High Ametropia

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 ( ).