Eye Optics and Refractive Errors: Examination Methods

Examination Methods

Refraction Testing

Refraction testing means measuring the additional refractive power required to produce a sharp image on the retina. Subjective and objective methods are used. Subjective methods require information from the patient.

Subjective refraction testing: 

This consists of successively placing variouscombinations of lenses before the patient’s eye until the maximum visual acuity is reached (see Correction of Refractive Errors).

Objective refraction testing: 

Objective testing is unavoidable when thepatient is unable to provide subjective information (for example with infants) or when this information is unreliable. This method also greatly accelerates subjective refractive testing.

Retinoscopy (shadow testing): The retina is illuminated through the pupil.The examiner observes the optical phenomena in the patient’s pupil while moving the light source (Fig. 16.7).

Refractometry.The measuring principle is based on ophthalmoscopic obser-vation of a test image projected on to the patient’s retina. The distance between the test figure and the eye is changed until the image appears in focus on the retina. Refraction can then be calculated from the measured values. An alternative to changing of the distance is to place various lenses in the path of the light beam.

Automated refractometry.The method measures refraction automaticallywith the aid of light-sensitive detectors and a computer until a focused image appears on the retina. These systems operate with infrared light.

Any objective measurements of refraction should be verified by subjec-tive testing whenever possible.

Testing the Potential Resolving Power of the Retina in the Presence of Opacified Ocular Media

Special examination methods are indicated in the presence of opacification of the ocular media of the eye (such as a cataract) to determine the potential visual acuity of the retina. This permits the ophthalmologist to estimate whether optimizing the refractive media with techniques such as cataract surgery or corneal transplantation would achieve the desired improvement.

Laser interference visual acuity testing: 

Lasers are used to project inferencestrips of varying widths on to the retina. The patient must specify the direc-tion in which these increasing narrower strips are aligned. This examination can no longer be performed where there is severe opacification of the optic media such as in a mature cataract. The preliminary examination then con-sists of evaluating the pattern of the transilluminated retinal vasculature.

Fig. 16.7 With the retinoscope, the examiner moves a light source (a beam of yellow light) across the pupil (dark spot) at a distance of about 50 cm from the patient. This pro-duces a light reflex (red spot) in the patient’s eye. It is important to note how this light re-flex (red spot) behaves as the light source of the retinoscope is moved. There are two possibilities:

a “With” motion: the light reflex in the pupil (red spot) moves in the same direction (redarrows) as the light source of the retinoscope (yellow arrows). This means that the far point of the eye is behind the light source. b “Against” motion. The light reflex in the pupil moves in the opposite direction (red arrows) to the light source of the retinoscope (yellow arrows). This means that the far point of the eye lies between the eye and the light source. The examiner places appropriate lenses in front of the patient’s eyes (plus lenses for “with” motion and minus lenses for “against” motion) until no further motion of the light reflex is observed. The motion of the retinoscope will then only elicit an infinitely fast reflex (neutral point). This method is used to determine the proper lens for correct-ing the refractive error.