Introduction
This chapter examines contact lens fitting for higher magnitudes of myopia, astigmatism and hyperopia. Prescribing contact lenses for high astigmatism ( Chapter 16, Chapter 17, Chapter 25 ) and hyperopia ( Chapter 27 ) is also covered in detail elsewhere.
High Myopia
In an epidemiological context, high myopia is defined as a spherical equivalent refraction of at least 5.00 D ( ) or 6.00 D ( ). High myopia can occur in isolation, in association with other ocular conditions such as retinopathy of prematurity ( ) and x-linked retinitis pigmentosa ( ) or as part of a syndrome (e.g. Ehlers–Danlos syndrome type VII, Stickler syndrome type V, Marfan syndrome, oculocutaneous albinism type II) ( ). High myopia (in particular a longer axial length) is associated with an increased risk of ocular disease (e.g. cataract, glaucoma, retinal detachment) ( ) and reduced visual acuity due to central pathological changes such as maculopathy and choroidal neovascularisation ( ). The use of contact lenses to correct high myopia may increase in the coming years since it has been estimated that between 2.2% and 8.6% of the population will have myopia >5.00 D by 2050 ( ), and myopia >10.00 D has increased in China in recent years ( ).
In addition to improved cosmesis, the visual benefits of contact lenses compared to spectacles in high myopia include a larger field of fixation, improved contrast sensitivity ( ) and an increased retinal image size. This increase in retinal image size can result in a 1–2 line improvement in visual acuity in myopes of approximately ≥15.00 D without macular pathology ( ), dependent upon the spectacle lens vertex distance ( Fig. 26.1 ). During near tasks however, contact lens corrected high myopes have a greater convergence and accommodation demand compared to their spectacle correction (due to the loss of the base in prismatic effect that arises during convergence with spectacles) ( Chapter 3 ).
Overarching Fitting Principles
When designing high-powered soft and corneal rigid minus lenses, it is desirable to is to minimise the mid-peripheral thickness to optimise oxygen delivery to the cornea and aid lens centration. For scleral lenses, the fluid reservoir thickness is the primary factor that limits oxygen delivery ( ). It is also necessary to correct for the vertex distance along both the flat and steep meridians to determine the required lens power (for all high ametropias) ( Appendix C ).
Corneal Rigid Lenses
High minus powered lenses with a thick mid-periphery can ride high due to lid attachment. If this occurs unilaterally, a vertical prismatic imbalance can be induced, along with discomfort and inferior corneal desiccation staining. Conversely, for an interpalpebral fit, the upper lid can force the lens downward due to its interaction with the thicker mid-periphery. The mid-peripheral lens thickness can be minimised through polishing or customised anterior surface lathing. A lenticular or aspheric design may be required to minimise the junctional thickness for comfort and stability ( ). This is demonstrated in ( Fig. 26.2 ) where a higher-powered minus lens has a substantially thicker mid-periphery compared to a lower-powered minus lens with the same front optic zone diameter. Fig. 26.3 highlights the reduction in mid-peripheral thickness for a high-powered minus lens by lathing a steeper anterior lenticular radius.
If centration cannot be improved by altering the lens design, a piggyback system may be of benefit, with a rigid lens fitted over a low-powered highly oxygen-permeable soft lens. In addition to hybrid lenses, a customised soft lens with a central depression in the front surface (a cut-out region ranging from 6.5 to 12.5 mm in diameter) that can accommodate any rigid lens design is also available (Flexlens Piggyback, X-Cel Contacts). These fitting approaches are often used in the correction of keratoconus to take advantage of the rigid lens optics, improve comfort and minimise corneal insult, but can also be utilised to correct high myopia.
Soft Lenses
The range of soft contact lens powers available for the correction of high myopia vary between manufacturers and lens designs. In general, many disposable lenses (daily, fortnightly and monthly replacement) are available in 0.50-D steps from −6.00 to −12.00 D and customised lenses (planned replacement) are often available up to −20.00 to −40.00 D, many of which utilise aspheric designs.
Silicone-hydrogel lens materials are preferable to minimise potential hypoxic complications associated with thick high minus powered soft lenses. For thicker lenses, there is the potential for the enhanced retention of preservatives from disinfecting solutions, which could lead to a toxicity or hypersensitivity response as absorbed chemicals gradually leach into the tears during wear. Similarly, hydrogen peroxide may be absorbed deep within a thick lens and require a longer period of time to achieve neutralisation ( ). The fit of a thick, soft lens can tighten over time due to lens dehydration that causes an increase in sagittal depth and a steepening of the back optic zone radius. An increase in the refractive index of the lens material with dehydration can also cause an increase in lens power (more minus) ( ).
Scleral Lenses
Highly oxygen permeable scleral lenses are a useful contact lens correction for high myopia since lens movement due to blinking or eye movements is minimal, and the central back surface lens profile can be altered to generate a minus powered postlens fluid reservoir without compromising the lens fit, which is largely dictated by the alignment of the scleral landing zone. During the 1980s myopia was the primary indication for scleral lens fitting in ~19% of patients attending Moorfields Eye Hospital, but has gradually diminished over time (9% in 1996 and 2% in 2003) ( ) as other contact lens options have become available for the correction of a wider range of refractive errors.
Partial Correction
For a number of reasons (e.g. lens parameter availability, comfort), it may be practical to correct only some of the myopia with contact lenses and the remainder of the residual refractive error with spectacles (including astigmatism or presbyopia). This approach can still significantly improve cosmesis and visual outcomes and allows patients the choice of a wider range of spectacle frames due to the reduced spectacle prescription.
Although the maximum target myopia correction for overnight orthokeratology is approximately 4.00 or 5.00 D ( ), some products have been approved for the correction of myopia up to 6.00 D (e.g. Paragon CRT, CooperVision [Europe]). A challenge with fitting high levels of myopia with orthokeratology is that as the target correction increases, the treatment zone diameter decreases ( ), which results in a relatively small region of appropriately flattened cornea, and can induce a multifocal optical effect. A small number of studies have examined the myopia control efficacy of partial orthokeratology corrections for high myopia [i.e. correcting only 4.00 D ( ) or 6.00 D ( ) of myopia using orthokeratology and any residual refractive error with single vision spectacles]. These studies suggest that the myopia control efficacy is retained when only using a partial orthokeratology correction for higher degrees of myopia (~−6.00- to −8.00-D spherical equivalent) ( ). Currently, the partial correction of high myopia using orthokeratology is considered an off-label treatment for many lens designs.
Other High Minus Lens Applications
A high minus powered contact lens can be used as the eyepiece of a Galilean contact lens telescope in conjunction with a plus powered spectacle lens objective lens to create a hands-free monocular low vision aid ( ). Both soft ( ) and corneal rigid lenses ( ) have been used for this application but are prone to decentration or excessive lens movement, which creates an apparent movement of the visual field. Consequently, scleral lenses provide the optimal eyepiece for a contact lens telescope due to their stability during blinking.
There is some evidence from retrospective cases series to support the use of corneal rigid lenses in unilateral high myopia (highly myopic anisometropic amblyopia without strabismus) ( ), potentially in conjunction with occlusion therapy of the fellow nonamblyopic eye. Contact lens correction of the highly myopic eye results in less aniseikonia and suppression compared to a spectacle correction, which may lead to improved visual outcomes and binocular function ( ).
High Hyperopia
The advantages of contact lenses compared to spectacles for the correction of high hyperopia include a greater field of view, reduced peripheral distortion or aberrations and enhanced cosmesis. In contrast to contact lens corrections for high myopia outlined above, for hyperopes, the accommodation and convergence demand is reduced for near work compared to spectacles, however, visual acuity may also be compromised due to a reduction in retinal image size ( Fig. 26.1 ).
Overarching Fitting Principles
The overarching aim when fitting contact lenses for high hyperopia, which requires thicker lenses with a more anteriorly positioned centre of gravity, is to modify the lens fit to minimise lens movement and decentration and optimise oxygen delivery to the cornea. Lens stability can be improved for both soft and corneal rigid lenses by increasing the total diameter and utilising a lenticular design ( Fig. 26.4 ) to move the lens centre of gravity posteriorly ( ). Lenticular designs can incorporate a regular (parallel) carrier for an interpalpebral fit or a minus carrier for a lid attachment fit ( Fig. 26.5 ).
Fitting Considerations
A range of contact lens corrections and fitting considerations for infantile aphakia are outlined below, since this is the ocular condition for which the highest plus power lenses are likely to be prescribed [mean spherical equivalent refraction of ~+31.00 D in aphakic infants ( ) compared to ~+12.00 D in aphakic adults ( )], and the potential fitting challenges discussed may also be encountered for lower degrees of hyperopia. For high plus powered contact lenses, front surface aspheric designs are required to minimise the amount of positive spherical aberration induced, which can degrade visual acuity. Based on theoretical modelling, the optimum front surface eccentricity for rigid and soft lenses for the correction of infant aphakia are approximately 0.7 and 0.5, respectively ( ).
Following the Infant Aphakia Treatment Study ( ), most paediatric cataract surgeons (84%) utilise contact lenses as the initial optical correction in infants with unilateral aphakia less than 6 months of age, with equal preference for contact lenses (47%) and intraocular lenses (42%) for older infants ( ). Contact lenses play an important role in the correction of infantile aphakia, since they can be modified frequently to accommodate the growing eye during neonatal emmetropisation; provide the full hyperopic correction, which may not be possible with spectacles in some cases; and provide the greatest potential for binocular function by minimising aniseikonia and interocular prismatic differences in unilateral aphakia (see Chapter 27 ).
General considerations for the correction of infantile aphakia include that in the absence of a crystalline lens, substantially more ultraviolet light can reach the retina, so contact lenses with a UV tint should be considered. Infantile aphakic glaucoma is also relatively common (15%–45% incidence) ( ) and often requires filtration surgery to control the intraocular pressure (e.g. trabeculectomy, drainage implant), which can alter the topography of the conjunctiva and peripheral cornea and potentially impact contact lens fitting ( ). Patients with conjunctival filtering blebs should be monitored closely and contact lenses should not impinge on these blebs excessively. Topical antiglaucoma medications are usually still required following filtration surgery and should be instilled before and after lens wear. It should be noted that patients (or caregivers of infants) may have a preference for a particular lens design for ease of handling or lens application (e.g. the increased lens modulus of silicone elastomer material, the rigidity of a corneal rigid lens or the larger diameter of a scleral lens).
Soft Lenses
Soft lenses can be manufactured in a wide range of parameters customised to the eyes of infants and older adults and provide good initial comfort. Hydrogel lenses for the correction of aphakia result in a range of significant ocular complications related to corneal hypoxia (e.g. intracorneal haemorrhage) ( ). Determination of the average oxygen transmissibility of soft lenses of various power requires knowledge of the lens thickness profile; however, contact lens manufacturers typically only cite centre thickness values. Appendix F tabulates the average thickness of soft lenses of given centre thickness and lens power, which can facilitate average oxygen transmissibility calculation.
Hydrogel lenses can dehydrate on the eye resulting in a reduction in the effective power of the lens ( ). Consequently, the correction of high hyperopia (greater than 15.00 D) using hydrogel lenses requires a substantially greater back vertex power to compensate for this dehydration effect, which further reduces oxygen delivery. Therefore customised silicone-hydrogel lenses are the preferred soft lens option for the correction of high hyperopia to minimise corneal hypoxia. Noncustomised planned replacement silicone-hydrogel lenses may also be prescribed dependent upon the magnitude of hyperopia (e.g. maximum powers for planned replacement high plus lens designs can vary from +15.00 to +30.00 D dependent upon the lens design and manufacturer).
Silicone Elastomer Lenses
A silicone elastomer lens (Silsoft, Bausch & Lomb) (elastofilcon A, Dk 350 barrers), which provides substantially greater oxygen delivery compared to silicone-hydrogel materials, is FDA approved for 30-day-extended wear for the correction of aphakia. While lens dehydration is not a significant issue with this material ( ), the lenses can become heavily deposited and require regular replacement ( ). Other limitations include the restricted range of parameters (steepest BOZR of 7.50 mm and BVP range of +23 to +35 D in 3.00-D steps) which may not be sufficient in some cases of infantile aphakia.
Corneal Rigid Lenses
The advantages of corneal rigid lenses in the correction of high hyperopia include improved oxygen delivery through tear exchange, the ability to correct astigmatism using the postlens tear layer and a more stable optical correction since rigid lenses do not dehydrate on eye. The smaller total diameter and rigidity compared to soft lenses may allow easier parental handling and insertion. Disadvantages include reduced comfort during the initial adaptation period and the potential for trapped foreign bodies in the postlens tear layer.
Scleral Lenses
In the 1960s, scleral lenses were often preferred over corneal rigid lenses for the correction of aphakia to minimise insult to the limbal surgical site and provide greater stability to avoid vertical prismatic imbalance in unilateral cases ( ). A potential disadvantage however, is that scleral lenses increase the retinal image size compared to corneal rigid lenses due to the thicker lens material and fluid reservoir, which can prevent fusion in monocular lens wearers ( ). Gould recommended fenestrated moulded scleral lenses for the correction of aphakia when corneal astigmatism exceeded 4 D, if the central cornea was relatively flat (flatter than 8.5 mm), and in cases of persistent corneal staining associated with corneal rigid lens wear ( ). Scleral lenses also play a role in the correction of aphakia in the presence of highly irregular corneal optics following ocular trauma when a stable lens fit with a corneal rigid lens may not be possible ( ).
Despite recent advances in scleral lens practice such as improved material oxygen permeability and surface coatings and anterior segment imaging ( ), aphakia currently accounts for a very small proportion of scleral lens fits (<1%–3%) ( ) compared to 32% in the 1980s ( ). This may be related to the increased cost or chair time associated with scleral lenses compared to corneal rigid lenses.
Contact Lens Modifications and Replacement
During the first 2 years of life, frequent lens modifications are required to ensure the fit remains appropriate for the growing eye and the desired refractive correction is achieved as hyperopia reduces with axial growth. The refractive error of an aphakic one month old can range from +18 to +40.00 D and decreases by ~4.00 D during the first year and slightly less each subsequent year, plateauing by around age three ( ). The central cornea also flattens by ~1.70 D (0.28 mm) over the first year ( ). Therefore the back vertex power must be periodically reduced, the BOZR flattened, and the overall diameter increased gradually over time. For corneal rigid lenses, regular modifications are required up to two years of age; approximately every 2–3 months for the back vertex power, every 3–4 months for the BOZR, and every 8 months for the overall diameter ( ).
Since aphakic infants cannot accommodate, they are initially over corrected by +2.50 to +3.00 D to allow clear vision at near ( ) which is gradually reduced to +1.00 to +1.50 D over correction by 18–24 months ( ). By 3–4 years of age, bifocal or multifocal spectacles are worn over the contact lens containing the optimal distance prescription. Multifocal contact lenses are typically not used due to excessive movement with blinking and poor near acuities (in adults) ( ). Other reasons for frequent lens replacement for infantile aphakia include loss due to lens dehydration and eye rubbing (soft lenses) or high levels of deposition (silicone hydrogels) ( ).
High Astigmatism
While a small amount of astigmatism (<0.75 D) is very common, astigmatism greater than 3.00 D is rare ( ). For example, in a large cohort of myopic refractive surgery candidates (over 11,000 eyes), less than 1% displayed astigmatism greater than 2.25 D ( ). Uncorrected astigmatism of ~3.00 D throughout infancy and early childhood can lead to meridional amblyopia ( ) and high levels of astigmatism [typically with-the-rule astigmatism ( )] have also been associated with both higher levels of myopia ( ) and hyperopia ( ). Since contact lenses have minimal effect on image magnification compared to spectacles, the correction of high levels of astigmatism with contact lenses results in substantially less spatial distortion than astigmatic spectacle corrections. In cases of high irregular astigmatism (nonorthogonal principal meridians), rigid contact lens corrections may be the only nonsurgical solution that can provide adequate visual acuity.
Overarching Fitting Principle
The aim is to minimise the total ocular astigmatic refractive error by neutralization of anterior corneal astigmatism using the postlens fluid layer with a spherical or toric back surface rigid lens (with or without a toric front surface to correct residual astigmatism) or correction of the ocular astigmatism using a soft toric contact lens (with a toric front or back surface). For toric lens designs the orientation of the lens is critical to achieve a physiologically acceptable fit and optimise vision, particularly as the magnitude of astigmatism increases. This is achieved by lens modifications to stabilize and align the lens at the desired orientation for the specific eyelid anatomy and blinking characteristics of the individual (see Chapter. 9, Chapter. 16 ).
Regular Astigmatism
The vast majority of disposable soft toric contact lens designs provide a maximum astigmatic correction of 1.75–2.25 D at a limited number of axis orientations (e.g. 10° increments or 90° and 180°±20°), while planned replacement HEMA and silicone-hydrogel lenses cater for a wider range of astigmatic corrections (e.g. up to 10.00–12.00 D of astigmatism at any axis even for high spherical ametropia). The thickness profile of soft lenses to correct high levels of astigmatism can impact oxygen delivery due to the modifications required to ensure lens stability (e.g. prism). Corneal rigid lenses can be used to enhance tear exchange and oxygen delivery but require a back surface toric design typically when the anterior corneal toricity exceeds 2.50 D to optimise the lens fit ( Chapter 16 ). Scleral lenses can also be used for the correction of regular astigmatism, and since lens orientation and stability is primarily affected by landing zone alignment, a toric optic zone is generally not required. The use of orthokeratology to correct high levels of regular astigmatism is limited. Most toric orthokeratology lenses can correct with-the-rule astigmatism less than 2.00 D, with one double reservoir lens design that claims to correct up to 4.00 D of astigmatism for any axis orientation ( ).
Irregular Astigmatism
Irregular astigmatism typically arises from corneal ectasia, surgery or injury. The correction of irregular astigmatism in keratoconus ( Chapter 25 ) and following penetrating keratoplasty ( Chapter 29 ) are also discussed in detail elsewhere.
Rigid Lenses
Rigid lens materials are the preferred optical correction for irregular astigmatism, including hybrid or corneal lenses since the postlens tear layer can neutralise the majority of anterior corneal astigmatism. However, as the magnitude of irregular corneal astigmatism increases, it can be difficult to obtain an acceptable lens fit with a hybrid or corneal rigid lens due excessive lens decentration or ejection from the eye, and in such cases scleral lenses may be required. There is some evidence to suggest that as the magnitude of irregular corneal astigmatism increases, corneoscleral and scleral lenses can provide improved visual outcomes compared to corneal rigid lenses in keratoconus ( ) and following penetrating keratoplasty ( ), most likely due to the enhanced stability and centration achieved.
Custom Soft Lenses
In keratoconic patients with varying degrees of irregular astigmatism, soft hydrogel contact lenses that wrap to the anterior corneal shape provide comparable visual acuity outcomes compared to spectacles (a 400-µm centre thickness soft toric) ( ) but poorer performance than corneal rigid lenses (35- to 200-µm centre thickness soft spheres) ( ). However, several toric silicone-hydrogel lenses are available with an increased centre thickness (up to ~600 µm) to partially mask anterior corneal irregularities, and a toric front surface and an aspheric back surface to correct residual irregular astigmatism (e.g. HydroCone Toris K, KeraSoft Thin, NovaKone). These lens designs cater for a wide range of sphere powers (±30.00 or 40.00 D) and a maximum of 8.00–15.00 D of cylinder, with the option for quadrant specific fitting modifications. Based on clinical retrospective analyses, on average, these customised soft toric lenses can improve high contrast visual acuity by 3–4 lines compared to optimal spectacle correction ( ) but provide comparable ( ) or slightly worse high contrast visual acuity compared to rigid lens corrections ( ). Consequently, these customised soft lens designs are a potential option for patients with irregular astigmatism who cannot tolerate corneal rigid lenses due to discomfort or do not have high level visual demands that require rigid lens optics.
Practical Considerations
Patients with bilateral high ametropia may have difficulty with lens insertion without a refractive correction in place. Magnifying mirrors can assist in this regard, and various devices that include a light source to guide lens insertion are available (e.g. See-Green lens inserter). A monocular spectacle correction may be required in cases of bilateral aphakia to facilitate lens insertion in the fellow eye. Handling tints can provide additional contrast for patients with high ametropia in the case of a lost lens during handling.
Conclusion
Contact lenses offer numerous advantages compared to spectacles for patients with high ametropia. Practitioners should be aware of these cosmetic and optical benefits, and the different lens modalities, designs (e.g. lenticular forms), lens modifications and materials available to ensure appropriate visual outcomes and a physiologically acceptable lens fit.
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References
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