Ординатура / Офтальмология / Английские материалы / Pediatric Ophthalmology Current Thought and A Practical Guide_Wilson, Saunders, Trivedi_2008

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Contents

10.2.1Cover Tests  . . . . . . . . . . . . .   114

10.2.2Ocular Rotations  . . . . . . . . . . .   120

10.2.3Other Tests of Motor Function  . . . . .   120

10.2.4Assessment of Control  . . . . . . . .   122

10.3The Sensory Evaluation  . . . . . . . .   123

10.3.1The History  . . . . . . . . . . . . .   124

10.3.2Sensory Fusion  . . . . . . . . . . .   125

10.4Orthoptic Treatment  . . . . . . . . .   136

Core Messages

•The sensorimotor examination clarifies the patient’s symptoms and expectations, precisely quantifies the ocular deviation, and determines the potential for binocular single vision in a focused, efficient manner.

•The goals of orthoptic therapy are to minimize the risk of post-operative overor under-correction, to improve comfortable control over a non-surgical deviation, and to alleviate symptoms of diplopia and visual confusion.

10.1 Overview

In the broadest sense, the purpose of the sensorimotor exam is to efficiently gather information on eye align-

aid both the clinician and the patient in forming reasonable treatment goals and expectations.

A preand post-operative sensorimotor exam is recommended for all patients undergoing strabismus surgery. It is mandated for any patient with persistent binocular vision complaints (such as diplopia, visual confusion, or asthenopia) neither attributable to defects in the ocular media nor ameliorated with appropriate refractive correction. The sensorimotor exam can be worthwhile pre-operatively for patients scheduled for cataract or refractive surgery. But the orthoptic evaluation, particularly the sensory component, is invaluable in any surgical candidate beyond the age of visual maturity with a history of long-stand- ing strabismus. It is this subgroup of patients that is most likely to suffer the consequences of pre-existing anomalous binocular vision, resulting in diplopia in spite of alignment of the visual axes.

This chapter is not intended to be a step-by-step comprehensive tutorial on how to perform the various steps and tests in an orthoptic exam, nor is it intended to present an exhaustive menu of all possible tests and methods used in the orthoptic evaluation. A familiarity with commonly used tests is assumed. Rather, while some new methods will be presented, the balance of the chapter covers the finer points of some universally used tests, avoiding common sources of error, presenting new ways to use standard tests, and alternative ways to interpret the results. The focus is on exam efficiency: learning the most from the exam in the least amount of time, utilizing a minimum number of well-known methods.

This chapter is organized in what is hoped the reader will find to be a logical fashion, covering the Motor Evaluation first, followed by the Sensory

Evaluation, and concluding with Treatment. Some clinicians feel strongly that sensory testing should be done prior to any other exam component with the exception of the History. They argue, in spite of evidence to the contrary, that even testing vision has the potential to “break fusion,” dissociate the eyes, and destroy any trace evidence of a fragile binocularity[29] . However, there are several sound reasons for, at minimum, quantifying visual acuity and eye alignment in primary position at distance and near before any sensory testing is done. Without some information on acuity and alignment, it is difficult to select the appropriate fixation target or sensory test, and even more so to interpret the result obtained.

By doing the sensory evaluation first, the examiner works in an open-loop system, without appropriate input or feedback guiding clinical judgment. To avoid inadvertently omitting some critical test, one is forced to perform every test! This is an inefficient approach to the sensorimotor exam, which at best results in wasted time, at worst in lost cooperation from the patient. In short, the motor exam can direct and even predict the course of the sensory exam, but the converse is not true.

Itisconceivablymoreimportanttofindevidenceof strong, but anomalous fusion in a pre-operative sensorimotor exam, than any vestige of a brittle binocularity in the post-operative evaluation. Deep-rooted fusion ability will not vanish irretrievably with brief occlusion during vision testing or even with alternate cover testing. Moreover, binocularity that is as tenuous as to be unrecoverable following vision testing may be of no consequence to the patient and should not impact any management decisions.

10.2 The Motor Evaluation

While most clinicians equate an orthoptic evaluation with the sensory exam alone, the motor exam is an equally important and necessary component, requiring expertise and attention to detail. The purpose of the motor evaluation is threefold: (1) to detect and quantify an eye misalignment; (2) to evaluate the function of each extraocular muscle; and (3) to assess control over the deviation. Results of the motor exam will determine if the patient is a candidate for surgical vs non-surgical therapy and, for the surgical candidate, which muscles should be targeted and how much should be done. There are many case-selective tests that may be used in the motor evaluation, some requiring special equipment, but the two basic methods used universally are the cover tests and version/ duction testing.

10.2.1 Cover Tests

The cover tests in all their variations are the cornerstone of the motor exam. They are very familiar even to the dabbler in strabismus, yet they are deceptively

difficult and easily done incorrectly. Performed well, they may obviate the need for other motor tests. But under the right circumstances, small errors in execution may lead to large errors in measurement! A review of proper technique is therefore appropriate. There are three primary sources of error in cover testing: (1) errors in fixation target selection; (2) errors in prism placement; and (3) errors in occlusion.

An ideal fixation target is one that both stimulates and controls accommodation. For a patient with 20/20 vision in both eyes, this means a target no larger than

20/40 [39]. For those with reduced vision in one or both eyes, a target no larger than two Snellen lines above the visual threshold for the poorer-seeing eye must be used.

Once the target has been identified by the patient, it will lose much of its accommodative power. Staring at the same 20/30 letter for minutes at a time does not control accommodation! The ideal target is a computer screen with multiple, randomized lines of 20/30 letters that can be refreshed frequently as the patient reads aloud during the cover test. Modern computerized vision testing has made this easy to accomplish for distance fixation.

Near fixation is more problematic, as most reduced

Snellen cards or sticks present optotypes over a wide

scale from 20/400 to 20/20, with relatively few lines qualified to be in the “accommodative target” range. In answer to this, the Snow fixation stickers were developed (by J. Snow, C.O., Penn State Hershey Medical Center, Hershey, Pa.). These stickers feature near targets with multiple lines of 20/30 to 20/60 letters (Fig. 10.1). They can be applied to a tongue depressor, to the bridge of the examiner’s glasses, or directly onto the examiner’s nose for fixation. The latter two methods are favored because they do not require the patient to hold the fixation target, which can be difficult for both the very young and the very elderly, yet they free both of the examiner’s hands for prism and cover testing.

When examining the very young it is even more important to have one’s hands free! It is also more difficult to find a true accommodative target that will capture the child’s attention long enough for testing. Finger puppets, small toys, and most reward stickers tend to be too large to be considered accommodative. Small, detailed stickers affixed to the examiner’s nose are ideal for this purpose. These may be found in gift, card, or stationery stores (Fig. 10.2).

The second common error in cover testing is incorrect prism placement. These errors include inadvertent rotation of the prism around the x-, y-, or

z-axis, or errors in vertex distance in which the prism is held too far away from the eye.

The strength of any given prism is determined by the index of refraction of the optical medium and the angle formed at the apex of the prism. The labeled value on the prism is the minimum deviation of a light ray that can be produced by that particular prism; however, the prism diopter value stamped on the base is only accurate if the prism is held in the position for which it was calibrated. The effective power of a prism can be altered by rotating the prism before the eye to increase or decrease the angle at which light rays will strike the surface. The larger the

angle at the apex of the prism, the greater the potential error in measurement with only slight alterations in prism position.

Most loose prisms in use today are plastic and calibrated for use in the position of minimum deviation (PMD; Fig. 10.3). This is the position in which the same amount of refraction will occur at both the anterior and posterior surfaces of the prism. When a plastic prism is rotated away from the PMD, the effective power of the prism is increased, and the deviation being measured will be underestimated. Rotation of a prism around the z-axis will cause measurement errors in esoand exodeviations, and rotation around

the x-axis causes errors in vertical strabismus measurement.

Unfortunately, the PMD is awkward as it requires the examiner to hold the prism in a slight rotation that is difficult to estimate quickly and repeat through frequent prism changes during cover testing. Holding the prism in the frontal plane position (FPP), in which the posterior face of the prism is parallel to the orbital rim, is easier to locate, more comfortable for the examiner, and is close enough to the PMD that it is used interchangeably. Errors in prism rotation may be minimized by using a prism bar or rotary prism.

Measurement errors due to accidental prism rotation are particularly common when horizontal deviations are measured in right or left gaze, or when vertical deviations are measured in up or down gaze.

Care must be taken not to rotate the plastic prism with the head into these secondary positions of gaze, as this will move the prism out of the frontal plane (Fig. 10.4). This error occurs with particular fre-

quency when measuring large exodeviations in side gazes, and may reveal a false lateral incomitance [34].

When measuring large deviations in the secondary positions of gaze, it is also important to remember that the globe does not have unlimited freedom of rotation due to the considerable elastic and stabilizing forces within the orbit. Underestimation of the deviation may occur if the prism is placed in such a way that it forces the eye to rotate beyond its capacity in order to take up fixation. The eye will stop shifting with repeated alternating cover because the eye has reached its limit of rotation, not because the deviation has been completely neutralized. The larger the deviation, the more of a problem this may cause. To avoid this testing artifact, place the prism over the eye with the greatest room to move. For example, base-in prism should be placed over the adducted eye when measuring large exodeviations in side gazes. The important exception to this rule

is found in the case of restrictive or paralytic strabismus, in which the prism must be placed over the affected eye regardless of the gaze being measured. Again, the examiner may not observe a shift during cover testing because the affected eye simply cannot move into the position of gaze being tested. Putting the prism over the restricted eye moves the image to meet the eye, rather than forcing the eye to move to the image.

Inadvertent rotation of a horizontal or vertical prism around the y-axis will not only overestimate the deviation, but will also create the appearance of, or exacerbate, a multiplanar deviation. The examiner must be acutely mindful of y-axis rotation when measuring a deviation in head tilts, as only small changes in measurement of a vertical strabismus are often used to determine if the result is positive or negative.

The prism base should remain parallel with the floor of the orbit, not the floor of the room, with head rotation to the shoulder (Fig. 10.5).

The effective power of a prism will also change with a change in the distance of the prism to the eye. Though obviously not possible, the ideal position for the prism would be the center of rotation of the eye. In lieu of this, the prism should be held as close as is reasonable to the eye: typically no more than 1−2 cm away from the cornea. The distance must be appropriate but also consistent throughout the cover test. It is helpful to brace the side of your middle finger against the patient’s brow bone, while holding the prism between the index finger and thumb. If the patient wears

spectacles, the prism can be held directly against the frame of the glasses.

Errors in prism position may also occur when measuring deviations that exceed the largest prism available. Two plastic prisms with the base in the same direction cannot be stacked back to back as this completely alters the angle of incidence and, consequently, the angle of refraction. To some extent, the errors induced by stacking can be minimized by splitting the power with one prism before each eye; however, angles measured in prism diopters are not additive, and so even this method is not entirely accurate. The deviation measured by the sum of two prisms will always be greater than the sum of the calibrated values. There are tables that display the total value for prisms held in this manner [38].Alternatively, the true deviation in prism diopters can be calculated by multiplying the tangent of the total angle of deviation in degrees by 100.

In general, all measurement miscalculations due to inaccurate prism placement have the potential of being exaggerated at near fixation. Because of the shorter working distance, the often larger deviations, and the natural inward rotation of the visual axes, errors can be magnified.

Inadequate occlusion is the final, and possibly the most common, type of error leading to inaccurate measurement of strabismus. In order to reveal the entire deviation, the eyes must be maximally dissociated.

To accomplish this, an opaque or translucent occluder held close to the eye is recommended. This is prefer-

able to the popular “thumb” occluder (Fig. 10.6), or other variations held away from the face. Occlusion methods such as these may be adequate for vision testing, because they do successfully obstruct the fovea, but they do not sufficiently block the peripheral visual field. This is necessary in order to minimize peripheral motor fusion. Motor fusion (also known as fusional or disparity vergence), the ability to align the eyes in such a manner as to image the object of regard on corresponding retinal points to allow sensory fusion, is driven primarily by matching input from the overlapping areas of our visual fields. Non-foveal input from the central and peripheral retina has the most powerful influence on vergence, simply because there are more retinal elements to match [18]. Foveal input has comparatively little influence on motor fusion. This can be easily demonstrated by occluding the fovea of one eye using a remote thumb, and introducing a 14−16 base-out prism over one eye. Even in the absence of foveal input, fusional convergence will be observed in the subject with average fusional vergence ability as long as adequate peripheral input remains. In contrast, if the subject now fixates through a single pinhole mounted in a trial frame as base-out prism is introduced, convergence is not induced and diplopia results.

A good cover test requires patience and repetition, particularly in the case of intermittent or latent deviations, or in cases with decreased acuity. The occluder should be held in place for a few seconds to ensure that the patient is able to achieve fixation before it is

rapidly transferred to the other eye without allowing a period of binocularity. It must be kept in mind that high-powered prisms will degrade the vision of the eye viewing through it. The occluder may have to be left in place over each eye a bit longer to allow the patient time to fixate through a high-powered prism.

Measuring to reversal, meaning the prism and alternate cover test is continued, adding prism past the point where no shift in the visual axes is observed, is also recommended. The full deviation is then the last prism value that produced no shift in fixation.

Variations of the cover test used in certain specific situations include the cover−uncover test, the simultaneous prism and cover test, and diagnostic occlusion. The cover−uncover test is often used not only to detect a manifest deviation, but to assess control over an intermittent deviation by estimating how easily fusion is disrupted and how quickly it is regained once the cover is removed (see Sect. 10.2.4).

The simultaneous prism and cover test is used to quantify the manifest deviation when a latent deviation is also present. As the name of the test suggests, it is important to simultaneously place the prism over the deviating eye as the cover is introduced over the fixating eye. In other words, it is not equivalent to holding a prism over the deviating eye during a cover−uncover of the fixating eye. By definition, those who have both a manifest and a latent component to their deviation have motor fusion (fusional vergence). By maintaining a prism over the deviating eye while the patient is binocular, motor fusion is re-

cruited in order to maintain the image of the fixation target on the preferred perifoveal retinal point. If performed in this way, the test results will over-estimate the manifest deviation.

Diagnostic occlusion (Marlow occlusion [29] or the Patch Test), is prolonged monocular occlusion and is, essentially, a protracted cover test. The patch is kept in place over a period of 30−60 minutes, but it may be continued over 24−48 hours. The better the patient’s control over the deviation, the longer the period of diagnostic occlusion necessary in order to fully appreciate the deviation. Diagnostic occlusion is most commonly used pre-operatively in cases of exotropia suspected to be of the pseudo-divergence excess type. It is also valuable in the symptomatic patient with a phoria, or the child whose parent sees a deviation that is not apparent with standard cover testing. Rules of fixation targets, prism placement, and adequate occlusion apply to these cover-test modifications.

10.2.2 Ocular Rotations

The motor exam includes the evaluation of the function of each individual extraocular muscle by observing version and duction movements. The standard method of grading function compares muscle action relative to its yoke and is based on a -4 to +4 scale. If mobility is restricted such that the eye is not even able to move into primary position, negative numbers larger than -4 may be used. Investigating ocular rotations in this manner may be technically easy to do, though the results are sometimes difficult to interpret. Variations in soft tissue or orbital anatomy may result in the strong illusion of muscle imbalance, particularly in the secondary and tertiary positions of gaze. In addition, this system assesses comparative muscle function. Limitation of movement may be underestimated if it is bilateral (whether symmetric or asymmetric), except in the case of obvious and severe under-action. Though there is little inter-observer reproducibility with this method, it is the one most commonly used and is probably adequate for most cases. Certainly in the case of muscle “over-action,” this method is clinically useful.

Another method of quantifying ocular rotations estimates function in degrees based on the same scale

used for the Hirschberg test (1 mm ≈ 7º). As the examiner holds a transilluminator or penlight held at the midline of the face, the patient follows a fixation target as it moves into the diagnostic positions of gaze. The corneal light reflex should remain in approximately the same location in each eye if there is no overor under-action present. If a limitation is present, the reflex in the affected eye will become displaced. The amount of limitation in degrees can be estimated based on the amount of displacement of the light reflex in millimeters. This method may be a bit more precise but suffers from the same drawbacks as the conventional method, though perhaps to a lesser degree.

For the purposes of measuring change over time, more meticulous and reproducible techniques are preferred. One of these methods uses the cervical range of motion (CROM) device, an instrument originally designed for the purpose of measuring the range of motion of the cervical spine. The apparatus consists of three meters representing position with respect to the x- (chin elevation or depression), y- (head tilt), and z- (face turn) axes, mounted in a spectacle frame and secured to the patient’s face with Velcro straps. Rather than comparing function to the yoke muscle, the CROM quantifies the degrees of excursion or range of movement with the fellow eye occluded.

The head is rotated into the primary field of action of each of the extraocular muscles as the patient fixates an accommodative target. When the limits of rotation are reached, the image will slip off the fovea, causing blur. The degree of excursion into that field can then be read directly off of the dials on the apparatus. This method has proven to be reliable and repeatable [24].

10.2.3 Other Tests of Motor Function

While the motor evaluation typically starts with cover testing and observation of versions, results of these tests may indicate the need for further motor evaluation in some cases. Additional tests may include prism adaptation, diagnostic occlusion, calculation of the AC/A ratio, tests for torsion, and Lancaster redgreen or Hess/Lees screen testing.

Prism adaptation is accomplished by offsetting the manifest deviation with temporary prisms. After a period of hours to days, the alignment and binocular