Table 12-2: Yoke muscles in cardinal positions of gaze.

Development of Binocular Movement

The neuromuscular system of an infant is immature, so that it is not uncommon in the first few months of life for ocular alignment to be unstable. Transient esodeviations are most common and may be associated with immaturity of the accommodation-convergence system. Gradually improving visual acuity together with maturation of the oculomotor system allows a more stable ocular alignment by age 2 months. Any ocular misalignment after this age should be investigated by an ophthalmologist.

2. SENSORY ASPECTS

Binocular Vision

In each eye, whatever is imaged on the fovea is seen subjectively as being straight ahead. Thus, if two dissimilar objects were imaged on the two foveas, the two objects would be seen superimposed, but the dissimilarities would prevent fusion into a single impression. Because of the different vantage point in space of each eye, the image in each eye is actually slightly different from that in the other. Sensory fusion and stereopsis are the two different physiologic processes that are responsible for binocular vision.

Sensory Fusion & Stereopsis

Sensory fusion is the process whereby dissimilarities between the two images are not appreciated. On the peripheral retina of each eye, there are corresponding points that in the absence of fusion localize stimuli in the same direction in space. In the process of fusion, the direction values of these points can be modified. Thus, each point of the retina in each eye is capable of fusing stimuli that strike sufficiently close to the corresponding point in the other eye. This region of fusible points is called Panum's area.

Fusion is possible because subtle differences between the two images are ignored, and stereopsis, or binocular depth perception, occurs because of the cerebral integration of these two slightly dissimilar images.

Sensory Changes in Strabismus

Up to age 7 or 8, the brain usually develops responses to abnormal binocular vision that may not occur if the onset of strabismus is later. These changes include diplopia, suppression, anomalous retinal correspondence, and eccentric fixation.

A. Diplopia:

If strabismus is present, each fovea receives a different image. The objects imaged on the two foveas are seen in the same direction in space. This process of localization of spatially separate objects to the same location is called visual confusion. The object viewed by one of the foveas is imaged on a peripheral retinal area in the other eye. The foveal image is localized straight ahead, while the peripheral image of the same object in the other eye is localized in some other direction. Thus, the same object is seen in two places (diplopia).

B. Suppression:

Under binocular viewing conditions, the images seen by one eye become predominant and those seen by the other eye are not perceived (suppression). Suppression takes the form of a scotoma in the deviating eye only under binocular viewing conditions. (A scotoma is an area of reduced vision within the visual field, surrounded by an area of less depressed or normal vision.) Suppression scotomas in esotropia are usually approximately elliptical in shape, extending on the retina from just temporal to the fovea to the point in the peripheral retina where the object of regard for the other eye is imaged. In exotropia, the suppression area tends to be larger and extends from the fovea to usually the entire temporal half of the retina. When fixation shifts to the other eye, the suppression scotoma also switches to the newly deviating eye. In the absence of strabismus, a blurred image in one eye may also lead to suppression. The lack of simultaneous perception in the central retina prevents fine stereopsis, though crude stereopsis from the peripheral retina may still be present.

C. Amblyopia:

Prolonged abnormal visual experience in a child under the age of 7 years may lead to amblyopia (reduced visual acuity in the absence of detectable organic disease in one eye). The two clinical contexts in which amblyopia occurs are strabismus and any disorder that causes a blurred retinal image in one or both eyes, eg, a significant refractive difference between the eyes (anisometropia) or visual deprivation such as that due to congenital cataract.

In strabismus, the eye used habitually for fixation retains normal acuity and the nonpreferred eye often develops decreased vision (amblyopia). If spontaneous alternation of fixation is present, amblyopia does not develop. Suppression and amblyopia are different processes. Amblyopia is present when the affected eye is tested alone. Suppression occurs under binocular conditions and is a process in which the brain "ignores" a portion of the image received from the deviating eye so that the patient avoids diplopia. This visual field defect is termed a facultative scotoma, since no visual deficit can be demonstrated when the suppressing eye is tested alone.

D. Anomalous Retinal Correspondence:

Anomalous retinal correspondence is a sensory adaptation that occurs in strabismus under binocular viewing conditions. Heterotropia leads to suppression in the nonfixating eye and a shift in the visual direction of the deviated eye. This shift in visual direction offsets the amount of motor deviation and prevents the perception of diplopia. This binocular phenomenon allows some form of binocular cooperation to occur in patients with strabismus, but stereopsis will remain abnormal.

E. Eccentric Fixation:

In eyes with sufficiently severe amblyopia, an extrafoveal retinal area may be used for fixation under monocular viewing conditions. It is always associated with severe amblyopia and unstable fixation. The eccentric fixation point is often not displaced in a direction appropriate to the direction of strabismus (eg, the nasal retina in esotropia). Gross eccentric fixation can be readily identified clinically by occluding the dominant eye and directing the patient's attention to a light source held directly in front. An eye with gross eccentric fixation will not point toward the light source but will appear to be looking in some other direction. More subtle degrees of eccentric fixation can be detected by an ophthalmoscope that projects a small fixation target onto the retina. If any area other than the macula is selected for fixation by the patient, the presence of eccentric fixation has been established.

EXAMINATION

History

A careful history is important in the diagnosis of strabismus.

A. Family History:

Strabismus and amblyopia are frequently found to occur in families.

B. Age at Onset:

This is an important factor in long-term prognosis. The earlier the onset of strabismus, the worse the prognosis for good binocular function.

C. Type of Onset:

The onset may be gradual, sudden, or intermittent.

D. Type of Deviation:

The misalignment may be in any direction. It may be greater in certain positions of gaze, including the primary position for distance or near.

E. Fixation:

One eye may constantly deviate, or alternating fixation may be observed.

Visual Acuity

Visual acuity should be evaluated even if only a rough approximation or comparison of the two eyes is possible. Each eye is evaluated by itself, since binocular testing will not reveal poor vision in one eye. For the very young child, it may only be possible to establish that an eye is able to follow a moving target. The target should be as small as the child's age, interest, and level of alertness allow. Fixation is described as being normal if it is centrally (foveally) fixated and maintained while the eye follows a moving object. One technique for quantitatively measuring visual acuity in younger children is forced-choice preferential looking.

By the age of 21/2-3 years, it is possible to perform recognition visual acuity testing using the Allen pictures. By age 4 years, many children will understand the Snellen tumbling "E" game and the HOTV recognition test. By age 5 or 6 years, most children can respond to Snellen alphabet visual acuity testing. At this age, single optotype Snellen acuity has normally developed fully, but Snellen acuity to a line of multiple optotypes (linear acuity) may not develop fully for another 2 years.

Determination of Refractive Error

It is important to determine the cycloplegic refractive error by retinoscopy (see Chapter 20). The standard drug for producing complete cycloplegia in children under age 2 years is atropine, which may be given as 0.5% or 1% eye drops or ointment instilled twice a day for 3 days. Atropine should not be used in older children, since prolonged cycloplegia lasting up to 2 weeks will interfere with near vision. After age 2, cyclopentolate 1% or 2% is the preferred cycloplegic.

Inspection

Inspection alone may show whether the strabismus is constant or intermittent, alternating or nonalternating, and variable or constant. Associated ptosis and abnormal position of the head may also be noted. The quality of fixation of each eye separately and of both eyes together should be noted. Nystagmoid movements indicate unstable fixation and often reduced visual acuity.

Prominent epicanthal folds that obscure all or part of the nasal sclera may give an appearance of esotropia (pseudoesotropia). Although this entity is confusing to lay persons as well as some physicians, these children have a normal corneal light reflection test. Prominent epicanthal folds gradually disappear by 4 or 5 years of age.

Determination of Angle of Strabismus (Angle of Deviation)

A. Prism and Cover Tests:

(Figure 12-3.) Cover tests consist of four parts: (1) the cover test, (2) the uncover test, (3) the alternate cover test, and (4) the prism cover test. In all four tests, the patient looks intently at a target, which may be in any direction of gaze at distance or near.

1. Cover test-

As the examiner observes one eye, a cover is placed in front of the other eye to block its view of the target. If the observed eye moves to take up fixation, it was not previously fixating the target, and a manifest deviation (strabismus) is present. The direction of movement reveals the direction of deviation (eg, the eye moves outwardly if there is esotropia).

2. Uncover test-

As the cover is removed from the eye following the cover test, the eye emerging from under cover is observed. If the position of the eye changes, interruption of binocular vision has allowed it to deviate, and heterophoria is present. The direction of corrective movement shows the type of heterophoria.

3. Alternate cover test-

The cover is placed alternately in front of first one eye and then the other. This test reveals the total deviation (heterotropia plus heterophoria if also present).

4. Prism plus cover testing-

To quantitatively measure the deviation, an increasing strength of prism is placed in front of one or both eyes until there is neutralization of eye movement on alternate cover testing. For example, to measure full esodeviation, the cover is alternated while prisms of increasing base-out strength are placed in front of one or both eyes until the horizontal refixation movement of the deviated eye is neutralized.

B. Objective Tests:

Prism and cover measurements are objective in the sense that no report of sensory observations is required from the patient. However, cooperation and some degree of vision are required. Clinical determinations of eye position that require no sensory observation by the patient (objective tests) are considerably less accurate, although still useful at times. Two methods commonly used depend on observing the position of the corneal reflection of a light. Results by both methods must be modified by allowing for the angle kappa.

1. Hirschberg method-

The patient fixates a light at a distance of about 33 cm (13 inches). Decentering of the light reflection is noted in the deviating eye. By allowing 18^Δ for each millimeter of decentration, an estimate of the angle of deviation can be made.

2. Prism reflex method (Krimsky test)-

The patient fixates a light. A prism is placed before the deviating eye, and the strength of the prism required to center the corneal reflection measures the angle of deviation.

Ductions (Monocular Rotations)

With one eye covered, the other eye follows a moving target in all directions of gaze. Any decrease of rotation indicates weakness in the field of action of that muscle.

Versions (Conjugate Ocular Movements)

Hering's law states that yoke muscles receive equal stimulation during any conjugate ocular movement. Versions are tested by having the eyes follow a light in the nine diagnostic positions: primary-straight ahead; secondary-right, left, up, and down; and tertiary-up and right, down and right, up and left, and down and left (Table 12-2). Apparent rotation of one eye relative to the other is noted as overaction or underaction. By convention, in the tertiary positions, the oblique muscles are said to be overacting or underacting with respect to the yoke rectus muscle. Fixation in the field of action of a paretic muscle results in overaction of the yoke muscle, since greater innervation is required for contraction of the underacting muscles (Figure 12-4). Conversely, fixation by the normal eye will lead to underaction of the paretic muscle.

Disjunctive Movements

A. Convergence:

(Figure 12-5.) As the eyes follow an approaching object, they must turn inward in order to maintain alignment of the visual axes with the object of regard. The medial rectus muscles are contracting and the lateral rectus muscles are relaxing under the influence of neural stimulation and inhibition. (Neural pathways of supranuclear control are discussed in Chapter 14.)

Convergence is an active process with a strong voluntary as well as involuntary component. An important consideration in evaluating the extraocular muscles in strabismus is convergence.

To test convergence, a small object is slowly brought toward the bridge of the nose. The patient's attention is directed to the object by saying, "Keep the image from going double as long as possible." Convergence can normally be maintained until the object is nearly to the bridge of the nose. An actual numerical value is placed on convergence by measuring the distance from the bridge of the nose (in centimeters) at which the eyes "break" (ie, when the nondominant eye swings laterally so that convergence is no longer maintained). This point is termed the near point of convergence, and a value of up to 5 cm (2 inches) is considered within normal limits.

The ratio of accommodative convergence to accommodation (AC/A) ratio is a way of quantitating the relationship of convergence to accommodation. Accommodative convergence is elicited by viewing an accommodative target, ie, one that has resolvable contours or letters that stimulate accommodation. The result is commonly expressed as prism diopters of convergence per diopter of accommodation. The AC/A ratio is useful as a research tool to further investigate and clarify this relationship and has contributed significantly to our understanding and therefore to the treatment of accommodative eso-tropia-particularly in using bifocals and miotics, as described later in this chapter.

B. Divergence:

Electromyography has established that divergence is an active process, not merely a relaxation of convergence. Clinically, this function is seldom tested except in considering the amplitudes of fusion.

Sensory Examination

While many tests of the status of binocular vision have been devised, only a few need be mentioned here. The tests are for stereopsis, suppression, and fusion potential. All require the simultaneous presentation of two targets separately, one to each eye.

A. Stereopsis Testing:

Many stereopsis tests are done with targets and Polaroid glasses to separate the stimuli. The monocularly observed targets have nearly imperceptible clues of depth. Random dot stereograms have no monocular depth clues. A field of random dots is seen by each eye, but the dot-to-corresponding-dot correlation between the two targets is such that if stereopsis is present, a form is seen in three dimensions.

B. Suppression Testing:

The presence of suppression is readily demonstrated with the Worth four-dot test. Glasses containing a red lens over one eye and a green lens over the other are placed on the patient. A flashlight containing red, green, and white spots is viewed. The color spots are markers for perception through each eye, and the white dot, potentially visible to each eye, can indicate the presence of diplopia. The separation of the spots and the distance at which the light is held determine the size of the retinal area tested. Foveal and peripheral areas may be tested at distance and near.

C. Fusion Potential:

In individuals with a manifest deviation, the status of binocular fusion potential can be determined by the red filter test. A red filter is placed over one eye. The patient is directed to look at a distance or near fixation light target. A red light and a white light are seen. Prisms are placed over one or both eyes in an attempt to bring the two images together. If fusion potential exists, the two images come together and are seen as a single pink light. If no fusion potential exists, the patient will continue to see one red and one white light.

OBJECTIVES & PRINCIPLES OF THERAPY OF STRABISMUS

The main objectives of strabismus treatment in children are (1) reversal of the deleterious sensory effects of strabismus (amblyopia, suppression, and loss of stereopsis) and (2) best possible alignment of the eyes by medical or surgical treatment. In all cases, the psychologic benefit of cosmetically straight eyes cannot be overestimated.

Timing of Treatment in Children

A child can be examined at any age, and treatment for amblyopia or strabismus should be instituted as soon as the diagnosis is made. Neurophysiologic studies in animals have shown that the infant brain is quite responsive to sensory experience, and the quality of function possible later in life is greatly influenced by early life experiences. It has been shown that overall results are favorably influenced by early alignment of the eyes, preferably by age 2. Good eye alignment can be achieved later, but normal sensory adaptation becomes more difficult as the child grows older. By age 8, the sensory status is generally so fixed that deficient stereopsis and amblyopia cannot be effectively treated.

Medical Treatment

Nonsurgical treatment of strabismus includes treatment of amblyopia, the use of optical devices (prisms and glasses), pharmacologic agents, and orthoptics.

A. Treatment of Amblyopia:

The elimination of amblyopia is crucial in the treatment of strabismus and is always one of the first goals. The strabismic deviation may lessen-rarely enlarge-following the treatment of amblyopia. Surgical results are more predictable and stable if there is good visual acuity in each eye preoperatively.

1. Occlusion therapy-

The mainstay of amblyopia treatment is occlusion. The sound eye is covered with a patch to stimulate the amblyopic eye. Glasses are also used if there is a significant refractive error.

Two stages of successful amblyopia treatment are identified: initial improvement and maintenance of the improved visual acuity.

a. Initial stage-

Full-time occlusion is the standard initial treatment. In some cases only part-time occlusion is used if the amblyopia is not too severe or the child is very young. As a guideline, full-time occlusion may be done for as many weeks as the child's age in years without risk of reduced vision in the sound eye. Occlusion treatment is continued in some form as long as visual acuity improves (occasionally up to a year). It is not worthwhile continuing to patch for more than 4 months if there is no improvement.

Amblyopia is functional (ie, there is no identifiable organic lesion, although the adaptation must be cerebral). In most cases, if treatment is started soon enough, substantial improvement or complete normalization of visual acuity can be achieved. Occasionally, there is no improvement even under ideal conditions. Poor compliance with treatment (peeking around a patch or inadequate enforcement of patching by the parents) can always be a factor.

b. Maintenance stage-

Maintenance treatment consists of part-time patching continued after the improvement phase to maintain the best possible vision beyond an age when amblyopia is likely to recur (about age 8).

2. Atropine therapy-

A few children are intolerant to occlusion therapy. In such cases that have moderate or high hyperopia, atropine therapy may be effective. Atropine causes cycloplegia and therefore decreased accommodative ability. The sound eye is atropinized, and glasses are used to focus that eye for distance or near fixation only. This forces use of the amblyopic eye at all other times. Atropine 1%, 1 drop every few days, is usually sufficient for sustained cycloplegia.

B. Optical Devices:

1. Spectacles-

The most important optical device in the treatment of strabismus is accurately prescribed spectacles. The clarification of the retinal image produced by glasses allows the natural fusion mechanisms to operate to the fullest extent. Small refractive errors need not be corrected. If there is significant hyperopia and esotropia, the esotropia probably is at least partially due to the hyperopia (accommodative esotropia). The prescription compensates for the full cycloplegic findings. If bifocals permit sufficient relaxation of accommodation to allow for near fusion, they should be used.

2. Prisms-

Prisms produce optical redirection of the line of sight. Corresponding retinal elements are brought into line to eliminate diplopia. Correct sensory alignment of the eyes is also a form of antisuppression treatment. Used preoperatively, prisms can simulate the sensory effect that will follow successful surgery. In patients with horizontal deviation, prisms will show the patient's ability to fuse a simultaneous small vertical deviation, thus indicating whether surgery also needs to be done for the vertical component. In children with esotropia, prisms can be used preoperatively to predict a postoperative shift in position that might nullify the surgical result, and the planned surgery can be modified accordingly (prism adaptation test).

Prisms can be implemented in several ways. A particularly convenient form is the plastic Fresnel press-on prism. These plastic membranes can be placed on the glasses without the need for an optician and are very useful for diagnostic and temporary therapeutic purposes. For permanent wear, prisms are best ground into the spectacle prescription, but the amount is limited to about 5^Δ per lens since prismatic distortion becomes prominent at higher strengths.

C. Pharmacologic Agents:

1. Miotics-

Echothiophate iodide and isofluro-phate inactivate acetylcholinesterase at the neuromuscular junction and thus potentiate the effect of every nerve impulse. Accommodation becomes more effective relative to convergence than before treatment. Since accommodation controls the near reflex (the triad of accommodation, convergence, and miosis), less convergence will occur with reduced accommodation and the angle of deviation will be significantly reduced, often to zero.

Miotics have been used extensively for diagnosis and treatment of accommodative esotropia with or without an accompanying high accommodative convergence-to-accommodation (AC/A) ratio. In children who present with acquired esotropia and who have less than +3.00 spherical hyperopia, miotics can be used diagnostically. If after 4-6 weeks the esodeviation is eliminated, the diagnosis of accommodative esotropia is established. Miotic treatment can be continued, or fully corrected hyperopic glasses can be prescribed. Miotics may also be used in association with single vision glasses to avoid bifocals in many patients with a high AC/A ratio. Long-term use of miotics in children can be associated with development of iris cysts; this can be prevented by coadministration with phenylephrine solution.

2. Botulinum toxin-

The injection of botulinum toxin type A (Botox) into an extraocular muscle produces a dose-dependent duration of paralysis of that muscle. The injection is given under electromyographic positional control using a bipolar electrode needle. The toxin is tightly bound to the muscle tissue. The doses used are so small that systemic toxicity does not occur. The desired length of paralysis is dependent upon the angle of deviation. The larger the angle of deviation, the longer the duration of paralysis required. Paralysis of the muscle shifts the eye into the field of action of the antagonist muscle. During the time the eye is deviated, the paralyzed muscle is stretched, whereas the antagonist muscle is contracted. As the paralysis resolves, the eye will gradually return toward its original position but with a new balance of forces that permanently reduces or eliminates the deviation. Two or more injections are often necessary to obtain a lasting effect.

D. Orthoptics:

An orthoptist is trained in methods of testing and treating patients with strabismus. Orthoptists offer significant help to the ophthalmologist, particularly in diagnosis and to a lesser extent in treatment. Evaluation of the sensory status may be very helpful in determining the fusion potential. An orthoptist may be able to aid in preoperative treatment, especially with patients who have amblyopia. At times, orthoptic training and instructions for "exercises" to be used at home can supplement and solidify surgical treatment.

Surgical Treatment (  Figure 12-6)

A. Surgical Procedures:

A variety of changes in the rotational effect of an extraocular muscle can be achieved with surgery.

1. Resection and recession-

Conceptually, the simplest procedures are strengthening and weakening. A muscle is strengthened by a procedure called resection. The muscle is detached from the eye, stretched out longer by a measured amount, and then resewn to the eye, usually at the original insertion site. The small amount of extra length is trimmed off. Recession is the standard weakening procedure. The muscle is detached from the eye, freed from fascial attachments, and allowed to retract. It is resewn to the eye a measured distance behind its original insertion.

The superior oblique is strengthened by tucking or advancing its tendon. This can be done by a graded amount. Superior oblique weakening is accomplished by a tenectomy (complete or partial division of the tendon) or one of several lengthening procedures. There is no effective strengthening procedure on the inferior oblique. The inferior oblique can be weakened by disinsertion, myectomy, or recession, with generally equivalent results.

2. Shifting of point of muscle attachment-

In addition to simple strengthening or weakening, the point of attachment of the muscle can be shifted; this may give the muscle a rotational action it did not previously have. For example, a vertical shift of both horizontal rectus muscles on the same eye affects the vertical position of the eye. Vertical shifts of the horizontal rectus muscles in opposite directions affect the horizontal eye position in upgaze and downgaze. This is done for A or V patterns, in which the horizontal deviation is more of an esodeviation in upgaze or downgaze, respectively.

The torsional effect of a muscle can also be changed. Tightening of the anterior fibers of the superior oblique tendon, known as the Harada-Ito procedure, gives that muscle enhanced torsional action.

3. Faden procedure-

A special operation for muscle weakening is called the posterior fixation (Faden) procedure (Figure 12-7). In this operation, a new insertion of the muscle is created well behind the original insertion. This causes mechanical weakening of the muscle as the eye rotates into its field of action. When combined with recession of the same muscle, the Faden operation has a profound weakening effect on the muscle without significant alteration of the primary position of the eye. The procedure can be effective on vertical rectus muscles (dissociated vertical deviation) or horizontal muscles (high AC/A ratio, nystagmus, and other rare incomitant muscle imbalances).

B. Choice of Muscles for Surgery:

The decision concerning which muscles to operate on is based on several factors. The first is the amount of misalignment measured in the primary position. Modifications are made for significant differences in distance and near measurements. The medial rectus muscles have more effect on the angle of deviation for near and the lateral rectus muscles more effect for distance. For esotropia greater at near, both medial rectus muscles should be weakened. For exotropia greater at distance, both lateral rectus muscles should be weakened. For deviations approximately the same at distance and near, bilateral weakening procedures or unilateral recession/resection procedures are equally effective.

Surgical realignment affects only the muscular or mechanical part of a neuromuscular imbalance. Although most individuals respond in a predictable manner, variable responses may be due to differing mechanical properties of the muscles and surrounding tissues as well as variable innervational input. For these reasons, more than one operation may be required to obtain a satisfactory result.

C. Adjustable Sutures:

(Figure 12-8.) The development of adjustable sutures offers a great advantage in muscle surgery, particularly for reoperations and incomitant deviations. During the operation, the muscle is reattached to the sclera with a slip knot placed so that it is accessible to the surgeon. After the patient has recovered sufficiently from the anesthesia to cooperate in the adjustment process, a topical anesthetic drop is placed in the eye and the suture can be tightened or loosened to change the eye position as indicated by cover testing. Adjustable sutures can be used on any rectus muscle for either recession or resection and on the superior oblique muscle for correction of torsion. Although any patient willing to cooperate is suitable, the method is usually not applicable for children under age 12.

		"A" and "V" patterns Hypertropia UNDEFINED: SIDEBARCONTENT

 
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