Retinal nerve fiber layer measurements using scanning laser polarimetry after photorefractive keratectomy

To assess the effects of corneal depth changes on retinal nerve fiber layer (RNFL) postoperative measurements in myopic patients who had undergone photorefractive keratectomy (PRK).

A total of 120 myopic patients underwent PRK for myopia (range -2 to -10 diopters) and were divided into three groups according to their myopic correction: lower than 3 diopters (low myopia group), between 3 and 6 diopters (medium myopia group), over 6 diopters (high myopia group). RNFL parameters were evaluated preoperatively and 5 days, 3 months and 6 months after surgery, using a GDx NFA II scanning laser ophthalmoscope.

Significant changes were seen in the symmetry, superior maximum, and average thickness, comparing baseline with six-month measurements (p=0.008, 0.027, 0.015 respectively). Dividing the sample according to attempted myopic correction, it was found that mean postoperative RNFL thickness was significantly lower after PRK only in the high myopia group. Mean RNFL thickness did not change with time (p=0.884). Ablation depth was correlated with a change in RNFL thickness by the sixth postoperative month for each group. These variables were significantly related only in the high myopia group (p=0.003).

As polarised light penetrates the ablation area, morphological and functional changes might affect Gdx NFA II measurements. It must always be borne in mind that RNFL thickness can decrease either in cases of glaucoma or after PRK for high myopia, so close attention must be paid to interpreting these measurements in patients who have undergone PRK.