Astigmatism is divided into regular and irregular.
In regular astigmatism the curvature of the cornea is greater in one meridian than in another, whereas in irregular astigmatism the curvature varies in the different sectors of the same meridian.
Regular astigmatism is divided into simple hyperopic, compound hyperopic, simple myopic, compound myopic and mixed.
The meridians of maximum and minimum curvatures are always at right angles to each other, and most usually are the vertical and horizontal. They are known as the principal meridians, the vertical being generally that of the greatest curvature. The intermediate meridians between the principal meridians are of regularly intermediate refracting power. The effect on a pencil of rays passing through an astigmatic eye whose vertical meridian is that of the greatest curvature is shown in Fig.18 by various sections supposed to be thrown on a screen placed at varying distances from the cornea.
Remembering that the rays passing through the vertical meridian are most sharply refracted, we have at A, not a round section, but a horizontal oval. At B the rays passing through the vertical meridian have come to a point, and those from the horizontal meridian form a horizontal line. Beyond this, the vertical rays diverge, having crossed at the focus, while the horizontal diffusion decreases, giving rise to the section at C and later, when the two are equal, a circle as at D. The figure next becomes a vertical oval, as at E and later, when the horizontal rays come to a focus, a vertical line at F. Finally a vertical oval as at G.
The interval between the foci of the two principal meridians is called the focal interval of Sturm.
The position of the retina with reference to the two principal foci designates the kind of astigmatism. Thus in simple hyperopic astigmatism one focus is situated upon the retina and the other behind; in compound hyperopic both are behind; in simple myopic one is situated upon and the other in front; in compound myopic both are in front; in mixed one in front and the other behind.
CAUSES.-The seat of astigmatism is usually the cornea but it may also be present in the lens, and when this is the case it may neutralize some of the corneal astigmastism. Sometimes, however, it augments it. Lentil astigmatism is often compensatory and is produced by localized contractions of the ciliary muscle. Astigmastism may also be produced by an oblique position of the lens. Operations upon the cornea frequently produce it by the contraction of the cicatrix formed by the healing of the incision.
SYMPTOMS.-From what has been said it will be easy to understand the difficulties under which an astigmatic individual labors in appreciating horizontal or vertical lines, depending upon the kind of astigmatism present. As letter-press is composed for the most part of horizontal and vertical lines, and as the astigmatic eye is unable to clearly recognize at the same moment both kinds of lines in the same plane, considerable difficulty in reading letters is experienced because the circles of diffusion which form in one direction cover the distinct images which are formed in the other.
If parallel rays from a point enter an eye which is emmetropic in the vertical meridian and hyperopic in the horizontal, those rays which enter the former meridian will focus at a point on the retina, while those which enter the latter will form horizontal diffusion lines at either side. As a line is made up of an infinite number of points such an eye would appreciate horizontal lines much clearer than vertical ones, because the lines of diffusion would not materially affect horizontal lines except to elongate them.
These facts are utilized in the diagnosis of astigmatism by the use of the ordinary “clock face” test-type.
They also explain why astigmatic persons often partly close their eyelids to shut out the rays from one meridian and incline their heads to one side or the other to bring the other principal meridian to correspond to the slit-like palpebral opening. For like reasons a stenopaic slit improves the vision of astigmatic individuals.
Persons with hyperopic astigmatism frequently bring objects at which they may be looking very near their eyes to increase the visual angle. Astigmatism is the cause of a very large percentage of headaches and gives rise to a number of nervous troubles of a reflex nature. Chorea and epilepsy have been cured by correcting it with proper glasses.
Frequently the weaker degrees give rise to more of these troubles than the higher, owing to the constant efforts of the ciliary muscle to overcome the error.
The presence of astigmatism can often be determined by the ophthalmoscopic appearance. An observer has difficulty in seeing both vertical and horizontal vessels simultaneously, and must alter his accommodation to see first one and then the other. The optic papilla, instead of being circular, appears oval. In the direct examination the long axis corresponds to the meridian of greatest curvature and in the indirect to the least.
TREATMENT.-As spherical lenses refract light equally in all meridians, it is evident that they cannot correct the differences in the refractive powers of the two principal meridians in astigmatism. This can only be corrected by cylindrical lenses, which are sections of cylinders parallel to their axes. Such lenses refract light in one direction only, viz., that at right angles to their axes. Thus simple hyperopic and myopic astigmatism, where one meridian is emmetropic and the other hyperopic or myopic, are corrected by convex or concave cylinders with their axes corresponding to the emmetropic meridian.
Cases of compound hyperopic and compound myopic astigmatism, where both foci are either behind or in front of the retina, are corrected by convex or concave sphericals which render one meridian emmetropic and partially correct the other, combined with convex or concave cylinders which correct the remainder.
Mixed astigmatism, where the retina is situated between the two foci, requires a combination of a convex and a concave cylinder placed at right angles to each other which set back one focus and advance the other.
A variety of combinations of spherical with cylindrical lenses is possible which are optical equivalents.
In testing the compound forms of astigmatism, it is the rule to correct as much as possible with spherical lenses and the balance with cylinders.
As in simple myopia and hyperopia, the weakest concave and the strongest convex glasses which render distant vision most distinct represent the degree of the error. In prescribing glasses, it is the general rule to fully correct the astigmatism with cylinders, but the sphericals may be weakened to suit the accommodation. The general rules governing their selection in hyperopia and myopia apply in astigmatism. In simple hyperopic or myopic astigmatism, the strongest convex and weakest concave cylinders which improve distant vision most are selected. In compound hyperopic the spherical may be weakened, and in compound myopic this is frequently necessary, especially for near work.
Mixed astigmatism ordinarily receives the full correction. As a general rule, all cases of astigmatism ought to be thoroughly tested with the accommodation paralyzed.
Irregular Astigmatism.-A low degree of this defect occurs in the majority of eyes. This is often more manifest when the pupil is dilated, or when the eye is being tested under atropine. It will be found impossible to bring the vision up to what it was before the mydriatic was instilled.
Higher degrees reduce the vision very much. The stenopaic hole increases vision, but such spectacles are impracticable on account of their small field. Sometimes one meridian of regular curvature can be found, and, if so, the vision is benefited, by means of a cylindrical lens, which can be prescribed. Irregular astigmatism is frequently produced by the cicatrices of ulcers of the cornea. The congenital form is due to irregular refracting power in different parts of the lens.
Anisometropia.-By this term is meant a difference in the refraction of the two eyes, one being more hyperopic or myopic than its fellow, or a different form of ametropia existing in each eye.
When the different is slight, it is usually possible to fully correct each eye. When the difference is considerable, an attempt may be made to do so, but if it is impossible the stronger glass should be weakened. Sometimes the choice of eyes to be corrected lies with the vision, the best eye receiving the proper correction. Again, it may be advisable to correct one for distance and the other for near.
Each case is usually a law unto itself, and should be dealt with accordingly. The difficulties are usually due to the absence of binocular vision and the prismatic effects of the correcting lenses.
Presbyopia.-There is a diminution in the amplitude of accommodation, which commencing at an early age, progresses with advancing years. It is caused chiefly by a progressive loss of elasticity of the lens, and the different layers becoming more homogeneous. Late in life the ciliary muscle becomes less powerful, and this adds to the difficulty. The effect of this progressive diminution is to cause the near point to recede from the eye. From the tenth year there is a steady decline in the dynamic refraction and a relative recession of the near point. This is shown diagramatically in Fig. 19 devised by Donders.
Professor of Ophthalmology in the College of the New York Ophthalmic Hospital; Surgeon to the New York Ophthalmic Hospital. Visiting Oculist to the Laura Franklin Free Hospital for Children; Ex-President American Homoeopathic Ophthalmological, Otological and Laryngological Society. First Vice-President American Institute of Homoeopathy : President Homoeopathic Medical Society of the State of New York ; Editor Homoeopathic Eye. Ear and Throat Journal : Associate Editor. Department of Ophthalmology, North American Journal of Homoeopathy, etc.