Topography-guided PRK reduces astigmatism after PK

The technique corrects pre-existing irregular astigmatism that results from post-PK changes to the corneal shape, surgeon says.

Issue: October 25, 2013

Customized PRK corrected irregular astigmatism after previous penetrating keratoplasty, according to a study.

Best corrected visual acuity improved in almost half of all eyes, Simon P. Holland, MD, said at the American Society of Cataract and Refractive Surgery annual meeting in San Francisco.

Topography-guided PRK after previous PK corrects irregular astigmatism more effectively and predictably than glasses or toric contact lenses, Holland said in a subsequent interview.

“In terms of irregular astigmatism, it is hard to correct with either a glasses correction or a toric contact lens,” Holland said.

Simon P. Holland

Irregular high astigmatism decreases BCVA, he said.

“With the topography-guided treatment, the idea is to improve the shape of the cornea in addition to resolving the astigmatism. There are two aspects to it. Topography-guided gives you the opportunity to treat the irregular part of the astigmatism,” he said.

Two-step procedure

Topography-guided PRK is performed in two steps, according to Holland.

“The first step is just to correct the irregular astigmatism,” he said. “It doesn’t do anything to correct the power of either the pre-existing refractive error of the patient or the induced refractive error by the shape change. So, this technique compensates for both of those aspects, so we can correct for the power changes induced by the topography by improving the shape. Then, we can correct the pre-existing refractive error, in this case astigmatism.”

Without topographic neutralization, two treatments would be required, Holland said.

“First of all, you have to bring the patient back when it’s stable — maybe 6 months — and then do the refractive treatment; that’s how topography-guided [PRK] was used initially. We combined it into a single procedure,” he said.

Patients, procedure and results

The retrospective case series included 49 eyes that underwent topography-guided PRK after previous PK.

Surface ablation was performed with the WaveLight Allegretto excimer laser (Alcon). The custom topography neutralization technique was used to modify manifest refraction based on refractive changes predicted from plano topographically guided correction.

After treatment, mitomycin C 0.02% was applied to all eyes and a standard post-PRK management protocol was used.

Corrected and uncorrected distance visual acuity, refraction, keratometry, topography and haze were evaluated at 3, 6 and 12 months after surgery; 39 eyes completed at least 6 months of follow-up.

Uncorrected visual acuity improved to 20/40 or better in 18 eyes (46%).

BCVA improved in 16 eyes (42%); eight eyes (21%) gained one line and eight eyes (21%) gained two or more lines. Five eyes (13%) lost one line and three eyes (8%) lost two or more lines.

Postoperative cylinder ranged from 0 D to 6 D. The re-treatment rate was 5.76%.

Clinical observations

Holland and colleagues were unable to gauge the effect of the graft-host interface or PK incision on keratometry and the induction of irregular astigmatism.

Patients who underwent topography-guided PRK had less postoperative haze than those who underwent standard PRK; the reasons for that are not completely understood, according to Holland.

“The problem with the standard treatment is that you create trenches in the cornea. With topography-guided [treatment], it gives you a better corneal surface, so maybe that has something to do with it,” Holland said. “Haze hasn’t been a significant problem with this technique. We do get haze, but it’s nothing like what we used to get.”

Previously, Holland and colleagues performed LASIK on patients who underwent previous PK, but the results were unpredictable; PRK is safer than LASIK for post-PRK patients, he said.

“[It’s] less invasive. I’ve seen poor flaps made on transplants with high astigmatism and very steep corneas. For us, it’s a safer procedure to do the PRK, and the results have been good,” he said. “It might be different if we were using femtosecond flaps. Obviously, some people do femtosecond LASIK after transplants, but I don’t have that experience yet.” – by Matt Hasson

Simon P. Holland, MD, can be reached at Pacific Laser Eye Centre, 1401 West Broadway, Vancouver, BC V6H 1H6; 604-736-2625; email: simon_holland@telus.net.
Disclosure: Holland is a member of the speakers bureau for Allergan and Bausch + Lomb and receives research funding from Alcon.