Skiascopy

SKIASCOPY, or the "shadow test" (from the Greek skia—shadow and skopeo—I examine), is an objective method for determining the refraction of the eye, proposed in 1873 by Cuignet under the name "keratoscopy"; it is also called "retinoscopy." Due to its simplicity and accuracy, this method is of great practical importance, allowing refraction to be determined regardless of the patient's own reports. Therefore, skiascopy is especially valuable when examining the refraction of children, malingerers, and subjects with reduced intelligence. The essence of skiascopy is as follows. If a beam of light is directed into the pupil of the eye being examined using a flat or concave mirror, it will illuminate a certain area of the fundus, corresponding to the shape and size of the pupil. To the observer, the pupil will appear uniformly red. If one now, while continuing to observe the pupil through the aperture of the mirror, slowly rotates the latter around its vertical axis, the illuminated spot on the fundus will shift to the side, which will manifest as the appearance of a crescent-shaped shadow from the opposite edge of the pupil, which will gradually expand upon further rotation of the mirror until the entire pupil becomes dark. At the same time, if the rotational movements of the mirror are performed sufficiently quickly, it will seem to the observer that some kind of shadow is running across the pupil, which in some cases moves in the same direction as the mirror is rotating, and in others—in the opposite direction. The method is based on observing the nature of this shadow's movement, since the direction of this movement itself is closely dependent on three factors: 1) the refraction of the eye being examined, 2) the properties of the mirror used, and 3) the distance between the subject and the examiner.

The dependence of the nature of the shadow's movement on the refraction and the properties of the mirror used can be formulated by the following basic principles: 1. When examining an eye with a flat mirror, the shadow moves in the opposite direction if the far point of clear vision lies in the space between the subject and the examiner's eye; it moves in the same direction as the mirror if the far point of clear vision of the eye being examined lies either behind it or behind the examiner's eye. 2. When examining with a concave mirror, the results are the opposite of those stated. 3. If the examiner's eye is placed exactly at the far point of clear vision of the subject, the appearance of a shadow is not observed at all, or it is of an indeterminate nature, regardless of which mirror is used for the examination. The theoretical justification for skiascopy can be presented as follows: if the examination is conducted using a concave mirror with a focal length of 15–20 cm and from a distance of 1 m, the rays falling from the light source onto the mirror are reflected from the latter and converge in front of it at the focus, where an aerial, inverted image of the light source is obtained, which in turn becomes the source of illumination for the eye being examined. The light rays emerging from here pass through the round aperture of the pupil and illuminate a part of the fundus in the form of a small circle. When the mirror is turned in one direction or another, the image of the flame located at the focus will move in the same direction, while the illuminated area on the fundus, due to the inversion of optical refraction, will move in the opposite direction, and a shadow will advance into its place. This is what happens in reality; to the examiner, however, the movement of light and shadow will appear differently depending on the refraction of the eye being examined. The path of the rays in myopia is shown in Fig. 1. The rays coming from the light source A fall on the mirror, which is in position 1, are reflected from it, and converge at its focus a, from where they go into the eye being examined, where they illuminate a certain area b. Since the eye being examined is myopic, the light rays, upon exiting the eye from b, will take on a converging direction and converge at the far point of clear vision of this eye, for example at c. From c, they will then go in the form of a diverging beam through the aperture of the mirror into the observer's eye and illuminate an area d on its retina. If the examiner now moves the mirror from left to right (one must imagine all parts of the drawing arranged not in a vertical, but in a horizontal plane), i.e., gives it position 2, the following will occur: the image of the flame a will move to a', the illumination in the eye being examined will move from b to b', the aerial image of the illuminated spot from c to c', and the image of the illuminated spot in the observer's eye (due to optical refraction) from d to d'. Projecting the movement of light (and shadow) that occurred in the observer's eye into space, the latter will, as always, be in the opposite direction, i.e., to c', as a result of which it will seem to him that the shadow (illumination) is moving in the same direction as the mirror. Such a case corresponds exactly to the situation where the far point of clear vision of the eye being examined lies between the subject and the examiner, which is possible only with myopia of the subject greater than 1.0 D (Fig. 2). Figure 2 illustrates the path of rays in emmetropia, hypermetropia, and myopia weaker than 1.0 D.

Figure 1. Scheme of skiascopy. The eye being examined is myopic. 6 71

the observer will, as always, project it in the opposite direction, i.e., from right to left to d'. Consequently, it will seem to him that the shadow (illumination) is moving in the direction opposite to the movement of the mirror. Such a case corresponds to the situation where the far point of clear vision of the eye being examined lies entirely outside the space between the two eyes. Then one of three possibilities remains: either the far point of the eye being examined lies behind the examiner at infinity, if the subject is emmetropic, or it lies behind the examiner at a distance further than 1 m, if the subject has myopia of less than 1.0 D, or finally, it lies in negative space behind the eye being examined, if it is hypermetropic. When using a flat mirror, all relationships are reversed. Practically, skiascopy is performed under the same conditions as ophthalmoscopy (see Ophthalmoscope). It is only necessary to relax the accommodation of the eye being examined (atropinization is usually used), and a flat mirror is used more often. To determine the degree of refraction, it is necessary to keep in mind the basic principle of skiascopy: the disappearance of the shadow occurs if the examiner's eye is at the far point of clear vision of the subject. Based on this, skiascopy is performed either by the so-called neutralization of shadows or by finding the neutral point. In the first case, skiascopy is conducted from a certain distance, most often 1 m. At a distance of 1 m, the disappearance of shadows can be observed either if the subject has myopia of 1.0 D or by bringing any of their refraction to myopia of 1.0 D by placing appropriate optical lenses. In the latter case, the degree of refraction is determined by calculating the power of the placed lenses and correcting for the distance (for myopia over 1.0 D, 1.0 D is added, and for other types of refraction, it is reduced by 1.0 D). The "neutral point finding" method is used less frequently. Its essence boils down to the fact that the disappearance of shadows is achieved here not from a fixed distance, but, on the contrary, by changing the distance from the subject, seeking to find the point at which the shadow disappears. This method is convenient for determining myopia only of weak and moderate degrees, and therefore, in cases of high myopia, one resorts to preliminary partial correction with concave lenses. The degree of refraction is also determined by calculating the power of the placed lens and the distance. Astigmatism is determined by skiascopy.

Figure 2. Scheme of skiascopy. The eye being examined is hypermetropic or emmetropic.

At the initial position of the mirror at 1, the rays from the light source, as in the previous case, converge at a and from there go into the eye being examined, where they illuminate area b. If the eye being examined is hypermetropic, the light rays, upon exiting it, will take on a diverging direction; if the subject is emmetropic, the rays will take on a parallel direction, and thus in both cases, the rays will not converge anywhere in the space between the subject and the examiner. If, however, the eye being examined has myopia weaker than 1.0 D, the rays, upon exiting it, although they will take on a converging direction, will be able to converge only behind the examiner's eye. From this, it follows that in these three positions, an aerial image of the illuminated area cannot be formed in the space between the mirror and the eye being examined: the rays will go directly into the observer's eye and illuminate area d there. If the examiner now moves the mirror from left to right, i.e., to position 2, then a will move to a', b to b', and d to d'.

Figure 3. Skia-

In the first position of the mirror at 1, the rays from the light source, as in the previous case, converge at a and from there go into the eye being examined, where they illuminate area b. If the eye being examined is hypermetropic, the light rays, upon exiting it, will take on a diverging direction; if the subject is emmetropic, the rays will take on a parallel direction, and thus in both cases, the rays will not converge anywhere in the space between the subject and the examiner. If, however, the eye being examined has myopia weaker than 1.0 D, the rays, upon exiting it, although they will take on a converging direction, will be able to converge only behind the examiner's eye. From this, it follows that in these three positions, an aerial image of the illuminated area cannot be formed in the space between the mirror and the eye being examined: the rays will go directly into the observer's eye and illuminate area d there. If the examiner now moves the mirror from left to right, i.e., to position 2, then a will move to a', b to b', and d to d'.

by way of studying the scanto-refraction in each principal meridian. For performing S., ophthalmoscopes with concave or flat mirrors are usually used, attaching optical glasses from eye sets when determining the strength of refraction. In addition, for this same purpose there also exists a whole series of special instruments—skiascopes—mirrors with measuring tapes, with sets of correcting glasses in special frames, mounts. Such are the skiascopes of Antonelli-Lavrentiev (Antonelli) (fig. 3), Roth, Hess, etc.