PRESCRIBING FOR ASTIGMATISM
“IRREGULAR” APPLICATIONS OF TORIC GP LENSES
BRITNEY KITAMATA-WONG, OD; GRACE LIAO, OD; & TIMOTHY B. EDRINGTON, OD, MS, FAAO
The use of bitoric corneal lenses for managing keratoconus is widely debated. While some practitioners routinely prescribe bitoric corneal GP lenses for keratoconus, others argue that it is seldom indicated and not very successful when attempted (Shovlin, 2001; Blackmore et al, 2006). Keratoconus typically presents with large amounts of irregular corneal toricity. Though significant corneal toricity may seem like an appropriate indication for a toric-back-surface lens design, corneal irregularity can lead to poor lens stability, especially rotational stability.
In many cases of mild-to-moderate keratoconus, the fluorescein pattern observed with a spherical corneal GP lens often resembles a with-the-rule cornea. Typically, we observe minimal peripheral clearance at the 3 o’clock and 9 o’clock positions and excessive peripheral clearance superiorly and inferiorly (Figure 1). These patients may benefit from a toric-peripheral-curve lens design to create a more uniform peripheral clearance (Figure 2). This uniformity tends to improve lens centration, enhance wearing comfort, and decrease lens ejection.
Figure 1. A spherical corneal GP lens on a keratoconic cornea. Note the minimal peripheral clearance at 3 o’clock and 9 o’clock and the excessive peripheral clearance at 6 o’clock.
Figure 2. A toric-back-surface corneal GP lens on a keratoconic cornea. Note the improved uniformity of peripheral clearance.
If the residual astigmatism is not adequately corrected with a spherical GP lens, consider a bitoric or a front-surface-toric corneal GP. Because the cornea has both regular and irregular astigmatism, lens rotation might not be adequately stabilized. We tend to prescribe the amount of base curve toricity by interpreting the amount of toricity observed in the fluorescein pattern and not by the amount of corneal toricity measured by manual or sim K readings.
We Know, There Are Also Scleral Options
For cases in which corneal GP lenses are not indicated or successful, scleral GP lenses should be recommended. The majority of scleral GP labs offer toric-peripheral-curve designs to optimize the fit when a spherical design isn’t optimal. A scleral lens with a toric periphery can aid in lens centration, improve patient comfort, decrease blanching (Figure 3), and minimize unwanted bubbles beneath the base curve.
Figure 3. A scleral GP lens exhibiting conjunctival blanching.
Additionally, many scleral lens manufacturers now provide the option of incorporating a front-surface toric to correct excessive residual refractive astigmatism. Currently, these front-surface-toric scleral lens designs use prism-ballast or toric peripheral curves to achieve the correct axis orientation and to stabilize lens rotation.
The complexity of a toric GP fit may initially seem daunting; however, many practitioners have great success with these lens designs. CLS
For references, please visit www.clspectrum.com/references and click on document #251.
Dr. Kitamata-Wong is a Cornea and Contact Lens Resident at the Southern California College of Optometry (SCCO) at Marshall B. Ketchum University (MBKU). She is a member of the American Optometric Association (AOA), the California Optometric Association (COA), and the San Mateo County Optometric Society.
Dr. Liao is a Cornea and Contact Lens Resident at the SCCO at MBKU. She is a member of the AOA, the COA, and the American Academy of Optometry.
Dr. Edrington is a professor at the Southern California College of Optometry at Marshall B. Ketchum University. He has received honoraria for the STAPLE program. You can reach him at email@example.com.