2.Brilliance. The reflex is dull when the far point is distant from the examiner; it becomes brighter as neutrality is approached. Against reflexes are usually dimmer than with reflexes.
3.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.
Figure 3-7 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 b y C. H. Wooley.)
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 3-8). The power of the correcting lens (or lenses) neutralizing the reflex helps determine the patient’s refractive error.
Figure 3-8 Observation system at neutralization.
The examiner determines the refractive error at the distance from which he or she is working. 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. Because a
common working distance is 67 cm, many phoropters have a 1.50 D (1.00/0.67 m) “working-distance lens” for use during retinoscopy (however, this lens can produce bothersome reflexes).
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. For example, suppose that 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.
Finding Neutrality
In against movement, the far point is between the examiner and the patient. Therefore, to bring the far point to the peephole of the retinoscope, a minus lens is placed in front of the patient’s eye. Similarly, in the case of with movement, a plus lens is placed in front of the patient’s eye. This procedure gives rise to the simple clinical rule: If with movement is observed, add plus power (or subtract minus power); if against movement is observed, add minus power (or subtract plus power) (Fig 3-9).
Figure 3-9 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 b y C. H. Wooley.)
Because it is easier to work with the brighter, sharper with movement image, one should “overminus” the eye and obtain a with reflex; then reduce the minus power (or add plus power) until neutrality is reached. Be aware that the slow, dull reflexes of high-refractive errors may be confused with the neutrality reflex. Media opacities may also produce dull reflexes.
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.
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:
1.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 3-10). The break disappears (ie, the line appears continuous) when the projected streak is rotated to the correct axis.
2.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 3-11).
3.Intensity. The intensity of the line is brighter when the streak is on the correct axis.
4.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 3-12). The reflex and streak move in the same direction when the streak is aligned with 1 of the principal meridians.
Figure 3-10 Break. The retinal reflex is discontinuous with the intercept when the streak is off the correct axis (dashed
lines). (Illustration b y C. H. Wooley.)
Figure 3-11 Width, or thickness, of the retinal reflex. The examiner locates the axis where the reflex is thinnest (dashed
lines). (Illustration b y C. H. Wooley.)
Figure 3-12 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 b y C. H. Wooley.)
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 3-13).
Figure 3-13 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.
This axis can be confirmed through a technique known as straddling, which is performed with the estimated correcting cylinder in place (Fig 3-14). 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. This technique is often more accurate than subjective cross-cylinder axis refinement.
Figure 3-14 Straddling. The straddling meridians are 45° off the correcting cylinder axis, at roughly 35° and 125°. As the