Topography-Guided PRK for Keratoconus

Combining corneal cross-linking (CXL) with topography-guided photorefractive keratectomy (TPK) can greatly improve visual acuity in patients with keratoconus, often allowing correction with soft contact lenses or spectacles rather than gas-permeable hard lenses. But how and when to combine them is a matter of some debate, said Eric D Donnenfeld MD.

Combining the two procedures on the same day is the preferred method of A John Kanellopoulos MD, who pioneered the practice now known as the Athens Protocol. But Dr Donnenfeld follows a different approach.

CXL alters the corneal refractive index in sometimes unpredictable ways, Dr Donnenfeld pointed out. So, it is better to see where the cross-linked cornea settles before performing a refractive ablation.

Healing is also an issue, Dr Donnenfeld said. “My experience in cross-linking and laser ablation is when you do them at the same time, you end up having more healing problems—there’s more damage, there’s more risk of scarring. I almost always [perform] cross-linking and then wait three months [before] topographic ablation.”

Severity matters

The severity of corneal irregularities affects what kind of ablation to use, Dr Donnenfeld noted. For mild cases, it may be possible to conduct a wavefront-guided ablation, which has the advantage of considering both the anterior and posterior corneal surfaces.

However, in the majority of more severe cases, it is not possible to get a usable wavefront image, so topographic ablation is extraordinarily helpful.

“This changes lives,” Dr Donnenfeld said. “We’ve prevented hundreds of corneal transplants—we’ve moved people from gas-permeable lenses to soft lenses.”

However, he emphasised topography-guided procedures rely on a successful topographic image capture.

“We are looking for topography where you see a steep area and a flat area, and this has to coincide with the Topolyzer wavefront evaluation.”

The steep area receives a hyperopic ablation treatment and the flat area a myopic treatment.

“I find you can treat up to 10.0 D irregularity pretty regularly,” Dr Donnenfeld said.

Yet stromal thickness can limit tissue ablation. Dr Donnenfeld likes to leave behind at least 350 microns to ensure stability. If the ablation zones are too deep, the optical zone can be shrunk to remove less tissue.

Also, patients must be informed the goal is to achieve emmetropia.

“Never promise the patient they will get rid of glasses,” Dr Donnenfeld said. “You are trying to improve visual acuity, and the results can be very unpredictable.”

Alternative treatments

However, it’s not always possible to generate a usable topographic image. For example, pellucid marginal degeneration corneas tend not to do well with topographic imaging. If a treatable topographic image cannot be taken, patients do well with intrastromal ring implants followed by cross-linking, he added.

For patients who cannot have topical ablation (usually those with K values exceeding 55), Dr Donnenfeld recommends employing the volcano technique, which involves removing thickened epithelium that can add steepness. Ablating where it is thickest can reduce irregularity by up to 5.0 D. This can be enough to get patients into soft contact lenses. Alternatively, after six months, a topographic ablation may be possible.

“This is anything but cosmetic. This is visually rehabilitating some of the most difficult patients we see in our practices,” Dr Donnenfeld concluded. “While this is a wonderful treatment today, I’m absolutely certain the technology will continue to evolve.”

Dr Donnenfeld made his comments at Refractive Surgery Day at AAO 2024.

Eric D Donnenfeld MD is clinical professor of ophthalmology at New York University, US; trustee of Dartmouth Medical School in Hanover, New Hampshire, US; and in private practice in New York State, US. ericdonnenfeld@gmail.com

1. A Nattis, et al. JCRS, 46(4): 507–516.