AccessLange: General Ophthalmology
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Chapter 12: Strabismus
Under normal binocular viewing conditions, the image of the object of regard falls simultaneously on the fovea of each eye (bifoveal fixation) and the vertical retinal meridians are both upright. Either eye can be misaligned, so that only one eye at a time views the object of regard. Any deviation from perfect ocular alignment is called "strabismus." Misalignment may be in any direction-inward, outward, up, or down. The amount of deviation is the angle by which the deviating eye is misaligned. Strabismus present under binocular viewing conditions is manifest strabismus, heterotropia, or tropia. A deviation present only after binocular vision has been interrupted (ie, by occlusion of one eye) is called latent strabismus, heterophoria, or phoria.
Strabismus is present in about 4% of children. Treatment should be started as soon as a diagnosis is made in order to ensure the best possible visual acuity and binocular visual function. There is no such thing as "outgrowing" strabismus.
Angle kappa: The angle between the visual axis and the central pupillary line. When the eye is fixing a light, if the corneal reflection is centered on the pupil, the visual axis and the central pupillary line coincide and the angle kappa is zero. Ordinarily, the light reflex is 2-4 degrees nasal to the pupillary center, giving the appearance of slight exotropia (positive angle kappa). A negative angle kappa gives the false impression of esotropia.
Conjugate movement: Movement of the eyes in the same direction at the same time.
Ductions: (Figure 12-1.) Monocular rotations with no consideration of the position of the other eye.
Adduction: Inward rotation.
Abduction: Outward rotation.
Supraduction (elevation): Upward rotation.
Infraduction (depression): Downward rotation.
Fusion: Formation of one image from the two images seen simultaneously by the two eyes. Fusion has two aspects:
Motor fusion: Adjustments made by the brain in innervation of extraocular muscles in order to bring both eyes into bifoveal and torsional alignment.
Sensory fusion: Integration in the visual sensory areas of the brain of images seen with the two eyes into one picture.
Heterophoria (phoria): Latent deviation of the eyes held straight by binocular fusion.
Esophoria: Tendency for one eye to turn inward.
Exophoria: Tendency for one eye to turn outward.
Hyperphoria: Tendency for one eye to deviate upward.
Hypophoria: Tendency for one eye to deviate downward. (See Hypotropia.)
Strabismus: Manifest deviation of the eyes that cannot be controlled by binocular fusion.
Esotropia: Convergent manifest deviation ("crossed eyes").
Exotropia: Divergent manifest deviation ("wall-eyes").
Hypertropia: Manifest deviation of one eye upward.
Hypotropia: Manifest deviation of one eye downward. By convention, in the absence of specific causation to account for the lower position of one eye, vertical deviations are designated by the higher eye (eg, right hypertropia, not left hypotropia, when the right eye is higher).
Incyclotropia: Inward rotation of one eye about its anteroposterior axis (ie, clockwise right eye, counterclockwise left eye).
Excyclotropia: Outward rotation of one eye about its anteroposterior axis (ie, counterclockwise right eye, clockwise left eye).
Orthophoria: The absence of any tendency of either eye to deviate when fusion is suspended. This state is rarely seen clinically. A small phoria is normal.
Primary deviation: The deviation measured with the normal eye fixing and the eye with the paretic muscle deviating (Figure 12-2).
Prism diopter (Δ): A unit of angular measurement used to characterize ocular deviations. A 1-diopter prism deflects a ray of light toward the base of the prism by 1 centimeter at 1 meter. One degree of arc equals approximately 1.7Δ.
Secondary deviation: (Figure 12-2.) The deviation measured with the paretic eye fixing and the normal eye deviating.
Torsion: Rotation of the eye about its anteroposterior axis (Figure 12-1).
Intorsion (incycloduction): Rotation of the 12 o'clock meridian of the eye toward the midline of the head.
Extorsion (excycloduction): Rotation of the 12 o'clock meridian of the eye away from the midline of the head.
Vergences (disjunctive movements): Movement of the two eyes in opposite directions.
Convergence: The eyes turn inward.
Divergence: The eyes turn outward.
Versions: Binocular rotations of the eyes in qualitatively the same direction.
1. MOTOR ASPECTS
Individual Muscle Functions (Table 12-1)
Each of the six extraocular muscles plays a role in positioning the eye about three axes of rotation. The primary action of a muscle is the principal effect it has on eye rotation. Lesser effects are called secondary or tertiary actions. The exact action of any muscle depends on the direction of the eye in space.
The medial and lateral rectus muscles adduct and abduct the eye, respectively, with little effect on elevation or torsion. The vertical rectus and oblique muscles have vertical rotation and torsional functions. In general terms, the vertical rectus muscles are the main elevators and depressors of the eye, and the obliques are mostly involved with torsional positioning. The vertical effect of the superior and inferior rectus muscles is greater when the eye is abducted. The vertical effect of the obliques is greater when the eye is adducted.
Field of Action
The position of the eye is determined by the equilibrium achieved by the pull of all six extraocular muscles. The eyes are in the primary position of gaze when they are looking straight ahead with the head and body erect. To move the eye into another direction of gaze, the agonist muscle contracts to pull the eye in that direction and the antagonist muscle relaxes. The field of action of a muscle is the direction of gaze in which that muscle exerts its greatest contraction force as an agonist, eg, the lateral rectus muscle undergoes the greatest contraction in abducting the eye (Table 12-1).
Synergistic & Antagonistic Muscles (Sherrington's Law)
Synergistic muscles are those that have the same field of action. Thus, for vertical gaze, the superior rectus and inferior oblique muscles are synergists in moving the eye upward. Muscles synergistic for one function may be antagonistic for another. For example, the superior rectus and inferior oblique muscles are antagonists for torsion, the superior rectus causing intorsion and the inferior oblique extorsion. The extraocular muscles, like skeletal muscles, show reciprocal innervation of antagonistic muscles (Sherrington's law). Thus, in dextroversion (right gaze), the right medial and left lateral rectus muscles are inhibited while the right lateral and left medial rectus muscles are stimulated.
Yoke Muscles (Hering's Law)
For movements of both eyes in the same direction, the corresponding agonist muscles receive equal innervation (Hering's law). The pair of agonist muscles with the same primary action is called a yoke pair. The right lateral rectus and the left medial rectus muscles are a yoke pair for right gaze. The right inferior rectus and the left superior oblique muscles are a yoke pair for gaze downward and to the right. Table 12-2 lists the yoke muscle combinations.
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
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.
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).
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.
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.
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.
One eye may constantly deviate, or alternating fixation may be observed.
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 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 18Δ 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.
(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.
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.
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.
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:
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.
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 5Δ per lens since prismatic distortion becomes prominent at higher strengths.
C. Pharmacologic Agents:
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.
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.
ESOTROPIA (Convergent Strabismus, "Crossed Eyes")
Esotropia is by far the most common type of strabismus. It is divided into two types: paretic (due to paresis or paralysis of one or both lateral rectus muscles) and nonparetic (comitant). Nonparetic esotropia is the most common type in infants and children; it may be accommodative, nonaccommodative, or partially accommodative. Paretic strabismus is uncommon in childhood but accounts for most new cases of strabismus in adults. Most cases of childhood nonaccommodative esotropia are classified as infantile esotropia, with onset by age 6 months. The remainder occur after age 6 months and are classified as acquired nonaccommodative esotropia.
A. Infantile Esotropia:
Nearly half of all cases of esotropia fall into this group. In most cases, the cause is obscure. The convergent deviation is manifest by age 6 months. The deviation is comitant, ie, the angle of deviation is approximately the same in all directions of gaze and is usually not affected by accommodation. The cause, therefore, is not related to the refractive error or dependent upon a paretic extraocular muscle. It is likely that the majority of cases are due to faulty innervational control, involving the supranuclear pathways for convergence and divergence and their neural connections to the medial longitudinal fasciculus. A smaller number of cases are due to anatomic variations such as anomalous insertions of horizontally acting muscles, abnormal check ligaments, or various other fascial abnormalities.
There is also good evidence that strabismus does occur on a genetically determined basis. Esophoria and esotropia are frequently passed on as an autosomal dominant trait. Siblings may have similar ocular deviations. An accommodative element is often superimposed upon comitant esotropia, ie, correction of the hyperopic refractive error reduces but does not eliminate all of the deviation.
The deviation is often large ( 40Δ). Abduction may be limited but can be demonstrated. Vertical deviations may be observed after 18 months of age as a result of overaction of the oblique muscles or dissociated vertical deviation. Nystagmus, manifest or latent, may be present. The most common refractive error is low to moderate hyperopia.
The eye that appears to be straight is the eye used for fixation. Almost without exception, it is the eye with better vision or lower refractive error (or both). If there is anisometropia, there will probably be some amblyopia as well. If at various times either eye is used for fixation, the patient is said to show spontaneous alternation of fixation; in this case, vision will be equal or nearly equal in both eyes. In some cases, the eye preference is determined by the direction of gaze. For example, with large-angle esotropia, there is a tendency for the right eye to be used in left gaze and the left eye in right gaze (cross fixation).
Infantile esotropia is treated surgically. Preliminary nonsurgical treatment may be indicated to ensure the best possible result. It is essential that amblyopia be fully treated prior to surgery. Glasses should be tried in hyperopic refractive errors of 3 D or more to determine if reducing accommodation has a favorable effect on the deviation. A myotic may be used successfully as an alternative to glasses if the refractive error is not above approximately 4 D.
Surgery is usually indicated after medical therapy and treatment of amblyopia have been completed. Once reproducible measurements are obtained, surgery should be performed since there is ample evidence that sensory results are better the sooner the eyes are aligned. Many procedures have been recommended, but the two most popular are (1) weakening of both medial rectus muscles, and (2) recession of the medial rectus and resection of the lateral rectus on the same eye.
B. Acquired Nonaccommodative Esotropia:
This type of esotropia develops in childhood, usually after the age of 2 years. There is little or no accommodative factor. The angle of strabismus is often smaller than in infantile esotropia but may increase with time. Otherwise, clinical findings are the same as for congenital esotropia. Treatment is surgical and follows the same guidelines as for congenital esotropia.
Accommodative esotropia occurs when there is a normal physiologic mechanism of accommodation with an associated overactive convergence response but insufficient relative fusional divergence to hold the eyes straight. There are two pathophysiologic mechanisms at work, singly or together: (1) sufficiently high hyperopia, requiring so much accommodation (and therefore convergence) to clarify the image that esotropia results; and (2) a high AC/A ratio, which is accompanied by mild to moderate hyperopia (1.5 D or more).
A. Accommodative Esotropia Due to Hyperopia:
Accommodative esotropia due to hyperopia typically begins at age 2-3 but may occur earlier or later. Deviation is variable prior to treatment. Glasses with full cycloplegic refraction allow the eyes to become aligned
B. Accommodative Esotropia Due to High AC/A Ratio:
In accommodative esotropia due to a high ratio of accommodative convergence to accommodation (AC/A ratio), a deviation is greater at near than at distance. The refractive error is hyperopic. Treatment is with glasses with full cycloplegic refraction plus bifocals or miotics to relieve excess deviation at near.
Partially Accommodative Esotropia
A mixed mechanism-part muscular imbalance and part accommodative/convergence imbalance-may exist. Although antiaccommodative therapy decreases the angle of deviation, the esotropia is not eliminated. Surgery is performed for the nonaccommodative component of the deviation with the choice of surgical procedure as described for infantile esotropia.
Incomitant strabismus results from paresis or restriction of action of one or more extraocular muscles. Incomitant esotropia is usually due to paresis of one or both lateral rectus muscles as a result of unilateral or bilateral abducens nerve palsy (see Chapter 14). Other causes are fracture of the medial orbital wall with entrapment of the medial rectus muscle, dysthyroid eye disease with contracture of the medial rectus muscles, and Duane's retraction syndrome (see below). Abducens nerve palsy is most frequently seen in adults with systemic hypertension or diabetes, in which case spontaneous resolution usually begins within 3 months. Abducens palsy may also be the first sign of intracranial tumor or inflammatory disease. Associated neurologic signs are then important clues. Head trauma is another frequent cause of abducens palsy.
Incomitant esotropia is also seen in infants and children, but much less commonly than comitant esotropia. These cases result from birth injuries affecting the lateral rectus muscle directly, from injury to the nerve, or, less commonly, from a congenital anomaly of the muscle or its fascial attachments.
Esotropia is characteristically greater at distance than at near and greater to the affected side. Paresis of the right lateral rectus causes esotropia that becomes greater on right gaze and, if paresis is mild, little or no deviation on left gaze. If the lateral rectus muscle is totally paralyzed, the eye will not abduct past the midline.
Acquired abducens palsy is initially managed by occlusion of the paretic eye or with prisms. Botulinum toxin type A injections into the antagonist medial rectus muscle may provide symptomatic relief but do not appear to influence the final outcome. If lateral rectus function in incomplete palsies has not recovered after 6 months, medial rectus botulinum toxin type A injections may be used on a long-term basis to allow fusion-and hence abolition of diplopia-in straight-ahead gaze or to facilitate prism therapy. Horizontal rectus muscle surgery, involving resection of one or both lateral recti and recessions of the medial recti, may also be valuable. Adjustable sutures are useful in achieving the largest possible area of binocular single vision. In complete palsies that have failed to improve after 6 months, surgical transposition of the insertions of the superior and inferior rectus muscles to the insertion of the lateral rectus muscle, combined with temporary paralysis of the medial rectus muscle by botulinum toxin type A, produces the best results. Abduction cannot be restored, but fusion in primary position, with or without the aid of prisms, and a reasonable field of binocular single vision can usually be achieved.
Pseudoesotropia is the illusion of crossed eyes in an infant or toddler when no strabismus is present. This appearance is usually caused by a flat, broad nasal bridge and prominent epicanthal folds that cover a portion of the nasal sclera, giving the impression that the eyes are crossed. This very common condition may be differentiated from true misalignment by the corneal light reflection appearing in the center of the pupil of each eye when the child fixates a light. With normal facial growth and increasing prominence of the nasal bridge, this pseudoestropic appearance gradually disappears. Of course, true esotropia may be present in association with this common infantile facial configuration.
EXOTROPIA (Divergent Strabismus)
Exotropia is less common than esotropia, particularly in infancy and childhood. Its incidence increases gradually with age. Not infrequently, a tendency to divergent strabismus beginning as exophoria progresses to intermittent exotropia and finally to constant exotropia if no treatment is given. Other cases begin as constant or intermittent exotropia and remain stationary. As in esotropia, there may be a hereditary element in some cases. Exophoria and exotropia (considered as a single entity of divergent deviation) are frequently passed on as autosomal dominant traits, so that one or both parents of an exotropic child may demonstrate exotropia or a high degree of exophoria.
Alternative Classification of Exotropia
Constant or intermittent exotropia can also be classified on a descriptive basis as being an excess of divergence or an insufficiency of convergence. These descriptive terms do not imply that the cause of the deviation is understood.
A. Basic Exotropia:
Distance and near deviations are approximately equal.
B. Divergence Excess:
Distance deviation is significantly larger than near deviation.
C. Convergence Insufficiency:
Near deviation is significantly larger than distance deviation.
D. Pseudodivergence Excess:
Distance deviation is significantly larger than near deviation: however, use of a +3 diopter lens for near measurement will cause the near deviation to become approximately equal to the distance deviation.
Intermittent exotropia accounts for well over half of all cases of exotropia. The onset of the deviation may be in the first year, and practically all have presented by age 5. The history often reveals that the condition has become progressively worse. A characteristic sign is closing one eye in bright light (Figure 12-10). The manifest exotropia first becomes noticeable with distance fixation. The patient usually fuses at near, overcoming moderate to large angle exophoria. Convergence is frequently excellent. There is no correlation with a specific refractive error.
Since a child fuses at least part of the time, there is usually no gross sensory abnormality. For distance, with one eye deviated, there is suppression of that eye and normal retinal correspondence with little or no amblyopia.
A. Medical Treatment:
Nonsurgical treatment is largely confined to refractive correction and amblyopia therapy. If the AC/A ratio is high, the use of minus lenses may delay surgery for a while. Occasionally, antisuppression or convergence exercises may be of temporary benefit
B. Surgical Treatment:
Most patients with intermittent exotropia require surgery when their fusional control deteriorates. Deterioration of control is documented over time by an increasing percentage of time the manifest exotropia is observed, an enlarging angle of deviation, decreasing control for near fixation, and worsening in the patient's measured distance and near stereoscopic abilities. Surgery may also alleviate diplopia or other asthenopic symptoms.
The choice of procedure depends on the measurements of the deviation. Bilateral lateral rectus muscle recession is preferred when the deviation is greater at distance. If there is more deviation at near, it is best to undertake resection of a medial rectus muscle and recession of the ipsilateral lateral rectus muscle. Surgery on one or even two additional horizontal muscles may be necessary for very large deviations (> 50Δ). It is desirable to obtain slight overcorrection in the immediate postoperative period for best long-term results.
CONSTANT EXOTROPIA (Figure 12-11)
Constant exotropia is less common than intermittent exotropia. It may be present at birth or may occur when intermittent exotropia progresses to constant exotropia. Because infantile exotropia is commonly seen in children with underlying neurologic impairment, pediatric neurologic consultation is indicated in all such cases. Some cases have their onset later in life, particularly following loss of vision in one eye. Except for cases due to loss of vision, the underlying cause is usually not known.
Constant exotropia may be of any degree. With chronicity or poor vision in one eye, the deviation can become quite large. Adduction may be limited, and hypertropia also may be present. There is suppression if the deviation was acquired by age 6-8; otherwise, diplopia may be present. If exotropia is due to very poor vision in one eye, there may be no diplopia. Amblyopia is uncommon in the absence of anisometropia, and spontaneous alternation of the fixating eye is frequently observed.
Surgery is nearly always indicated. The choice and amount are as described for intermittent exotropia. Slight overcorrection in an adult may result in diplopia. Most patients adjust to this, especially if they have been forewarned of the possibility. If one eye has reduced vision, the prognosis for maintenance of a stable position is less favorable, with the strong possibility that the deviating eye will gradually become more exotropic. Botulinum toxin type A injections can be useful as primary treatment in small deviations or as supplementary treatment in significant surgical overcorrections or undercorrections.
A & V PATTERNS
A horizontal deviation may be vertically incomitant, ie, the deviation is different in upgaze versus downgaze (A or V pattern). An A pattern shows more esodeviation or less exodeviation in upgaze compared to downgaze. A V pattern shows less esodeviation or more exodeviation in upgaze compared to downgaze. An A pattern is diagnostically significant when greater than 10Δ and a V pattern when greater than 15Δ. These patterns are frequently associated with overaction of the oblique muscles, inferior obliques for V patterns and superior obliques for A patterns.
When surgically treating an A or V pattern, oblique muscle overaction must be treated if present. If little or no oblique overaction exists, vertical offsets of one tendon width of the horizontal muscles are utilized to collapse the pattern. The insertions of the medial rectus muscles are displaced toward the narrow end of the pattern (in V esotropia, recessed medial rectus muscles are moved down), and lateral rectus muscles are displaced toward the open end (in V exotropia, the insertions of the recessed lateral rectus muscles are moved up).
HYPERTROPIA (Figure 12-12)
Vertical deviations are customarily named according to the high eye, regardless of which eye has the better vision and is used for fixation. Hypertropias are less common than horizontal deviations and are usually acquired after childhood.
There are many causes of hypertropia. Congenital anatomic anomalies may result in muscle attachments in abnormal locations. Occasionally, there are anomalous fibrous bands that attach to the eye. Closed head trauma may produce paresis of the superior oblique muscle. Orbital trauma or tumors, brain stem lesions, and systemic diseases such as myasthenia gravis, multiple sclerosis, and Graves' disease can all produce hypertropias. Many of these specific entities are discussed in Chapter 14.
The clinical findings may vary, depending on the cause. The history is particularly important in diagnosis of hypertropias. Prism and cover measurements in primary and cardinal positions and head tilts are the mainstay of the clinical evaluation and may often be diagnostic. Observation of ocular rotations for limitations can also be of great value.
Diplopia is almost invariably present if strabismus develops past age 6-8. As in other forms of strabismus, sensory adaptation occurs if the onset is before this age range. Suppression and anomalous retinal correspondence may be present in gaze directions where there is strabismus. In gaze directions without strabismus, there may be no suppression and normal stereopsis.
There may be head tilt, turn, or abnormal posture of the head. The deviation may be of any magnitude and usually changes with the direction of gaze. Most hypertropias are incomitant. The deviation tends to be greatest in the field of action of one of the four vertically acting muscles. There may be an associated cyclotropia, especially with superior oblique dysfunction. To measure a cyclotropia, the double Maddox rod test is used. In a trial frame, a red and white Maddox rod are aligned vertically, one over each eye. With the patient's head held straight and fixing a light, one rod is gradually turned until the observed lines are parallel to each other and to normal horizontal orientation. The angle of tilt is then read from the angular scale on the trial frame.
The superior oblique is the most commonly paretic vertical muscle. The vertical rectus muscles are commonly involved in trauma, as with entrapment of the inferior rectus in an orbital floor fracture, and in thyroid eye disease, in which the inferior rectus becomes hypertrophied, inelastic, and fibrotic, which pulls the eye downward.
Paresis of the superior oblique is usually present with hypertropia on the involved side with a head tilt to the opposite side. Other motility patterns can be seen when the deviation is of long standing, with contractures of other vertically acting muscles.
The Bielschowsky head tilt test (Figure 12-13) is useful to confirm the diagnosis of superior oblique paresis. The test exploits the differing effects of each vertical muscle on torsion and elevation. Thus, with a paretic right superior oblique when the head is tilted to the right, the superior rectus and superior oblique contract to intort the eye and maintain the position of the retinal vertical meridian as much as possible. The superior rectus elevates the eye, and the superior oblique depresses the eye. Because of weakness of the superior oblique muscle, the vertical forces do not cancel out as they normally would, and right hypertropia increases. In head tilt to the left, the intorting muscles for the right eye relax and the inferior oblique and inferior rectus both contract to extort the eye. Both the paretic superior oblique and the superior rectus relax, and hypertropia is minimized. Hypertropia should be measured by prism plus cover with the head tilted to either side.
A. Medical Treatment:
For smaller and more comitant deviations, a prism may be all that is required. For constant diplopia, one eye may need to be occluded. Systemic disease must be treated if suspected to be the underlying cause.
B. Surgical Treatment:
Surgery is often indicated if the deviation and diplopia persist. The choice of procedure depends on quantitative measurements. The use of adjustable sutures (Figure 12-8) is frequently a great help in fine-tuning the effect of vertical muscle surgery.
DUANE'S RETRACTION SYNDROME
Duane's retraction syndrome is typically characterized by marked limitation of abduction, mild limitation of adduction, retraction of the globe and narrowing of the palpebral fissure on attempted adduction, and, frequently, upshoot or downshoot of the eye in adduction. Usually it is monocular, with the left eye more often affected. Most cases are sporadic, although some families with dominant inheritance have been described. A variety of other anomalies may be associated, such as dysplasia of the iris stroma, heterochromia, cataract, choroidal coloboma, microphthalmos, Goldenhar's syndrome, Klippel-Feil syndrome, cleft palate, and anomalies of the face, ear, or extremities. The causes of the motility defects are varied, and some anomalies of muscle structure have been found. Most cases can be explained by inappropriate innervation to the lateral rectus and sometimes to other muscles as well. Sherrington's law of reciprocal innervation is not obeyed, because nerve fibers to the medial rectus may also go to the lateral rectus. This accounts for simultaneous contraction of the medial and lateral rectus muscles (co-contraction), causing retraction of the globe. Cases with proved absence of the abducens nucleus and nerve have been documented.
Only when a primary position misalignment or a significant compensatory head turn exists is surgical treatment indicated. The goal is to obtain straight eyes in the primary position and to horizontally expand the field of single vision. Recession of the medial rectus on the affected side is performed if any esotropia is present in the primary position. For more severe cases, temporal transposition of the vertical rectus muscles accompanied by weakening of the medial rectus muscle, either by adjustable recession or botulinum toxin A, is often indicated.
DISSOCIATED VERTICAL DEVIATION
Dissociated vertical deviation is frequently associated with congenital esotropia and rarely with an otherwise normal muscle balance. The exact cause is not known, though it is logical to assume it is from faulty supranuclear innervation of extraocular muscles.
Each eye drifts upward under cover, frequently with extorsion and a small exotropic shift, and then returns to its resting binocular position when the cover is removed. Occasionally, the upward drifting will occur spontaneously, causing a noticeable vertical misalignment. Most cases are bilateral, though asymmetry of involvement is common. There are usually no symptoms.
Treatment is indicated if the frequency of the intermittent manifest vertical deviation is unacceptable. Nonsurgical treatment is limited to refractive correction to maximize the potential of motor fusion and therapy for amblyopia. Surgical results have been variable and can be disappointing. Currently, the most popular and successful procedures are very large recession of the superior rectus or recession of the superior rectus combined with the Faden procedure. A new procedure that involves transposing anteriorly the insertion of the inferior oblique muscle has also been effective.
BROWN'S SYNDROME (Superior Oblique Tendon Sheath Syndrome)
Brown's syndrome is due to fibrous adhesions in the superior nasal quadrant involving the superior oblique tendon and trochlea, which mechanically limit elevation of the eye. Limitation of elevation is most marked in the adducted position, and improvement in elevation occurs gradually as the eye is abducted. Differential diagnosis is concerned mainly with paresis of the inferior oblique muscle. Forced duction testing is diagnostic, since there is an upward restriction to elevation in adduction when Brown's syndrome is present. The condition is usually unilateral and idiopathic, though rarely it may be due to trauma or inflammation.
Surgical treatment is limited to those cases where there is an abnormal head position to compensate for hypotropia or cyclotropia of the involved eye. The objective is to free the mechanical adhesions and weaken the superior oblique muscle. Although controversial as to its timing, weakening of the ipsilateral inferior oblique may compensate for the induced fourth nerve palsy. Normalization of the head position may occur, but restoration of full motility is seldom achieved.
Heterophoria is deviation of the eyes that is held in check by binocular vision. Almost all individuals have some degree of heterophoria, and small amounts are considered normal. Larger amounts may cause symptoms depending on the level of effort required by the individual to control latent muscle imbalance.
The symptoms of heterophoria may be clear-cut (intermittent diplopia) or vague ("eyestrain" or asthenopia). Diplopia may come on only with fatigue or with poor lighting conditions, as in night driving. Usage requirements for the eyes and personality type are additional factors. Thus, there is no degree of heterophoria that is clearly abnormal, though larger amounts are more likely to be symptomatic. Except for hyperopia, high AC/A ratios, and mild cases of muscle paresis not resulting in frank heterotropia, the fundamental causes of heterophorias are unknown.
Asthenopia is sometimes caused by uncorrected refractive errors as well as by muscle imbalance. One possible mechanism is aniseikonia, in which an image seen by one eye is a different size and shape from that seen by the other eye. Spectacles with unequal lens powers in the two eyes can cause asthenopia by creating prismatic displacement of the image in one eye for gaze away from the optic axis that is too large to control (induced prism). Another mechanism that may produce symptoms is a change in spatial perception due to the curvature of the lenses or astigmatic corrections. (See Chapter 20.)
The symptoms encountered in asthenopia take a wide variety of forms. There may be a feeling of heaviness, tiredness, or discomfort of the eyes, varying from a dull ache to deep pain located in or behind the eyes. Headaches of all types occur. Easy fatigability, blurring of vision, and diplopia, especially after prolonged use of the eyes, also occur. Symptoms are more common for near visual work than for distance. Frequently, an aversion to reading develops. Symptoms can be brought on by fatigue or illness or following the ingestion of medications or alcohol.
The diagnosis of heterophoria is based on prism and cover measurements. Relative fusional vergence amplitudes are measured. While the patient views an accommodative target at distance or near, prisms of increasing strength are placed in front of one eye. The fusional vergence amplitude is the amount of prism the patient is able to overcome and still maintain single vision. Measurements are done with base-out, base-in, base-up, and base-down prisms. The important feature is the size of the amplitudes in comparison to the angle of heterophoria. While one cannot give exact norms for normal relative fusion vergence, guidelines for typical normal findings are as follows: at distance, convergence is 14Δ, divergence is 6Δ, and vertical is 2.5Δ; at near, convergence is 35Δ, divergence is 15Δ, and vertical is 2.5Δ.
Heterophoria requires treatment only if symp-tomatic. Untreated heterophoria or asthenopia does not cause any permanent damage to the eyes. Treatment methods are all aimed at reducing the effort required to achieve fusion or at changing muscle mechanics so that the muscle imbalance itself is reduced.
A. Medical Treatment:
1. Accurate refractive correction-
Occasionally, poor visual acuity is found in the presence of symp-tomatic heterophoria. Spectacles providing clear vision are sometimes all that is needed to alleviate symptoms. The clearer image allows the patient's fusional capacity to function to its fullest.
2. Manipulation of accommodation-
In general, esophorias are treated with antiaccommodative therapy and exophorias by stimulating accommodation. Plus lenses often work well for esophoria, especially if hyperopia is present, by reducing accommodative convergence. A high AC/A ratio may be effectively treated with plus lenses, sometimes combined with bifocals or miotics.
The use of prisms requires the wearing of glasses; for some patients, this is unacceptable. A trial of plastic Fresnel press-on prisms should be made before ground-in prisms are ordered. For optical reasons, larger amounts of prismatic correction produce visual distortions limiting the use of prisms in higher strengths. Furthermore, very thick lenses can result. The usual practice is to prescribe about one-third to one-half of the measured deviation, which often allows fusion to occur. Prisms can be useful for esophoria, exophoria, and vertical phorias as well.
4. Botulinum toxin type A (Botox) injection-
This treatment is well suited to producing small to moderate shifts in ocular alignment and has been used as a substitute for surgical weakening of one muscle. The main disadvantage is that the resulting effect may be variable or wear off completely months later.
B. Surgical Treatment:
Surgery should be done only after medical methods have failed. Muscles are chosen for correction according to the measured deviation at distance and near in various directions of gaze. Sometimes only one muscle needs adjustment. Adjustable sutures can be very helpful (Figure 12-8).REFERENCES
List of Figures
List of Tables
AccessLange: General Ophthalmology
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