CHAPTER 3
Clinical Refraction
Objective Refraction Technique: Retinoscopy
Although autorefractors are easily accessible, retinoscopy remains an important skill and tool for the ophthalmologist to objectively determine the spherocylindrical refractive error of the eye. A retinoscope can also help the examiner detect optical aberrations, irregularities, and opacities, even through small pupils. Retinoscopy is especially useful for examinations of infants, children, and adults unable to cooperate.
Most retinoscopes in current use employ the streak projection system developed by Copeland. The illumination of the retinoscope is provided by a bulb with a straight filament that forms a streak in its projection. The light is reflected from a mirror that is either half silvered (Welch Allyn model) or totally silvered around a small circular aperture (Copeland instrument) (Fig 3-1). The filament light source can be moved in relation to a convex lens in the system. If the light is slightly divergent, it appears to come from a point behind the retinoscope, as if the light were reflected off a flat mirror (ie, a plano mirror setting) (Fig 3-2).
Figure 3-1 Observation system: light path from patient’s pupil, through mirror, to observer’s retina. (Illustration b y C. H.
Wooley.)
Figure 3-2 Illumination system: position of source (S) with plano mirror (M) effect.
Alternatively, when the distance between the convex lens and the filament is increased by moving the sleeve on the handle, convergent light is emitted. In this situation, the image of the filament appears between the examiner and the patient, as if the light were reflected off a concave mirror (Fig 3-3). Early retinoscopes actually used flat and concave mirrors to achieve these effects.
Figure 3-3 Illumination system: position of source with concave mirror effect.
Retinoscopy is usually performed using the plano mirror setting. We restrict our discussion to the plano mirror effect; recall that in the concave mirror effect, the direction of motion is opposite that of the plano mirror effect. Not all retinoscopes employ the same sleeve position for the plano mirror
setting. For example, the original Copeland retinoscope is in plano position with the sleeve up; the Welch Allyn instrument is in plano position with the sleeve down. The axis of the streak is rotated by rotating the sleeve.
Positioning and Alignment
Ordinarily, the examiner uses his or her right eye to perform retinoscopy on the patient’s right eye, and the left eye for the patient’s left eye. Doing so prevents the examiner’s head from moving into the patient’s line of sight and thus inadvertently stimulating accommodation. If the examiner looks directly through the optical centers of the trial lenses while performing retinoscopy, reflections from the lenses may interfere. In general, if the examiner is too far off-axis, unwanted spherical and cylindrical errors may occur. The optimal alignment is just off center, where the lens reflections can still be seen between the center of the pupil and the lateral edge of the lens.
Fixation and Fogging
Retinoscopy should be performed with the patient’s accommodation relaxed. The patient should fixate at a distance on a nonaccommodative target. For example, the target may be a dim light at the end of the room or a large Snellen letter (20/200 or 20/400 size). Children typically require pharmacologic cycloplegia.
The Retinal Reflex
The projected streak illuminates an area of the patient’s retina, and this light returns to the examiner. By observing characteristics of this reflex, the examiner determines the refractive status of the eye. If the patient’s eye is emmetropic, the light rays emerging from the patient’s pupil are parallel to one another; if the eye is myopic, the rays are convergent (Fig 3-4); and if the eye is hyperopic, the rays are divergent. Through the peephole in the retinoscope, the emerging rays are seen as a red reflex in the patient’s pupil. If the examiner (specifically, the peephole of the retinoscope) is at the patient’s far point, all the light leaving the patient’s pupil enters the peephole and illumination is uniform. However, if the far point of the patient’s eye is not at the peephole of the retinoscope, only some of the rays emanating from the patient’s pupil enter the peephole, and illumination of the pupil appears incomplete.
Figure 3-4 Observation system for myopia.
If the far point is between the examiner and the patient, the emerging rays will have focused and then diverged. The border between the dark and lighted portions of the pupil will move in a direction opposite to the motion (sweep) of the retinoscope streak (known as against movement) as it is moved across the patient’s pupil. If the far point is behind the examiner, the light moves in the same direction as the sweep (known as with movement; Fig 3-5).
Figure 3-5 Retinal reflex movement. Note movement of the streak from face and from retina in with versus against
movement. (Illustration b y C. H. Wooley.)
The condition in which the light fills the pupil and does not move is known as neutrality (Fig 3- 6). The far point is moved with placement of a correcting lens in front of the patient’s eye. At neutrality, if the examiner moves forward (in front of the far point), with movement is seen; if the examiner moves back and away from the far point, against movement is seen.
Figure 3-6 Neutrality reflex. Far point of the eye is conjugate with the peephole of the retinoscope. (Illustration b y C. H.
Wooley.)
Characteristics of the reflex
The moving retinoscopic reflex has 3 main characteristics (Fig 3-7):
1.Speed. The reflex seen in the pupil moves slowest when the far point is distant from the examiner (peephole of the retinoscope). As the far point is moved toward the peephole, the speed of the reflex increases. In other words, large refractive errors have a slow-moving reflex, whereas small errors have a fast reflex.