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 4-4); and if the eye is hyperopic, the rays are divergent. Through the peephole in the retinoscope, the emerging light is 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.
If the far point is between the examiner and the myopic 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 4-5).
The state in which the light fills the pupil and apparently does not move is known as neutrality (Fig 4-6). 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. The far point may be moved with placement of a correcting lens in front of the patient’s eye.
Figure 4-4 Observation system for myopia.
Figure 4-5 Retinal reflex movement. Note movement of the streak from face and from retina in with versus against movement.
(Illustration by C. H. Wooley.)
Figure 4-6 Neutrality reflex. Far point of the eye is conjugate with the peephole of the retinoscope.
(Illustration by C. H. Wooley.)
Characteristics of the reflex
The moving retinoscopic reflex has 4 main characteristics (Fig 4-7):
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.
Brilliance. The reflex is dull when the far point is distant from the examiner; it becomes brighter as neutrality is approached.
Width. When the far point is distant from the examiner, the streak is narrow. As the far point is moved closer to the examiner, the streak broadens and, at neutrality, fills the entire pupil. This situation applies only to with movement reflexes.
Regularity. An irregular reflex indicates a media problem that should be further explored in examination.
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.