Fitting the Postoperative Corneal Transplant Patient
ZADNIK, O.D., Ph.D.
For most of us, prescribing RGP contact lenses after penetrating keratoplasty isn't an everyday occurrence, but when the opportunity does present, it can be among the most professionally gratifying of contact lens prescribing experiences.
As many as 50 percent of patients who undergo corneal transplantation require contact lenses postoperatively. Although some of those lenses are therapeutic or bandage hydrogels, many are rigid lenses prescribed for optical reasons because the cornea has high, regular astigmatism, irregular astigmatism or high anisometropia.
Unfortunately, patients are often not counseled prior to surgery that they might need rigid lenses postoperatively, and elderly post-graft patients are often not happy about the contact lens fitting. Postoperative keratoconus graft patients for whom rigid lenses have been a fact of life are usually more amenable to contact lens wear.
The greatest challenge for practitioners who don't routinely prescribe post-transplant rigid lenses is deciding on a starting point for the fitting. The following list covers most of the factors involved.
THE POST-GRAFT MANTRA: BIG IS GOOD
If there are no other specifications regarding lens parameters, the lens should be larger than the diameter of the corneal graft -- considerably larger. If the lens is too small, it may bear heavily over the host-graft interface and cause epithelial staining or abrasions in the area of the sutures. Lenses of 9.5mm to 10.5mm in overall diameter are the norm. If you don't have lenses in these diameters, your RGP laboratory can lend you a fitting set.
FIG. 1: RIGID LENSES CAN PROVIDE COMFORTABLE, STABLE VISION FOR POSTOPERATIVE CORNEAL TRANSPLANT PATIENTS.
CALL ON THE USEFUL BITORICS
High, irregular astigmatism may be the reason you are prescribing rigid
lenses, so spherical lenses will likely be too unstable on the eye. You
may need a bitoric, either spherical or cylindrical power effect, for a
stable fit and adequate vision.
If you try a spherical lens first, choose a base curve near the average of the flat and steep keratometric readings. Bitoric lenses should undercut the amount of corneal toricity by about one-third and should be slightly flatter than the keratometric readings.
TO CENTER OR NOT TO CENTER MAY NOT BE THE QUESTION
In general, on any cornea, rigid lenses decenter to the steepest portion of the cornea. Fortunately, in normal corneas this is the corneal apex and is near the visual axis. In post-grafted eyes, however, the steepest portion of the corneal graft can be anywhere, and all the usual tricks to improve lens centration prove futile. But with large lenses and a high level of visual tolerance from transplant patients, the decentered lens is not a significant problem as long as it provides adequate tear exchange and the optic zone of the lens covers part of the pupil.
PROVIDE COMPREHENSIVE POSTOP CARE
In assuming responsibility for the postoperative graft patient's rigid lens fitting, you also assume responsibility for the short-term and long-term viability of the graft itself. The patient may come to your office first with what sounds on the telephone like a contact lens-related problem, perhaps a foreign body sensation with lens insertion, blurry vision or an abrasion. Sometimes, it may be difficult to discern a contact lens-related problem from a graft-threatening problem such as graft rejection. Graft rejections can manifest as subepithelial infiltrates, low-grade uveitis, an epithelial rejection line and/or a full-blown Khodadoust line (endothelial rejection line).
In general, treat any problem that you cannot attribute to the contact lens as a graft rejection until you can prove otherwise. CLS
Dr. Zadnik is an assistant professor at The Ohio State University College of Optometry in Columbus.