Recovery of Corneal Subbasal Nerve Density After PRK and LASIK

Recovery of Corneal Subbasal Nerve Density After PRK and LASIK

Severing the corneal nerves during refractive surgery may have short- or long-term effects on the health of the cornea. Erie et al. measured and compared the return of corneal innervation up to five years after PRK and LASIK in a prospective, nonrandomized clinical trial.

Eighteen eyes of 12 patients received PRK to correct a mean refractive error of –3.73, and 16 eyes of 11 patients received LASIK to correct a mean refractive error of –6.56. Corneas were examined by confocal microscopy before and at one, two, three and five years after the procedures. Subbasal nerve fiber bundles were measured to determine density (visible length of nerve/frame area) and were expressed as micrometers
per square millimeter.

After PRK, mean subbasal nerve density was reduced by 59 percent at one year when compared with preoperative density. By two years, subbasal nerve density was not significantly different from density before PRK and remained unchanged to five years. After LASIK, subbasal nerve density was reduced by 51 percent, 35 percent and 34 percent at one, two and three years, respectively. By five years, subbasal nerves had returned to densities that were not significantly different from densities before LASIK.

Corneal subbasal nerve density does not seem to recover to near-preoperative densities until five years after LASIK, as compared with two years after PRK.

Retinal Nerve Fiber Thickness vs. Optic Disc Algorithms for Detecting Glaucoma

Retinal Nerve Fiber Thickness vs. Optic Disc Algorithms for Detecting Glaucoma
The early detection of glaucoma would result in better preservation of visual field in most patients. Manassakorn et al. compared the performance of the retinal nerve fiber layer (RNFL) thickness and optic disc algorithms as determined by optical coherence tomography (OCT) to detect glaucoma in an observational cross-sectional study at an academic tertiary-care center.

The authors considered one eye from each of 42 control subjects and 65 patients with open-angle glaucoma with visual acuity of > 20/40, and no other ocular pathologic condition was selected.

Two OCT algorithms were used: “fast RNFL thickness” and “fast optic disc.” Area under the receiver operating characteristic curves and sensitivities at fixed specificities were determined. Discriminating ability of the average RNFL thickness and RNFL thickness in clock-hour sectors and quadrants was compared with the parameters that were derived from the fast optic disc algorithm. Classification and regression trees were used to determine the best combination of parameters for the detection of glaucoma.

The average visual field mean deviation was 0.0 and –5.3 decibels in the control and glaucoma groups, respectively. The RNFL thickness at the 7 o’clock sector, inferior quadrant, and the vertical cup/diopter ratio had the highest area under the receiver operating characteristic curves. At 90 percent specificity, the best sensitivities from each algorithm were 86 percent for RNFL thickness at the 7 o’clock sector and 79 percent for horizontal integrated rim width (estimated rim area). The combination of inferior quadrant RNFL thickness and vertical cup/diopter ratio achieved the best classification (misclassification rate, 6.2 percent).

The fast optic disc algorithm performed as well as the fast RNFL thickness algorithm for discrimination of glaucoma from normal eyes. A combination of the two algorithms may provide enhanced diagnostic performance.