A study to determine the use of cross cylinder in conjunction with the cross grid at distance as an alternative method for the duochrome technique amongst University of Limpopo optometry students

Abstract

BACKGROUND The measurement of the refractive end point plays a major role in the assessment of the refractive status of the patient. Currently, there are very few techniques if not one that can measure the refractive end point of the patient. The current method which is the duochrome technique is based on chromatic aberration. Until now there is no test for refractive end point that is not based on chromatic aberration. Therefore, it was against this background that the current study was undertaken to investigate an alternative for the duochrome technique to measure the refractive end point.

AIM OF THE STUDY The aim of this study was to establish the use of cross grid in conjunction with Jackson crossed cylinders at distance as a monocular refractive end point technique.

METHODOLOGY A cross-sectional analytic and descriptive study design was used. Sixty-four subjects (31 males and 33 females) were included in this study. Their ages ranged from 18 to 37 years with a mean of 20.75 years (SD = ± 2.67 years). The participants were University of Limpopo Optometry students. Ethical approval to perform this study was obtained from the Turfloop Research Ethics Committee (TREC) of University of Limpopo. All participants were made aware of the purpose of the study, and signed consent was obtained from each participant. All investigations and measurements adhered to the tenets or principles, belief and requirements of the Declaration of Helsinki. Optometric procedures performed included visual acuity, subjective measurement of refractive error, monocular refractive end point using duochrome technique and monocular refractive end point measurement using gross grid in conjunction with the cross grid. Data analysis was done using the Statistical Package for Social Sciences (SPSS) Version 23. Paired sample t-test was performed on all the procedures.

RESULTS The uncorrected distance VA for the right eyes (OD) ranged from -0.30 LogMAR to 1.00 LogMAR with a mean of -0.25 LogMAR (SD = ±0.32). The nearest equivalent spherical powers (NSE = sphere +0.5 cylinder) for the right eye ranged from -4.25 to +4.25D (mean = -0.13, SD= ±1.09D) and from -4.50D to +1.00D (mean = +0.07 ±0.94D) for the left eye. After the removal of outliers, the refractive end point measurements with the duochrome technique for the right eyes ranged from -4.50D to 3.50D with a mean of -0.269, SD (±1.037) in bright illumination and from -4.50D to 4.50D with a mean of -1.914 (±1.101) in dim illumination. Subsequently, the NSE with the crossed cylinder in conjunction with the cross grid technique for the right eyes in bright illumination ranged from -4.75D to 4.75D with a mean of -0.481, SD (±1.037) and from -4.25D to 0.50D with a mean of -0.427 (±0.860) in dim illumination.

Sample Pearson correlation coefficients for end point with duochrome and crossed cylinder found that the highest correlation is between the duochrome in bright and cross grid in dim illumination (r = 0.701, p < 0.05), while duochrome in dim and cross grid in bright illumination had the lowest correlation (r = 0.659). The Bland–Altman plots showed that there was a good agreement between the duochrome methods and between the crossed cylinder method methods.

CONCLUSION the refractive end point measurements obtained from duochrome and cross grid are well correlated and comparable, suggesting that they could be used interchangeably in most clinical settings. However, caution is needed when using measurements obtained by cross grid method in dim illumination.