centration causing the lens to drop less (see Fig. 9.3 ).
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The junction thickness needs to be adequate to prevent the lens from flexing or breaking at the junction.
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Fitting the lenses minimally steep will also help centration but not so steep as to cause bubbles to become lodged under the back optic zone.
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Larger total diameter (TD) can improve comfort and stabilise acuity.

Large-diameter lenses (See Chapter 9 )

By increasing the diameter of a lens, the centration can be improved.

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Diameters are typically between 10.50 and 13.50 mm, which aids centration and reduces movement on blinking ( Fig. 21.5 ).
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The BOZD is typically 8.5–10.0 mm, and the lens is usually made up as a bicurve with the second curve at least 0.7 mm flatter than the back optic zone radius (BOZR). If tear exchange is found to be inadequate, a third peripheral curve can be added later.
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In very large–diameter lenses (more than 13.5 mm), fenestration is occasionally carried out in order to facilitate tear exchange and lens removal.
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Lenses can be made up in very high–Dk materials to reduce hypoxia.
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Large-diameter lenses are particularly useful in cases of traumatic aphakia, when the cornea is irregular and the pupil abnormal.

Soft lenses (see Chapter 10 )

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The junction must not be too thin or the carrier is liable to evert on lens insertion or to break with lens handling.
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As with rigid lenses, large TDs are used to aid centration.
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High-power soft lenses exhibit bending effects when the lens is on the eye, which can cause fluctuations in the vision ( ).
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Thicker custom-made lenses do not drape the cornea as well as thin lenses. A greater range of lens parameters is therefore needed in order to achieve a satisfactory fit. Trial lenses may be necessary.

Lens trial

After inserting the trial lens, it should be allowed to settle for at least 30 minutes. Thick soft lenses take longer than thin lenses to equilibrate. Carry out an overrefraction, measure the BVD and calculate the power to order.

If single vision lenses are fitted, reading spectacles should be prescribed.

Troubleshooting

Poor Centration – Lenses Drop

Soft Lenses

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Fit larger TD.
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Make lenses thinner:
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  Reduce FOZD.
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  Fit aspheric lenses.
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Fit spherical disposable lenses (Coopervision Proclear monthlies centre thickness of +20.00 is 0.35 mm), and overcorrect with spectacles.

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