Prescribing for Astigmatism
For Ghosting With Scleral Lenses, Call in the Experts
By Brooke Messer, OD, & Timothy B. Edrington, OD, MS, FAAO
It has been rewarding for both our patients and ourselves to prescribe scleral GP contact lenses for patients who have been unsuccessful with other lens designs due to lens intolerance or the severity of their corneal condition. Patients typically rave about the comfort and vision provided by their new scleral lenses. But on occasion, a patient will complain of “annoying ghost images.” When there's something strange in the neighborhood—residual astigmatism—it's time to call in the “ghostbusters.”
Discussing challenging contact lens cases with colleagues and laboratory consultants is an excellent way to ensure success when learning to prescribe a new lens modality. We have learned a few tips on managing residual astigmatism from consulting with some of the top ghostbusters in the scleral lens field.
Some Expert Advice
Residual astigmatism is not uncommon in contact lens prescribing. It's defined as the astigmatic refractive error present when a rigid lens is placed on the cornea to correct the existing ametropia. Causes include lenticular cylinder, tilt of the crystalline lens, and GP lens flexure.
The first step in managing residual astigmatism is to determine the type and cause of the residual cylinder. Consider performing keratometry or topography over the anterior surface of the contact lens to determine whether the over-refractive cylinder is due to lens flexure or true physiologic cylinder. If a significant amount of flexure is present, Greg DeNaeyer, OD, FAAO, president of the Scleral Lens Society and a practitioner at Arena Eye Surgeons, and Eric Marshall, vice president of operations, Visionary Optics, agree that increasing the scleral lens' center thickness by 0.15mm to 0.20mm should decrease or eliminate the unwanted flexure and reduce the amount of residual astigmatism.
Marshall and Keith Parker, president of Advanced Vision Technologies, both noted that about 10 percent to 15 percent of scleral lens wearers benefit from the correction of their residual astigmatism after correcting for lens flexure. Scleral lenses can be prescribed with a front-surface toric design to improve the patient's vision. Like any toric lens design, rotational stability is critical to visual success. Marshall explained that front-surface toric scleral lenses may be designed with a double slab-off peripheral curve system to optimize rotational stability (Figure 1).
Figure 1. Markers at 3 o'clock and 9 o'clock assist practitioners in assessing lens rotation and rotational stability.
Front-surface toric lenses are a great option for correcting residual astigmatism in patients who have true physiologic astigmatism. Parker offered a simple, but underutilized, option for correcting residual astigmatism: prescribing a pair of glasses for wear over the contact lenses. This is a great option for presbyopic patients needing a progressive lens, or for those patients desiring optimal vision during specific tasks, such as driving.
Improve Vision, Quality of Life
Contact lens patients who have residual astigmatism will appreciate your efforts to improve their quality of vision and hence their quality of life. So the next time your patient says that they're seeing “something weird, and it don't look good,” you know who to call… CLS
Dr. Messer is a 2010 graduate of the Southern California College of Optometry and is currently the Cornea and Contact Lens Resident at SCCO. Dr. Edrington is a professor at SCCO. He has also served as an advisor to B+L. E-mail him at email@example.com.