Page 30 - Practical-Refraction-English

Supplement: Supplement Estimating the Refractive The Jackson Cross Cylinder Error from the Level of Determining astigmatism by the cross cylinder method was made popular at the beginning of the 20th century Uncorrected Distance Vision by American ophthalmologist Edward Jackson (1856- the spherical equivalent of which is plano. The principle may be estimated from their level of uncorrected dis- of the technique is to place the cross cylinder in front of 4 The spherical equivalent of a patient’s refractive error 1942). The cross cylinder is a sphero-cylindrical lens, tance vision. Although attributed in France to the the patient’s eye and study the variations in vision that English physicist and optometry teacher William Swaine (1894-1986) and called Swaine’s Rule, the rule is not result from the combination of the astigmatism of the known by this name in the English-speaking world. The eye and that of the cross cylinder placed in different positions. rule stipulates that the level of vision is reduced by one step on an inverse scale (1/1, 1/2, 1/3, ¼, etc.) for each Practical Refraction A cross cylinder is a lens that combines two plano-cylin- drical lenses of identical powers but with opposite signs. Level of Vision Level of Vision Expected Ametropia The cylinders’ axes are perpendicular to each other (decimal) (inverse scale) (spherical equivalent) (hence the name ‘cross cylinder’). A ±0.25 cross cylin- 1.00 1/1 0.25 D der (that is, a cross cylinder created by combining +0.25D and -0.25D cylinders) is a +0.25/-0.50 lens; a 0.50 1/2 0.50 D ±0.50 cross cylinder is a +0.50/-1.00 lens. This lens is 0.33 1/3 0.75 D mounted in a special frame, the handle of which bisects 0.25 1/4 1.00 D the cylinder axes, such that the positive and negative axes of the cross cylinder may be easily swapped, by 0.20 1/5 1.25 D twisting the handle (Figure 27). 0.16 1/6 1.50 D 0.14 1/7 1.75 D When placed in front of the patient’s eye and so combi- ned with the astigmatic eye, the cross cylinder accen- 0.12 1/8 2.00 D tuates or reduces the astigmatism and consequently 0.11 1/9 2.25 D causes variation in the level of the patient’s vision. The 0.10 1/10 2.50 D cross cylinder is twisted and so its two positions presen- ted to the patient, who is then asked to indicate which 0.25DS of spherical refractive error. Thus a myopic position gives the better vision. The cross cylinder is patient with refractive error of -0.50 D is expected to used in two different aspects of subjective refraction: have uncor rected vision of approximately 1/2 (0.5), a determining the cylinder axis and the cylinder power of myopic patient with refractive error -0.75 D uncorrected the patient’s refraction. vision of 1/3 (0.3) and so forth (see table). Detailed procedures for using the cross cylinder are des- This rule allows the practitioner to estimate the patient’s cribed above. refractive error from their level of uncorrected distance vision and hence have an idea of the expected prescrip- tion before starting objective and subjective refraction. Also, during refraction using the fogging method, (which involves rendering the patient effectively myopic by the addition of a plus lens), the rule can be used to assess the spherical blur created and thus anticipate the value of the final ametropia of the subject. For example, if during the initial fogging, the patient's vision is 1/6 (0.16), it can be estimated that the patient’s ametropia is equal to the value of the fogging sphere – (6 x 0.25D) = +1.50 – (1.50D) = plano; if vision is 1/5 (0.20), the patient’s ametropia is ~ +1.50 – (5 x 0.25D) = +1.50 - 1.25 = +0.25D. The rule allows changes in vision to be monitored during fogging and removal of the fog. This rule is most effective for myopic refractive errors and less consistent for hypermetropia or astigmatism. It is not always a precise rule but may be used as a good guide, allowing the practitioner to assess the consisten- cy of the expected level of the patient’s ametropia with the final refraction found. Figure 27: Jackson Cross Cylinders 30 Copyright © 2008 ESSILOR ACADEMY EUROPE, 13 rue Moreau, 75012 Paris, France - All rights reserved – Do not copy or distribute.