The Ocular Motor Examination
Kenneth W. Wright
EVALUATION OF THE STRABISMIC PATIENT
The goals of the strabismus examination are to (1) diagnose amblyopia; (2) establish a strabismus diagnosis (e.g., pseudo- esotropia, congenital esotropia, cranial nerve palsy, or restric- tive strabismus); (3) assess the binocular status (e.g., bifoveal fusion, monofixation–peripheral fusion, anomalous retinal cor- respondence, no fusion–large suppression, diplopia, and fusion potential); and (4) measure and characterize the deviation. A well-focused, goal-oriented evaluation helps prevent a laborious exhaustive examination that results in patient fatigue, examiner fatigue, and the collection of spurious data. Even after a full eval- uation, a patient’s strabismus may not clearly fall into a specific category, and the diagnosis may be nebulous. In these cases, the patient can still be appropriately managed if evaluated for amblyopia, sensory status, size of the deviation, and the possi- bility of an underlying neurological problem or systemic disease.
As in all aspects of medicine, the combination of detailed history and careful physical examination provides the foundation for making the correct diagnosis and taking the appropriate action.
HISTORY
The character and onset of the strabismus provides information about binocular fusion potential. The earlier the onset and longer the duration of the strabismus, the worse the prognosis for binocular vision. Older children with congenital strabismus
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who have never experienced fusion have a poor fusion progno- sis, whereas an acquired strabismus of a few months duration usually indicates the patient has developed at least some binoc- ular vision and has the potential to recover binocular fusion. A history of compensatory head posturing speaks for binocular fusion, as does a history of an intermittent strabismus. In the real world of clinical practice, however, these general rules do not hold 100% of the time as some patients will obtain binoc- ular fusion after late surgery for a presumed congenital strabis- mus. The history of an acquired strabismus is important because it may indicate a neurological or systemic disease, especially when the strabismus is incomitant and associated with limited ductions. An unexplained acquired incomitant strabismus re- quires neurological evaluation. Examining baby photographs, the family album, or a patient’s driver’s license under magnifi- cation can facilitate documenting the onset and type of strabis- mus. Additionally, patients should be questioned about the presence of diplopia, as diplopia usually indicates acquired stra- bismus with onset usually after 4 to 6 years of age.
History regarding birth weight, complications of birth, the health of the child, and developmental milestones are also an integral part of a complete evaluation. Finally, the family history is very important. Although the exact hereditary pattern of stra- bismus is unclear, most types of strabismus are familial.
PHYSICAL EXAMINATION
Try to obtain as much information as possible by inspection without touching the child. Use toys to play with the child to observe eye alignment and eye movements. Save the more intru- sive parts of the examination for last. The steps for the strabis- mic examination are listed in order in Table 5-1. Traditionally, binocular sensory testing is performed before tests that require occluding one eye, such as visual acuity testing and the cover/
uncover tests. Covering one eye dissociates the eyes and may disrupt fusion in a patient with latent strabismus (large phoria or intermittent tropia). This may be more of a theoretical con- sideration, as Biedner et al.2found no significant difference in stereopsis tested at the beginning versus at the end of the exam.
This author prefers testing vision early, before sensory testing, as knowledge of the visual acuity sets the tone for the rest of the examination.
ORDER OF EXAMINATION
Inspection
The physical examination actually starts as the patient enters the room. While taking the history, it is important to observe the patient’s visual behavior, eye alignment, eye movements, fixation, and head posturing. By the time the history is recorded, a good observer will often have established a preliminary differ- ential diagnosis. An initial differential diagnosis helps guide the direction of the physical examination and minimize extraneous test. However, be careful not to overanticipate: keep an open mind.
Much can be learned about the patient’s sensory status from simple inspection. Do the eyes appear straight? Is there a face turn or head posturing? The presence of straight eyes with a face turn in a patient with strabismus can indicate the presence of binocular fusion even if this cannot be demonstrated by sensory testing. Often, patients with weak fusion will break down to a tropia after even a brief cover test or during sensory tests. There- fore, it is important to observe the patient’s alignment and face posturing before formal testing.
Amblyopia Assessment/Visual Acuity
Techniques for diagnosing amblyopia are covered in Chapter 4.
Always try to document visual function even in neonates or developmentally delayed children who are unable to cooperate with standard testing. Preverbal children can be tested for amblyopia by examining the quality of monocular fixation or binocular fixation preference. When evaluating amblyopia in older cooperative children, use linear acuity because single opto- 140 handbook of pediatric strabismus and amblyopia
TABLE 5-1. Order of Examination.
1. Inspection: Evaluation and measurement of face turns 2. Amblyopia assessment/visual acuity
3. Sensory tests (see Chapter 6) 4. Ductions and versions 5. Measurement of deviation
6. Special tests for identifying restriction and paresis 7. Cycloplegic refraction
8. Fundus examination (objective torsion)
type presentation lacks crowding and slightly underestimates the degree of amblyopia. In young preliterate children, however, single optotype testing is quicker, easier, and probably more accurate than linear optotypes. Crowding bars around an opto- type or using the Wright figures have inherent crowding and may be useful for diagnosing amblyopia with single optotypes. There are many ways to test visual acuity in preschool children includ- ing Wright figures, Allen picture cards, HOTV, and the illiterate E game.
Sensory Tests
A description of sensory tests is provided in Chapter 6. Evalua- tion of the sensory status should be part of every strabismus examination and usually includes a haploscopic fusion/suppres- sion test (e.g., Worth 4-dot test) and a test for stereoacuity (e.g., Titmus).
Ductions and Versions
Ductions test monocular movements and are examined with one eye occluded, forcing fixation to the eye being examined.
Ductions evaluate the ability for the eye to move into extreme fields of gaze. Figure 5-1 shows both normal and limited abduc- tion; this is a scale of 0 to 4, with 1 limitation meaning slight limitation and 4 indicating severe limitation with inability of the eye to move past midline. This scale can be used to measure horizontal and vertical ductions.
Versions test binocular eye movements and show how well the eyes move together in synchrony. Versions will detect subtle imbalance of eye movements and oblique muscle dysfunction missed on ductions. Evaluation of versions should include eye movements through the nine cardinal positions of gaze: from primary position to straight right, straight left, straight up, straight down, up to the right, up to the left, down to the right, and down to the left (Fig. 5-2). Abnormal versions can be noted on a scale of 4 to 4 with 0 indicating normal and 4 indi- cating maximum overaction (Fig. 5-3A), whereas 4 indicates severe underaction (Fig. 5-3B). It is important to remember, when observing for oblique dysfunction, to make sure the abducting eye is fixing so the adducting eye is free to manifest the oblique dysfunction; this can be accomplished by partially occluding the adducting eye (with your thumb or occluder) and
142 handbook of pediatric strabismus and amblyopia
A
B C
D E
FIGURE 5-1A–E. Ductions are monocular eye movements: (A) normal abduction; (B)1 limitation to abduction; (C) 2 limitation to abduc- tion; (D)3 limitation to abduction; (E) 4 limitation to abduction. This scheme is used to quantitate limitation of duction movements and can be used for abduction or vertical ductions as well.
FIGURE 5-2. Versions are binocular eye movements: dextroversion, rightgaze; levoversion, leftgaze; superversion, upgaze; infraversion, downgaze.
looking around the occluder to see if the eye manifests the oblique dysfunction (Fig. 5-4).
Measuring Ocular Deviation
The methods for measuring the angle of strabismus have been divided into the following categories: light reflex tests, cover tests, and subjective tests. Light reflex tests are the easiest to perform on young children and infants. These tests, however, are not as precise as other tests such as the cover tests. The Lancaster red-green test is useful in adult patients with diplopia and an incomitant deviation.
Most methods for measuring ocular deviations involve prisms. There is a basic discussion on the use of prisms in stra-
A
B
FIGURE 5-3A,B. (A) Versions showing overaction of the oblique muscles:
upper drawing, right inferior oblique overaction 3; lower drawing, right superior oblique overaction 3. (B) Versions showing underaction of the oblique muscles: upper drawing,3 underaction of the right inferior oblique; lower drawing,3 underaction of the right superior oblique.
bismus in Chapter 3. For prism neutralization of a deviation, remember to orient the prism so the apex points in the same direction as the deviated eye. Esotropia is corrected with a base- out prism, exotropia with a base-in prism, and hypertropia by a base-down prism.
When measuring a deviation, it is critical that the patient is fixating and appropriately accommodating on the fixation target.
Accurate measurements cannot be obtained if the patient is gazing around the room or daydreaming and not accommodat- ing on the fixation target. An accommodative target is a target that has fine detail that requires accurate accommodation to be seen. A penlight, for example, is a poor accommodative target as there is no fine detail and accommodation is not required to see the light. One of the best accommodative targets for adults, in the distance or near, is Snellen letters at a size close to visual threshold. By having the patient read the letters, the examiner knows that the patient is accommodating on the fixation target.
For young children, small detailed toys or small pictures with fine detail can be used at near and a children’s video or animated toys in the distance.
LIGHTREFLEXTESTS HIRSCHBERGTEST
The Hirschberg test, or corneal light reflex test, assesses eye alignment by the noting the location of the corneal light reflex within the pupil. The term corneal light reflex is a misnomer, as it is not a reflex off the cornea. What we perceive as the light reflex is actually the first Purkinje image, which is a virtual 144 handbook of pediatric strabismus and amblyopia
FIGURE 5-4. Versions showing an occluder is placed in front of the right eye to ensure that the patient fixates with the left eye. When patient is fixing left eye, it allows the inferior oblique overaction to become obvious and manifest. If the patient were to fix with the right eye, however, one would not see a downshoot because of Hering’s law.
image located behind the pupil. The Hirschberg test should be performed by holding a light source (muscle light or penlight) in front of the examiner’s eye and directing the light into the patient’s eyes. Have the patient look at the light, then assess the location of the light reflex in each eye. Hirschberg testing is only valid if the patient fixes on the light source. The examiner must view the light reflexes from a position directly behind the light source. Therefore, for practical purposes, the Hirschberg test can only be performed at near. An accommodative fixation target can be placed next to the light source to attract the patient’s attention and provide an accommodative target.
With normal orthotropic alignment, the light reflexes are slightly decentered nasally, but they are symmetrically located within each pupil. Slight symmetrical nasal displacement of approximately 5° is normal and is a “physiological” positive angle kappa (see Angle Kappa, below). Patients with strabismus will have an eccentric light reflex in the deviated eye. Tempo- ral displacement of the light reflex indicates esotropia, nasal displacement indicates exotropia, and inferior displacement indicates hypertropia (Fig. 5-5). One can estimate the size of an ocular deviation by the amount of light reflex displacement within the pupil. Temporal displacement of the light reflex to the pupillary margin indicates an esotropia of 15° (ET 30 PD), displacement to the temporal midiris indicates esotropia 30° (ET 60 PD), and temporal displacement to the limbus indicates an esotropia of 40° (ET 80 PD). Another way to estimate the angle of deviation is to multiply the millimeters of light displacement by 15 PD to give the deviation in prism diopters.3,6,7Thus, 2 mm of nasal displacement of the reflex from its normal location when viewing monocularly indicates an exotropia of 30 PD.
These are relatively gross estimates and, as a rule, are not used to determine the amount of surgery.
ANGLEKAPPA
Angle kappa measures the angle between the line of sight and the corneal–pupillary axis. The line of sight is a line from the fixation target to the fovea, and the corneal–pupillary axis is a line from the center of the pupil that is tangential to the cornea.
Angle kappa is a monocular measurement of monocular align- ment to a visual target. Angle kappa does not have any rela- tionship to the fellow eye and does not measure strabismus. If the fovea is located directly behind the pupil, then the line of sight would be in line with the corneal–pupillary axis and the
angle kappa would be 0°. On the other hand, a fovea located off center (not directly behind the cornea) creates a discrepancy between the line of sight and the corneal–pupillary axis, result- ing in an angle kappa (Fig. 5-6). A positive angle kappa is asso- ciated with a temporally displaced fovea (Fig. 5-6A). With the fovea displaced temporally, the eye must abduct to put the image on the fovea, which causes a nasal displacement of the Hirschberg light reflex and gives an exo-appearance. Figure 5-7A shows a patient with a positive angle kappa, left eye, caused by a dragged left macula secondary to retinopathy of prematurity.
This patient appears to have an exotropia but is actually orthotropic. When the left eye is covered, the right eye remains 146 handbook of pediatric strabismus and amblyopia
FIGURE 5-5. Hirschberg test: top drawing, normally centered reflex;
second drawing, esotropia, right eye, with the light reflex deviated tem- porally; third drawing, exotropia, right eye, with the light reflex deviated nasally; bottom drawing, right hypertropia, with the light reflex deviated inferiorly.
abducted, as this is the position necessary to view the target (Fig.
5-7B). Remember, angle kappa relates to the eye position during monocular viewing and is associated with central fixation by an ectopic fovea. Note that patients with a positive angle kappa and esotropia look as if the eyes are straight. A positive angle kappa can occur congenitally or, if the fovea is dragged temporally, by retinal fibrosis occurring with diseases such as retinopathy of prematurity or a temporal retinal scar from Toxocara canis. A negative angle kappais caused by nasal displacement of the fovea toward the optic nerve (see Fig. 5-6B); this results in a turning in of the eye, a temporal shift of the Hirschberg light reflex, and an eso-appearance. Nasal macular displacement can be secondary to a retinal scar located between the fovea and optic nerve or can occur congenitally without a specific etiology.
A B
FIGURE 5-6A,B. (A) Positive angle kappa. The eye turns out to pick up fixation under monocular viewing conditions as the fovea (F) is displaced temporally. Note that the line of sight differs from the corneal pupillary axis. (B) Negative angle kappa. The eye deviates nasally to pick up fixa- tion as the fovea is displaced nasally, close to the optic nerve. Again, the line of sight is not parallel with the corneal pupillary axis.
148 handbook of pediatric strabismus and amblyopia
A
B
FIGURE 5-7A,B. Clinical photograph of a patient with retinopathy of prematurity. Both foveas are dragged temporally. (A) The patient appears exotropic and both eyes are exotropic, unlike true exotropia where the fixing eye is straight and the nonfixing fellow eye is deviated. This patient has a positive angle kappa and is actually fusing with both eyes deviated temporally to align the foveas. (B) There is no ocular shift when the left eye is covered: no exotropia.
Most normal patients have a physiological positive angle kappa, as the fovea is located 5° temporal to the center of the posterior pole; this is why we usually see the Hirschberg light reflex as being slightly decentered nasally in orthotropic patients. An angle kappa can be distinguished from a tropia by the cover/uncover test (see following). If the light reflex is dis- placed from the center of the pupil with monocular viewing, this indicates an abnormal angle kappa, not strabismus (Fig. 5-7B).
KRIMSKYTEST
The Krimsky test adds use of a prism to the Hirschberg test to measure a strabismus. This test is indicated to estimate the devi- ation size in uncooperative patients and patients with sensory strabismus and poor vision of 20/400 or worse. A prism is placed in front of one eye, with the base oriented appropriately (esotropia, base-out; exotropia, base-in; hypertropia, base-down) to neutralize the deviation. A penlight is then shone into both eyes as described for the Hirschberg test. The patient is directed to fixate on an accommodative target juxtaposed to the penlight.
The prism is increased or decreased until the reflex from each eye becomes equally and symmetrically centered in the pupil.
Placing a prism over the fixing eye in a patient with a tropia will cause a version movement in which both eyes move in the direc- tion of the apex of the prism, which moves the light reflex in the deviated eye (Fig. 5-8). Placing the prism over the nonfixing eye directly moves the light reflex to the center of the pupil without a version shift. One can place the prism over either eye, except in cases of a restriction or paresis. In these patients, measure the primary deviation by placing the prism over the eye with limited rotations and mesure the secondary deviation by placing the prism over the eye with full ductions (see primary versus secondary deviation, following).
BRÜCKNERREFLEXTEST
The Brückner reflex test is performed by using the direct oph- thalmoscope to obtain a red reflex from both eyes simultane- ously. Make sure that the patient is looking at the light during the Brückner test; if the patient looks to peripheral targets, the test is invalid. In patients with strabismus, the Brückner test will show asymmetrical reflexes with a brighter reflex coming from the deviated eye. There is less pigment in the peripheral retina, so the deviated eye will reflect more light. This is a screening test that identifies strabismus and pathology that
change the normal red reflex including anisometropia, gross retinal pathology, large retinal detachment, and corneal, lentic- ular, or vitreous opacities (see Chapter 4; Figs. 4-13, 4-14).
COVERTESTS
COVER/UNCOVERTEST
The cover/uncover test is designed to detect the presence of a tropia in patients who appear to have straight eyes and may be fusing. The idea is to test for a tropia without dissociating an existing phoria. Cover/uncover testing is performed by very briefly covering the eye that is thought to be the fixing eye while observing the eye suspected of deviating for a tropia shifts as the eye picks up fixation. If there is no shift, then perform cover/uncover testing on the opposite eye. If there is no shift of either eye after covering and uncovering each eye, then there is no manifest tropia and the eyes are straight, that is, orthotropia.
If briefly covering one eye produces a refixation shift of the fellow eye, then a manifest tropia is present. A nasal to tempo- 150 handbook of pediatric strabismus and amblyopia
A B
FIGURE 5-8A,B. The Krimsky test in a patient with esotropia. (A) The light reflex is deviated temporally in the esotropic left eye. (B) A base-out prism is presented in front of the right eye, which is fixing. The patient continues to fixate with the right eye, but the right eye turns in to pick up fixation. The left eye, because of Hering’s law, moves temporally. The left eye is now centered and the right eye is turned in; however, the light reflex would be centered in both eyes; this is the neutralization point, and the amount of prism needed to achieve the neutralization point measures the angle of deviation.
ral refixation movement indicates an esotropia, a temporal to nasal shift indicates an exotropia, and a downward shift indi- cates a hypertropia (Fig. 5-9).
Be sure to have the patient fixate on an accommodative target during testing, as the test is invalid if fixation is uncon- trolled and wandering. Cover the fixing eye for 1 to 2 s, just long enough to see if there is a shift of the uncovered eye to midline.
The cover/uncover test can be dissociating and can manifest an underlying phoria if the test is performed improperly by cover- ing one eye for several seconds. Prolonged occlusion of one eye will break up fusion, and the patient will manifest a phoria that may erroneously be called a tropia because it is associated with the cover test. Remember to briefly cover one eye for only 1 or
A
B C
FIGURE 5-9A–C. Cover/uncover test. (A) Esotropia. Left eye is fixing.
When the left eye is covered, the right eye moves out to pick up fixation.
This outward movement indicates that the right eye is esotropic. (B) Exotropia. Left eye is fixing. When the left eye is covered, the right eye turns in to pick up fixation. The inward movement indicates that the right eye is exotropic. (C) Right hypertropia. Left eye is fixing. Covering the left eye causes the right eye to come down to pick up fixation. Move- ment of the right eye indicates a right hypertropia.
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).
ALTERNATECOVERTESTING
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.
INTERPRETINGRESPONSES TOCOVERTESTS
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 alter- nate 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, 152 handbook of pediatric strabismus and amblyopia
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 (esopho- ria), 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.
B
C
D
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tropia plus phoria. Comparing cover/uncover testing to alternate cover testing is a good way to diagnose the monofixation syn- drome, even in children who are too young to cooperate with sensory testing. Remember, the presence of a phoria is an indi- cation of binocular fusion. Table 5-2 shows the clinical findings of phorias, tropias, and monofixation.
PRISMALTERNATECOVERTEST
Prism alternate cover testingdetermines 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 anticipa- tion 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 accept- able to stack horizontal over vertical, but stacking prisms of the same orientation results in underestimation of the angle size.
COMMONCAUSES FORVARIABLEMEASUREMENTS 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 meas- urements. Have a string measured at 1/3 meter to measure the near working distance.
154 handbook of pediatric strabismus and amblyopia
TABLE 5-2. Clinical Findings of Phorias, Tropias, and Monofixation.
Corneal light Cover/ Alternate
reflex uncover cover Fusion
Orthotropia Straight No shift No shift Yes
Phoria Straight No shift Shift Yes
Tropia 10 PD Deviation Shift Shift No
Monofixation10 PD Small deviation Small shift Larger shift Yes
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. Solu- tion: 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 eleva- tion, or chin depressions during the exam will change the devia- tion 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 meas- urements 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 stra- bismus and vision of 20/400 or worse, use Krimsky to measure the deviation.
MEASURING IN THECARDINALPOSITIONS OFGAZE There are nine cardinal positions of gaze; however, in most clin- ical 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 posi- tions of gaze are very helpful in identifying and quantifying incomitance.
SIMULTANEOUSPRISMCOVERTEST
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-
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 move- ment, 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 156 handbook of pediatric strabismus and amblyopia
B A
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.
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
Undercorrected with nasal to temporal shift
No shift
20
40
40
40
C
D
No movement
F F
F F
20
40
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.
158 handbook of pediatric strabismus and amblyopia
A
B
C
D
for a typical monofixator with a small-angle esotropia would read ET 6 PD–E 15 PD.
MEASURINGINCOMITANTDEVIATIONS
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 sec- ondary 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 devi- ation 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 deviat- ing eye while simultaneously covering the fixing eye. If the prism is suf- ficient 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 neu- tralizes the 10 prism diopter esotropia.
A
B
C
FIGURE 5-13A–C. Primary versus secondary deviation. Top figure, esotropia secondary to a tight medial rectus muscle. Left, diagram of the primary deviation with the nonrestricted eye (left eye) fixing and a 20 prism diopter prism placed in front of the restricted right eye. With the prism in front of the restricted right eye, the image is on the fovea with the eye resting in esotropic position. Note that the fixing eye is always the eye without the prism, regardless of which eye is actually viewing. The three drawings to the right show the secondary deviation (right eye fixing): (A) A 20 prism diopter prism is placed in front of the left eye. The left eye picks up fixation by adducting, and causes the right eye to abduct because of Hering’s law. (B) The amount of force required to move the unrestricted left eye is minimal, so the right eye gets minimal innervational force. With a 20 prism diopter base-out prism over the left eye, the right eye does not abduct sufficiently to place the image on the fovea. (C) A 40 diopter base- out prism causes the left eye to deviate greatly, moving the restricted right eye enough to place the image on the fovea. Secondary deviation equals 40 prism diopters, whereas the primary deviation is 20. Note that, with the prism over the left eye, the restricted right eye must come to primary posi- tion to fixate.
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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 fix- ation 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 dis- tance 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, con- vergence 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 het- erophoria method and the lens gradient method. These tests are based on changing the patient’s accommodation and then meas- uring the associated change in convergence. Accommodation is changed by either changing the fixation distance (heterophoria
method) or by changing the amount of accommodation needed for a specific fixation distance by introducing various numbers of plus and minus spherical lenses (lens gradient method). A third method that does not actually measure the AC/A ratio but measures the relationsip between the distance and near devia- tion is the clinical distance–near relationship. The clinical dis- tance–near relationship provides information about the overall change in convergence when one looks from distance to near, including the effects of accommodation and proximal conver- gence. Most clinicians use either the clinical distance–near rela- tionship method or the lens gradient method to determine the accommodation to convergence relationship.
When measuring the AC/A ratio for any of these methods, it is important to use accommodative targets, have the patient wear their full optical correction, use alternate cover testing to measure the deviation, and control the fixation target distance.
By convention, 6 m (20 ft) is used for distance and 1/3 m (14 in.) for near. Normal AC/A ratio for the heterophoria method and lens gradient method is 4:1 and 5:1, and ratios of 6:1 or more are considered high. For calculations of the AC/A ratio, esodevia- tions are represented as positive numbers and exodeviations as negative numbers.
Heterophoria Method The heterophoria method compares the distance and near deviation to determine the AC/A ratio. It requires measurement of the distance and near deviation in prism diopters and the interpupillary distance in centimeters.
The following formula is used to calculate the AC/A ratio by the heterophoria method, where IPD is interpupillary distance (cm), D is distance deviation (PD), N is near deviation (PD), and D A is diopters of accommodation for near fixation (1/3 m 3 diopters):
Formula: AC/A IPD Example 1.
Distance ET 31 Near ET 40
Interpupillary distance 50 mm
Nearest target distance 1/3 m 3 D accommodation AC/A 5 (40 31)3 8 (high AC/A ratio)
N D
D A 162 handbook of pediatric strabismus and amblyopia
LENSGRADIENTMETHOD
The lens gradient method determines the AC/A ratio by meas- uring the change in ocular deviation associated with a specific change in lens-induced accommodation. This test changes accommodation by having the patient view an accommodative target through supplemental plus or minus spherical lens. A plus lens relaxes accommodation so that with less accommodation there is less convergence. A minus lens causes increased accom- modation, increased convergence, and an eso-shift. The AC/A ratio is calculated by measuring the deviation at a set distance, with and without supplemental spherical lenses, and dividing the difference by the lens power used. Measurements are usually made in the distance to minimize proximal convergence, and a
3.00 diopter lens is usually used.
The formula for the gradient method is AC/A
Example 1.
Deviation without lens ET 40 Deviation with 3.00 lens ET 10
AC/A 10 (high AC/A ratio) Example 2.
Deviation without lens XT 4 Deviation with 3.00 lens ET 14
AC/A 6 (normal AC/A ratio)
Another useful calculation is to estimate the effect of a spec- tacle lens on a deviation, given an estimated AC/A ratio, as shown in Examples 3 and 4:
Example 3.
If a child is assumed to have a normal AC/A ratio (5) and an exophoria of 10 PD, what is the effect of changing the patient’s spectacle correction by 2.00 diopters? As the minus 2.00 lens increases accommodation by 2.00 diopters, and convergence is increased by a ratio of 5 to 1 (AC/A ratio 5), the 2.00 lens overcorrection would result in 10 PD of convergence and orthophoria.
4 14
3
40 10
3
Deviation without lens Deviation with lens Lens in diopters
Example 4.
A child has a 4.00 refractive error and a 30 PD esotropia.
Assuming the AC/A ratio is high normal (6), what will be the effect of the full hypermetropic correction on the deviation? A
4.00 diopter lens will cause 24 PD of divergence; thus, the devi- ation with glasses will be esotropia of 6 PD. Thus, prescribing the hypermetropic glasses would have a good chance of correct- ing the deviation with only a small residual deviation.
Clinical Distance–Near Relationship
The clinical distance–near relationship does not specifically measure the accommodative convergence nor is it a ratio. It is a simple comparison of the deviation in the distance to the devi- ation at near. One can figure the clinical distance–near rela- tionship by subtracting the distance deviation from the near deviation. A distance–near difference within 10 PD is considered normal whereas differences greater than 10 PD are considered high. This clinical distance–near relationship is a simple, but very useful, method for identifying patients with a high AC/A ratio.
N D clinical distance–near relationship
D distance deviation viewing target at 6 m (20 ft) N near deviation viewing target at 1/3 m Example 1: D ET 20
N ET 40
AC/A relationship: 40 20 20 (high AC/A ratio) Example 2: D XT 10
N ET 20
AC/A relationship: 20 (10) 30 (high AC/A ratio) Example 3: D ortho
N XT 15
AC/A relationship: 15 0 15 (low AC/A ratio)
Lancaster Red-Green Test
The Lancaster red-green test is a fovea-to-fovea test with two fixation targets, one that the examiner controls and one con- trolled by the patient. This test is very useful for measuring incomitant strabismus in patients with diplopia and NRC. The 164 handbook of pediatric strabismus and amblyopia
fixation targets are red- and green-colored linear streaks of light that are projected on a screen. The patient wears red-green glasses (usually red over right eye) and holds one light (green light in Fig. 5-14) while the examiner holds the second light (red light in Fig. 5-14). The examiner projects the red light on the screen, and the patient is directed to look at the red light.
Because the patient’s right eye with the red filter only sees the examiner’s red light, the right eye (fovea) aligns with the exam- iner’s light. Thus, the right eye becomes the fixing eye and its position is controlled by where the examiner places the red light.
Next, the patient is directed to aim the green light (which they are holding) over the examiner’s red light. Because the left eye only sees the green light, the patient moves the green light over the red light by orienting the green light so it falls on the left fovea. The patient now sees the two lights superimposed, as both lights fall on the fovea of each eye. The patient in Figure 5-14 has a left esotropia, so with the green filter over the left eye, the patient directs the green light to the right of the red light. Patients with orthotropia will place the lights on top of each other, whereas a patient with a left exotropia will point the green light to the left of the red light.
The Lancaster red-green test directly shows the examiner where the eyes (foveas) are pointing, which is just the opposite of diplopia tests. The amount of deviation is measured by the amount of separation between the two projected lights on the screen. With the Lancaster red-green test, the eye that sees the examiner’s light is the fixing eye, so the examiner can move the target to various positions on the screen to measure the deviation in eccentric fields of gaze. Primary versus secondary deviations can be measured by the examiner trading lights with the patient. Torsion can also be assessed in various positions of gaze by observing the tilt of the lines on the screen. Nasal dis- placement of the top of the line indicates intorsion, and tempo- ral displacement of the top of the line indicates extorsion.
TORSION
MADDOXROD ANDTORSION
The line seen with the Maddox rod can be used to determine subjective torsion, with a single lens (single Maddox rod test) or a lens over each eye (double Maddox rod test). With the double Maddox rod test, the patient is asked to make the two streaks of the Maddox rod parallel. If the eyes are straight, a prism can
166 handbook of pediatric strabismus and amblyopia
be used to induce a deviation either horizontally or vertically to separate the lines of the Maddox rod. Patients without torsion see parallel lines (Fig. 5-15A), those with intorsion see the 12 o’clock position turned nasally (Fig. 5-15B), and those with extorsion see the 12 o’clock position turned temporally (Fig. 5- 15C). Note that the Maddox rod tests, and most subjective torsion tests for that matter, do not localize the eye with the torsion; they only measure the relative difference in torsion between the two eyes. One often finds a monocular torsion with the subjective Maddox rod testing but detects bilateral torsion by objective testing with indirect ophthalmoscopy because the eye that the patient perceives to have torsional misalignment depends on which eye is fixing (ocular dominance). To find the total torsion with the double Maddox rod, add the torsion of the two eyes together.
TORSIONALDIPLOPIA INFREEVIEW
Patients with retinal intorsion view the world as being extorted, and retinal extorsion cause objects to be perceived as being intorted. A person with intorsion sees the top of a vertical line tilted temporally, and extorsion will cause the top of a vertical line to appear to be shifted nasally.
FIGURE 5-14. Lancaster red-green test in a patient with normal retinal correspondence (NRC), esotropia, and diplopia; this is a fovea-to-fovea test. The patient fixates on the streak of light projected by the examiner.
The patient then directs the other light to align with the examiner’s light.
Patient will perceive a single streak of light as each light falls on the cor- responding fovea, even though the streaks are separated.
A B C
FIGURE 5-15A–C. Double Maddox rod test with the patient perceiving the streak of light vertically. Remember, this is not a localizing test, and change in torsion is relative to the fellow eye. (A) Normal patient with no torsion. The Maddox rods are aligned at the zero position for each eye.
(B) A patient with right incyclotorsion 15°, relative to the left eye, with the right Maddox rod turned clockwise. (C) Patient with bilateral extor- sion, 15° each eye. Total extorsion is 30°.
OBJECTIVERETINALTORSION
Objective retinal torsion is used to estimate the relationship of the fovea to the optic disc. In normal patients, the fovea is located between the midpoint and the lower border of the optic nerve. Patients with torsion will have a shift in the position of the fovea relative to the optic disc. With extorsion, the fovea is shifted below the inferior border of the optic disc, whereas intor- sion shifts the fovea higher than the midpoint of the optic nerve.
In actuality, the fovea is the center of vision and the optic nerve actually rotates around the fovea. Remember that the indirect ophthalmoscopic view is inverted, so extorsion is viewed when the fovea is above the upper pole of the disc, and intorsion is viewed when the fovea is below the midpoint of the disc. See Chapter 3 (Fig. 3-14) for an example of objective retinal torsion.
Special Tests for Identifying Restriction and Paresis
Tests for identifying restriction and paresis include forced duction testing, generated forced duction testing, and saccadic velocity measurement. Restriction and paresis can coexist, espe- cially in cases of long-standing muscle paralysis such as a long- standing sixth nerve palsy. In these cases, the antagonist of the paretic muscle (i.e., the medial rectus muscle in the case of a sixth nerve palsy) contracts and becomes stiff, thus adding a component of restriction to the paralytic condition.
FORCED-DUCTIONTESTING
Forced ductions are indicated if there is evidence of restricted ductions. Forced ductions is somewhat invasive, however, but can be performed on most cooperative adults. In patients who are scheduled for surgery, forced ductions are performed at the time of surgery. The technique for rectus muscles is to grasp the eye at the limbus and slightly proptose the eye, then rotate the eye into the field of limited ductions. If the eye is inadver- tently pushed posteriorly during testing, the rectus muscles will slacken, which may cause the examiner to possibly miss a rectus muscle restriction. When examining awake patients, be sure to ask the patient to look in the direction of the forced ductions to relax the muscle that is being tested. The tightness of oblique muscles can be assessed by a retropulse maneuver called the exaggerated traction test, developed by Guyton.4
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ACTIVEFORCED-GENERATIONTEST
Active forced-generation testing assesses rectus muscle strength. The eye is anesthetized with a topical anesthetic, and the eye is grasped with forceps at the limbus in the same fashion as forced-duction testing. The patient is asked to look into the field of limitation while the eye is held in primary position (Fig.
5-16). This author prefers to use a cotton tip applicator instead of forceps, as forceps can tear the conjunctiva. The examiner feels the force generated by the muscle and compares this with the fellow, nonaffected eye. This test is useful in assessing the amount of muscle function associated with any palsy such as sixth nerve paresis or double elevator palsy.
SACCADICVELOCITYMEASUREMENT
There are various ways to measure saccadic velocities. Clinical estimation is available to all clinicians and is simply the obser- vation of fast eye movements. Fast eye movements can be elicited by having the patient look quickly from side to side or by using an optokinetic nystagmus (OKN) drum. An OKN drum is very useful in young children. Patients with rectus palsies will not be able to generate saccades. Quantitation of eye movements can be made by special equipment such as the electro-oculogram (EOG), which measures the velocity of eye movements. Figure 5-17 shows an EOG tracing of a patient with a sixth nerve paresis. The initial part of the tracing shows a vertical spike indi- cating adduction movement; however, the end of the tracing shows a mild slope indicating slow abduction. Clinically, if the patient is able to generate a saccadic eye movement in the direc- tion of the eye limitation, then the limitation is restrictive and not secondary to paralysis. Normal saccadic velocity depends on the size of the saccadic eye movement. Large eye movements have higher peak velocities. Normal saccadic velocities range from 200 to 700 degrees per second (°/s).1
RESTRICTION
Forced-duction testing is a useful test for identifying restric- tions. If the eye can not be easily rotated into the field of limited ductions, then a restriction is present. Another sign of restric- tion is the “dog on a leash” eye movement. A patient with restrictive strabismus and good muscle function will show normal saccadic (fast) eye movements until the eye reaches the
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A
B
FIGURE 5-16A,B. Forced-generation test on patient with a right sixth nerve palsy. (A) Patient viewing in primary position with the right eye anesthesized and a dry cotton-tipped applicator placed to the temporal limbus. (B) The patient looks to the right and attempts to abduct the right eye. Pressure by the cotton-tipped applicator pushing the eye nasally, pre- vents the right eye from moving. The examiner can feel the amount of force exerted by the right lateral rectus through the cotton-tipped appli- cator. Normally, the applicator could not hold the eye in adduction when the patient is actively abducting.
