M.S. Rahhal

Intraobserver reproducibility of retinal nerve fiber layer measurements using scanning laser polarimetry and optical coherence tomography in normal and ocular hypertensive subjects.

Purpose To evaluate quantitatively the intraobserver reproducibility of measurements of the retinal nerve fiber layer (RNFL) in healthy subjects and an ocular hypertensive population using two nerve fiber analyzers. Methods Sixty eyes of normal (n=30) and ocular hypertensive subjects (n=30) were consecutively recruited for this study and underwent a complete ophthalmologic examination and achromatic automated perimetry. RNFL were measured using scanning laser polarimeter (GDx-VCC) and optical coherence tomography (OCT Model 3000). Reproducibility of the RNFL measurements obtained with both nerve fiber analyzers were compared using the coefficient of variation. Results In both groups the authors found fair correlations between the two methods in all ratio and thickness parameters. The mean coefficient of variation for measurement of the variables ranged from 2.24% to 13.12% for GDx-VCC, and from 5.01% to 9.24% for OCT Model 3000. The authors could not detect any significant differences between healthy and ocular hypertensive eyes, although in normal eyes the correlations improved slightly. Nevertheless, the test–retest correlation was slightly better for GDx-VCC than for OCT Model 3000 (5.55% and 7.11%, respectively). Conclusions Retinal mapping software of both nerve fiber analyzers allows reproducible measurement of RNFL in both healthy subjects and ocular hypertensive eyes, and shows fair correlations and good intraobserver reproducibility. However, in our study, GDx showed a better test–retest correlation.

Assessment of applanation tonometry after hyperopic laser in situ keratomileusis.

Purpose. To determine the reliability and identify pitfalls in intraocular pressure measurement by Goldmann applanation tonometry after hyperopic laser in situ keratomileusis (LASIK). Methods. Prospective non-masked case series at University of Valencia, Faculty of Medicine and Rahhal Ophthalmology Clinic, Valencia, Spain. One hundred three patients (103 eyes) treated with hyperopic LASIK were evaluated. The main treatment was hyperopic LASIK using a microkeratome Chiron Hansatome (Chiron Vision Corp, Claremont, CA) and the excimer laser Chiron Technolas 217-C LASIK (Chiron Technolas GmbH, Dornack; Germany). Central Goldmann applanation tonometric readings before surgery and 1, 3, and 6 months after surgery were analyzed. Results. After hyperopic LASIK, a significant decrease in intraocular pressure was observed in the postoperative controls. In the low hyperopia patients (range: 1.00–3.00 D) a decrease of 2.43 mmHg was observed at the 6-month follow-up (p < 0.001). In the moderate hyperopia group (range: 3.25–6.00 D) a decrease of 2.05 mmHg was observed at the 6-month follow-up (p < 0.001). There were no significant differences between low and moderate hyperopia (p = 0.812). There was no statistically significant correlation between the magnitude of decrease in tonometry and gender, age, treated spherical equivalent, pachymetry, and anterior chamber depth (ACD). Conclusions. This clinical study displays that postoperative tonometry after hyperopic LASIK was significantly lower than the preoperative, hence modifying the reliability of Goldmann tonometry, and causing an intraocular pressure underestimation. A correcting factor should be applied when using applanation tonometry to measure postoperative intraocular pressure in patients who have undergone hyperopic LASIK.

Anatomic study of the corneal thickness of young emmetropic subjects

Purpose To study the corneal thickness of young emmetropic subjects. Methods One thousand eyes of 1000 young healthy emmetropic subjects were analyzed with the Orbscan Topography System II (Orbscan, Inc, Salt Lake City, UT) from January 2001 to May 2003. The age of the subjects ranged from 20 to 30 years old (mean ± SD = 27.12 ± 2.86). The mean of 5 consecutive measurements of the corneal thickness in the center of the cornea and at temporal, superotemporal, inferotemporal, nasal, inferonasal, and superonasal cornea were recorded. Results The corneal thickness at the following areas ranged as follows: 518 to 589 μm center; 603 to 678 μm nasal; 620 to 689 μm superonasal; 600 to 669 μm inferonasal; 571 to 639 μm temporal; 601 to 669 μm superotemporal; and 572 to 647 μm inferotemporal. In each individual the difference between the central thickness and the maximum paracentral thickness ranged from 85 to 107 μm (mean ± SD, 99.21 ± 3.80). The difference between the central thickness and the minimum paracentral thickness ranged from 36 to 59 μm (48.97 ± 4.23 μm). The difference between the minimum paracentral corneal thickness and the maximum paracentral corneal thickness ranged from 37 to 58 μm (50.24 ± 4.30). The tonometry was statistically correlated with the corneal thickness (P < 0.05 at each corneal location analyzed). Conclusions In emmetropic corneas the difference between the minimum paracentral thickness and the maximum paracentral thickness was similar to the difference between the central thickness and the minimum paracentral thickness.

Quantitative anatomical differences in central corneal thickness values determined with scanning-slit corneal topography and noncontact specular microscopy

Purpose: This study was designed to analyze the differences in central corneal thickness values determined with noncontact specular microscopy and scanning-slit corneal topography. The measurements were performed on the same eye. Methods: We analyzed the central corneal thickness values of 93 patients (n = 93) by means of noncontact specular microscopy (Topcon SP-2000P noncontact specular microscope, Topcon Corp., Tokyo, Japan) and scanning-slit corneal topography (Orbscan Topography System II, Orbscan Inc., Salt Lake City, UT). One experienced physician performed 3 consecutive central corneal thickness measurements with both devices. Results: The central corneal thickness values obtained by means of Orbscan pachymetry were 17 ± 2.7 (range, 12-24) μm greater. A significant correlation was observed between scanning-slit corneal topography and noncontact specular microscopy (Pearson correlation coefficient, r = 0.976; P < 0.001). Conclusions: Researchers should know of the existence of this difference between noncontact specular microscopy and Orbscan pachymetry when interpreting central corneal thickness values.

Differences in corneal anatomy in a pair of monozygotic twins due to continuous contact lens wear

Purpose. To study the differences in the corneal anatomy in a pair of 31-year-old monozygotic female twins, one of whom has been a contact lens wearer for the past 15 years and the other is a nonwearer. Methods. We analyzed the corneal thickness and the corneal endothelial cell density of the monozygotic twins and of two age-matched control groups, one made up of non–contact lens wearers (35 eyes of 35 patients; mean age, 29.32 ± 2.83 years) and the other made up of soft contact lens wearers (30 eyes of 30 patients; mean age, 33.08 ± 3.72 years; mean duration of contact lens use, 10.62 ± 4.32 years). Five corneal thickness measurements were carried out with the Orbscan Topography System II. Three consecutive measurements of the corneal endothelial cell density were carried out with the Topcon SP-2000P noncontact specular microscope. Results. Lower central corneal endothelial cell densities were found in both eyes of the monozygotic contact lens–wearing twin. The greater corneal thickness was shown at superonasal cornea except in the right cornea of the monozygotic contact lens twin. Moreover, significant differences in corneal thickness values were found between the monozygotic twins. Conclusions. The differences between the monozygotic twins seem to confirm that daily use of soft contact lenses can modify normal corneal anatomy.

Corneal thickness differences between type 2 diabetes and non-diabetes subjects during preoperative laser surgery examination.

AIMS To evaluate the differences in corneal thickness between type 2 diabetes subjects with HbA1c under 7.0% and non-diabetes subjects during their preoperative laser surgery examinations. METHODS The mean of five consecutive corneal thickness measurements at the central and mid-peripheral cornea was obtained by means of noncontact scanning-slit corneal topography (Orbscan Topography System II; Orbscan, Inc., Salt Lake City, UT, USA) in 35 myopic non-insulin dependent type 2 diabetes subjects (17 males and 18 females) and 48 healthy myopic controls (23 males and 25 females). RESULTS The corneal thickness values at the central and mid-peripheral cornea were significantly higher in the diabetic group (p<.001). The diabetic subjects presented the highest thickness value in the superior cornea (n=22; 62.9%) followed by the nasal (n=9; 25.7%) and the temporal (n=4; 11.4%) cornea, but never in the inferior cornea. The control subjects presented the highest thickness value in the superior cornea (n=19; 39.6%) followed by the nasal (n=18; 37.5%), the inferior (n=6; 12.5%), and the temporal (n=3; 6.3%) cornea. The central corneal thickness (CCT) of the diabetes patients was not statistically correlated with their HbA1c (r2=.078; p=.104), body mass index (r2=.007; p=.633), and time from diagnosis of diabetes (r2=.025; p=.363), but it was correlated with their corneal endothelial cell density values (r2=.543; p<.001). CONCLUSIONS Diabetes subjects with HbA1c under 7.0% who are candidates for laser refractive surgery present thicker corneas than their age-matched control subjects. In these patients, there is a correlation between their CCT values and their corneal endothelial cell density values, so when higher CCT values were found, lower corneal endothelial cell density values were observed.