When you hear someone mention custom LASIK, you assume he's talking about myopic treatments. But what about custom hyperopic ablations? Surgeons at work on these challenging cases say it may be some time before they're at the level of accuracy of their myopic counterparts, if that's even possible. In this article, refractive surgeons share their results with custom hyperopic treatments and explain what makes them so difficult.

The Double Threat
Expert surgeons say two aspects of hyperopic treatments have the potential to undermine a successful outcome: centration and healing.

Houston surgeon Jack Holladay says that, since almost all refractive surgeons center their treatments on the pupil, but the pupillary center is actually nasal to the center of the cornea, they're treating on the nasal side of the cornea's "hill." The effect of this can be more pronounced in hyperopia.

"The treatment is symmetrical, but the cornea isn't symmetrical in that position, so we end up with an asymmetric result," he explains. He says this is why hyperopic results start to degrade above around 4 D, because the treatment begins to induce an irregular astigmatism in the cornea. 

In the realm of wound healing, Rochester, N.Y., surgeon Scott MacRae has a theory that may explain why hyperopic treatments tend to regress. He says that the more "transition points" you impose on the cornea, or points at which you go from an untreated area to a treated one or from one degree of treatment to another, the more the surface cells want to smooth the cornea back to its original shape. "When you correct myopia, there's only one transition point for each half of the cornea where you go from normal untreated cornea to treated cornea, and all you're doing is flattening," he says. "When you treat hyperopia, you have three transition points: one in the mid periphery at the deepest part of the treatment, one at the edge of the treatment in the far periphery, and one in the center of the cornea."

He says that, since we blink over 6,000 times per day, the cornea's cells tend to try to smooth out these three curves with each blink. "Since there are three curves with hyperopia as opposed to one with myopia, it becomes three times harder to treat hyperopia," he says. "This has been proven out: You can treat around 4 D of hyperopia where you can treat 12 D of myopia before you hit the 'wall.' " 

Can Customization Help?
Dr. Holladay says that surgeons can deliberately slightly decenter an ablation nasally to try to encompass the visual axis with the ablation, but to overcome the difficulty of centration, the laser makers will need to factor this decentration into their programming. "You have to modify the ablation profile to put the right shape on this asymmetric surface and make it come out right," he says. "You can't do that without a topographic map and without knowing how much decentered you are." He says LaserSight, for which he is medical director, is doing this in European patients with Aleksandar Stoyanovic, MD, of Tromsoe, Norway.

Dr. Stoyanovic says that, "for +3 D and below, the results are almost as good as custom myopic ablations, and I'm more satisfied with custom hyperopes' results than with standard treatment." He echoes Dr. MacRae when he considers the effect of regression on custom hyperopic treatments. "I think regression has as much to do with constructing a transition area as not treating too much hyperopia," he says.

Dr. MacRae thinks that improvements in laser technology, not just in wavefront, will eventually lead to better results. "There are several things converging with wavefront," he says. "Laser spot sizes are getting smaller and ablations are at a finer precision, so ablation patterns may become more elegant, in which case the healing response and secondary tissue response may be less."

Current Outcomes
Laser makers are facing the challenge of custom hyperopic treatments in international studies. Here are some of the latest results.

Houston's Steven Slade, MD, is currently performing a randomized study in Mexico comparing custom hyperopic surgery with the Bausch & Lomb Zyoptix system to the company's conventional PlanoScan. Forty-five patients with up to 6 D of hyperopia were randomized to receive a custom treatment in one eye and a conventional one in the other. At one month, 85 percent of the custom eyes see 20/32, compared to 57 percent of the conventional ones. He says there has been a 20-percent improvement in best-corrected vision with Zyoptix, and that he didn't run into problems centering over the pupil. "That's been a big dispute, whether to center over the visual axis or the pupil," he says. "The B&L laser centers on the pupil, but, if you want, you can adjust this with the tracker by 0.1 mm at a time."

Toronto surgeon Omar Hakim and other investigators have studied Alcon's CustomCornea in a small number of hyperopes (15), comparing it to conventional LadarVision treatment. The average hyperopia treated was low, around 1.98 D, with a range from +0.5 to +3.25 D. At one month, 80 percent of the custom eyes see at least 20/20 uncorrected, compared to 53 percent of the conventional, and all custom eyes see 20/40 or better, versus 87 percent of the conventional ones. Also, none of the custom patients who could see 20/20 best-corrected before surgery lost any lines of vision, while 7 percent of the 20/20 conventional eyes did.

As far as aberrations, CustomCornea induces some, but still fewer than conventional LASIK. "We're inducing a third of what we induce with conventional," says Dr. Hakim.

Ottawa, Ont., surgeon W. Bruce Jackson is heading up the investigation of Visx's custom hyperopic trial, and he says he's pleased with the results so far. "The initial group, the one that won approval for custom hyperopia in Canada, had hyperopia up to +3 D," he says. "We kept the correction low because the procedures take a long time, and we didn't want them to take longer than 100 seconds in order to avoid corneal drying. We're starting another study up to +5 or +6 D, but now the laser is running at 20 Hz instead of 10 Hz, which will keep the time down."

At one month, 69 percent of 16 eyes saw 20/20 and no eyes lost more than two lines of vision. Five months later, however, due to regression, the refraction had crept up to +0.24 D. "The nomogram has been adjusted accordingly, and that's what's been released commercially," says Dr. Jackson, who adds that the results are better than what he's experienced with conventional treatments thus far.

Milan's Paolo Vinciguerra, MD, has treated 20 hyperopic patients with the Nidek Advanced Vision Excimer Laser System (NAVEX), which combines data from Nidek's OPD-Scan and the ARK-10000 aberrometer and feeds it into the company's Final Fit software. The software then helps plan the ablation.

Though he wouldn't discuss specific results, waiting instead to discuss them at the Refractive On-line meeting in Milan on September 11, 2003, he says the system is useful for knowing how much of a patient's cornea he can ablate safely, and which patients he shouldn't even operate on. It does this by quantifying aspects of the patient's cornea, such as asphericity. "It gives numerical values that are easier to interpret than just relying on an interpretation of the appearance of the topography or wavefront," he says. "Knowing to never increase the postop asphericity greater than 1.1 is easy to remember."
He thinks the upper limit of the hyperopic treatment depends more on the asphericity of the cornea rather than dioptric error. "If it's highly aspherical to start with, you don't have that much room to correct hyperopia, because in doing so you have to increase an asphericity value that's already near the limit," he says. "If it's more spherical, though, you have a lot of room to correct it."