The effects of ablation diameter on the outcome of excimer laser photorefractive keratectomy. A prospective, randomized, double-blind study
D. P. O'Brart, M. C. Corbett, C. P. Lohmann, M. G. Kerr Muir and J. Marshall
Department of Ophthalmology, St Thomas' Hospital, London, England.
OBJECTIVE: To determine the effects of the ablation diameter on the outcome
of excimer laser photorefractive keratectomy. DESIGN: Eighty patients were
randomized to either a 5.00-mm or a 6.00-mm treatment group and within
these groups underwent either a -3.00-diopter (D) or a -6.00-D correction
based on their preoperative refraction. A Summit Omnimed excimer laser was
used throughout the study. RESULTS: In eyes treated with a 6.00-mm-diameter
ablation, the initial hyperopic shift was reduced, with significant
differences at 1 week with -3.00-D corrections and at 1 and 4 weeks with
-6.00-D corrections (P < .01). At 6 months, the refractive changes were
greater and closer to that intended with 6.00-mm-diameter ablations. The
predictability of photorefractive keratectomy was significantly improved
with 6.00-mm zones, with a reduction in variance of the refractive changes
at all stages postoperatively (P < .05 to P < .001). With -3.00-D
corrections, objective measurements showed significantly less anterior
stromal haze in eyes treated with 6.00-mm zones at 6 months (P < .05).
With -6.00-D corrections, haze was significantly reduced at 1, 3, and 6
months in the eyes treated with 6.00-mm zones (P < .05). Five eyes
treated with 5.00-mm zones experienced severe regression of the correction,
with marked corneal haze and a reduction of 3 or more lines of best
corrected Snellen visual acuity at 6 months. No eyes treated with 6.00-mm
zones were similarly affected. Computerized measurements of "night" halo
were significantly lower in the 6.00-mm treatment groups at 1 week and at 1
and 6 months in the eyes with -3.00-D corrections and at 1 week and at 1
month in the eyes with -6.00-D corrections (P < .05). At 6 months, seven
patients treated with 5.00-mm zones complained of severe disturbances of
night vision. No eyes in the 6.00-mm group were similarly affected.
CONCLUSIONS: Treatment with 6.00-mm ablation diameters precipitated less
initial overcorrection, greatly improved the predictability of
photorefractive keratectomy, and was associated with a reduction in
complications impairing postoperative visual performance.
