Ординатура / Офтальмология / Английские материалы / Handbook of Pediatric Strabismus and Amblyopia_Wright, Spiegel, Thompson_2006

2 s, and to remove the cover for several seconds before covering the fellow eye to allow for reestablishment of fusion. A properly performed cover/uncover test will only identify the tropia without disclosing an underlying phoria. A phoria is detected by alternate cover testing (see discussion that follows).

ALTERNATE COVER TESTING

The alternate cover test is used to dissociate binocular fusion to determine the full deviation, including both tropia and phoria. The test is performed by alternately occluding each eye, then observing for a refixation shift of the uncovered eye to midline. It is important to hold the occluder over one eye for several seconds to dissociate fusion, then rapidly move the occluder to the fellow eye making sure one eye is always occluded. The direction of the refixation shifts of the eyes to alternate cover testing is interpreted the same as for cover/uncover testing, described previously and shown in Figure 5-9.

INTERPRETING RESPONSES TO COVER TESTS

No shift to alternate cover testing indicates orthophoria. A refixation shift to alternate cover testing indicates a strabismus is present, either a tropia, a phoria, or a tropia with a phoria (monofixation syndrome). Patients with a tropia and no fusion and no phoria will show the same shift on cover test and alternate cover test. Patients with a phoria have straight eyes by Hirschberg light reflex, and no refixation shift to cover/uncover testing, but do show a shift to the alternate cover test (Fig. 5-10). Patients with a small-angle strabismus and peripheral fusion (monofixation syndrome) will have both a phoria and a tropia. Monofixators, therefore, demonstrate a small shift to cover/uncover testing and a larger shift to alternate cover testing. Cover/uncover testing discloses the tropia, and alternate cover testing breaks down the phoria to show the full deviation,

FIGURE 5-10A–D. Alternate cover test in a patient with an esophoria.

(A) Eyes are straight; however, the patient has a tendency to cross (esophoria), but fusional divergence maintains proper alignment. (B) Left eye is covered, dissociating fusion and allowing the left eye to manifest the esophoria. Note that the left eye turns in under the cover. (C) The cover is quickly shifted from the left eye to the right eye without allowing binocular fusion. Now the left eye moves out as the right eye turns in under the cover. (D) The cover is removed and the right eye moves out by fusional divergence to allow the patient to regain fusion.

tropia plus phoria. Comparing cover/uncover testing to alternate cover testing is a good way to diagnose the monofixation syndrome, even in children who are too young to cooperate with sensory testing. Remember, the presence of a phoria is an indication of binocular fusion. Table 5-2 shows the clinical findings of phorias, tropias, and monofixation.

PRISM ALTERNATE COVER TEST

Prism alternate cover testing determines the amount of prism necessary to neutralize the full deviation tropia and any latent phoria. This is the test used to measure a deviation in anticipation of strabismus surgery. The test is performed by first using the alternate cover test to estimate the size of the deviation. A prism is then placed over one eye, oriented appropriately, in an attempt to neutralize the deviation. Alternate cover testing is then performed with the prism in place. If there is a residual refixation shift with the prism in place, the prism is changed (either increased or decreased) to neutralize the deviation (Fig. 5-11). When changing prisms, be sure to always keep one eye covered to maintain binocular dissociation. Also, be sure not to stack prisms of the same orientation (horizontal over horizontal or vertical over vertical) to increase the prism power. It is acceptable to stack horizontal over vertical, but stacking prisms of the same orientation results in underestimation of the angle size.

COMMON CAUSES FOR VARIABLE MEASUREMENTS

1.Poor control of accommodation. Solution: use targets that require full accommodation to be seen. Targets with small detail close to visual threshold are the best.

2.Variable working distance (usually at near). Solution: control working distance to 1/3 meter at near, and standardize working distance at distance; this is more critical for near measurements. Have a string measured at 1/3 meter to measure the near working distance.

3.Tonic fusion not suspended; usually seen in patients with intermittent exotropia and accommodative esotropia. Solution: keep binocular vision dissociated by prolonged occlusion with alternate cover testing. Make sure one eye is always covered when changing prisms (this is why prism bars are helpful).

4.Physiological redress fixation movements; commonly associated with large-angle strabismus. Even when the deviation is neutralized, there is an overshoot of the refixating eye. Solution: move occluder away from the patient’s face to allow peripheral vision of the occluded eye. Also, judge the point of neutralization as the point when redress movement is equal to the refixation movement. Finally, bracket the deviation by intentionally overcorrecting with too much prism, then reduce prism until the best neutralization is achieved.

5.Incomitant deviation (A- or V-patterns and lateral gaze incomitance). Small changes of face turns, head tilts, chin elevation, or chin depressions during the exam will change the deviation measured if the deviation is incomitant. Solution: control the patient’s head position for primary position and cardinal fields of gaze. Consistent head positioning is critical if reproducible measurements are to be obtained.

6.Poor vision. Solution: patient should wear full optical correction during measurements. Use optotypes or fixation targets that the patient can see. For patients with sensory strabismus and vision of 20/400 or worse, use Krimsky to measure the deviation.

MEASURING IN THE CARDINAL POSITIONS OF GAZE

There are nine cardinal positions of gaze; however, in most clinical situations,5 measuring the deviation in primary position, upgaze, downgaze, rightgaze, and leftgaze is sufficient (see Fig. 5-2). The positions of gaze are usually measured with the patient fixing on a distance target. Sidegaze measurements are obtained by moving the head up, down, right, left, and then in the oblique axes. Measurement of the deviation in primary position should also be done at near (1/3 m). Measurements in the cardinal positions of gaze are very helpful in identifying and quantifying incomitance.

SIMULTANEOUS PRISM COVER TEST

The simultaneous prism cover test is used to measure the tropia component of the monofixation syndrome without dissociating the phoria and is therefore used only in patients with small-

A

B

FIGURE 5-11A–B. Prism alternate cover test to measure esotropia. (A) Left eye is esotropic; right eye fixing. (B) Base-out prism is placed before the deviated left eye, and the retinal image moves closer to fovea, but the deviation is still undercorrected.

angle strabismus. The test is performed by first estimating the size of the tropia with corneal light reflex testing. A prism, as determined by estimating the size of the tropia, is then presented in front of the nonfixing eye (i.e., deviated eye) to neutralize the tropia while the fixing eye is simultaneously covered by an occluder (Fig. 5-12A). If the prism neutralizes the tropia, the deviated eye will stay in its deviated position and there will be no refixation shift. If the deviated eye shows a refixation movement, a residual tropia is present. The prism and occluder are withdrawn from the eyes and, after several seconds, a different prism is presented to the deviated eye while the fixing eye is

No movement

F F

40

40

40

No shift

D

FIGURE 5-11C–D. (C) Alternate cover testing is performed, covering first right eye, then left eye. Because the prism undercorrects the deviation, there is an outward shift of the uncovered eye. (D) Larger prism (40 prism diopters, PD) is placed in front of the left eye. Alternate cover testing now reveals there is no shift in eye position, as the deviation is completely neutralized.

simultaneously covered. It is important to allow several seconds to elapse before repeating the test so the patient can regain binocular fusion. This process is repeated until there is no shift of the deviated eye when the fixing eye is covered (Fig. 5-12B–D). In patients with the monofixation syndrome, the amount of tropia can be measured with simultaneous prism cover testing, and the alternate prism cover test can be used to measure the total angle, tropia plus phoria. The notation in the clinic chart

for a typical monofixator with a small-angle esotropia would read ET 6 PD–E 15 PD.

MEASURING INCOMITANT DEVIATIONS

If the deviation is comitant and ductions are full, a prism can be placed in front of either eye or even split between the eyes to measure a deviation. However, when measuring patients with an incomitant deviation secondary to ocular restriction or muscle paresis, one must consider the primary versus the secondary deviation (see Chapter 3, Fig. 3-21). In accordance with Hering’s law, the deviation is larger when the eye with limited ductions is fixing (secondary deviation) than when the eye with full ductions (primary deviation) fixes. When measuring a deviation with prisms, remember that the eye without the prism is considered to be the fixing eye, and the eye looking through the prism is the nonfixing eye, regardless of fixation preference or the presence of amblyopia; this is because the eye without the prism must come to primary position to fixate during alternate cover testing. So, to measure the primary deviation, place a prism over the eye with limited ductions and measure the secondary deviation by placing the prism over the good eye (Fig. 5-13).

The clinically accepted notation for primary and secondary deviation in Figure 5-13 is

Left eye fixing 20 PD (primary deviation)

Right eye fixing 40 PD (secondary deviation)

FIGURE 5-12A–D. Simultaneous prism cover test for small-angle esotropia. This test is useful for measuring the tropia in a patient with a tropia and a phoria (monofixation syndrome). (A) Right esotropia with left eye fixing. Estimate the deviation, then present a prism over the deviating eye while simultaneously covering the fixing eye. If the prism is sufficient to neutralize the esotropia, the eye behind the prism (in this case the right eye) does not move. (B) Trying a 5 PD prism; it is too small and the right eye moves out to pick up fixation. (C) A 10 PD prism is now used to neutralize the esotropia. In this case, the prism neutralizes the esotropia. (D) The left eye is covered as the 10 diopter prism is placed in front of the right eye. No shift occurs because the 10 diopter prism neutralizes the 10 prism diopter esotropia.

Measuring the Accommodative Convergence to Accommodation (AC/A) Ratio

To understand the AC/A ratio, we must review measurements of accommodation and convergence. Accommodation is the increase in lens power to clearly focus at near. The closer the fixation target, the more accommodation is needed to keep the image focused on the retina. Accommodation is measured in diopters. The number of diopters of accommodation needed to focus at a specific near point is the reciprocal of the fixation distance in meters. For example, if the fixation target is at 1/3 m, then an emmetropic patient has to accommodate 3.00 diopters to put the image in focus, 2.00 diopters at 1/2 m, and 1.00 diopter at 1 m. Note, that a 2.00 hypermetrope without correction would have to accommodate 5.00 diopters at 1/3 m (2.00 diopters for the hypermetropia and 3.00 diopters for near fixation).

Convergence keeps the eyes aligned on the approaching targets. Because convergence is linked to accommodation, convergence increases as accommodation increases. Additionally, the farther apart the eyes, the more convergence is required to keep the eyes aligned at near. The amount of convergence needed to keep the eyes aligned on a target at a specific distance is the reciprocal of the fixation distance in meters times the interpupillary distance in centimeters. For example, if the patient has an interpupillary distance of 50 mm and the target is at 1/3 m, the patient must converge 15 prism diopters to keep ocular alignment on the near target (3 diopters 5 cm). If the interpupillary distance is 40 mm for the same working distance of 1/3 m, the required convergence would be 12 prism diopters.

AC/A RATIO

Accommodative convergence to accommodation ratio (AC/A ratio) is the amount of change in convergence for a specific amount of change in accommodation. A high AC/A ratio means the eyes overconverge for a given amount of accommodation (eso-shift at near), whereas a low AC/A ratio means there is underconvergence per diopter of accommodation (exo-shift at near).

Two methods for measuring the AC/A ratio are the heterophoria method and the lens gradient method. These tests are based on changing the patient’s accommodation and then measuring the associated change in convergence. Accommodation is changed by either changing the fixation distance (heterophoria