Refractometry is a method of ophthalmology for objective visual acuity testing (visual acuity testing). It involves determining what additional refractive power is needed to obtain a sharp image on the retina. The human eye is shaped approximately like a sphere and contains a complex optical system. In emmetropia (normal vision), the eyeball is about 24 mm long and the total refractive power of the accommodative (set for distance vision) eye is about 58 dpt, of which the greater part is accounted for by the refractive power of the cornea (43 dpt) and the lens (20 dpt). Light rays emanating from a fixed point in the environment are focused by the optical refractive media of the eye and imaged as accurately as possible on the fovea centralis (point of sharpest vision on the retina). In case of deviations from the normal state, such as short or long eyeball or refractive power changes, ametropia (defective vision) occurs. The image point constructed by the eye is shifted in front of or behind the retinal plane, resulting in a blurred image on the retina, and the patient thus perceives a reduction in visual acuity. In principle, there are subjective and objective methods for determining visual acuity. The subjective methods always involve the cooperation of the patient and are therefore difficult or impossible to perform in young children or uncooperative patients.This is where objective methods such as refractometry find their use.
Indications (areas of application)
Refractometry is indicated for visual acuity testing and, if necessary, determination of ametropia (defective vision). Ametropia (myopia (nearsightedness); hyperopia (farsightedness); astigmatism (astigmatism)) can be caused either by deviations in the axial length of the eye (axial ametropia) or by changes in refractive power (refractive ametropia). Refractometry is an objective method of visual acuity testing because it can be performed independently of patient information. Thus, it has the following applications:
- Performed prior to subjective fine-tuning of spectacle prescription. The doctor can thus estimate the ametropia in advance and limit himself to a narrow range of lenses when fitting glasses, which is, among other things, very time-saving.
- Children with strabismus (strabismus) or with suspected strabismus.
- Persons with unreliable information
In cooperative patients, refractometry alone is not sufficient as a method of visual acuity testing. A subsequent subjective method is always more accurate and allows systematic optimization of lens power using the patient’s information.
Contraindications
There are no known contraindications to performing refractometry.
Before the examination
Children cannot voluntarily relax the ciliary muscle (contraction of this parasympathetically innervated ciliary muscle causes accommodation). Therefore, they should be given cyclopentolate eye drops before the examination to eliminate accommodation (adjustment of the refractive power of the eye).
The procedure
The principle of refractometry is based on observation of a test figure projected onto the patient’s retina. If this is seen to be in focus by the examiner, it is assumed that the patient can also perceive it as being in focus. Manual refractometer:
- A test figure is imaged through the pupil on the patient’s retina.
- The examiner views the retina through an ophthalmoscope (eye mirror).
- The image of the test figure is brought into focus on the retina. This can be achieved in two different ways: Changing the distance between the test figure and the eye or by placing lenses in front of the beam path.
- The determined values (distance of the test figure or lens power) determine the refraction.
Automatic refractometer:
- The focusing of the image on the retina is done automatically with the help of a computer.
- Nowadays, automatic devices are used almost exclusively.
Possible complications
No complications are expected with refractometry. When using cyclopentolate eye drops, adverse drug reactions or contraindications should be considered.
