contact lens primer
That's All, Folks!
BY TIMOTHY B. EDRINGTON, OD, MS, FAAO, & JOSEPH T. BARR, OD, MS, FAAO
This is the final installment of the Primer series. Despite the recent advances in corneal topography technology, fluorescein pattern interpretation is still invaluable in fitting challenging corneal contours. Judge the fit of the contact lens by observing where the lens is bearing or touching the cornea. An excessive amount of residual fluorescein or fluorescein on the front surface of the lens may otherwise mislead you.
Other GP Options
All aspherics are not created equal. Aspheric contact lenses may be prescribed to enhance the contact lens-to-cornea fitting relationship (low eccentricity, back-surface aspheric designs); improve vision by reducing spherical aberrations (front surface aspheric designs); and provide a multifocal correction for presbyopia (generally high eccentricity back-surface aspheric designs). Because each laboratory's aspheric lens designs tend to be unique, work closely with the lab's consultants to fit and to problem-solve.
Reverse geometry lenses are often indicated in fitting post-RK and post-penetrating keratoplasty (PK) patients who have flat central corneal contours.
Figure 1. Dimple veiling (bubbles)
behind a GP lens optic zone.
Large overall diameter GP contact lenses may improve lens centration when fitting patients with pellucid marginal degeneration and some post-PK patients. The challenge with large diameter lenses is to avoid areas of harsh touch or excessive clearance. In other words, don't fit 'em too flat or too steep. Excessive areas of clearance may lead to bubble formation causing decreased vision or dimple-veil staining (See Figure 1).
Disposable multifocal soft contact lenses allow the practitioner and patient a quick and inexpensive means of exploring the possibility of multifocal contact lenses. Prescribing pearls, such as using loose spectacle trial lenses to over-refract, and helpful advice is available from the manufacturers. Even if the outcome is single vision monovision, the patient no longer has to go through life wondering "what if" about bifocal contact lenses. Also, you can put them on a mailing list to try new multifocal designs as they become available.
Lens centration is critical when prescribing most soft multifocal lenses. Fight off the tendency to over-minus the lens power to address distance vision complaints and to over-plus to address near vision complaints. Consider modified monovision to resolve the "if it could just be a little better at..." complaints.
Corneal distortion may be present with long-term wear of GP contact lenses. This may be confirmed by topography changes, keratometry mire distortion or symptoms of spectacle blur. Even though it is tempting to deprive patients of contact lens wear until the cornea is rehabilitated, it is not practical or popular with the patient. Temporarily refitting these patients into disposable soft sphere or toric contact lenses is an option. However, the patient might need to camp out in your reception area to keep up with the changes to his refraction. Also, if the corneal distortion is severe, the vision obtained with soft contact lenses might not be acceptable. Refitting the patient into a higher Dk GP lens with an alignment fitting relationship will provide the patient with optimal vision while the cornea rehabilitates.
Dr. Edrington is a professor and in the contact lens service at the Southern California College of Optometry. E-mail him at email@example.com.
Dr. Barr is editor of Contact Lens Spectrum and assistant dean for clinical affairs at The Ohio State University.