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

Retinoscopy of Regular Astigmatism

Most eyes have some regular astigmatism. In such cases, light is refracted differently by the 2 principal astigmatic meridians. Let us consider how the retinoscope works in greater detail and apply it to astigmatism.

Figure 4-9 Approaching neutrality. Change in width of the reflex as neutrality is approached. Note that working distance remains constant, and the far point is pulled in with plus lenses.

(Illustration by C. H. Wooley.)

Sweeping the retinoscope back and forth measures the power along only a single axis. Moving the retinoscope from side to side (with the streak oriented at 90°) measures the optical power in the 180° meridian. Power in this meridian is provided by a cylinder at the 90° axis. The convenient result is that the streak of the retinoscope is aligned with the axis of the correcting cylinder being tested. In a patient with regular astigmatism, one seeks to neutralize 2 reflexes, 1 from each of the principal meridians.

Finding the cylinder axis

Before the powers in each of the principal meridians can be determined, the axes of the meridians must be determined. Four characteristics of the streak reflex aid in this determination:

Break. A break is observed when the streak is not oriented parallel to 1 of the principal meridians. The reflex streak in the pupil is not aligned with the streak projected on the iris and surface of the eye, and the line appears broken (Fig 4-10). The break disappears (ie, the line appears continuous) when the projected streak is rotated to the correct axis.

Width. The width of the reflex in the pupil varies as it is rotated around the correct axis. The reflex appears narrowest when the streak, or intercept, aligns with the axis (Fig 4-11).

Intensity. The intensity of the line is brighter when the streak is on the correct axis.

Skew. Skew (oblique motion of the streak reflex) may be used to refine the axis in small cylinders. If the retinoscope streak is off-axis, it moves in a slightly different direction from that of the pupillary reflex (Fig 4-12). The reflex and streak move in the same direction when the streak is aligned with 1 of the principal meridians.

When the streak is aligned at the correct axis, the sleeve may be lowered (Copeland instrument) or raised (Welch Allyn instrument) to narrow the streak, allowing the axis to be determined more easily (Fig 4-13).

Figure 4-10 Break. The retinal reflex is discontinuous with the intercept when the streak is off the correct axis (dashed lines).

(Illustration by C. H. Wooley.)

Figure 4-11 Width, or thickness, of the retinal reflex. The examiner locates the axis where the reflex is thinnest (dashed lines).

(Illustration by C. H. Wooley.)

Figure 4-12 Skew. The arrows indicate that movements of the reflex (single arrow) and intercept (2 arrows) are not parallel. The reflex and intercept do not move in the same direction but are skewed when the streak is off axis. Dashed lines indicate the on-axis line.

(Illustration by C. H. Wooley.)

Figure 4-13 Locating axis on the protractor. A, First, determine the astigmatic axis. B, Second, adjust the sleeve to enhance the intercept until the filament is observed as a fine line pinpointing the axis.

(Illustration by C. H. Wooley.)

Figure 4-14 Straddling. The straddling meridians are 45° off the correcting cylinder axis, at roughly 35° and 125°. As the examiner moves back from the eye while comparing the meridians, the reflex at 125° remains narrow (A) at the same distance that the reflex at 35° has become wide (B). This dissimilarity indicates an axis error; the narrow reflex (A) is the guide toward which the examiner must turn the correcting cylinder axis.

(Illustration by C. H. Wooley.)

This axis can be confirmed through a technique known as straddling, which is performed with the estimated correcting cylinder in place (Fig 4-14). The retinoscope streak is turned 45° off-axis in both directions, and if the axis is correct, the width of the reflex should be equal in both off-axis positions. If the axis is not correct, the widths are unequal in these 2 positions. The axis of the correcting plus cylinder should be moved toward the narrower reflex and the straddling repeated until the widths are equal.

Finding the cylinder power

After the 2 principal meridians are identified, the previously explained spherical techniques are applied to each axis:

With 2 spheres. Neutralize 1 axis with a spherical lens; then neutralize the axis 90° away. The difference between these readings is the cylinder power. For example, if the 90° axis is neutralized with a +1.50 sphere and the 180° axis is neutralized with a +2.25 sphere, the gross retinoscopy is +1.50 +0.75 × 180. The examiner’s working distance (ie, +1.50) is subtracted from the sphere to obtain the final refractive correction: plano +0.75 × 180.

With a sphere and cylinder. Neutralize 1 axis with a spherical lens. To enable the use of with reflexes, neutralize the less plus axis first. Then, with this spherical lens in place, neutralize the axis 90° away by adding a plus cylindrical lens. The spherocylindrical gross retinoscopy is read directly from the trial lens apparatus.

It is also possible to use 2 cylinders at right angles to each other for this gross retinoscopy. Video 4-2 demonstrates the plus cylinder technique, and Video 4-3 demonstrates the minus cylinder technique.

VIDEO 4-2 Retinoscopy: plus cylinder technique.

Animation developed by Thomas F. Mauger, MD.

VIDEO 4-3 Retinoscopy: minus cylinder technique.

Animation developed by Thomas F. Mauger, MD.

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

Retinoscopy of Regular Astigmatism

Figure 4-9 Approaching neutrality. Change in width of the reflex as neutrality is approached. Note that working distance remains constant, and the far point is pulled in with plus lenses.

Finding the cylinder axis

Figure 4-10 Break. The retinal reflex is discontinuous with the intercept when the streak is off the correct axis (dashed lines).

Figure 4-11 Width, or thickness, of the retinal reflex. The examiner locates the axis where the reflex is thinnest (dashed lines).

Figure 4-12 Skew. The arrows indicate that movements of the reflex (single arrow) and intercept (2 arrows) are not parallel. The reflex and intercept do not move in the same direction but are skewed when the streak is off axis. Dashed lines indicate the on-axis line.

Figure 4-13 Locating axis on the protractor. A, First, determine the astigmatic axis. B, Second, adjust the sleeve to enhance the intercept until the filament is observed as a fine line pinpointing the axis.

Finding the cylinder power

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