Classic drawbacks of PRK succumb to new strategies

By Cheryl Guttman

PHILADELPHIA - Innovative approaches addressing the classic drawbacks of PRK have led to a rebirth of interest in the surface ablation procedure.
When LASIK appeared on the scene, many refractive surgeons were glad to abandon PRK with its associated problems of postoperative pain, delayed visual recovery and haze.

In a session entitled "PRK Reborn" at the annual ASCRS Symposium on Cataract, IOL and Refractive Surgery, researchers reviewed advances in PRK which might encourage surgeons to take a second look.

David Edmison MD pointed out that discomfort could now be well-controlled with the use of a bandage contact lens and postoperative topical nonsteroidal anti-inflammatory drugs.

In addition, he noted some surgeons are using diluted solutions of a topical anaesthetic safely without risk of impeding re-epithelialisation.
Postoperative haze has been the other unwanted effect of PRK. However,
mitomycin-C has now gained acceptance as a valuable therapeutic modality for this problem, he noted.

"One myth surrounding PRK is that once haze develops, subsequent interventions only worsen the situation. In fact, we realise now that this is only rarely the case," Dr Edmison said.

Indeed, new ablation techniques have dramatically reduced the incidence of haze. In addition, some surgeons are using mitomycin-C successfully for haze prophylaxis in high-risk patients. Other novel techniques are also being explored.

Dieter G. Dausch MD described the value of a modified PRK technique designed to avoid significant increases in corneal temperature for hastening visual recovery and minimising postoperative pain and haze.

The procedure combines a modification of the laser shot distribution pattern to avoid summation of thermal effects with vapour and liquid cooling of the cornea.
He reported that in a series of 35 eyes of 31 patients undergoing PRK for myopic astigmatism with the MEL 70 excimer laser (Asclepion Meditec), this approach was associated with minimal pain, minimal or no haze and rapid recovery of BCVA.

On the first day after surgery, all patients were asked to rate the worst pain they had experienced on a scale of zero (no pain) to 10 (severe pain). The mean score was 2.24, representing very minimal to minimal pain comparable to any discomfort associated with LASIK.

Re-epithelialisation occurred after a mean of 2.95 days and recovery of BCVA was relatively fast, being present within seven days after the procedure.
"Previous studies using the same pain rating scale we employed yielded mean results ranging from 3.0 to 6.28, and sub-epithelial haze formation along with slow visual recovery have also been discouraging limitations associated with PRK.

"All of those events might be attributed to laser-induced elevations in corneal temperature, but by using multimodal strategies to minimise thermal injury, we have been successful in minimising those problems.

"However, to further corroborate the benefits of this treatment approach, we would like to undertake a contralateral comparison study using one of the latest generation lasers for the PRK," Dr Dausch said.

The rationale for this approach derives from observations that the corneal temperature during PRK, as measured with non-contact, infrared thermography, rises to 42oC, which is near to the point of protein degradation.

To mitigate that change, Dr Dausch and his colleagues developed an online cooling system that allows for continuous exposure of the treatment area to a mist of room-temperature water. The vapour is generated by an atomiser and expelled at a flow rate of 3.0 m/sec. The moisture is continuously removed by vacuum through a second tube.

The cooling system is an experimental device being manufactured by Asclepion-Meditec. It will be available in the near future as an update to the MEL 70 laser system.
Evaluation of the cooling system in experimental animal models demonstrated that it reduced the maximum temperature increase during a -5.0 D PRK correction from 21.4oC to only 13.7oC, Dr Dausch reported.

To further cool the cornea, a metal ring is placed on the ocular surface once the ablation is completed and the inner region overlying the treatment zone is filled with BSS chilled to -8oC. The liquid is removed with a sponge after a period of five to 10 seconds and this is repeated two to three times. Postoperatively, the eye is covered for two to three days with a bandage contact lens.

"Use of the metal ring is important for preventing conjunctival exposure to the chilled BSS that will result in pain after the procedure," Dr Dausch said.
Aashish Bansal MD presented his preliminary experience indicating that an amniotic membrane graft may be an effective modality for preventing haze after PRK or PTK.

He reported a series of 14 eyes, including four eyes undergoing repeat PRK for haze and regression and ten treated with PTK for anterior stromal dystrophy and corneal scars.
At the six-month follow-up, 12 of the 14 eyes showed improvements in both best-corrected vision and haze.

"The development of sub-epithelial haze can be a significant vision-limiting problem after corneal surface ablation procedures. A variety of techniques have been used to treat or prevent such haze, but none have consistently yielded unequivocally good results.
"Amniotic membrane transplant helps to reduce the haze after PRK. It accelerates epithelial healing by preventing keratocyte apoptosis and by preventing the inflammatory response to the surgical trauma, thus minimising the activation of keratocytes," Dr Bansal said.

He added that the development of sub-epithelial haze can be a significant vision-limiting problem after corneal surface ablation procedures.
"A variety of techniques have been used to treat or prevent such haze, but none have consistently yielded unequivocally good results. Our experience has been promising, but longer-term follow-up in a larger series of eyes is needed to better evaluate its role," Dr Bansal said.

In his study, the ablation procedures were performed with a Nidek EC-5000 laser using a 4.5 mm to 7.0 mm optical zone and a 5.0 mm to 7.5 mm transition zone.
Once the ablation was completed, the amniotic membrane was placed on the cornea epithelial side down and secured with five tangential 10-0 nylon sutures.

The eye was covered with a bandage contact lens and patients were instructed to use fluormetholone 0.1% for two to three weeks and ciprofloxacin for five days.
The sutures were removed as they became loose, which occurred between five and 21 days after surgery. The graft is removed when the sutures become loose.

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