RGP insights

Fitting Sagging Cones in Keratoconus Patients

BY LORETTA SZCZOTKA, OD, MS
June 1999

What are usually considered simple RGP fitting requirements, such as achieving total pupillary coverage, preventing lens adherence and decreasing lens awareness, become challenging in keratoconus patients with sagging, or in keratoconus patients with sagging, or oval cones.

These challenges stem from the fact that the steepest portion of the cornea is located considerably more inferior than the corneal apex of regular corneas or the peak of the cone in central keratoconus. Because RGPs tend to decenter themselves over the steepest portion of any cornea, they naturally drift inferiorly. This can create a suboptimal fitting relationship if the RGP optical zone bisects the pupil and results in glare and monocular diplopia. Trying to fit with apical clearance often makes things worse, since a classic combination of inferior lens displacement with a steep base curve-to-cornea fitting relationship often results in RGP lens adherence (Fig. 1).

FIG. 1: Fluorescein stain highlighting corneal impression ring and diffuse epithelial staining secondary to an adhered RGP (which has been removed in a keratoconus patient.)

Detection

Sagging cones can be detected by slit lamp evaluation or during RGP fitting by fluorescein pattern interpretation and the assessment of lens decentration. Corneal topography can also be helpful in inferior cone detection, although viewing the wrong map may be misleading.

Tangential maps should be used for their accurate localization of the corneal apex. Axial maps tend to "spread out" topographic patterns, since the averaging nature of the algorithms creates global representations of corneal shapes. Consequently, the majority of keratoconus patients appear to have inferior steepening, which extends to the limbus on axial map displays. However, what's thought to be a sagging cone on an axial map may turn out to be a slightly decentered central cone on a tangential map (Fig. 2).

FIG. 2: A 4-map overview of a slightly decentered central cone in keratoconus.

Fitting Tips

If you encounter a true sagging cone, use the following tips for a successful fit.

• Try fitting the lens flatter than flat K to achieve superior corneal alignment and attempt upper lid attachment.
• Accept inferior lens stand-off if the lens is fit acceptably to the central cornea. High inferior edge lift usually results in minimal lens awareness against the lower lid margin.
• Use large overall lens diameters (9.0mm to 9.5mm are common) and optical zone diameters to ensure full pupil coverage.
• If decentration is unavoidable, use a wide and flat peripheral curve system to prevent adherence.
• Consider using a piggyback soft contact lens, such as the Flexlens from Paragon Vision Sciences, to promote centration.

Dr. Szczotka is an assistant professor at Case Western Reserve University Dept. of Ophthalmology and Director of the Contact Lens Service at University Hospitals of Cleveland.