Concomitant Exodeviations



Concomitant Exodeviations


Paul R. Mitchell

Marshall M. Parks



Visual axes that form a divergent angle as they proceed outward constitute an exodeviation. Diverged visual axes kept latent by single binocular vision constitute exophoria; when manifest, the misaligned visual axes constitute exotropia. Many patients with divergent deviations vacillate between phoria and tropia; this fluctuating condition is termed intermittent exotropia.


ETIOLOGY

A disturbance in the tonic horizontal vergence is usually considered the cause of most primary divergent deviations. However, given that so little is known about the total spectrum of vergence innervations that disjugately drive the eyes, other than accommodative convergence and fusional vergence, such speculation is rather meaningless. There appears to be some disruption in the usual balance of tonic vergence innervations; this results in either a deficiency of convergence innervation or an excess of divergence innervation. Whatever the cause of such innervational disruption, it seems to be genetically influenced; hereditary aspects of the primary divergent deviation are inescapably obvious to the ophthalmologist.

Secondary anatomic factors probably occur within and surrounding the extraocular muscles; they account for a gradual increase in the divergent angle in both intermittently and constantly exotropic patients. Excess innervation to the lateral rectus muscles possibly results in their hypertrophy. In addition, in long-standing constant exotropia, the elastic fibers surrounding the fascial and conjunctival tissues may add their force to the divergent pull of the visual axes, further increasing the exotropic angle. Therefore, it may be difficult to determine which part of the cause is primarily innervational and which is secondarily anatomic. The interplay of innervational and anatomic forces is displayed in a patient who sustains a traumatic unilateral cataract. Although the eyes were orthophoric before injury, within a few weeks following disruption of binocular vision by the acquired lens opacity, the injured eye usually begins to diverge in adults and in children older than 5 years of age. In younger children, the injured eye usually converges. Over the years, the divergent injured eye becomes increasingly exotropic, and eventually, the traction test reveals resistance to passive adduction and shortening of the temporal conjunctiva and Tenon capsule extending between the temporal limbus and the outer canthus. Apparently, when ungoverned by binocular vision, tonic divergence prevails over tonic convergence, and over time, the original innervational alteration is secondarily reinforced by anatomic changes in the extraocular muscles and surrounding tissues.

Anatomic causes of exotropia may be the primary factor in, rather than the consequence of, prior innervational imbalance to the extraocular muscle. Surgical overcorrection of esotropia,1 trauma to the orbit causing either a palsy of the medial rectus or an adhesive cicatricial pull on the temporal aspect of the globe,2 and osteologic disorders of the face and cranium (e.g., the shallow diverged orbits in Crouzon disease) are examples of anatomic abnormalities that produce exotropia. Basmak and coworkers3 showed that exotropic patients have higher angle kappa values when compared with esotropic patients, and that the angle kappa must be considered when performing Krimsky and Hirschberg tests on young and uncooperative patients in order to improve surgical results.


CLINICAL CHARACTERISTICS


Exophoria

Small amounts of exophoria in adults and even relatively large amounts in youngsters are usually asymptomatic. Discomfort or visual disturbances are the usual
symptoms. The discomfort is manifest by vague asthenopia, which gradually increases during prolonged detailed visual tasks and dissipates with rest. The visual symptoms are momentary diplopia, vacillating with momentary blurred vision. Symptoms occur when the fatigue threshold of fusional convergence is exceeded. As the patient finds it more difficult to maintain fusional convergence, accommodative convergence may be substituted, which is necessarily associated with accommodation. As the accommodation relaxes, allowing the vision to clear, the accommodative convergence, which was substituted for the exhausted fusional convergence, also decreases. The latent divergent deviation becomes manifest, and while it persists, heteronymous diplopia is present. The response to the diplopia stimulus is again to apply the synkinetic near reflex, namely, overaccommodation and convergence, thus blurring the vision and eliminating the diplopia. Repeating this cycle creates the symptoms of momentary blurred vision vacillating with momentary diplopia.

The symptoms of exophoria occur for distant-visual tasks if the exodeviation angle is significant for distance viewing and insignificant for near, and the reverse is true for the opposite distance-near exodeviation measurements. Convergence insufficiency, manifest by a greater near than distance exodeviation, is more prevalent and accounts for the bulk of symptomatic exophoria. Students in the upper grades of school and adults involved in pursuing prolonged near-visual tasks can be troubled by the recurrence of these symptoms. Many exophoric patients can anticipate the onset of their symptoms after having read either for a certain number of minutes or a certain number of pages of text. They usually describe the symptoms of “burning and watering eyes,” followed soon thereafter with the “doubling and blurring” of the print if they attempt to persevere with reading. Most exophoric patients learn to solve the latter symptoms, if they continue reading, by closing or covering one eye. This is followed by a soporific state that they describe as “sleepiness,” and the reading they attempted is interrupted either by sleep or the distraction of daydreaming. Poor health and chronic fatigue hasten the onset of the symptoms after the visual tasks are started. These exophoric symptoms probably play a greater role in subtly determining the patient’s vocation than the patient is aware of; they impose a significant hardship on the patient’s genuine intent and desire to complete a competitive academic program. Stavis and coworkers4 reported that school children with mini-convergence insufficiency type exophoria showed objective improvement in reading comfort, speed, accuracy, and comprehension when treated with base-in prism eyeglasses.


Intermittent Exotropia

Intermittent exotropia is a divergent deviation, the onset of which is usually during infancy or early childhood. It may be noted as early as the first few months of life. The parents are often convinced of the repeated outward turning of one or both of the infant’s eyes by the sixth month. In a study of 1,219 infants in a normal newborn nursery, Nixon and coworkers5 found that 22% (268) had intermittent exodeviations, 11% (130) had constant exodeviations, 1.4% (17) had intermittent esodeviations, 2% (23) had variable deviations, 49% (593) had orthophoria, and 15% (188) were not able to be classified. In a study of 2,271 neonates from birth to 10 months of age, Sondhi and associates6 found constant deviations in 54%, intermittent deviations in 16%, and orthophoria in 30%. The most common was exodeviation in 67%, and the least common was esodeviation in 1%. By 6 months of age, 97% of the infants were orthophoric; and after 6 months of age, exotropia was rare. In a prospective study,7 the same authors performed 6,228 examinations in 4,211 infants, finding a predominance of exodeviation early in infancy, with a gradual decrease in prevalence and in the amount of the exodeviation. Congenital esotropia developed in three infants who were exotropic or orthophoric at birth, and large-angle exotropia developed in two infants, both of whom were exotropic at birth, as are many visually normal neonates. The characteristic large deviation developed between 2 and 4 months of life, when most normal infants are developing binocular vision. The relatively low frequency of infantile exotropia was reported in a retrospective study of five pediatric ophthalmic practices.8 Only 13 patients were identified with large-angle, constant exotropia before 1 year of age, without other neurologic or pathologic findings. All required surgical correction, with recession of the lateral rectus of each eye. Two reoperations were required. In some children, however, it may never be noticed until they attain a much later age, perhaps, the seventh or eighth year of life. All but a small percentage of the total number of cases are manifest during the early years of childhood. After the intermittent exotropia is manifest, it either remains unchanged or gradually deteriorates to a constant exotropia; only rarely does it undergo partial disappearance. Hiles and colleagues9 reported on long-term observations, up to 22 years, in a small group of 48 patients with intermittent exotropia. Deterioration does not occur in time with all intermittent exotropias; some patients may improve both in the quantitative and qualitative aspects of their exodeviation. In some patients with small degrees of intermittent exotropia, surgery can be deferred unless deterioration occurs. The course of intermittent exotropia
during a 20-year period in a population-based cohort was evaluated by Nusz et al.10 Of 138 patients, the deviation resolved in 4% with a median follow-up of 9.2 years, while the Kaplan-Meier rate of increasing by 10 prism diopter (PD) or more was 23% at 5 years and 53% at 20 years. For 55 patients who had surgery, the distance deviation increased by a median of 5 PD during a mean follow-up of 3.2 years, while there was a zero PD median change during a mean follow-up of 7.1 years in the 83 patients who did not require surgery. Children who had surgery in this population were significantly more likely to have demonstrated an increase in their deviation during the preoperative period.

Some cases have been documented of patients who manifested intermittent exotropia during early childhood and then, without receiving treatment, converted to exophoria in late childhood; they seemed to continue to be asymptomatic during their early adulthood. Possibly, these patients are those who supposedly have the onset of intermittent exotropia in adulthood. Because patients’ memories fade as they grow older and because they have extremely varied attitudes toward the visual and cosmetic aspects associated with this disorder, the clinical course of intermittent exotropia is difficult to document in those few patients who apparently improve spontaneously with growth. If one disregarded the patient’s statements and ignored the matter of relative frequency of exophoria and exotropia, but plotted the alternate cover measurements of the exodeviation over the patient’s lifetime, probably, only rarely does the patient show a significant permanent reduction in the exoangle with increasing age. Essentially, the exoangle is primarily a motor innervational disorder established during infancy and kept latent in most afflicted people most of the time by the sensorimotor innervation of binocular vision. The interplay of the motor etiology and the sensorimotor compensation allows for a spectrum of possible clinical manifestations of intermittent exotropia. After the tropic aspect of this divergent deviation occurs, it usually recurs according to a pattern unique to each patient throughout the remainder of life. The pattern of recurrence of the exotropia that varies widely is the principal feature distinguishing one case from another. Romanchuk and coworkers11 retrospectively studied the natural history of surgically untreated intermittent exotropia with a minimum of 5 years follow-up. They followed up 109 patients a mean of 9 years (range, 5 to 25 years) and found no significant change in the mean angle of deviation from initial (20.6 PD) to final visit (20.9 PD). However, the initial to final angle decreased by more than 10 PD in 19%, remained stable in 58%, and increased by more than 10 PD in 23%, suggesting a nearly chance occurrence for the distance deviation to progressively improve or worsen, possibly explaining why there is difficulty prognosticating about the long-term stability of intermittent exotropia.

The tropic phase of intermittent exotropia is aggravated by many factors. Because binocular vision is the essential compensatory sensorimotor factor, the patient must be visually alert to the environment and willing to make the effort to maintain binocular vision. Daydreaming, visual distraction induced by thinking and speaking, fatigue, illness, and drowsiness on awakening are the usual factors that cause exotropia to replace exophoria. Distance viewing rather than near viewing is frequently more productive of exotropia. In some patients, exposure to bright illumination, such as sunlight, is a striking aggravating factor; when exotropia occurs, this exposure causes sufficient annoyance to precipitate the immediate reflex closure of one eye. The unilateral contraction of the orbicularis may be so obvious that the parent presents the child to the ophthalmologist for this reason and is unaware of a divergent deviation. A study by Jung and Lee12 comparing the clinical characteristics of intermittent exotropia in children and adults revealed that the chief complaint in a group of 360 children and 38 adults was outward eye deviation, followed by photophobia in the pediatric group and diplopia in the adult group. The rate of fusion as measured with the Worth 4 dot test at distance and near was higher in the pediatric group. The mean distance and near deviation were 23.4 and 23.6 PD in the pediatric group and 28.3 and 30.7 PD in the adult group.

Campos and Cipolli13 described a weakening of binocular sensory status and deterioration of fusional amplitude during light exposure in patients with intermittent exotropia. There was no eye closure in their control patients, suggesting that the intermittent eye closure was specific to intermittent exotropia. However, von Noorden14 described monocular eye closure also in patients with constant exotropia, esotropia, and in patients without strabismus.

The tropic phase tends to increase in many affected patients, possibly even in most. The increase is by way of changes in frequency and duration. Instead of occurring only once or twice daily as it originally did, the tropia tends to recur many times per day with increasing age. It also tends to remain for many minutes, instead of the original momentary duration. In some patients, increased recurrence and duration of tropia eventually seem almost to merge into a continuous exotropia. In patients whose sensorimotor compensation for the divergent deviation has deteriorated to this extent, the last stage of their intermittent exotropia is usually characterized by retention of the exophoric phase only for near viewing; they are constantly exotropic for distance viewing. Other patients tend
to retain the same frequency and duration of intermittent exotropia for years. In a prospective study of 65 patients with intermittent exotropia, Dadeya and coworkers15 investigated whether the prism adaptation test (PAT) was useful preoperatively. Satisfactory motor alignment was achieved in only 53.8% of the group with a negative response to the PAT, but 88.8% in the PAT positive response group, suggesting that the PAT may be helpful in achieving a favorable outcome in intermittent exotropia surgery.

Stathacopoulos and coworkers16 studied stereoacuity in normal patients and in patients with intermittent exotropia. Satisfactory near stereoacuity was found in normal patients and in patients with intermittent exotropia, but significantly worse distance stereoacuity was found in intermittent exotropia as compared with that in normal patients. This reduction in distance stereoacuity is valuable as an objective measure of poor control of intermittent exotropia and may assist in determining the timing for surgery. After surgery for intermittent exotropia, O’Neal and colleagues17 reported improved distance stereoacuity in 75% with contour circles, and 45% with the random dot E test, using the Mentor BVAT II Video acuity tester (Mentor O & O, Inc, Norwell, MA) preoperatively and postoperatively. They suggested distance stereoacuity could be restored with surgical realignment of intermittent exotropia, and that distance stereoacuity provided valuable information in patient evaluation. Yildirim and coworkers18 evaluated the near and distance stereoacuity and central and peripheral fusion in patients with intermittent exotropia, before and after surgery, in a prospective study. Successful outcome was achieved in 69%, with exotropia of 10 PD or less at distance, at least 1 year after surgery. Peripheral fusion was demonstrated in all patients with intermittent exotropia, but 35% demonstrated central suppression both preoperatively and postoperatively. The preoperative distance stereoacuity significantly diminished when compared with that in normal controls, but improved 58% postoperatively. Improvement in distance stereoacuity postoperatively was more likely to occur in those patients who achieved successful surgical alignment than in those who failed to achieve successful alignment. They concluded that successful surgery may improve distance stereoacuity, and that those with better preoperative distance stereoacuity and central fusion were likely to achieve higher rates of surgical success. Ball and associates19 found excellent stereoacuity of 40 to 60 seconds of arc in a series of patients with long-standing horizontal strabismus. They suggested that good vision in both eyes and excellent postoperative alignment can produce highgrade stereoacuity in some cases, even though the preoperative measurements would suggest otherwise.

Occasionally, some patients have exacerbations and remissions of the tropic phase related to their general health, state of rest, and various disturbances that may occur in the daily routine. Finally, a few patients may manifest the tropic phase intermittently for several months to years when they are young and then improve spontaneously for several months to years, becoming constantly exophoric and never exotropic. Some of these patients suffer recurrences of the exotropia when they are older, but probably others do not. Most of these latter patients eventually become lost to follow-up, and their exophoria-exotropia status in later life remains unknown.


Exotropia

The clinical characteristics of constant exotropia are determined by its cause, which may include deteriorated intermittent exotropia or a poor seeing or blind eye. In congenital constant exotropia, binocular vision has never had an opportunity to develop, and the misaligned eyes constantly manifest abnormal motor innervation to the extraocular muscle. This is a rare disorder compared with congenital esotropia, but in a study conducted with additional colleagues, we have documented its existence in more than 25 infants younger than 6 months of age (J.D. Baker, personal communication, 1974; S.F. Maddox, personal communication, 1974.; R.R. Strome, personal communication, 1974).8, 20

Congenital esotropia may indirectly become the cause of constant exotropia, inasmuch as the absence of the ability to develop binocular vision is characteristic of congenital esotropes, unless the eyes were straightened by surgery while the patient was still young. The congenital esotropia soon becomes too old to initiate the cortical neurophysiologic pathways required by binocular vision, even though surgery has straightened the eyes. Although the eyes may appear well aligned for a few years after surgery, increasing exotropia may gradually evolve in those without binocular vision, despite normal visual acuity in each eye. In a series of 88 patients with congenital esotropia, Stager and coworkers21 described delayed consecutive exotropia in 27% (24 of 88) after a 7-mm recession of the medial rectus muscle in each eye. In infants who had surgery before they were 7 months of age, the rate was 38% (8 of 21); in infants with surgery at 7 to 12 months, 20% (10 of 49); and at 13 months or older, 33% (6 of 18). Because of the delayed appearance of consecutive exotropia, averaging 26.8 months postoperatively, the authors suggested a prospective study to compare 7-mm recession with three- and four-muscle surgery, with long-term follow-up observation.

In addition, some congenitally esotropic patients gradually improve or change to exotropia without surgery,
a common trend among children with cerebral palsy.22, 23, 24, 25 and 26 Senior and Chandna26 evaluated 14 patients who developed spontaneous consecutive exotropia and found common characteristics of age of onset of esotropia at 24 months or less, hypermetropic refractive errors of +5 diopters (D) mean spherical equivalent (range, +1.75 to +9.75), and lack of single binocular vision. Amblyopia was not an important characteristic.


OBJECTIVE SIGNS

The accommodative convergence to accommodation (AC/A) ratio determines the difference between the exodeviation measured at distance and that at near fixation. Patients with exodeviations that measure similarly at 6 and 0.33 m while the examiner controls the patient’s accommodation have a normal AC/A ratio. Greater deviations at distance or at near are the product of an abnormal AC/A ratio; a high AC/A ratio causes greater exodeviation at distance, and a low AC/A ratio causes greater exodeviation at near. According to Duane’s classification,27 exodeviation greater at distance is divergence excess; exodeviation greater at near is convergence insufficiency. Common usage of Duane’s classification since 1896 has served the clinician well by classifying exodeviation into three categories. The third category is called basic deviation, an exodeviation with neither divergence excess nor convergence insufficiency, in that the exodeviation is the same for distance and near. Variations in these three categories of deviation are found in all types of exodeviations, regardless of whether they represent constant exophoria, intermittent exotropia, or constant exotropia.


Exophoria

Exophoria is a latent deviation with good fusional vergence amplitude and no amblyopia. Bifixation is maintained despite the exophoria, and normal stereoacuity demonstrates its existence.


Intermittent Exotropia

Intermittent exotropia is also associated with normal fusional vergence amplitudes and no amblyopia. In exophoric patients, the fusional vergence amplitudes are used to maintain bifixation; this is sufficient to prevent amblyopia. The stereoacuity is almost always normal when the patient is exophoric, but stereopsis is not present when the patient is exotropic.

Young patients quickly adapt sensorially to the exotropia phase by developing suppression and anomalous retinal correspondence (ARC), thus freeing themselves of diplopia and visual confusion. The suppression scotoma projecting into space soon enlarges, and the involved temporal retina extends from the hemiretinal line to beyond the locus at which the object of regard is projected as an image. Profound as the sensorial adaptations become, the moment the eyes converge to straight, normal retinal correspondence occurs, with bifixation functioning at a peak level. To gain the advantage of stereopsis, visual alertness and willingness to make the effort to attain this visual reward are required. With inattention or reduction in the desire for maintaining stereoscopic viewing, the effort to keep the eyes straight is terminated, and the moment exotropia ensues, suppression and ARC viewing replace the stereoscopic view. Hatt and coworkers28 assessed stereoacuity in children with intermittent exotropia with the Frisby Davis Distance test and the Distance Randot Test at distance, and the Frisby and Preschool Randot tests at near. Testing was repeated three to four times each day, with at least 2 hours between tests. Nearly half of the children with intermittent exotropia demonstrated marked changes in stereoacuity over the course of a single day. In some cases, stereoacuity changed from measurable stereoacuity on one assessment to negative stereoacuity on another. Therefore, when based on isolated testing, an apparent change in distance stereoacuity between visits should be interpreted with caution. Children 10 years of age and older who have not developed the sensorial adaptations when the eyes diverge are beyond the age at which they can develop them. Hence, they experience persistent heteronymous diplopia, while the exotropia prevails. Although the diplopia is annoying, perhaps, it is more acceptable than the asthenopia produced by struggling to keep the eyes straight when fatigued. However, these patients experience vexing symptoms while they are exotropic during certain visual circumstances; for example, while the patient is driving, the road seems to split into two roads, and oncoming automobile headlights become diplopic. The intermittent exotropic patient has instantaneous suppression and ARC, whereas a tropic patient is much less annoyed visually than her or his counterpart who lacks these sensorial adaptations.

Measurements of the exodeviation while the distance and near accommodations are controlled are best obtained by the cover test, using prisms to quantitate the exoangle. A muscle light should not be used as a fixating target for either distance or near accommodation because the exotropic patient frequently overaccommodates subconsciously using the extra accommodative convergence to reduce the exoangle. The blurred image of the muscle
light offers no deterrence to this method of compensating for the divergent deviation; however, the images of small symbols that are fixated demand an accurate accommodative response to be identified, allowing the examiner to obtain valid measurements of the total exoangle. The exodeviation may also vary in upgaze and downgaze (A or V pattern) and in right and left gaze from the primary position just as the distance and near measurements may vary. These various gaze positions are recorded for distance viewing, as well as for the straight-ahead near position. An increase in the angle of intermittent exotropia at distance may occur after fixating on an outdoor distant target or after 1 hour of monocular occlusion. Kushner29 evaluated the hypothesis that surgery should be performed when a larger deviation is discovered by one of these methods. His prospective study revealed a satisfactory outcome in 86% (43 of 50) of patients who underwent surgery for the largest angle, compared with only 62.5% (25 of 40) of patients in the control group, who had had surgery for the initial measurement of the exotropia at 6 m. A satisfactory outcome was declared if the examination occurred at least 1 year after surgery (range, 12 to 15 months) and if the final measurement was between 10 PD of exophoria and 5 PD of esophoria. Kushner concluded that surgery should be performed on the largest exotropia angle measured, both while fixating on a distant outdoor target and after 1 hour of monocular occlusion. Prism adaptation was not investigated in this study.

Versions often reveal inferior or superior oblique muscle dysfunction, and this should always be recorded. Long-standing exodeviations have a relatively high percentage of associated oblique muscle dysfunctions, and because A and V patterns are frequently associated with the oblique muscle dysfunctions, long-standing intermittent exotropic patients have an increased incidence of A and V patterns. Wilson and Parks30 studied retrospectively 456 patients with strabismus. Of 148 patients with intermittent exotropia, 32% had primary overaction of the inferior oblique muscles, when observed for 5 or more years, with an average age at onset of 5.2 years. Dissociated vertical deviation was found in only 3% of the patients with intermittent exotropia. Incidence of inferior oblique overaction was not related to the age at onset of the strabismus, onset of time of surgery, age at surgery, or ocular alignment decompensation.

A high AC/A ratio is more frequently encountered in intermittent exotropia than a low AC/A ratio is, and younger children with intermittent exotropia tend to have a significantly higher AC/A ratio than older children. Exodeviations measuring 20 to 35 PD at distance and 5 to 10 PD at near are common in young children, but the near measurement tends to increase with age, causing the distance and near deviation to approximate each other. Viewing the near accommodative target through +3.00 PD lenses increases the near exodeviation measurement in most patients, regardless of the nature of the AC/A ratio; those with a high AC/A ratio experience the greatest effect. However, the near prism and alternate cover measurements obtained through +3.00-D lenses have not provided more helpful information for management of exodeviation than simply the distance and near accommodation-controlled prism and alternate cover measurements.

Burian and Spivey31 have a different opinion and use the +3.00-D lens near prism and alternate cover measurement to determine the muscle for surgical correction of the exodeviation. Their reasoning is based on the dual premise that a central nervous system center controls divergence and that divergence innervation flows from this center when looking at distance and ceases when fixating at near. Therefore, excessive divergence innervation causes greater exodeviation at distance than at near. They also assume that the lateral rectus muscles diverge and the medial muscles converge. From this conclusion, they deduce that the diverging muscle should be weakened in patients with an excess of divergence. From these premises, they reason that patients with similar distance and near exodeviation measurements possess some unknown factor affecting the extraocular muscles and surrounding fascia, causing the divergence of the visual axes; whatever this factor, the exodeviation does not result from excessive innervational flow from the divergence center to the lateral rectus muscles. They call this basic exodeviation, given that excessive contraction of the lateral rectus muscles is not considered to be the primary cause of basic exodeviation. They advise recession of the lateral rectus muscle and resection of the medial rectus muscle of the same eye, presuming that this surgery produces a similar effect on straightening the diverged visual axes for both distance and near. They also reason that the innervational dispatch of the convergence center to the medial rectus muscles is deficient in patients with convergence insufficiency; they advise resection of the medial rectus muscles for this disorder.

If one subscribes to this line of reasoning, prism and alternate cover near measurements through +3.00-D lenses can provide some changes that make a particular case difficult to classify. For example, a patient’s divergence excess measurements may be converted with +3.00-D lenses to basic exodeviation measurements; consequently, this patient is reclassified as having pseudodivergence excess. Other patients initially classified as having basic exodeviation are changed by +3.00-D lenses and are reclassified as having convergence insufficiency measurements, and
presumably they are also reclassified as having pseudobasic exodeviation. Patients with convergence insufficiency cannot have their initial diagnosis changed by +3.00-D lenses; the only possible change is an increased exodeviation measurement at near.

Two weaknesses in the reasoning given here create unnecessary confusion. The first is the assumption that a patient with divergence excess, a greater exodeviation at distance than at near, is etiologically unrelated to another patient with basic exodeviation, a similar distance and near exodeviation. It is relatively more appealing to theorize that the distance exodeviation in all patients results solely from disturbance in the balance of opposing tonic horizontal vergence innervations, regardless of whether the near exodeviation is identical to, greater than, or less than the distance exodeviation. Congenital esotropia is theorized to result from the opposite disturbance in tonic horizontal vergences, and no consideration is given to whether the near esotropia is identical or dissimilar to the distance esotropia. The second weakness in this reasoning is the reluctance to use the synkinetic near reflex to explain the differences in the distance-near exodeviation measurements. The AC/A ratio is used to explain differences in distance-near esodeviation measurements and is a well understood and accepted method used in the diagnosis and treatment of esodeviations. Therefore, we believe it should also be used to explain the differences in distance-near exodeviation measurements: a normal AC/A ratio causes similar distance and near measurements, a high AC/A ratio causes greater distance exodeviation, and a low AC/A ratio causes greater near exodeviation. These two weaknesses are overcome by assuming that the interplay between the tonic vergence disturbance and the AC/A ratio explains all possible distance-near measurements encountered in the exodeviations. Therefore, no case needs to be classified as “pseudo” because of a near measurement obtained with +3.00-D spheres.

Children with divergent deviations tend to reveal less exodeviation at near than at distance by the prism and alternate cover test. This suggests that their AC/A ratio is pliable to the extent that it can be adjusted to assist offsetting the exodeviation for the benefit of maintaining single binocular vision. Because infants and young children seem to have a more pliable overall sensorimotor innervating system than older children and adults, this could account for the fact that the high-AC/A ratio occurs more frequently among exophoric and intermittently exotropic children. By this reasoning, the divergence excess exodeviation measurement results from an attempt to compensate for excessive divergence innervation rather than from excessive divergence innervation, as suggested in the preceding paragraphs. Kushner32, 33 attributes disparity between distance and near deviation in intermittent exotropia to proximal vergence aftereffects as well as to alterations in AC/A ratio. His term, “tenacious proximal fusion” describes the fusional aftereffect, which explains the distance-near disparity in patients previously classified as pseudodivergence excess type. With reduced angle of exotropia at near, these patients appear to have a slow-to-dissipate proximal fusion mechanism, which prevents them from manifesting their true near deviation during brief cover testing. By occluding one eye for 30 to 60 minutes, however, the “tenacious proximal fusion” can be suspended, and the true near deviation revealed. Kushner’s classification of intermittent exotropia includes: Basic Exotropia, 37%, equal distance and near measurements; Tenacious Proximal Fusion, 40%, distance measurements exceeds near, but near measurement increases after 60 minutes occlusion; Low AC/A Ratio, 11%, near measurement exceeds distance measurement; High AC/A Ratio, 5%, distance measurement exceeds near measurement; Proximal Convergence, 4%, distance measurement exceeds near measurement, even after 60 minutes occlusion, normal AC/A ratio; Fusional Convergence Insufficiency, less than 1%, near measurement exceeds distance measurements, poor fusional convergence amplitudes; Pseudoconvergence Insufficiency, less than 1%, near measurement exceeds distance measurement, but distance measurement increases with 60 minutes of occlusion.


Exotropia

Exotropia is a constant divergent deviation that is usually associated with either a normal or low AC/A ratio. There is seldom a high AC/A ratio except in the surgically overcorrected esotrope who had a high AC/A ratio before surgery because no sensory reward exists for the constantly tropic patient for evolving a high AC/A. Unless the fixation continues to be alternated between the two eyes, the patient who becomes constantly exotropic while younger than 10 years of age usually has amblyopia. In constant exotropia, suppression and ARC tend to be present unless there is no binocular vision, as encountered in either the congenital exotrope, some congenital esotropes who become exotropic, or the patient who unfortunately has a unilateral abnormal eye.


MANAGEMENT

The divergent deviations are treated by several methods: compensating for the misalignment with prisms, urging the patient to compensate for the misalignment with fusional convergence, subtly stimulating the patient to
compensate for the misalignment with accommodative convergence, or reducing the misalignment with surgery. The ophthalmologist’s selection of treatment depends on the nature of the exodeviation and experience with various treatment modalities. Clarke34 emphasizes that significant refractive errors, even hyperopic ones, should be corrected in intermittent exotropia in order to sharpen retinal images and improve fusion. Kushner35 has described paradoxical resolution of exotropia in some patients after correcting hyperopia with eyeglasses.


Exophoria

If exophoria is asymptomatic, it requires no treatment. Symptoms are rare in young children and tend to occur with increasing frequency in patients older than 10 years of age.

Fusional convergence training by orthoptics is successful in most affected patients and should be the first method used to alleviate the patient’s recurrent symptoms.36 The objective is to enhance the fusional convergence amplitude by various techniques. Treatment is emphasized for distance if the exodeviation is greater at distance than at near; near exercise is emphasized if the exodeviation is greater at near than at distance. While the patient does the exercise, the ophthalmologist should control the accommodation so that the fusional convergence is stimulated rather than the accommodative convergence. Small, detailed, distance targets are fixated for the distance exercises, and small print is read for the near exercise. Various combinations of loose prisms, 2, 3, 10, and 20 PD, can be arranged horizontally to offer the following stepwise increments in stimulus to fusional convergence: 2, 3, 5, 7, 8, 10, 12, 13, 15, 17, 18, 20, 22, 23, 25, 27, 28, 30, 32, 33, and 35 PD. The loose prism fusional convergence technique used in combination with a Polaroid screen fitted over the television while the patient wears Polaroid spectacle analyzers ensures that the patient is fusing at distance rather than suppressing as the base-out prism increments are made. Bar reading is an excellent method of ensuring against suppression at near as the increments of base-out prisms are made. Bar reading is accomplished by holding a narrow septum vertically midway between the eyes and the page; this obscures a different portion of the page for the right and left eye, but the entire page is seen undisturbed as long as the patient fuses.

Various stereoscopic techniques are also available for stimulating fusional convergence. One technique involves using a series of stereocards in a Wheatstone stereoscope; increasing convergence is demanded as the patient progresses through the series. Another method is built around the Polaroid vectographic technique: a series of Polaroid vectographic plates are viewed through Polaroid analyzers. As the patient proceeds through the series of plates, progressively more convergence is required to obtain the perception of stereopsis.

Whenever possible, the ophthalmologist should use accommodative convergence to compensate for exophoria in the subpresbyopic patient by undercorrecting hypermetropia and overcorrecting myopia. Forcing 2 to 3 D accommodation by appropriate spectacle power is usually well tolerated in young patients, but in older patients, this therapy may provoke symptoms of asthenopia similar to those it seeks to alleviate, and the benefit is thus nil. In a series of 35 children with intermittent exotropia who wore overcorrecting minus lenses, Caltrider and Jampolsky37 reported a qualitative change, with poorly controlled intermittent exotropia improving to well-controlled exophoria in 46%. A quantitative decrease of at least 15 D and latency of the exodeviation while wearing overcorrecting minus lenses was found in 26%. There were no significant qualitative or quantitative changes in 28%. Kushner38 found that overcorrecting minus lens therapy for intermittent exotropia did not appear to cause myopia in a series of 74 patients treated for 6 months and a subset of 34 patients treated for 5 years. dePaula and coworkers39 concluded that treatment of intermittent exotropia with overcorrecting minus lenses did not induce refractive error changes, taking into consideration age, treatment period, initial spherical equivalent, and magnitude of overcorrections used. Minus-lens therapy has also been advocated for control of postoperative exodeviation.40 In a series of 37 patients, preoperative diagnoses included 10 patients with esotropia and 27 with exotropia. Before treatment with minus lenses, over 75% had poor eye alignment, with manifest exotropia of 10 D or more, exophoria, or intermittent exotropia of 15 D or more. The overminus therapy improved results to excellent in more than 75%, with intermittent exotropia of less than 10 D, or a exophoria of 7 to 14 D, classified as satisfactory. After a 2-year follow-up without therapy, 50% had excellent to satisfactory alignment and 33% had no to 6 D of intermittent exotropia. In addition, medical therapy is available to stimulate accommodative convergence. By instilling 2% homatropine eye drops, the AC/A ratio is increased, which provides more convergence to offset the exophoria. However, the symptoms of asthenopia and blurred vision that accompany this therapy regardless of the patient’s age make it an impractical method for managing exophoria.

Base-in prism spectacles to compensate for the exophoria continue to have a place in the overall therapeutic regimen. Such spectacles are particularly useful in both presbyopic patients and subpresbyopic patients,
regardless of age, who are unable to increase their fusional convergence amplitude with orthoptic training. In this connection, there is an established clinical entity that is ideally treated by base-in prism spectacles. It occurs in young people, is lifelong, and causes symptoms during near vision manifested by blurred print and diplopia. Orthophoria or minimal exophoria is measured at distance, but a 12 to 18 PD exotropia is measured at near. Most significantly, the amplitude, or range, of accommodation is conspicuously less than the norm for the chronologic age. In addition, all fusional vergence amplitudes are obviously less than normal or almost nil, and, no matter how seriously the patient tries, these amplitudes cannot be expanded. This near-vision disorder presumably results from some deficiency in the optomotor reflexes. Accommodation, accommodative convergence, and fusional vergences are not there, and there is no way to improve them. Such patients should not receive prolonged orthoptic training. Presbyopic therapy with plus lens power is required for reading, along with base-in prisms to compensate for the near exotropia. The optic combination of these two therapies can be provided in reading spectacles; half-eye spectacles probably offer the best solution.

Surgery is seldom the answer for either distance or near exophoria, and it should be deferred until other methods of treatment have failed, unless the prism and alternate cover distance and near measurements are significant. Patients with either distance or near measurements who approach orthophoria are poor surgical candidates; an adult with approximately distant orthophoria and a symptomatic large angle of near exophoria is particularly of poor surgical risk. For such a patient with a low AC/A ratio, some ophthalmologists claim that resections of the medial rectus muscles improve the near exophoria without altering the preoperative distance orthophoria, but few have documented this claim of permanent improvement in near exophoria without a concomitant disturbing postoperative distance esotropia.

A study by von Noorden41 recommends resection of the medial rectus in persistent symptomatic convergence insufficiency but cautions that temporary consecutive esotropia occurs, requiring temporary prism control for some time after surgery.

The exophoric patient with a large distance deviation and approximately near orthophoria, who never slips into intermittent exotropia while viewing at distance, is also almost never troubled with symptoms and rarely requires treatment of any kind, including surgery. The patient with a large angle of exophoria at both distance and at near (i.e., in excess of 12 PD) and frequent symptoms does well with surgery, and it is justifiable to offer this treatment.


Intermittent Exotropia

Surgery is justified for intermittent exotropia if anisometropia or myopia has been adequately corrected with spectacles and if an eye continues to turn out intermittently. The patient has the best opportunity for a complete cure with elimination of the exoangle prior to the development of suppression and ARC. If the patient is 10 years of age or older and suppression and ARC are not present while the patient is exotropic, these sensorial adaptations to binocular vision will never develop, and the good prognosis for cure will continue unchanged. Until the patient reaches approximately 10 years of age, there is always the risk that suppression and ARC will be learned while exotropic; after these are learned, they can always be retained. These adaptations tend to be used and reused for any residual exodeviation, no matter how small, after surgery. Eventually, the small exodeviation tends to build back toward the preoperative levels. To prevent this discouraging result, the small residual postoperative exodeviation must be kept latent, as it is in patients without suppression and ARC. Attempts to prevent the suppression and ARC from ruining the immediate postoperative result by employing preoperative and postoperative orthoptics have usually failed. Chung and coworkers42 provided hyperopic correction when indicated in a prospective series of 114 patients with basic or divergence excess intermittent exotropia. Although the hyperopic correction resulted in a limited increase in exodeviation with a subnormal AC/A ratio, one-third of the patients developed a significant increase in their exodeviation. A trial of eyeglasses is recommended before considering surgery with patients having hyperopia and intermittent exotropia.

Cooper and Leyman43 compared the results of surgical and orthoptic treatment of intermittent exotropia. Their 673 patients were divided into four categories of treatment: (1) occlusion only, (2) operation only, (3) orthoptic training and operation, and (4) orthoptic training only. Orthoptic exercises were of two types: antisuppression techniques and convergence training. Most of the groups with only orthoptic training had exotropia less than 24 PD, whereas most patients in the two surgical groups measured 25 PD or more.

The highest percentage of good results, 59%, was in group 4, but this group also had smaller degrees of exotropia. Good results were obtained in 42% of group 2 and 52% of group 3. Cooper and Leyman encourage orthoptic training, when available, for small-angle exotropias and also to enhance the surgical results of exotropias above 25 PD.

An opposing view was presented by Moore,44 who reviewed the orthoptic treatment of intermittent exotropia
in 180 patients ranging in age from 3 to 18 years. None of these patients had good fusion on initial examination, 48 had fair fusion, and 132 had poor fusion. Treatment results were carefully recorded, and of the 57 patients in the surgery group, 33% were cured and 51% improved with a remaining intermittent deviation, for a total improvement of 84%. Of the 106 patients receiving surgery plus orthoptic training, 30% were cured and 42% were improved with a remaining intermittent deviation, for a total improvement of 73%. Moore concluded that orthoptic training did not appear to increase the likelihood of a cure in intermittent exotropia.

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Jul 10, 2016 | Posted by in OPHTHALMOLOGY | Comments Off on Concomitant Exodeviations
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