Retinoscopy
Barry Milder, MD
Introduction
Retinoscopy is an objective method of determining a patient's refractive error. Although some degree of patient cooperation is
needed, no subjective responses from the patient are necessary. The accuracy of the measurement depends on the retinoscopist's abilities.
Achieving any retinoscopic endpoint would be difficult if a patient's accommodative state is in flux, so accuracy requires that patients
fixate steadily on a distant target (6 M or beyond) or accommodation be stabilized with cycloplegic eye drops.
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Slide 1 |
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Slide 2 |
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The Retinoscope
The retinoscope is a light source with a view port - no more, no less. It is a convenient way to shine light into a patient's
eye and observe its reflection from the patient's retina. The retinoscope itself is not a factor in the determination of refractive
error, except as a light source.
Contemporary streak retinoscopes have variable light output which may be set to slightly divergent light (plano mirror effect)
or slightly convergent light (concave mirror effect) by adjusting a slide on the handle (Slide 1). The slide position that produces
each effect may differ on retinoscopes from different manufacturers, but can be easily determined. The plano mirror position
projects light that appears as a diffuse bar when directed at the patient's face (or your hand), whereas the concave mirror position
projects a more sharply defined bar of light. In this tutorial, it will be assumed that the retinoscope is used in the plano mirror
position, unless otherwise stated. Rotating the slide rotates the bar of light so that it may be projected at the patient at any axis.
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Slide 3 |
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Slide 4 |
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The light of the retinoscope projected into the patient's pupil is bounced off the retina (Slide 2). Thereafter,
one must consider the retina to be the origin of the light. It is refracted by the lens and cornea as it exits the eye.
The vergence of the light after passing through the cornea determines where it will be brought to a focus, at a position
known as the far point (Slide 3). The far point plane is conjugate to the patient's retina. Light originating at the far
point plane would be focused on the retina, just as light from the retina is focused at the far point.
The far point of a myopic eye is in front of the eye. The distance of the far point from the eye is defined by the amount
of ametropia (Slide 4). A 5.00-D myope would have a far point 1/5 m (20 cm) before the eye, assuming the patient was looking
through air and not water. The far point of a hyperopic eye lies behind the cornea and lens.
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Slide 5 |
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Slide 6 |
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The retinoscopist in front of the patient is looking at the reflected light coming from the pupil and sees that light as if it originated at
the far point (Slide 5). If the far point is between the retinoscopist and the patient, when the retinoscopist passes the band of light across
the patient's pupil, the bar of light within the pupil, or the "streak," will be seen as moving across the pupil in the opposite direction
from the passage of the bar of light across the external eye. This is known as "against motion" (Slide 6). This phenomenon occurs because the
light from the pupil has come to a focus at the far point before it reached the retinoscopist's eye, and the image of the streak is inverted,
but the light reflected from the patient's face is not inverted (Slide 7).
If the far point is behind the retinoscopist or behind the patient's eye, the streak in the pupil will not be inverted when
the retinoscopist sees it (Slide 8), and the streak in the pupil will move across the pupil in the same direction as the passage
of the bar of light across the external eye. This is known as "with motion" (Slide 9).
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Slide 7 |
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When the far point is dioptrically distant from the retinoscopist's eye, the streak in the pupil is dull and narrow. The closer the
far point is to the retinoscopist, the brighter and broader the streak becomes. If the far point is at the retinoscopist's eye, there
will be neither with nor against motion. The retinoscopist's eye is conjugate to the patient's retina, and the pupil will be filled with light.
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Slide 8 |
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Interposing lenses between the patient and the retinoscopist will change the vergence of the light, changing the position of the
far point before the light reaches the retinoscopist's eye. By watching the movement of the streak in the pupil as lenses are changed
before the eye, the retinoscopist can determine which lens placed the far point at his or her eye (Slide 10).
Identifying that "neutralizing lens" is the goal of retinoscopy. Theoretically, one lens power will leave a bit of against motion, while a change
of 0.25 D produces a pupil full of light, and another change of 0.25 D results in a bit of with motion. In real life, it is more
difficult to identify against motion than with motion, and the endpoint of "no motion" may be perceived at more than one lens power.
However, a good retinoscopist should have results reproducible to +/-0.25 D.
Since the lens that places the far point at the retinoscopist's eye makes that position conjugate to the patient's retina,
it would make the patient see clearly at that distance. Therefore, the power of the neutralizing lens will vary, not just with the
refractive error of the patient, but will also be dependent on the position of the retinoscopist. The lens that would focus the
patient for distance vision would push the Far Point out to optical infinity. The difference between the retinoscopist's position
and optical infinity is called the "working distance." The lens that compensates for the working distance will always be a minus
power lens and will be equal to the dioptric distance of the retinoscopist from the patient. If the retinoscopist is 50 cm from the
patient, the "working distance" factor would be -1 m/50 cm = -1/0.5 m = -2.0 D. If he or she has longer arms and uses the retinoscope
57 cm from the patient, then the working distance factor would be -1 m/0.57 m = -1.75 D.
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Slide 9 |
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Slide 10 |
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Astigmatism Retinoscopy
When a patient has astigmatism, the retinoscope's light reflected from the retina will be split into a conoid of
Sturm upon exiting the cornea and will have two "far lines" rather than a far point. As in any conoid of Sturm, each far
line will be perpendicular to the meridional power of the eye and parallel to the axis of the astigmatic meridian that created
the line. The dioptric position of each far line must be evaluated to determine the refractive error of the patient's eye. A
spherical lens placed before the patient's eye will move both far lines equally. A cylindric lens placed with its axis aligned
with the retinoscopic bar will move only the far line parallel to the bar.
The presence of astigmatism is recognized by several characteristics of the retinoscopic reflex seen in the patient's pupil.
If there is no astigmatism, the streak of light in the pupil will align with the bar of light reflected from the surrounding face
at any orientation of the external bar. When astigmatism is present, the streak will align with the external bar only when the
streak is at one of the two axes of the astigmatic error (Slide 12). At other orientations, the streak will not appear
colinear with the external bar ("break phenomenon"). When on axis, the streak will be narrower and more sharply defined
than when off axis. It will also be brighter when on axis. If on axis, the movement of the streak in the pupil mimics that
outside the pupil, but if off axis, the streak may be seen to move in a slightly different direction from the bar on the face
("skew"). Since most patients have regular astigmatism, when one axis is found by the characteristics of the retinoscopic
streak, the other axis should be found 90° away (Slide 13).
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Slide 11 |
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Once the positions of the two far lines have been found, the neutralizing lens for each may be determined with spherical lenses.
The amount of astigmatism is the difference in power of the two neutralizing spherical lenses. The plus axis of astigmatism is
the orientation of the retinoscopic bar of the more plus neutralizing lens. Alternatively, the first far line may be neutralized
with a sphere and the other with a cylindric lens, added over the sphere, with its axis at the orientation of the retinoscopic
bar when neutralizing the second far line.
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Slide 12 |
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Slide 13 |
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Technique of Retinoscopy
The retinoscopist positions himself/herself at an arm's length from the patient along the line of sight from the patient to a
nonaccommodative target at 6 m or beyond (Slide 14). The retinoscope is held in the clinician's right hand and in front of the
clinician's right eye when retinoscoping the patient's right eye. The left hand and eye should be used for the patient's left
eye. To do otherwise would place the retinoscopist between the patient and the fixation target. The retinoscopist passes the
bar of light across the patient's pupil perpendicular to the length of the bar.
If a patient has moderate refractive error, when the retinoscopist first views the illuminated pupil, the retinoscopic bar of light, or
"streak," will be easily identified (unless discontinuities in the optical media, such as cataract or corneal scarring, degrade the
quality of the reflex). However, when the patient has a refractive error higher than 4.00 D or -6.00 D, it may be difficult to see the
streak. If the ametropia is even higher power, the pupil may appear dull, grayish, or dark upon first look with the retinoscope. In such
circumstances, the retinoscopist should observe the retinoscopic reflex while moving closer to the patient's eye. If the streak becomes
brighter as the working distance is shortened, the patient is obviously a high myope, since the retinoscopist is getting closer to the
neutralization point by increasing the minus power factor of the working distance. In a high hyperope, the reflex will not be improved
by approaching the patient. After gaining this information, the retinoscopist returns to the arm's length working distance.
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Slide 14 |
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Slide 15 |
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When a far line is close to the retinoscopist's eye (i.e., close to neutralization), the streak aligned with the far line appears bright
but more diffuse and harder to define when in front of the retinoscopist, in "against" motion, than when it is behind the retinoscopist
in "with" motion. It is much easier to observe changes in the streak in "with" motion near neutralization. Therefore, the retinoscopic
endpoint is often more accurate if one works from "with" motion toward "against," as will be outlined below.
The first step in retinoscopy, therefore, is to create a condition in which both far lines show "with" motion, that is, neither is
between the retinoscopist and the patient. When the retinoscopist sits in front of the patient and observes the streak at multiple
positions "around the clock" (usually 90°, 180°, 45°, and 135°), if "against" motion is present at any axis, enough minus sphere
is added before the patient to create "with" motion at all positions. Both far lines are now behind the retinoscopist's eye (Slide 15).
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Slide 16 |
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Next the retinoscopist attempts to determine the axes of the two far lines by observing the phenomena noted above - skew, brightness, crispness.
If the axes can be identified, only these two orientations of the retinoscopic streak must be examined. All others may be ignored.
The streak position that is broader and less defined is the far line closer to the retinoscopist's eye, closer to neutralization. If
the axes are not clearly identified at this step, multiple streak positions must be evaluated until neutralization is achieved.
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Slide 17 |
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Flipping the streak orientation from one far line axis to the other, the retinoscopist adds plus sphere power before the patient.
In so doing, the far lines are brought from behind the retinoscopist toward the patient. When the more myopic (or less hyperopic)
far line is at the retinoscopist's eye (Slide 16), passing the streak across the pupil parallel to that far line results in an illuminated pupil
without a visible streak ("neutralization"). The streak 90° from that position still has "with" motion, as it is parallel to the far line
located behind the retinoscopist. With the streak parallel to this second far line, 90° from the first neutralized position, the
retinoscopist adds more plus sphere until this second far line is at the neutralization point, the retinoscopist's eye (Slide 17).
The additional amount of plus sphere required to neutralize the second far line is the amount of plus cylinder and the plus cylinder
axis is the position of the streak when neutralizing the second far line. Alternatively, to neutralize the second far line, the
retinoscopist can add plus cylinder power to the sphere power present when the first far line was neutralized, positioning the plus
axis parallel to the second far line (Slide 18).
When both far lines have been neutralized, the "working distance" compensation is added. This is a minus sphere equal to the dioptric distance from the retinoscopist to the patient, which is approximately 2.00 D.
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Slide 18 |
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Summary
Following are the steps required when using retinoscpy to determine a patient's refractive error:
- Position of far point, or far lines, is determined by the refractive error of the eye.
- A far point between the patient and retinoscopist indicates "against" motion. A far point behind the retinoscopist or behind the patient indicates "with" motion. No motion if far point is at retinoscopist's eye.
- When the streak is "on axis":
- streak in pupil aligns with streak outside pupil
- streak is more defined than when off axis
- streak is brighter than when off axis
- edges of streak are more parallel than when off axis
- It is easier to retinoscope from "with to against" than "against to with." Initiate retinoscopy by adding enough minus to put far lines behind retinoscopist.
- Attempt to identify astigmatic axes. If unable to do so, repeat attempt when near neutralization.
- Add plus sphere until first far line neutralizes.
- If using plus cylinder, add cylinder at axis parallel to second far line until it is neutralized.
- If using only spheres, continue adding plus sphere until second far line is neutralized.
- The amount of astigmatism is the difference between the two far lines. The plus cylinder
- axis is the position of the second far line, while the minus cylinder axis is the position of the first far line.
- Add -2.00 D as "working distance factor" unless personal experience has defined your working distance as another amount.
Bibliography
The American Academy of Ophthalmology. The Revised 2000-2001 Basic and Clinical Science Course. Section 3:
Optics, Refraction, and Contact Lenses. San Francisco, Calif: The Foundation of the American Academy of Ophthalmology.
Corboy J. The Retinoscopy Book, A Manual for Beginners. Thorofare, NJ: Slack; 1996.
Gettes B. Practical Refraction. New York, NY: Grune and Stratton; 1957.
Michaels D. Basic Refraction Techniques. New York, NY: Raven Press; 1998.
Rubin M. Optics for Clinicians, 2nd ed. Gainesville, Fla: Triad Publishing; 1993.
Sloane A, Garcia G. Manual of Refraction, 3rd ed. Boston, Mass: Little Brown and Co; 1979.
Tasman W, Jaeger E, eds. Duane's Foundations of Clinical Ophthalmology. Vol 1: Refraction and Clinical Optics. Philadelphia, Pa: Lippincott-Raven Publishers; 2000.