Abstract
Refractive error (an imperfection of ocular image formation) is one of the most common reasons that patients seek out optometrists. Uncompensated refractive error, which is related to second order wavefront aberrations, causes unwanted blurring of retinal images. Distance refractive error can be affected by the presence of active ocular accommodation. Young patients, in particular, have very active accommodative systems and when active during the measurement of refractive error, can lead to erroneous results. Accommodation theoretically should only be active when a patient regards a proximal or near object; however this is not always the case and therefore pharmaceuticals (cycloplegics) have been employed by ophthalmologists and optometrists to temporarily paralyze the accommodative system in order for refractive error to be evaluated at any distance more accurately.
The primary aim of this study is to use multivariate methods of analysis to evaluate and study the changes in refractive error occurring at distance (a stimulated six metre target) and at near (40 centimetres), as well as exploring the effects of accommodation and cycloplegia on the measured refractive errors. The instruments used included the NIDEK ARK-700 autorefractor and the NIDEK OPD-Scan ARK 10000 wavefront aberrometer. Ten subjects were recruited for this particular study and eight samples were taken per individual under four different experimental conditions (four samples per instrument per subject). These samples were so named; the Distance Samples Without Cycloplegia, the Distance Samples With Cycloplegia, the Near Samples Without Cycloplegia, and the Near Samples With Cycloplegia and are referred to as such throughout his dissertation. These samples were collected with both the autorefractor and the wavefront aberrometer mentioned above and converted, using matrix methods, into scalar coefficients of power and therefore the stigmatic and antistigmatic components are represented as FI (the stigmatic coefficient), FJ (the ortho-antistigmatic coefficient) and FK (the oblique-antistigmatic coefficient). The sample size for each experimental condition was 25 measurements. The samples without cycloplegia were obtained first on both the autorefractor and wavefront aberrometer, and once these samples had been collected, one drop of 1% cyclopentolate hydrochloride was added to each of the subject’s right eyes and once the accommodative system was seemingly paralyzed the distance and near samples under the influence of cycloplegia were taken once again with each instrument...
M.Phil. (Optometry)