Surgically induced refractive errors can compromise vision following penetrating keratoplasty.
New Orleans-A variety of methods can be used to correct induced refractive errors in eyes that have undergone penetrating keratoplasty (PKP), although choosing Descemet's stripping automated endothelial keratoplasty (DSAEK) as the graft procedure instead of PKP may help avoid the need for an enhancement procedure, said Deepinder K. Dhaliwal, MD, during refractive surgery subspecialty day at the annual meeting of the American Academy of Ophthalmology.
"It is very frustrating for corneal surgeons who have performed PKP with excellent technique and see the graft is crystal clear, but the patient has poor vision because of induced refractive errors," said Dr. Dhaliwal, associate professor of ophthalmology and chief, refractive surgery, University of Pittsburgh School of Medicine. "DSAEK works very well as an alternative to PKP in patients with endothelial disease because it avoids both the need to contend with sutures and refractive surprises. Therefore, we consider it the best method for reducing enhancement rates in eyes that need corneal transplantation."
For the patient who has undergone PKP and needs refractive correction, first be sure that the refraction is stable, the interface is secure, and the patient shows no sign of allograft rejection within the previous 6 months, she said.
Another factor to consider is whether the patient has any signs of cataract, because in that situation, lens-based surgery should be performed instead of cornea-based refractive surgery. It also is important to evaluate endothelial function with pachymetry and specular microscopy, and to remember that patients with a history of herpes simplex virus keratitis should receive prophylactic antiviral treatment, she said.
When enhancement is indicated, Dr. Dhaliwal said she prescribes glasses for a patient who can tolerate spectacles and proceeds with rigid gas-permeable contact lens fitting if a patient has irregular astigmatism or significant anisometropia.
Surgery is considered only if spectacles and contact lenses do not work. Dr. Dhaliwal noted that astigmatic keratotomy is the most commonly performed post-PKP enhancement procedure in her practice. She reminded attendees, however, that it corrects astigmatism only; spherical equivalent will remain unchanged.
Her technique involves making the incisions 0.5 mm inside the graft-host junction on the steep axis using a diamond blade set to the thinnest corneal thickness. She also uses the Casebeer-Lindstrom nomogram, Dr. Dhaliwal said. Because the results can vary, however, she initially aims to correct only half of the cylinder according to the nomogram.
The incisions also can be made with a metal blade at the slit-lamp using topography guidance or with a femtosecond laser, and they can be placed asymmetrically in eyes with asymmetric astigmatism.
Experience with LASIK indicates that it can provide good results in patients undergoing PKP to correct myopia, hyperopia, or astigmatism, although Dr. Dhaliwal noted that concern exists about the effect of decreasing endothelial cell count on long-term flap stability. Suitable candidates for LASIK include patients without dry eye who have adequate corneal thickness and a healthy endothelium. The goal is to improve spectacle tolerance with return to binocularity but not to provide excellent uncorrected vision.
"If a decision is made to perform LASIK, be sure to wait at least 6 months after suture removal to ensure corneal stability, and remember that the flap creation itself can alter the refractive error and reduce astigmatism," Dr. Dhaliwal said.
She does not perform photorefractive keratectomy (PRK), she said, because studies have shown that it is associated with more haze, regression, and induction of irregular astigmatism when performed in post-PKP eyes. Nevertheless, phakic IOLs appear to be a viable option and have several attractive features because they can correct very high degrees of myopia without compromising the ocular surface or corneal thickness and are removable or exchangeable.
"There are only a limited number of published case reports describing phakic IOL implantation after PKP," Dr. Dhaliwal said. "While the results have been favorable, accelerated endothelial cell loss after implantation is a potential concern in a PKP eye."
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