Prospective study analysis: Contrast sensitivity loss still a challenge after PRP

Mexico City-Practitioners who are looking to minimize contrast sensitivity (CS) loss following panretinal photocoagulation (PRP) in their patients with diabetes may choose to consider performing PRP with a pattern scan laser (PASCAL Photocoagulator, OptiMedica) system instead of a conventional argon laser. Although some loss of CS is likely using either laser treatment, eyes treated with the pattern scan laser showed less CS decrease in a study of 26 patients, although the difference was not statistically significant.

Jose Luis Guerrero-Naranjo, MD, and his colleagues from the retina department, Asociación para Evitar la Ceguera en México IAP, Mexico City, conducted a randomized, prospective, comparative, open-label study of 52 eyes in 26 patients who have severe non-proliferative diabetic retinopathy (DR) or proliferative DR.

PRP was performed on the patients' right eyes with an argon laser and on their left eyes with the pattern scan laser.

In search of solution to CS loss

A lack of advances in the way laser PRP has been performed since the 1970s prompted Dr. Guerrero-Naranjo and his colleagues to examine whether the pattern scan laser might offer a solution to CS loss. Because of its short pulses and highly focused energy pattern, the pattern scan laser may offer less choroidal heating and less energy diffusion to nearby tissue than an argon laser, they hypothesized. The patients in this study had not had previous laser treatments performed on either eye and were not receiving any anti-angiogenic drug therapies. Inclusion criteria included the patients being aged at least 25 years and having severe non-proliferative DR or proliferative DR, pupil mydriasis (5 mm), and clear optical media.

Patients were excluded from the study if they had macular edema before treatment, significant corneal opacities, or other ocular diseases.

PRP was performed in the same manner for both the conventional laser and pattern scan laser-treated eyes: one session of 750 burns, a 2-week interval, then a second session of 750 burns. CS was evaluated at baseline and at 28, 42, and 85 days postoperatively with the CVS-1000 test. All of the patients received a full PRP successfully, and 100% returned for all of the follow-up visits.

The patients' CS decreased in both eyes, and the researchers did not find a statistical difference between the two treatments.

"Although the eyes treated with the pattern scan laser system showed less CS loss than those treated with the argon laser, the result was not statistically significant at our targeted p value (p < 0.05)," Dr. Guerrero-Naranjo said.

The researchers noted that in previous work, they had been able to identify a significant difference in the wave-b amplitude between groups treated with pattern scan and argon lasers, so it came as somewhat of a surprise that the CS changes were the same in this study. Dr. Guerrero-Naranjo speculated that the findings could be attributed to the CS at baseline being extremely low, the test used lacking the specificity and sensitivity to detect small changes, and the small number of patients.

"We must remember that we are dealing with a distended retina," Dr. Guerrero-Naranjo said. "Diabetic retinas and ischemic retinas already have degradation of contrast sensitivity. This is illustrated by the low specificity at the baseline."

Dr. Guerrero-Naranjo said that he and his colleagues will continue to work on this research with larger groups of patients.