Skiascopy

Sciascopia (from the Greek scia - shadow, scope - I examine) is a method of objective investigation of clinical refraction based on observation of the movement of shadows obtained in the pupil area when the latter is illuminated by various methods.

Without going into the essence of the physical phenomena on which the sciascopy is based, the basic position of this technique can be formulated as follows: the motion of the shadow is not observed if the further point of clear vision coincides with the source of pupil illumination, i.e., in fact, with the position of the researcher.

Method of conducting

Skiascopy is carried out according to the following procedure.

The doctor sits opposite the patient (usually at a distance of 0.67 or 1 m), illuminates the pupil of the eye with the mirror of the ophthalmoscope and, turning the apparatus around the horizontal or vertical axis in one direction and the other, observes the nature of the shadow movement against the background of the pink reflex from the fundus in area of the pupil. With a skiascopia with a flat mirror from a distance of 1 m in the case of hypermetropia, emmetropia and myopia is less than -1.0 D, the shadow moves in the same direction as the mirror, and in case of myopia, more - 1.0 D, in the opposite direction. In the case of a concave mirror, the inverse ratio. The absence of motion of a light spot in the pupil area with a skiascopy from a distance of 1 m when using both a flat and concave mirror indicates that the examined myopia has a 1.0 dptr.

In this way, the type of refraction is determined. To establish its degree, the method of neutralizing the motion of the shadow is usually used. When myopia is greater than -1.0 D, negative lenses are attached to the eye, first weak, and then stronger (in absolute value) until the movement of the shadow in the pupil area ceases. In cases of hypermetropia, emmetropia and myopia less than -1,0 dntp, a similar procedure is performed with positive lenses. With astigmatism do the same separately in the two main meridians.

The sought value of refraction can be determined by the following formula:

R = C-1 / D.

Where R is the refraction of the eye in question (in diopters: myopia with the sign "-", hypermetropia - with the sign "+", C - the force of the neutralizing lens (in diopters), D - the distance from which the study is performed (in meters).

Some practical recommendations for performing a skiascopy can be formulated as follows.

• It is advisable to use an electrocascoscope, that is, a device with an installed light source if possible, or, in the absence of it, a flat ophthalmoscope mirror and an incandescent lamp with a transparent balloon (less the area of the light source). When studying with a flat mirror (in comparison with a concave mirror), the shadow is more pronounced and homogeneous, its movements are easier to assess, and smaller mirror turns are required to move the shadow.
• To neutralize the shadows can be used as a special skiascopic rulers, and lenses from the set, which are inserted into the trial frame. The advantage of the latter method, despite the increased study time, is due to the exact observance of the constant distance between the lenses and the apex of the cornea, as well as the possibility of using cylindrical lenses to neutralize the shadow with astigmatism (cylinder cylinder method). The use of the first method is justified in the examination of children, since in these cases the doctor, as a rule, is forced to keep the scapacroscopic rulers before the patient's eye.
• It is advisable to conduct a skiascopy from a distance of 67 cm, which is easier to maintain during the study, especially when determining refraction in young children.
• When examining the eye under conditions of cycloplegia, the examinee should look at the mirror opening, and in cases of safe accommodation - past the doctor's ear on the side of the eye being examined.
• When using a skiascopic ruler, you should try to keep it vertically and at a standard distance from the eye (about 12 mm from the top of the cornea).

In the absence of movement of the shadow when changing a number of lenses for the indicator for calculations, it is necessary to take the arithmetic mean value of the strength of these lenses.

When performing a skiascopia in conditions of medical cycloplegia, which, as noted, is accompanied by the dilatation of the pupil (mydriasis), the following difficulties are possible. The shadow can move in different directions, and the neutralization of the shadow is provided by different lenses in different areas of the pupil - the so-called scissors symptom. This fact indicates an incorrect astigmatism, most often due to the nonspherical shape of the cornea (for example, in keratoconus - corneal dystrophy, accompanied by a change in its shape). In this case, the diagnosis is refined with an ophthalmometer. If this establishes any regularity in the movement of the shadow, for example, a different character in the center and on the periphery of the pupil, then this motion should be neutralized, guided by the movement of the shadow in the central zone.

The unstable, changing nature of the shadow movement during the study, as a rule, indicates a lack of cycloplegia and the possible influence of the accommodation tension on the results of the skiascopy.

Difficulties can arise in the case of a skiascopic examination of the eye with a low visual acuity and, as a consequence, an unstable noncentral fixation. As a result of the constant movement of this eye during the study, the refraction of the non-central region of the retina, rather than the area of the macula, will be determined . In such cases, the leading eye is presented for fixation of an object, moves it and with the help of friendly movements, installs a poorly seeing eye in a position where the light block of an ophthalmoscope or a skiascope will be located in the center of the cornea.

To clarify the refraction with astigmatism, you can use bar-skiascopy, or banded skiascopy. The study is carried out with the help of special ski-scopes having a light source in the form of a strip, which can be oriented in different directions. By setting the light bar of the device in the desired position (so that when it goes to the pupil it does not change), they perform a skiascopy according to the general rules in each of the main meridians found, seeking to stop the movement of the banded shadow.

Cylindroskiascopy

The data obtained with a skiascopy can be refined by cylinder cylinders. Initially, the usual skiascopia with rulers, tentatively determine the position of the main meridians of the astigmatic eye and the power of the lenses, with the use of which stops the movement of the shadow in each of them. The patient is put on a trial rim and a spherical and astigmatic lens is placed in the nest opposite the eye being examined, which should ensure the cessation of the movement of the shade simultaneously in both main meridians, and carry out a skiascopy. The cessation of the movement of the shadow in one direction and the other indicates that the refractive index is correctly determined. If the shadow moves not in the direction of the axis of the cylinder or its active cross-section, but between them (more often approximately at an angle of 45 ° to them), then, the axis of the cylinder is set incorrectly. In this case, rotate the cylinder, placed in the frame until the direction of movement of the shadow is the same as the direction of the axis.

The main advantage of skiascopy is its accessibility, since complex research is not required for the study. However, certain skills, experience and qualifications are required to perform a skiascopia. In addition, in a number of cases (for example, with astigmatism with oblique axes), the informative nature of the technique may be limited.