Astigmatism and Errors of Accomodation MCQs

Cross cylinder examination (otherwise known as Jackson's cross cylinder, JCC) is an examination used to refine the astigmatic refraction by fine-tuning the axis and strength of astigmatism. While a rough estimate can be obtained via retinoscopy or automated refractometry, cross-cylinder examination remains invaluable in fine-tuning the refraction for best vision. It confirm the axis and power of cylinder both Eg of types of JCC 0.25 JCC = +0.25DS/-0.50DC X 90 0.50 JCC = +0.50DS/-1.00DC X 90 1.00 JCC = +1.00DS/-2.00DC X 90 Reference: Khurana Optics 2/e

Rather in uncorrected myopia presbyopia occurs late, as they already have a good near vision without glasses Reference: Khurana Optics 2/e

In the young, the condition may cause no symptoms. When symptoms are present or arise, they are chiefly referable to the abnormal amount of accommodation to which these eyes are subjected, and to the lack of balance between accommodation and convergence. A healthy youth has an ample reserve of accommodation, and if hypermetropic, can accommodate for distant and near objects without being conscious of the act. If the youth is in poor health or does much near work the perpetual overaction of the ciliary muscle is likely to produce symptoms; the condition is often called accommodative asthenopia or 'eye strain' Reference: Parson ch 8, P. 79

When eyewear prescriptions are written, they can be classified into different areas depending on the power or refractive error. There are seven categories to which prescriptions can fall: Simple Hyperopia, the Rx will be (+) sphere Simple Myopia, the Rx will be (-) sphere Simple Myopic Astigmatism, the Rx will be plano (-) Simple Hyperopic Astigmatism, the Rx will be (+) Compound Hyperopic Astigmatism, major meridian power will be (+)(+) on both 90 and 180 degree axis Compound Myopic Astigmatism, major meridian power will be (-)(-) on both 90 and 180 degree axis Mixed Astigmatism, major meridian powers will be opposites (-)(+) (+)(-) on both 90 and 180 degree axis

Astigmatism WTR is produced when the corneal curvature is steepest in the vertical meridian; conversely astigmatism ATR is produced when the steepest corneal meridian is horizontal. Reference: Khurana Optics 2/e

The cornea is at first transparent and the vision is impaired due to myopic astigmatism. The patient becomes myopic, but the error of refraction cannot be satisfactorily corrected with ordinary glasses owing to the parabolic nature of the curvature which leads to irregular astigmatism in the later stages. Reference: Parson’s diseases of the Eye-Chap 12 – Diseases of Cornea

If the astigmatic error is small (less than 0.5 D) and not associated with symptoms, spectacles are unnecessary unless the highest visual acuity is desired. However, in all cases in which astigmatism causes asthenopic symptoms, full optical correction should be ordered for constant use, that is, both for distant and near vision. Other measures are rigid contact lenses, Toric contact lenses, astigmatic LASIK (1.5-5 D), arcuate keratotomy and Toric phakic IOLs. Reference: Parson ch 8, P. 82

Circle of least confusion/diffusion: The smallest cross-section of a blur circle between the two focal lines formed by the astigmatic lens. Also defined as “At the dioptric midpoint of focal lines the cross-section of sturm's Conoid appears as a circular patch of light rays with best overall focus. If the retina is placed at the ” Circle of least diffusion” of Strum's conoid, the distant vision is often found to be surprisingly good with a relatively high degree of mixed astigmatism. Reference: Khurana Optics 2/e