In patients who have regular corneas, those who are most appreciative of contact lens wear often have high refractive error. While many of these patients can now be satisfactorily fit with standard or specialty spherical or toric soft lenses, the optics provided by GP lenses are still often superior and worth offering as a potential option.
Patients are familiar with the advantages of superior technology and crisp visuals; for example, many do not hesitate to purchase high-definition televisions or smartphones that have high-resolution screens. Interestingly, eyecare practitioners seem to readily recommend the most advanced spectacle lens designs and materials for all patients, yet they will prescribe soft lenses that they know are optically inferior without offering any alternatives. This happens particularly to patients who have astigmatism, with soft contact lens prescriptions being written as spherical equivalents that either undercorrect or ignore sometimes significant amounts of cylinder.
Many corneal GP lens materials offer improved oxygen permeability in higher refractive errors compared to soft lens materials, and the smaller overall diameter of corneal GPs allows for more tear exchange per blink than in soft lenses (Polse, 1979). Parameter availability in GP lens manufacturing is extensive and allows for most high ametropes to fully correct all aspects of their refractive error (including presbyopia) with less compromise.
Difficulties may arise with high-minus or high-plus GP lens fitting, usually due to extremes in lens weight or thickness. Proper base curve and diameter selection is critical in obtaining good lens centration with these types of refractive errors.
A lenticular design can be added to improve edge thickness and comfort: high-minus lenses may benefit from a plus lenticular or CN bevel, while high-plus lenses benefit from a minus carrier lenticular design. Many GP laboratories have proprietary designs for high refractive errors that minimize areas of excessive thickness and aid in fitting, often without the practitioner needing to request special manufacturing details.
Lid tension is important to note when fitting higher refractive errors. A loose upper lid may push down a thicker minus lens edge, while a tight lid might “grab” a minus edge and cause undesirable superior decentration (Figure 1).
High-plus lenses tend to be heavy and center inferiorly. Center thickness may be an important consideration as well, particularly if flexure is occurring on corneas that have increased toricity. If a lens is decentering and changes in base curve or overall diameter do not help (or cause other issues to arise), adjusting the optic zone diameter is frequently beneficial.
If a good fit in corneal GP lenses isn’t achievable, hybrid or scleral lenses may be considered to improve comfort and speed up adaptation, though these modalities may come with an increase in cost and more challenges with application and/or removal. However, highly active patients may also appreciate the increased security of these modalities, as corneal GP lenses risk occasional lens displacement and loss.
GP lenses are an excellent option for many patients, particularly those who have higher refractive errors and/or critical vision demands. Keeping in mind the physical properties of these lenses will help the practitioner troubleshoot any fitting issues that might arise. CLS
- Polse KA. Tear flow under hydrogel contact lenses. Invest Ophthalmol Vis Sci. 1979 Apr;18(4):409-13.