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High Prescriptions
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High Prescriptions LYNNE SPEEDWELL
CHAPTER CONTENTS
Introduction, 415 General Points, 415 Ordering High-Power Lenses, 415 Manufacture of High Prescriptions, 416 Aphakia, 416 Aphakic Lens Fitting, 417
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. Lens handling ■ 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. ■ Removal – Spectacles need to be to hand when lenses are being removed. ■ A mislaid lens is harder to find, so it is safer to work over a clean towel on a designated surface. ■ Thick (positive) lenses are easily scratched. ■ 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. ■ Vision ■ Spectacle aberrations are more obvious when alternating between spectacles and contact lenses, and the pincushion effect of aphakic spectacles is especially difficult to cope with. ■ Objects appear larger with hypermetropic or aphakic spectacles and smaller with myopic spectacles (see Chapter 7).
High Hypermetropia, 419 High Myopia, 419 High Astigmatism, 421 Unilateral Ametropia, 421 Aftercare, 422 Conclusion, 422
Distance judgement is likewise affected when alternating between the two forms of correction and patients who drive need to be warned about this. ■ 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. ■ Low-cost disposable lenses are available in limited powers only (although the range of parameters is improving all the time). ■ Where lenses are custom-made, errors are more likely to occur in their manufacture, and replacement lenses may not be identical. ■ Before reordering lenses that have been worn for some time, recheck parameters as they are liable to have altered. ■ 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. ■
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Helpful hints include the following: Order different tints to help differentiate left and right lenses. ■ 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: 415
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SECTION 6 • Specialist Lens Fitting
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. ■ 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. ■
LENS POWER Refract and measure the back vertex distance (BVD). 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/). ■ 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: high oxygen permeability ■ good wettability ■ good deposit resistance ■ good scratch resistance. ■
GENERAL POINTS The natural crystalline lens is absent, so more ultraviolet light is able to reach the retina. ■ The risk of retinal detachment and glaucoma is increased. ■ The cornea may be compromised due to previous surgery or trauma. ■ 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). ■
In addition, extra properties of RGP lenses are: high refractive index (for thinner lenses) low specific gravity (positive lenses less likely to drop).
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Extra properties of soft lenses are: bound water silicone hydrogel.
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Unfortunately, the properties required are not all available in a single material.
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: infants and children born with cataracts who are unable to have IOLs at the time of surgery (see Chapter 24) ■ young uveitis patients, when an IOL is risky ■ 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) ■ 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 ■
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. The lenses are all made in lenticular form (Fig. 21.1). 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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Carrier
Aphakia
Lenticular
Junction
Fig. 21.1 Lenticulated lens.
Fig. 21.2 Traumatic aphakia. An aphakic contact lens improves vision, but the cosmesis is poor and the eye is photophobic. A prosthetic lens would improve the appearance and reduce the photophobia.
21 • High Prescriptions
lens may then be required (see Chapter 25 and ‘Unilateral aphakia’ below) patients with explanted eyes, when an IOL had to be removed because of problems ■ those in whom a problem occurred at the time of the initial operation such that it was not possible to insert an IOL. ■
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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. 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).
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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 (Carpel & Parker 1985). Issues encountered with elderly patients include: handling difficulties poor tears ■ poor endothelium ■ lid problems: ■ epiphora ■ entropion/ectropion ■ loose lids: ■ poor lower lid support ■ ptosis ■ poor lid closure/blink. ■ other ocular pathology: ■ glaucoma ■ retinal detachment ■ corneal problems. ■ dystrophy ■ keratitis. ■ ■
Aphakic Lens Fitting Choose a material that incorporates an ultraviolet inhibitor as the natural protection from the crystalline lens has been removed. ■ 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). ■ Because of the lens thickness, aphakic lenses are liable to sit low (Fig. 21.3). ■ The lenses move more with blinking. This is especially a problem when only one eye is aphakic. ■ The oxygen permeability of the lens is poor compared with a low-power lens of the same material. ■ 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’ p. 415). ■ 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). ■
SURGERY OR TRAUMA Many aphakic eyes are particularly difficult to fit, for example: failed IOLs glaucoma surgery, particularly trabeculectomy, which produces a filtration bleb; or glaucoma filtration tubes, which may be irritated or affected by a contact lens ■ off-centre or enlarged pupil ■ multiple pupils (polycoria) ■ decentred corneal apex ■ induced astigmatism. ■ ■
ADVANTAGES OF CONTACT LENSES OVER SPECTACLES better field of vision ■ less peripheral aberrations ■ cosmetically more acceptable. ■
Fig. 21.3 Low-riding lens on aphakic eye with updrawn pupil. The lenticular portion does not provide adequate pupil cover resulting in variable vision. A larger total diameter (see Fig. 21.5) may provide a better fit.
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SECTION 6 • Specialist Lens Fitting
In very large–diameter lenses (more than 13.5 mm), fenestration is occasionally carried out in order to facilitate tear exchange and lens removal. ■ Lenses can be made up in very high–Dk materials to reduce hypoxia. ■ 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) The junction must not be too thin or the carrier is liable to evert on lens insertion or to break with lens handling. ■ As with rigid lenses, large TDs are used to aid centration. ■ High-power soft lenses exhibit bending effects when the lens is on the eye, which can cause fluctuations in the vision (Chaston & Fatt 1980). ■ 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. ■
Fig. 21.4 Aphakic lens showing deposits within the lenticular portion.
LENS TRIAL
Fig. 21.5 Large-diameter lens with a total diameter of 13.50 mm fitted to an aphakic eye to improve centration of a low-riding lens.
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 RGP LENSES The centre thickness is typically 0.30–0.40 mm. 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). ■ The junction thickness needs to be adequate to prevent the lens from flexing or breaking at the junction. ■ 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. ■ 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. Diameters are typically between 10.50 and 13.50 mm, which aids centration and reduces movement on blinking (Fig. 21.5). ■ 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. ■
Poor Centration – Lenses Drop Soft Lenses ■ Fit larger TD. ■ Make lenses thinner: ■ Reduce FOZD. ■ Fit aspheric lenses. ■ Fit spherical disposable lenses (Coopervision Proclear monthlies centre thickness of +20.00 is 0.35 mm), and overcorrect with spectacles. RGP Lenses ■ Reduce FOZD. ■ Fit a multicurve lens with increased edge lift. ■ Order a lens with a negative or parallel carrier (see Fig. 9.16). ■ Fit larger TD (see Fig. 21.5). ■ Change material: ■ higher refractive index ■ lower specific gravity.
Lenses Move Excessively on Blinking ■ Make lenses thinner. ■ Fit larger TD. Signs of Hypoxia ■ Higher Dk material ■ Thinner lens. ■ Change hydrogel to silicone hydrogel.
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Fit the nearest silicone hydrogel, and overcorrect with spectacles.
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Lenses Scratch Centrally With Handling RGP Lenses ■ Change to harder material (e.g. Boston ES). ■ Change storage case design. Soft and RGP Lenses ■ Replace lenses more frequently.
Protein Deposits ■ Regular protein remover tablets ■ Aspheric lenses: lenses have a more uniform thickness profile (Bleshoy & Guillon 1984), and the junction is not so well circumscribed, so deposits may build up less ■ More regular replacement. Handling Difficulties or Poor Stability of Image ■ Consider scleral lenses (see Chapter 14).
High Hypermetropia Unless the hypermetropia is extremely high (e.g. in nanophthalmos – see below and Chapter 24), the problems found when fitting high hypermetropes are similar, but to a lesser degree, to those found in aphakia. Various disposable lenses are available in these prescriptions and can be used to overcome some of the problems associated with aphakia. There are some differences, however.
GENERAL POINTS Eyes may be smaller and steeper than normal. Hypermetropia is often associated with strabismus and amblyopia. Extra care is needed when one eye is amblyopic. Rigorous hygiene is required to help prevent contact lens– induced infection as effects on the better eye will affect overall vision. ■ If the high refractive error is not corrected from a young age, varying degrees of bilateral amblyopia can occur and, even when fully corrected, normal acuity may not develop. ■ High hypermetropia is associated with closed-angle glaucoma. There is a theoretical possibility that tight soft lenses might increase the IOP by compression of the limbal drainage vessels. ■ ■
ADVANTAGES OF CONTACT LENSES OVER SPECTACLES Better field of view (see Fig. 7.9). Less accommodation and convergence required for close work (see Chapter 7).
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DISADVANTAGES OF CONTACT LENSES OVER SPECTACLES The visual acuity is not as good for both distance and near; this is especially a problem when the spectacle acuity is less than 0.0 logMAR (6/6 Snellen).
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Nanophthalmos is a genetic condition in which the eyes are small with a short axial length, a thick but weak sclera and abnormal collagen (Sowden & Taylor 2005), and there is extreme hypermetropia, often more than +20.00 D, resulting in amblyopia. With a steep corneal radius and microcornea, lenses can be fitted similar to those required by infant aphakes (see Chapter 24) but there is usually adequate accommodation so bifocal spectacles are not usually necessary.
High Myopia (see Chapter 28) There is a distinction between high myopia (>–5.00 D) and pathological myopia; the latter usually occurs in axially long eyes and is more likely to result in retinal disease, in particular choroidal neovascularisation and retinal detachment. In a systematic review, the prevalence of pathological myopia ranged from 0.9–3.1% in different populations (Wong et al. 2014).
GENERAL POINTS Prevalence figures vary for both bilateral and unilateral high myopia but appear to be increasing. A review by Holden et al. (2016) estimated that 2.7% of the world population had high myopia (>–5.00 D) and that this is likely to rise to 9.8% of the world population by 2050. ■ It may be dominantly transmitted (Coscas & Soubrane 1993). ■ There is a higher incidence in premature infants (Larsson et al. 2003). ■ It can be associated with other syndromes such as Stickler or Marfan syndrome. ■ Most high myopia is axial (Grosvenor & Scott 1991). ■ It is associated with serious ocular pathology often leading to loss of vision: ■ enlarged globe ■ exophthalmia ■ fundus changes: ■ peripapillary atrophy (myopic crescent) ■ straightening of the retinal vessels ■ tessellated background ■ chorioretinal degeneration ■ retinal detachment. ■
Other problems with enlarged exophthalmic eyes include: poor blink dry eyes ■ flat, large corneas ■ abnormal extraocular muscles and strabismus. ■ ■
ADVANTAGES OF CONTACT LENSES OVER SPECTACLES Better acuity Better field. Although the field is theoretically greater with spectacles than with contact lenses, the peripheral field is distorted so contact lenses provide a better usable field.
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SECTION 6 • Specialist Lens Fitting
Also, the spectacle frame limits the field (see Chapter 7 and Fig. 7.8).
DISADVANTAGES OF CONTACT LENSES OVER SPECTACLES More accommodation and convergence are exerted for close work (see Chapter 7). ■ Cosmetic effect may be poor if the eyes are exophthalmic (Fig. 21.6). Cosmesis may be improved by fitting the greater part of the prescription with contact lenses and the remainder in spectacles. For example, a –20.00 DS myope can be fitted with –15.00 DS contact lenses and –5.00 DS spectacles over the top. ■ High myopia combined with astigmatism requires lenses that are especially thick in one meridian. This frequently leads to neovascularistion, particularly with soft lenses. Correcting the astigmatism in spectacles may be preferable. ■ Where hypoxia is a problem and disposable silicone hydrogel lenses are not available in high enough prescriptions, a compromise lens can be fitted in a disposable lens and the remainder prescribed in spectacles. ■ High myopes with poor acuity may prefer to read unaided rather than using a low visual aid. This is not always convenient. ■
For presbyopic patients, supplementary bifocal spectacles still need to be prescribed. As with aphakia, bifocal contact lenses do not always work in high prescriptions. Early presbyopes may benefit from having a reduced prescription in contact lenses and the remainder in spectacles as they are then able to remove the glasses whenever they wish to read or do any close work.
CONTACT LENS FITTING The degree of myopia that is considered to be high can be anything over –5.00 D. For the purpose of this chapter, most contact lens considerations are for prescriptions over –10.00 D. All types of lenses can be fitted – RGP, soft, or scleral. Lenses are thin centrally and thick peripherally. ■ Oxygen is reduced at the limbus, and hypoxia can lead to neovascularistion. ■ Thin lenses break easily when handling, and the power can alter or become distorted with lens cleaning. ■
High-minus contact lenses are occasionally used as the eyepiece together with a high-plus lens in spectacles as a Galilean telescope (see Chapter 27) although they are rarely successful.
RGP Lenses Thick edges cause displacement: Upwards – the lid is hitched up with each blink (Fig. 21.7). This can cause desiccation of cornea below the lens. If the lens is hitched up too high, it can result in flare as the peripheral zone or even the lower edge of the lens fails to cover the lower part of the pupil adequately. ■ Downwards – an interpalpebral fitting lens can be pushed downwards with each blink as the upper lid pushes on the thick junction between the lenticular and the carrier zone. ■
Requirements ■ A thinned junction (see Chapter 9) – to prevent too much movement on blink, too much lid hitch or the lens being pushed down with tight lids. The contact lens laboratory can input this into the computer-controlled lathe to achieve this. It can also be achieved by careful polishing (Moore & Mandell 1989) (see Chapter 33). ■ Aspheric lenses – as the junction is not as pronounced. ■ A negative or parallel carrier (see Fig. 9.16 and further information available at: https://expertconsult.inkling .com/) – to help with lid hitch. ■ A material with good dimensional stability-reduces lens changes due to wear and tear on the thin optic zone and the thick peripheral zones. Fitting Lenses to Large Myopic Eyes ■ Lenses tend to drop if the cornea is particularly flat (although in moderate myopia of less than 10 D, the corneas may actually be steeper; Carney et al. 1997). ■ Large corneas require large TDs. ■ Small lenses can sometimes be successful as the midperiphery can be made thinner but if the lens fits interpalpebrally, it is likely to sit low. Many high myopes have large pupils, and small RGP lenses may result in flare; this is a problem especially with night driving.
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Fig. 21.6 Exophthalmos in high myopia. This patient would benefit from wearing spectacles with a low-minus prescription over the contact lenses to reduce the exophthalmic appearance.
Fig. 21.7 High-riding lens. The high-minus lens has thick edges, which cause it to be hitched up by the upper lid resulting in desiccation of the inferior cornea and poor pupil cover, giving rise to flare. (Courtesy of Tony Phillips.)
21 • High Prescriptions
Soft Lenses ■ Soft disposable lenses, if available and not contraindicated, are ideal but they are only available in limited parameters. Proclear and Biofinity lenses (both Coopervision) are available in powers up to –20.00 D and, as mentioned earlier, the prescription can be refined by overcorrecting with spectacles. ■ Custom-made lenses have similar problems to RGP lenses, a thick midperiphery and deposition. ■ The thick transition between the lenticular portion of the lens and the carrier results in excess movement as the lid pushes the lens downwards or hitches it up, and reduced oxygen to the peripheral cornea can result in neovascularisation even with silicone hydrogel materials. ■ Small TDs exacerbate this problem and large TDs of 15.00 mm or more may be necessary. ■ Large aspheric lenses may centre better. ■ A tight lens may still move excessively with each blink because of the thick edge.
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High Astigmatism GENERAL POINTS Astigmatism can be any or all of the following: corneal lenticular ■ retinal. ■ ■
It can be either regular or irregular, although most cases of the latter are related to keratoconus or trauma and are covered in Chapters 20 and 25. Where there is a high degree of regular astigmatism, it is common to find that the major component is corneal and that the axes of the astigmatism, as measured with a keratometer, correspond to the axes of ocular astigmatism. Where the astigmatism is congenital, if it is not corrected from a young age, meridional amblyopia is likely.
ADVANTAGES OF CONTACT LENSES OVER SPECTACLES
TROUBLESHOOTING
The field is less distorted.
Poor Centration – Lid Hitch (RGP Lenses) ■ Fit aspheric lens. ■ Design lens with a parallel carrier (see Fig. 9.16). ■ Fit larger soft lens.
DISADVANTAGES OF CONTACT LENSES OVER SPECTACLES
Poor Centration – Lenses Pushed Down by Action of Tight Lids on Thick Junction Soft and RGP Lenses ■ Increase TD. ■ Reduce FOZD. ■ Fit aspheric lenses.
Lens Fitting For comprehensive information about fitting toric lenses, see Chapter 11.
RGP Lenses ■ Fit a multicurve lens with increased edge lift. ■ Change material: ■ higher refractive index ■ lower specific gravity.
Lenses Move Excessively on Blinking ■ Make the junction thinner. ■ Refit with a smaller lenticular diameter. ■ Refit with an aspheric lens. ■ Fit larger TD. Poor Oxygen Permeability ■ Change to a higher Dk material. ■ Consider RGP if currently in silicone hydrogel. Protein Deposits Soft and RGP Lenses ■ regular protein remover tablets ■ aspheric lenses – the junction is not so well circumscribed in aspheric lenses, so deposits may build up less ■ more regular replacement. Handling Difficulties or Poor Stability of Image ■ Consider scleral lenses (see Chapter 14).
A small dislocation of the lens axes can cause a large visual effect.
Lens Requirements ■ stable lenses ■ good oxygen permeability ■ high refractive index (RGP). Contact lenses can provide good acuity, but it may be difficult to achieve a fit that is stable and at the same time causes no corneal compromise. Fitting with a rigid lens usually requires a bitoric design in a material that has good oxygen permeability and wets well. If lenses are not disposable, encourage weekly enzymatic cleaners from the outset, as protein deposits are likely to be attracted to certain areas of the lenses because there is no lens rotation with blink action. Lens stability is critical when fitting toric soft lenses, but this may lead to long-term corneal compromise. The thick areas of contact lens can cause meridional hypoxia and neovascularisation, even with silicone hydrogel material, and these lenses are prone to deposit formation in the thicker parts of the lens.
Unilateral Ametropia (see Chapter 7) MYOPIA In cases of unilateral ametropia or aphakia, there is a better chance of stereopsis with a contact lens than with a spectacle
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SECTION 6 • Specialist Lens Fitting
lens. Aniseikonia is reduced for both axial and refractive myopia (Winn et al. 1988). It is suggested that each retina has the same number of photoreceptors, and in the longer eye these are more widely spaced than in the shorter eye, as might be expected during the growth process. Thus the bigger retinal image in the longer eye is thought to cover the same number of receptors as the smaller retinal image in the shorter eye, giving rise to better binocular fusion. However, patients who have achieved some stereo acuity with spectacles may not appreciate any improvement when a contact lens is fitted, and their stereoacuity may be disrupted. When the unilaterally myopic eye is larger than the fellow eye, it may look better cosmetically to continue to wear all or part of the myopic correction in the form of a spectacle lens as the size of the eye will appear to be reduced.
refractive errors are at risk of concomitant pathology, and signs and symptoms of ocular pathology must not be mistaken for an adverse response to contact lenses. For example, corneal oedema can result from contact lens–induced hypoxia but may equally be caused by a closed-angle glaucoma attack; likewise, poor vision may be a contact lens problem but could also be from a retinal detachment. An adverse response is more likely with high-power lenses, and careful observation is necessary. Lens parameters and/ or lens type should be altered accordingly. However, where optimum fitting has been achieved and some small degree of ocular change is still evident, the practitioner must feel competent enough to decide whether it is acceptable for the patient to continue, thus, to reduce lens wear or even to stop it altogether.
APHAKIA
Conclusion
As discussed earlier, it is unusual nowadays to have a unilaterally aphakic eye resulting from planned cataract surgery but unilateral aphakia may follow trauma or congenital cataract surgery. Both frequently result in strabismus and in the case of traumatic aphakia, other components of the eye and orbit may have been damaged including extraocular muscles. Binocular single vision may be impossible, and intractable diplopia may result (see Chapter 25). However, with time and contact lens correction, diplopia may resolve as the eye straightens. Most unilateral aphakes are not able to achieve stereopsis especially if the cataract had been long-standing. Because the contact lens is in front of the nodal point of the eye, binocular vision is, at best, poor. Guillon and Warland (1986) found that most unilateral aphakes with straight eyes achieved only 140 seconds of arc, and 80% had intermittent suppression. If diplopia is not a problem, a contact lens may increase the field of vision and help to keep the eye straighter where it might otherwise converge or, more commonly, diverge.
SPORT The better field afforded by contact lenses to high ametropes is particularly beneficial in sport. However, in certain sports (e.g. squash and badminton), safety spectacles should always be worn over the lenses. Spectacle wearers who are fitted with contact lenses lose the protection of the spectacle frame and also the lenses if they are made of a toughened material. Swimming is particularly inadvisable in contact lenses. As well as a general risk of infection and irritation, there is an increased risk of Acanthamoeba keratitis (see Chapter 17), so either goggles over the lenses or preferably swimming goggles with the spectacle prescription should be worn. Rigid lens wearers can reduce lens loss by wearing daily disposable lenses over their lenses for sport.
Aftercare Aftercare is covered in Chapter 16, and the detail is the same whatever the power of the lenses. However, eyes with high
Fitting high degrees of ametropia can be especially rewarding. Many patients have been totally dependent on spectacles for as long as they can remember, and have no idea of what life will be like without them. Because of some of the difficulties encountered with high-prescription contact lenses, instruction on lens care and potential eye problems is even more important than with low prescriptions. The lens of choice may not be available in the required power, and some compromise may be necessary in order to enable the patient to wear lenses.
References Bleshoy, H., Guillon, M., 1984. Soft lens design – clinical results. J. Br. Contact Lens Assoc. 7, 41–47. Carney, L., Mainstone, J., Henderson, B., 1997. Corneal topography and myopia. A cross-sectional study. Invest. Ophthalmol. Vis. Sci. 38, 311–320. Carpel, E., Parker, P., 1985. Extended wear aphakic contact lens fitting in high risk patients. Contact Lens Assoc. Ophthalmol. J. 11, 231–233. Chaston, J., Fatt, I., 1980. The change in power of soft lenses. Optician 180, 12–21. Coscas, G., Soubrane, G., 1993. Myopie fort ou myopie maladie. Rev. Prat. 43, 1768–1772. Grosvenor, T., Scott, R., 1991. Comparison of refractive components in youth-onset and early adult-onset myopia. Optom. Vis. Sci. 68, 204–209. Guillon, M., Warland, J., 1986. Aniseikonia and binocular function in unilateral aphakes wearing contact lenses. J. Br. Contact Lens Assoc. 3, 36–38. Holden, B.A., Fricke, T.R., Wilson, D.A., et al., 2016. Global prevalence of myopia and high myopia and temporal trends from 2000 through 2050. Ophthalmology 123, 1036–1042. Larsson, E.K., Rydberg, A.C., Holmstom, G.E., 2003. A population-based study of the refractive outcome in 10-year-old preterm and full-term children. Arch. Ophthalmol. 121, 1430–1436. Moore, C., Mandell, R.B., 1989. The design of high minus lenses. Contact Lens Spectr. 11, 43–47. Sowden, J., Taylor, D., 2005. Disorders of the eye as a whole. In: Pediatric Ophthalomology and Strabismus, third ed. Edited by Taylor & Hoyt Pub. Elsevier. Winn, B., Ackerley, R.G., Brown, C.A., et al., 1988. Reduced aniseikonia in axial anisometropia with contact lens correction. Ophthalmic Physiol. Opt. 8, 341–344. Wong, T., Ferreira, A., Hughes, R., et al., 2014. Epidemiology and disease burden of pathologic myopia and myopic choroidal neovascularization: an evidence-based systematic review. Am. J. Ophthalmol. 157, 9–25.
High Prescriptions LYNNE SPEEDWELL
CHAPTER CONTENTS
Introduction, 415 General Points, 415 Ordering High-Power Lenses, 415 Manufacture of High Prescriptions, 416 Aphakia, 416 Aphakic Lens Fitting, 417
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. Lens handling ■ 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. ■ Removal – Spectacles need to be to hand when lenses are being removed. ■ A mislaid lens is harder to find, so it is safer to work over a clean towel on a designated surface. ■ Thick (positive) lenses are easily scratched. ■ 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. ■ Vision ■ Spectacle aberrations are more obvious when alternating between spectacles and contact lenses, and the pincushion effect of aphakic spectacles is especially difficult to cope with. ■ Objects appear larger with hypermetropic or aphakic spectacles and smaller with myopic spectacles (see Chapter 7).
High Hypermetropia, 419 High Myopia, 419 High Astigmatism, 421 Unilateral Ametropia, 421 Aftercare, 422 Conclusion, 422
Distance judgement is likewise affected when alternating between the two forms of correction and patients who drive need to be warned about this. ■ 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. ■ Low-cost disposable lenses are available in limited powers only (although the range of parameters is improving all the time). ■ Where lenses are custom-made, errors are more likely to occur in their manufacture, and replacement lenses may not be identical. ■ Before reordering lenses that have been worn for some time, recheck parameters as they are liable to have altered. ■ 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. ■
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Helpful hints include the following: Order different tints to help differentiate left and right lenses. ■ 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: 415
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SECTION 6 • Specialist Lens Fitting
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. ■ 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. ■
LENS POWER Refract and measure the back vertex distance (BVD). 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/). ■ 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: high oxygen permeability ■ good wettability ■ good deposit resistance ■ good scratch resistance. ■
GENERAL POINTS The natural crystalline lens is absent, so more ultraviolet light is able to reach the retina. ■ The risk of retinal detachment and glaucoma is increased. ■ The cornea may be compromised due to previous surgery or trauma. ■ 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). ■
In addition, extra properties of RGP lenses are: high refractive index (for thinner lenses) low specific gravity (positive lenses less likely to drop).
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Extra properties of soft lenses are: bound water silicone hydrogel.
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Unfortunately, the properties required are not all available in a single material.
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: infants and children born with cataracts who are unable to have IOLs at the time of surgery (see Chapter 24) ■ young uveitis patients, when an IOL is risky ■ 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) ■ 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 ■
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. The lenses are all made in lenticular form (Fig. 21.1). 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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Carrier
Aphakia
Lenticular
Junction
Fig. 21.1 Lenticulated lens.
Fig. 21.2 Traumatic aphakia. An aphakic contact lens improves vision, but the cosmesis is poor and the eye is photophobic. A prosthetic lens would improve the appearance and reduce the photophobia.
21 • High Prescriptions
lens may then be required (see Chapter 25 and ‘Unilateral aphakia’ below) patients with explanted eyes, when an IOL had to be removed because of problems ■ those in whom a problem occurred at the time of the initial operation such that it was not possible to insert an IOL. ■
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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. 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).
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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 (Carpel & Parker 1985). Issues encountered with elderly patients include: handling difficulties poor tears ■ poor endothelium ■ lid problems: ■ epiphora ■ entropion/ectropion ■ loose lids: ■ poor lower lid support ■ ptosis ■ poor lid closure/blink. ■ other ocular pathology: ■ glaucoma ■ retinal detachment ■ corneal problems. ■ dystrophy ■ keratitis. ■ ■
Aphakic Lens Fitting Choose a material that incorporates an ultraviolet inhibitor as the natural protection from the crystalline lens has been removed. ■ 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). ■ Because of the lens thickness, aphakic lenses are liable to sit low (Fig. 21.3). ■ The lenses move more with blinking. This is especially a problem when only one eye is aphakic. ■ The oxygen permeability of the lens is poor compared with a low-power lens of the same material. ■ 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’ p. 415). ■ 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). ■
SURGERY OR TRAUMA Many aphakic eyes are particularly difficult to fit, for example: failed IOLs glaucoma surgery, particularly trabeculectomy, which produces a filtration bleb; or glaucoma filtration tubes, which may be irritated or affected by a contact lens ■ off-centre or enlarged pupil ■ multiple pupils (polycoria) ■ decentred corneal apex ■ induced astigmatism. ■ ■
ADVANTAGES OF CONTACT LENSES OVER SPECTACLES better field of vision ■ less peripheral aberrations ■ cosmetically more acceptable. ■
Fig. 21.3 Low-riding lens on aphakic eye with updrawn pupil. The lenticular portion does not provide adequate pupil cover resulting in variable vision. A larger total diameter (see Fig. 21.5) may provide a better fit.
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SECTION 6 • Specialist Lens Fitting
In very large–diameter lenses (more than 13.5 mm), fenestration is occasionally carried out in order to facilitate tear exchange and lens removal. ■ Lenses can be made up in very high–Dk materials to reduce hypoxia. ■ 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) The junction must not be too thin or the carrier is liable to evert on lens insertion or to break with lens handling. ■ As with rigid lenses, large TDs are used to aid centration. ■ High-power soft lenses exhibit bending effects when the lens is on the eye, which can cause fluctuations in the vision (Chaston & Fatt 1980). ■ 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. ■
Fig. 21.4 Aphakic lens showing deposits within the lenticular portion.
LENS TRIAL
Fig. 21.5 Large-diameter lens with a total diameter of 13.50 mm fitted to an aphakic eye to improve centration of a low-riding lens.
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 RGP LENSES The centre thickness is typically 0.30–0.40 mm. 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). ■ The junction thickness needs to be adequate to prevent the lens from flexing or breaking at the junction. ■ 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. ■ 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. Diameters are typically between 10.50 and 13.50 mm, which aids centration and reduces movement on blinking (Fig. 21.5). ■ 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. ■
Poor Centration – Lenses Drop Soft Lenses ■ Fit larger TD. ■ Make lenses thinner: ■ Reduce FOZD. ■ Fit aspheric lenses. ■ Fit spherical disposable lenses (Coopervision Proclear monthlies centre thickness of +20.00 is 0.35 mm), and overcorrect with spectacles. RGP Lenses ■ Reduce FOZD. ■ Fit a multicurve lens with increased edge lift. ■ Order a lens with a negative or parallel carrier (see Fig. 9.16). ■ Fit larger TD (see Fig. 21.5). ■ Change material: ■ higher refractive index ■ lower specific gravity.
Lenses Move Excessively on Blinking ■ Make lenses thinner. ■ Fit larger TD. Signs of Hypoxia ■ Higher Dk material ■ Thinner lens. ■ Change hydrogel to silicone hydrogel.
21 • High Prescriptions
Fit the nearest silicone hydrogel, and overcorrect with spectacles.
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Lenses Scratch Centrally With Handling RGP Lenses ■ Change to harder material (e.g. Boston ES). ■ Change storage case design. Soft and RGP Lenses ■ Replace lenses more frequently.
Protein Deposits ■ Regular protein remover tablets ■ Aspheric lenses: lenses have a more uniform thickness profile (Bleshoy & Guillon 1984), and the junction is not so well circumscribed, so deposits may build up less ■ More regular replacement. Handling Difficulties or Poor Stability of Image ■ Consider scleral lenses (see Chapter 14).
High Hypermetropia Unless the hypermetropia is extremely high (e.g. in nanophthalmos – see below and Chapter 24), the problems found when fitting high hypermetropes are similar, but to a lesser degree, to those found in aphakia. Various disposable lenses are available in these prescriptions and can be used to overcome some of the problems associated with aphakia. There are some differences, however.
GENERAL POINTS Eyes may be smaller and steeper than normal. Hypermetropia is often associated with strabismus and amblyopia. Extra care is needed when one eye is amblyopic. Rigorous hygiene is required to help prevent contact lens– induced infection as effects on the better eye will affect overall vision. ■ If the high refractive error is not corrected from a young age, varying degrees of bilateral amblyopia can occur and, even when fully corrected, normal acuity may not develop. ■ High hypermetropia is associated with closed-angle glaucoma. There is a theoretical possibility that tight soft lenses might increase the IOP by compression of the limbal drainage vessels. ■ ■
ADVANTAGES OF CONTACT LENSES OVER SPECTACLES Better field of view (see Fig. 7.9). Less accommodation and convergence required for close work (see Chapter 7).
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DISADVANTAGES OF CONTACT LENSES OVER SPECTACLES The visual acuity is not as good for both distance and near; this is especially a problem when the spectacle acuity is less than 0.0 logMAR (6/6 Snellen).
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Nanophthalmos is a genetic condition in which the eyes are small with a short axial length, a thick but weak sclera and abnormal collagen (Sowden & Taylor 2005), and there is extreme hypermetropia, often more than +20.00 D, resulting in amblyopia. With a steep corneal radius and microcornea, lenses can be fitted similar to those required by infant aphakes (see Chapter 24) but there is usually adequate accommodation so bifocal spectacles are not usually necessary.
High Myopia (see Chapter 28) There is a distinction between high myopia (>–5.00 D) and pathological myopia; the latter usually occurs in axially long eyes and is more likely to result in retinal disease, in particular choroidal neovascularisation and retinal detachment. In a systematic review, the prevalence of pathological myopia ranged from 0.9–3.1% in different populations (Wong et al. 2014).
GENERAL POINTS Prevalence figures vary for both bilateral and unilateral high myopia but appear to be increasing. A review by Holden et al. (2016) estimated that 2.7% of the world population had high myopia (>–5.00 D) and that this is likely to rise to 9.8% of the world population by 2050. ■ It may be dominantly transmitted (Coscas & Soubrane 1993). ■ There is a higher incidence in premature infants (Larsson et al. 2003). ■ It can be associated with other syndromes such as Stickler or Marfan syndrome. ■ Most high myopia is axial (Grosvenor & Scott 1991). ■ It is associated with serious ocular pathology often leading to loss of vision: ■ enlarged globe ■ exophthalmia ■ fundus changes: ■ peripapillary atrophy (myopic crescent) ■ straightening of the retinal vessels ■ tessellated background ■ chorioretinal degeneration ■ retinal detachment. ■
Other problems with enlarged exophthalmic eyes include: poor blink dry eyes ■ flat, large corneas ■ abnormal extraocular muscles and strabismus. ■ ■
ADVANTAGES OF CONTACT LENSES OVER SPECTACLES Better acuity Better field. Although the field is theoretically greater with spectacles than with contact lenses, the peripheral field is distorted so contact lenses provide a better usable field.
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SECTION 6 • Specialist Lens Fitting
Also, the spectacle frame limits the field (see Chapter 7 and Fig. 7.8).
DISADVANTAGES OF CONTACT LENSES OVER SPECTACLES More accommodation and convergence are exerted for close work (see Chapter 7). ■ Cosmetic effect may be poor if the eyes are exophthalmic (Fig. 21.6). Cosmesis may be improved by fitting the greater part of the prescription with contact lenses and the remainder in spectacles. For example, a –20.00 DS myope can be fitted with –15.00 DS contact lenses and –5.00 DS spectacles over the top. ■ High myopia combined with astigmatism requires lenses that are especially thick in one meridian. This frequently leads to neovascularistion, particularly with soft lenses. Correcting the astigmatism in spectacles may be preferable. ■ Where hypoxia is a problem and disposable silicone hydrogel lenses are not available in high enough prescriptions, a compromise lens can be fitted in a disposable lens and the remainder prescribed in spectacles. ■ High myopes with poor acuity may prefer to read unaided rather than using a low visual aid. This is not always convenient. ■
For presbyopic patients, supplementary bifocal spectacles still need to be prescribed. As with aphakia, bifocal contact lenses do not always work in high prescriptions. Early presbyopes may benefit from having a reduced prescription in contact lenses and the remainder in spectacles as they are then able to remove the glasses whenever they wish to read or do any close work.
CONTACT LENS FITTING The degree of myopia that is considered to be high can be anything over –5.00 D. For the purpose of this chapter, most contact lens considerations are for prescriptions over –10.00 D. All types of lenses can be fitted – RGP, soft, or scleral. Lenses are thin centrally and thick peripherally. ■ Oxygen is reduced at the limbus, and hypoxia can lead to neovascularistion. ■ Thin lenses break easily when handling, and the power can alter or become distorted with lens cleaning. ■
High-minus contact lenses are occasionally used as the eyepiece together with a high-plus lens in spectacles as a Galilean telescope (see Chapter 27) although they are rarely successful.
RGP Lenses Thick edges cause displacement: Upwards – the lid is hitched up with each blink (Fig. 21.7). This can cause desiccation of cornea below the lens. If the lens is hitched up too high, it can result in flare as the peripheral zone or even the lower edge of the lens fails to cover the lower part of the pupil adequately. ■ Downwards – an interpalpebral fitting lens can be pushed downwards with each blink as the upper lid pushes on the thick junction between the lenticular and the carrier zone. ■
Requirements ■ A thinned junction (see Chapter 9) – to prevent too much movement on blink, too much lid hitch or the lens being pushed down with tight lids. The contact lens laboratory can input this into the computer-controlled lathe to achieve this. It can also be achieved by careful polishing (Moore & Mandell 1989) (see Chapter 33). ■ Aspheric lenses – as the junction is not as pronounced. ■ A negative or parallel carrier (see Fig. 9.16 and further information available at: https://expertconsult.inkling .com/) – to help with lid hitch. ■ A material with good dimensional stability-reduces lens changes due to wear and tear on the thin optic zone and the thick peripheral zones. Fitting Lenses to Large Myopic Eyes ■ Lenses tend to drop if the cornea is particularly flat (although in moderate myopia of less than 10 D, the corneas may actually be steeper; Carney et al. 1997). ■ Large corneas require large TDs. ■ Small lenses can sometimes be successful as the midperiphery can be made thinner but if the lens fits interpalpebrally, it is likely to sit low. Many high myopes have large pupils, and small RGP lenses may result in flare; this is a problem especially with night driving.
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Fig. 21.6 Exophthalmos in high myopia. This patient would benefit from wearing spectacles with a low-minus prescription over the contact lenses to reduce the exophthalmic appearance.
Fig. 21.7 High-riding lens. The high-minus lens has thick edges, which cause it to be hitched up by the upper lid resulting in desiccation of the inferior cornea and poor pupil cover, giving rise to flare. (Courtesy of Tony Phillips.)
21 • High Prescriptions
Soft Lenses ■ Soft disposable lenses, if available and not contraindicated, are ideal but they are only available in limited parameters. Proclear and Biofinity lenses (both Coopervision) are available in powers up to –20.00 D and, as mentioned earlier, the prescription can be refined by overcorrecting with spectacles. ■ Custom-made lenses have similar problems to RGP lenses, a thick midperiphery and deposition. ■ The thick transition between the lenticular portion of the lens and the carrier results in excess movement as the lid pushes the lens downwards or hitches it up, and reduced oxygen to the peripheral cornea can result in neovascularisation even with silicone hydrogel materials. ■ Small TDs exacerbate this problem and large TDs of 15.00 mm or more may be necessary. ■ Large aspheric lenses may centre better. ■ A tight lens may still move excessively with each blink because of the thick edge.
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High Astigmatism GENERAL POINTS Astigmatism can be any or all of the following: corneal lenticular ■ retinal. ■ ■
It can be either regular or irregular, although most cases of the latter are related to keratoconus or trauma and are covered in Chapters 20 and 25. Where there is a high degree of regular astigmatism, it is common to find that the major component is corneal and that the axes of the astigmatism, as measured with a keratometer, correspond to the axes of ocular astigmatism. Where the astigmatism is congenital, if it is not corrected from a young age, meridional amblyopia is likely.
ADVANTAGES OF CONTACT LENSES OVER SPECTACLES
TROUBLESHOOTING
The field is less distorted.
Poor Centration – Lid Hitch (RGP Lenses) ■ Fit aspheric lens. ■ Design lens with a parallel carrier (see Fig. 9.16). ■ Fit larger soft lens.
DISADVANTAGES OF CONTACT LENSES OVER SPECTACLES
Poor Centration – Lenses Pushed Down by Action of Tight Lids on Thick Junction Soft and RGP Lenses ■ Increase TD. ■ Reduce FOZD. ■ Fit aspheric lenses.
Lens Fitting For comprehensive information about fitting toric lenses, see Chapter 11.
RGP Lenses ■ Fit a multicurve lens with increased edge lift. ■ Change material: ■ higher refractive index ■ lower specific gravity.
Lenses Move Excessively on Blinking ■ Make the junction thinner. ■ Refit with a smaller lenticular diameter. ■ Refit with an aspheric lens. ■ Fit larger TD. Poor Oxygen Permeability ■ Change to a higher Dk material. ■ Consider RGP if currently in silicone hydrogel. Protein Deposits Soft and RGP Lenses ■ regular protein remover tablets ■ aspheric lenses – the junction is not so well circumscribed in aspheric lenses, so deposits may build up less ■ more regular replacement. Handling Difficulties or Poor Stability of Image ■ Consider scleral lenses (see Chapter 14).
A small dislocation of the lens axes can cause a large visual effect.
Lens Requirements ■ stable lenses ■ good oxygen permeability ■ high refractive index (RGP). Contact lenses can provide good acuity, but it may be difficult to achieve a fit that is stable and at the same time causes no corneal compromise. Fitting with a rigid lens usually requires a bitoric design in a material that has good oxygen permeability and wets well. If lenses are not disposable, encourage weekly enzymatic cleaners from the outset, as protein deposits are likely to be attracted to certain areas of the lenses because there is no lens rotation with blink action. Lens stability is critical when fitting toric soft lenses, but this may lead to long-term corneal compromise. The thick areas of contact lens can cause meridional hypoxia and neovascularisation, even with silicone hydrogel material, and these lenses are prone to deposit formation in the thicker parts of the lens.
Unilateral Ametropia (see Chapter 7) MYOPIA In cases of unilateral ametropia or aphakia, there is a better chance of stereopsis with a contact lens than with a spectacle
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SECTION 6 • Specialist Lens Fitting
lens. Aniseikonia is reduced for both axial and refractive myopia (Winn et al. 1988). It is suggested that each retina has the same number of photoreceptors, and in the longer eye these are more widely spaced than in the shorter eye, as might be expected during the growth process. Thus the bigger retinal image in the longer eye is thought to cover the same number of receptors as the smaller retinal image in the shorter eye, giving rise to better binocular fusion. However, patients who have achieved some stereo acuity with spectacles may not appreciate any improvement when a contact lens is fitted, and their stereoacuity may be disrupted. When the unilaterally myopic eye is larger than the fellow eye, it may look better cosmetically to continue to wear all or part of the myopic correction in the form of a spectacle lens as the size of the eye will appear to be reduced.
refractive errors are at risk of concomitant pathology, and signs and symptoms of ocular pathology must not be mistaken for an adverse response to contact lenses. For example, corneal oedema can result from contact lens–induced hypoxia but may equally be caused by a closed-angle glaucoma attack; likewise, poor vision may be a contact lens problem but could also be from a retinal detachment. An adverse response is more likely with high-power lenses, and careful observation is necessary. Lens parameters and/ or lens type should be altered accordingly. However, where optimum fitting has been achieved and some small degree of ocular change is still evident, the practitioner must feel competent enough to decide whether it is acceptable for the patient to continue, thus, to reduce lens wear or even to stop it altogether.
APHAKIA
Conclusion
As discussed earlier, it is unusual nowadays to have a unilaterally aphakic eye resulting from planned cataract surgery but unilateral aphakia may follow trauma or congenital cataract surgery. Both frequently result in strabismus and in the case of traumatic aphakia, other components of the eye and orbit may have been damaged including extraocular muscles. Binocular single vision may be impossible, and intractable diplopia may result (see Chapter 25). However, with time and contact lens correction, diplopia may resolve as the eye straightens. Most unilateral aphakes are not able to achieve stereopsis especially if the cataract had been long-standing. Because the contact lens is in front of the nodal point of the eye, binocular vision is, at best, poor. Guillon and Warland (1986) found that most unilateral aphakes with straight eyes achieved only 140 seconds of arc, and 80% had intermittent suppression. If diplopia is not a problem, a contact lens may increase the field of vision and help to keep the eye straighter where it might otherwise converge or, more commonly, diverge.
SPORT The better field afforded by contact lenses to high ametropes is particularly beneficial in sport. However, in certain sports (e.g. squash and badminton), safety spectacles should always be worn over the lenses. Spectacle wearers who are fitted with contact lenses lose the protection of the spectacle frame and also the lenses if they are made of a toughened material. Swimming is particularly inadvisable in contact lenses. As well as a general risk of infection and irritation, there is an increased risk of Acanthamoeba keratitis (see Chapter 17), so either goggles over the lenses or preferably swimming goggles with the spectacle prescription should be worn. Rigid lens wearers can reduce lens loss by wearing daily disposable lenses over their lenses for sport.
Aftercare Aftercare is covered in Chapter 16, and the detail is the same whatever the power of the lenses. However, eyes with high
Fitting high degrees of ametropia can be especially rewarding. Many patients have been totally dependent on spectacles for as long as they can remember, and have no idea of what life will be like without them. Because of some of the difficulties encountered with high-prescription contact lenses, instruction on lens care and potential eye problems is even more important than with low prescriptions. The lens of choice may not be available in the required power, and some compromise may be necessary in order to enable the patient to wear lenses.
References Bleshoy, H., Guillon, M., 1984. Soft lens design – clinical results. J. Br. Contact Lens Assoc. 7, 41–47. Carney, L., Mainstone, J., Henderson, B., 1997. Corneal topography and myopia. A cross-sectional study. Invest. Ophthalmol. Vis. Sci. 38, 311–320. Carpel, E., Parker, P., 1985. Extended wear aphakic contact lens fitting in high risk patients. Contact Lens Assoc. Ophthalmol. J. 11, 231–233. Chaston, J., Fatt, I., 1980. The change in power of soft lenses. Optician 180, 12–21. Coscas, G., Soubrane, G., 1993. Myopie fort ou myopie maladie. Rev. Prat. 43, 1768–1772. Grosvenor, T., Scott, R., 1991. Comparison of refractive components in youth-onset and early adult-onset myopia. Optom. Vis. Sci. 68, 204–209. Guillon, M., Warland, J., 1986. Aniseikonia and binocular function in unilateral aphakes wearing contact lenses. J. Br. Contact Lens Assoc. 3, 36–38. Holden, B.A., Fricke, T.R., Wilson, D.A., et al., 2016. Global prevalence of myopia and high myopia and temporal trends from 2000 through 2050. Ophthalmology 123, 1036–1042. Larsson, E.K., Rydberg, A.C., Holmstom, G.E., 2003. A population-based study of the refractive outcome in 10-year-old preterm and full-term children. Arch. Ophthalmol. 121, 1430–1436. Moore, C., Mandell, R.B., 1989. The design of high minus lenses. Contact Lens Spectr. 11, 43–47. Sowden, J., Taylor, D., 2005. Disorders of the eye as a whole. In: Pediatric Ophthalomology and Strabismus, third ed. Edited by Taylor & Hoyt Pub. Elsevier. Winn, B., Ackerley, R.G., Brown, C.A., et al., 1988. Reduced aniseikonia in axial anisometropia with contact lens correction. Ophthalmic Physiol. Opt. 8, 341–344. Wong, T., Ferreira, A., Hughes, R., et al., 2014. Epidemiology and disease burden of pathologic myopia and myopic choroidal neovascularization: an evidence-based systematic review. Am. J. Ophthalmol. 157, 9–25.