Short- and long-term repeatability of Visioptic Alcon EyeMap (Visioptic EH-270) corneal topographer on normal human corneas.

The purpose of this study was to examine the short-term and long-term (6 months) repeatability of the Alcon EyeMap (Visioptic EH-270)a Computerized Corneal Topographer on normal eyes. Three measurements were taken on both eyes of 39 subjects with the corneal topographer. Both eyes of seven of these subjects were measured 6 months later in order to evaluate long-term repeatability. The data showed that short-term reliability of the instrument was acceptable with a mean standard deviation (SD) < or = 0.5 D up to 3 mm (6-mm optical zone) from the center of the normal cornea. Beyond that distance, SD's of repeated measures increased systematically, making the results less useful. Long-term repeatability of the instrument over a 6-month period was acceptable clinically, although statistically significant differences were noted.

Monocular vs. stereoscopic measurement of cup-to-disc ratios.

Stereoscopic viewing of the optic nerve head and cup has been reported to provide more reliable cup-to-disc (C/D) ratio estimates. We investigated the variability in C/D ratio estimates among three experienced optometric clinicians using conventional direct ophthalmoscopy and indirect biomicroscopy with the Volk 90 D lens. The pupils of 21 subjects were dilated and the discs assessed randomly by each examiner for the determination of the C/D ratio. Analysis of variance indicates that the two methods are not significantly different from each other when comparing interexaminer reliability. However, for a particular examiner, one method may be more effective than the other.

The Hirschberg test: correlation with corneal radius and axial length.

The results of this study indicate that the axial length is not strongly associated with the Hirschberg formula, and therefore the axial length does not have to be considered when measuring an ocular deviation with the Hirschberg test. This study does demonstrate an association between the radius of curvature of the cornea and the Hirschberg formula. For patients with keratometer readings below 46 D, the use of the standard Hirschberg formula, 1 mm = 22 delta, is appropriate. For patients with keratometer readings greater than 46 D, and with deviations greater than about 30 delta, the use of the Hirschberg formula, 1 mm = 27 delta, is more appropriate. When performing the Hirschberg test on infants up to about 2 months of age, the use of the higher Hirschberg formula would probably be more appropriate because this age group often has higher keratometer readings.

Rigid lens base curve stability upon hydrogen peroxide disinfection.

Because of the possibility of transmitting communicable diseases, in particular the HIV virus, it has been recommended that all diagnostic contact lenses, including rigid lenses, be disinfected after each use. Hydrogen peroxide is a recommended disinfection agent, but its effect on rigid lens polymers is relatively unknown. We soaked 50 lenses of 5 different polymers in 3% hydrogen peroxide for 10 min and measured the base curves to determine if any changes occurred. Our results showed no statistically significant change in base curve for the lenses measured, but 22% of the lenses did exhibit a small amount of warpage (mean 0.0382 mm) of the base curve. We were not able to determine what caused these lenses, and not all the lenses, to warp. Under the conditions of this study, hydrogen peroxide does not appear to cause clinically significant parameter changes, but lenses should be checked for warpage before use.

The subjective quality (SQF) of Bausch and Lomb SoflensTM.

The visual acutiy of a person is examined in terms of his response to a unique target. This target is a sequence of black bars on a white background, where the spacing of the bars and their width is governed by a log periodic function. The generation of these targets is performed by a PDP-8 computer connected to an X-Y plotter. There are six sets of bars which vary in contrast and frequency. In a clinical situation, an observer notates the position where he can no longer resolve the bar pattern. A recroding of these positions produces a Subjective Quality Function (SQF) curve for the individual. On fitting with soft contact lenses, a new SQF curve is generated. A comparison of the SQF for soft lenses with that for spectacle corrections is given.

Corneal hydration changes during the normal menstrual cycle--a preliminary study.

PIP: A study is described that was undertaken to determine if significant changes in corneal hydration and curvature occur during the normal menstrual cycle over an extended period of time. The right eyes of 6 young women ranging in age from 19 to 22 years, were measured for corneal thickness and curvature over a 2-month period. 5 readings of corneal thickness and 3 readings of corneal curvature were taken 3 times per week on the same day and hour for each subject. Each subject maintained a calendar of menstrual activity for the 2-month period and submitted it on completion of the experiment. The testing days were converted to the days of the menstrual cycle by matching chronological dates. The mean menstrual cycle length was 27.5 days. The mean menstrual phase was 5.5 days. Results from the data collected showed that the mean corneal thickness curve during 1 menstrual cycle closely paralleled the bimodal nature of the estrogen levels in plasma. A relationship of the corneal curvature change to corneal thickness change, of the levels found here (approximately 9%), was lacking. This study demonstrated that there is a significant change in corneal hydration during the normal menstrual cycle. Apparently, the change is associated with the effects of estrogen more than with progesterone. The authors believe one might assume ready access of these hormones via the aqueous humor or tear film because of their high lipid solubility. They suggest that another possibility would be an indirect action of hormones on the cornea via their effects on tear film osmolarity.^ieng