2020–2021 BCSC Basic and Clinical Science Course™

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2020–2021 BCSC Basic and Clinical Science Course™

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3 Clinical Optics

Chapter 4: Clinical Refraction

Objective Refraction Technique: Retinoscopy

The Correcting Lens

When the examiner uses the appropriate correcting lenses (with either loose lenses or a phoropter), the retinoscopic reflex is neutralized. In other words, when the examiner brings the patient’s far point to the peephole, the reflex fills the patient’s entire pupil (Fig 4-8). The power of the correcting lens (or lenses) neutralizing the reflex is determined by the refractive error of the eye and the distance of the examiner from the eye (the working distance). Theoretically, the working distance should be at optical infinity but this does not practically allow for changing lenses in front of the eye or seeing the retinal reflex. The dioptric equivalent of the working distance (ie, the inverse of the distance) must be subtracted from the power of the correcting lens to determine the actual refractive error of the patient’s eye. Common working distances are 67 cm (1.50 D) and 50 cm (2.00 D), and many phoropters have a 1.50 D or 2.00 D “working-distance lens” for use during retinoscopy, which are then removed at the end of the retinoscopy (however, this lens can produce bothersome reflexes). If the examiner is not using the “built-in” working lens in the phoropter, he or she must algebraically subtract the appropriate amount of spherical power to move the neutralization point from the examiner to infinity (Clinical Example 4-2).

Figure 4-7 Characteristics of the moving retinal reflex on both sides of neutrality. The vertical arrows indicate the position of the retinoscope with regard to the point of neutrality.

(Illustration by C. H. Wooley.)

Figure 4-8 Observation system at neutralization.

For example, an examiner obtains neutralization with a total of +4.00 D over the eye (gross retinoscopy) at a working distance of 67 cm. Subtracting 1.50 D for the working distance yields a refractive correction of +2.50 D.

Any working distance may be used. If the examiner prefers to move closer to the patient for a brighter reflex, the working-distance correction is adjusted accordingly. Working without an explicit lens to correct for the working distance may allow the use of fewer lenses held in front of the eye, reducing distracting reflections from the lens surfaces.

CLINICAL EXAMPLE 4-2

What happens if the correcting lens is not removed from the refractor at the conclusion of retinoscopy and the subjective refraction is then performed?

First the patient will be corrected for the focal length of the correcting lens if the retinoscopy is accurate. That would be 67 cm if the correcting (retinoscopy lens is +1.50). If the patient’s vision following retinoscopy is very poor at distance suspect that the correcting lens is still in place.

If the subjective refraction is carried out, then 1 of 2 results may occur, depending on whether the retinoscopy lens built into the refractor was used or if the examiner simply dialed in +1.50 D of sphere.

If the built-in retinoscopy lens was used (and not removed before the subjective refraction it will be −1.50 off due to having to correct for the +1.50 in the correcting lens as well as the patient’s refraction.

If the +1.50 D was simply dialed into the refractor before starting retinoscopy and then not dialed out before the subjective refraction then the refraction should still be accurate (although difficult because of the +1.50 starting point error).

For this reason, many examiners choose to dial in the correcting lens rather than using the built-in retinoscopy lens in the refractor.

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.

3 Clinical Optics

Chapter 4: Clinical Refraction

Objective Refraction Technique: Retinoscopy

The Correcting Lens

Figure 4-7 Characteristics of the moving retinal reflex on both sides of neutrality. The vertical arrows indicate the position of the retinoscope with regard to the point of neutrality.

Figure 4-8 Observation system at neutralization.

CLINICAL EXAMPLE 4-2

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