McLaren, Jay W.

Purpose: To determine the long-term changes in epithelial, stromal, and corneal thickness after LASIK and photorefractive keratectomy (PRK). Methods: In two prospective observational case series, 11 patients (16 eyes) received LASIK and 12 patients (18 eyes) received PRK to correct myopia or myopic astigmatism. None of the corneas had retreatment procedures. Corneas were examined using confocal microscopy before and at 1 month, and at 1, 2, 3, 5, and 7 years after surgery. Central thicknesses were measured from reflected light intensity profiles recorded by confocal microscopy. Postoperative epithelial thickness was compared to preoperative, and postoperative stromal and corneal thicknesses were compared to thickness at 1 month after surgery. Results: In LASIK, epithelial thickness at 1 month (51±4 µ m, n=11) was greater than before surgery (41±4 µ m, n=16; P <.001) and remained thicker through 7 years (52±6 µ m, n=13; P <.001). Stromal and corneal thickness did not change between 1 month and 7 years after LASIK. After PRK, corneal thickness at 1 year (464±44 µ m, n=17) was greater than at 1 month (442±39 µ m, n=15; P =.001) and remained thicker at 7 years after PRK (471±45 µ m, n=17; P <.001). Conclusions: The early increase in central epithelial thickness after myopic LASIK persists for at least 7 years and is probably the result of epithelial hyperplasia. Central corneal thickness increases during the first year after PRK and remains stable thereafter up to 7 years. [ J Refract Surg . 2007;23:385–392.]

Purpose: Patients with Fuchs endothelial corneal dystrophy (FECD) often notice poor vision in the morning that improves as the day progresses. In this study, we determined changes in corneal optical properties associated with induced corneal edema. Methods: Twenty-three phakic eyes (23 participants) with FECD (grades 1–6, modified Krachmer scale) and 8 normal eyes (8 participants) were examined by Scheimpflug photography. Central corneal thickness, high-order aberrations from anterior and posterior corneal surfaces, and backscatter from the anterior, mid-, and posterior cornea were determined from the Scheimpflug images. A low-oxygen permeable contact lens was placed on the eye, and eyes were closed for 2 hours, after which the lens was removed and Scheimpflug photography was repeated for up to 5 hours to determine changes in backscatter and high-order aberrations. Results: Corneas swelled by 10% [95% confidence interval (CI), 9–10]. Backscatter from the anterior cornea increased by 416 scatter units (SU, 95% CI, 344–488; P < 0.001), independent of the presence and severity of FECD. Recovery of anterior backscatter was slower in advanced FECD (81 SU/h, 95% CI, 60–120) compared with normal (123 SU/h, 95% CI, 95–150; P = 0.019). Anterior and posterior corneal high-order aberrations, and mid and posterior backscatter, did not increase with induced swelling. Conclusions: Inducing corneal edema increases anterior corneal backscatter but not high-order aberrations. Subjective poor vision in the morning in FECD is probably caused by scattered light rather than by high-order aberrations, suggesting that these patients experience more disability glare than decreased visual acuity.

Purpose: Abnormal ocular biomechanical properties may be important for understanding the risk of glaucoma. However, there are no clinical methods for measuring standard material properties in patients. In this feasibility study we demonstrated proof-of-principle for a novel method, ultrasound surface wave elastography (USWE), to determine the in vivo Young’s modulus of elasticity of corneas in normal human eyes. Methods: In total, 20 eyes of 10 healthy subjects (mean age, 51.4±7.2; ±SD; range, 43 to 64 y) were studied. A spherical-tipped probe (3-mm diameter) was placed on closed eyelids and generated a gentle harmonic vibration at 100 Hz for 0.1 second. Wave speed propagation in the cornea was measured by USWE, and Young’s modulus was calculated from the wave speed. Associations between Young’s modulus and intraocular pressure (IOP), age, central corneal thickness, and axial length were explored by the Pearson correlation. Statistical significance was determined by using generalized estimating equation models to account for possible correlation between fellow eyes. Results: Mean IOP was 12.8±2.7 mm Hg. Mean wave speed in the cornea was 1.82±0.10 m/s. Young’s modulus of elasticity was 696±113 kPa and was correlated with IOP (r=0.57; P=0.004), but none of the other variables (P>0.1). Conclusions: USWE is a novel noninvasive technique for measuring ocular biomechanical properties. Corneal Young’s modulus in normal eyes is associated with IOP, consistent with measurements in cadaver eyes. Further work is needed to determine elasticity in other ocular tissues, particularly the sclera, and if elasticity is altered in glaucoma patients.