Skiascopy is used to determine objective refraction and is used primarily on children. The heart of the skiascope is a translucent mirror that casts an image onto the back of the eye. Prior to skiascopy, the ciliary muscle is paralyzed with medication.
What is skiascopy?
Skiascopy is used to determine objective refraction of the eye and is used primarily on children. Refraction is the refractive value of optical corrections that provides the eye with sharp images from an infinite distance from a viewed object entirely without accommodation. Thus, refraction is the reciprocal of the so-called focal length. In this context, normal vision or emmetropia is always referred to when the refraction is zero. For other values, the ophthalmologist speaks of ametropia. These forms of ametropia are caused by a refractive anomaly or a change in axial length. Skiascopy can be used to determine the refraction of the human eye. The procedure is also known as shadow testing and is used to determine refraction, especially in children. The heart of the skiascopy is a skiascope with a translucent mirror. The examiner uses this mirror to illuminate the pupil of the eyes. When he moves the mirror, the shadows move. As the light beam moves, the physician observes the shadow migration within the pupil. Skiascopy determines an objective refraction value and thus differs from the so-called adjustment or spectacle adjustment, in which the subjective refraction is determined. The subjective determination of refraction is much less time-consuming than the objective determination.
Function, effect and goals
There are different procedures for determining refraction, which are grouped under the umbrella term of refractometry. A majority of these procedures are used to determine the value objectively. In most cases, objective determination methods for refraction are based on infrared projection. An object is projected by the physician onto the back of the eye. This creates a visible and measurable image that can be focused by the physician using adjustable lenses. Skiascopy is also known as retinoscopy or shadow test in connection with objective refraction determination. In this procedure, the object to be imaged corresponds to a virtually infinite light source. When this image is focused, uniform illumination of the entire fundus of the eye occurs. Only simple means are required for skiascopy, mainly the skiascope and certain measuring glasses and measuring glass strips. The time required for the procedure is relatively long. In addition, great experience is required from the examiner. In addition to the skiascope, there are other instruments for determining objective refraction. One of them is the autorefractometer. These devices automatically project the image themselves and then focus it via photosensors, i.e. they are capable of independent image processing. Much less experience is required of the examiner when working with these devices than when working with skiascopes. Another advantage of autorefractometers is their speed. On the other hand, the high price of the devices is a disadvantage. Skiascopes are significantly less expensive. In addition to these instruments, manual refractometers can be used for objective refraction determination. They do not automatically focus the image marks and are much less expensive than automatic instruments. The result of accurate manual refractometers is relatively accurate. However, these instruments will probably cease to exist in the foreseeable future as automatic devices increasingly replace them. In any objective refraction measurement, accommodation of the eye is a potential source of error. Accommodation could sometimes severely distort the results. Therefore, a cycloplegia is usually performed before determining the objective refraction. This is the complete paralysis of the ciliary muscle, which deprives the eye of the ability to accommodate. Paralysis of the muscle is induced by medication. Cycloplegics administered with this goal correspond to drugs from the group of parasympatholytics. They inhibit the parasympathetic nervous system and thus cause enlargement of the pupil in addition to ciliary paralysis.
Risks, side effects and dangers
If the paralysis of the eye muscle is not included in the skiascopy, the results are not evaluable in many cases. However, adverse side effects do occur in some cases when using the medications. Sometimes the most common side effect of parasympatholytics is dry mouth, which is associated with all agents in this drug group. In studies, the incidence for this side effect is reported to be 30 percent. The substances used are either from the tertiary amine subgroup or the quaternary ammonium subgroup. Quaternary ammonium ions have a positive charge. This distinguishes them from tertiary amines, which, unlike quaternary ammonium ions, behave lipophilically instead of hydrophilically. Lipophilic drugs can pass through the blood–brain barrier and thus enter the cerebrospinal fluid. As a result, they can cause side effects in the central nervous system. These central nervous side effects of the substances include, above all, sleep disturbances. In addition, memory disorders, but also hallucinations and states of confusion are possible consequences of ingestion. In the case of quaternary ammonium compounds with hydrophilic properties and a positive charge, the blood–brain barrier represents an insurmountable barrier. However, if a patient suffers from damage to the blood–brain barrier from the outset, these substances also enter the central nervous system to an appreciable extent. In comparison, tertiary amines are subject to higher absorption from the gastrointestinal tract. Because the activity of the drug-degrading liver enzymes can be stimulated or inhibited by many substances, a potentiating or reducing effect may occur with concomitant use of parasympathetics and other drugs.