There are no serious deterioration of the visual acuity, and no symptoms of asthenopia and eyestrain, the smaller astigmatic errors do not require correction. But if either of those two conditions is present, the error should be corrected by lenses.
The correction of the larger astigmatic. errors necessarily produces appreciable difficulties. Anyasymmetry between the correction of two eyes leads to an artificial heterophoria on looking any dif-ectioh other than through the optical cerntres of the two lenses. The greatest discomfort will arise from the correction of astigmatism with oblique axis which must introduce differences in the disparities of the images of the two eyes. In some cases declinations and size-differences may be partially compensated by cyclotorsions of the eyes, aniseikonic compensation in fusion and a compensating psychological mechanism; in others the visual sensations are profoundly distressing.
A. Retinoscopy in astigmatism
In the hands of a skilled and careful exponent a very high degree of accuracy can be attained, in favourable circumstances of the order of 0.25D in power and 5 degrees in the axis of astigmatism. There are two methods; (1) The first method consists of using spheres exclusively. Each principal meridian is retinoscoped using a sphere and the findings are combined into sphero-cylinder form. (2) The second method uses spheres and cylinders. The most myopic (or least hyperopic) meridian neutralized with a sphere and then plus cylinders are used to correct the other principal meridian.
B. Cross cylinder test
The cross cylinder is a simple refinement and is of extreme practical value to check both the power and-axis of the correcting cylinder, and to determine the amount of astigmatism.
C. Astigmatic figures
In regular astigmatism lines which run in the same direction as the meridian of the greatest refractive error appear sharply defined; lines which run at right angles to this are blurred, and lines running intermediately vary progressively between the two extremes.
A line therefore appears sharply outlined when it is accurately refracted in the meridian at right angle to it. This principle is used in a test for astigmatism when the patient observes suitably constructed figures.
D. Stenopaic slit
In discovering astigmatism and finding the axis of the cylinder, a stenopaic slit may be useful. When it is rotated with the optimal spherical correction in place it will not affect the vision of a stigmatic eye, but will improve the acuity, particularly in one meridian, if astigmatism is present.
Since the purpose of the practice of refraction is to make patients comforable and efficient rather than to strive after theoretical optical perfection, an under-correction of cylinder may be advisable in case with a high degree of astigmatism has never worn glasses previously. In this endeavour, instead of eliminating the interval of Sturm and reducing the two focal lines to a point focus, the focal interval should be reduced so that the circle of least diffusion is reasonably small, and then the position of this circle should be moved so that it lies upon the retina. It is necessary to prescribe a partial equivalent of the astigmatism.
The determination of and correction for astigmatism should never be done in the presence of a chalazion, which may cause an abnormal curvature of the cornea. The axis of the astigmatism may also differ slightly when the cycloplegic findings are compared with those of the postcycloplegic test or on accommodation. In case of different between cycloplegic and noncycloplegic findings, a compromise should be arrived at depending upon the reliability of the subjective refraction.
Irregular astigmatism caused by corneal scars, conical cornea and the like may be detected by the retinoscopic image, the irregular cornea reflecton of the concentric circles of Placido´s disc. or the keratometric image. Such astigmatism can best corrected by the use of contact lens.
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