Microtropia Research Articles

Fusion after Surgical Alignment of Longstanding Strabismus in Adults

Fusion and the development of the monofixation syndrome are well recognized after surgical alignment of congenital esotropia to within 8 delta of orthotropia. However, in adults with longstanding strabismus and a history of congenital esotropia who are not surgically aligned within the first few years of life, the prognosis for the development of fusion is believed to be poor. Twenty-four adults, 12 with a history of congenital esotropia (onset before 6 months age), 1 of congenital exotropia, and 11 with the onset of esotropia between 6 months and 2 years, were included in this study. All patients had longstanding strabismus, and none had been surgically aligned within the first 2 years of life. No patient had visual acuity worse than 20/30 in the worse eye. Preoperatively, no patient demonstrated fusion with the red glass test, Worth four-dot, or Titmus test. Postoperatively, all were aligned to within 8 delta of orthotropia, and all demonstrated peripheral fusion with the Worth four-dot at near. Twelve patients (50%), 8 of whom were in the congenital group, achieved stereopsis of 200 seconds of arc or better using the Titmus stereo test. These findings suggest that surgery in this group of patients not only eliminates the ocular alignment deformity but can confer the functional benefits of fusion and increased field of binocular vision.

Compliance in antiamblyopia occlusion therapy.

Antiamblyopia occlusion therapy relies on compliance. We retrospectively reviewed the charts of 496 amblyopic subjects. Measures of non-compliance included patient reporting and patient records of broken appointments. Out of 496 subjects, 92 (18%) failed to follow the occlusion regimen. Compliance with treatment was analyzed by age group, refractive error and type of strabismus. The failure rate was 82.6% for the unilateral high myopia group and 37.5% for the monofixation syndrome group. The risk for non-compliance appeared to be higher in the 1 to 2-year-old group (37%). The chi 2-test showed the differences to be highly significant (P less than 0.0001). Children having lower initial visual acuity were also significantly less complaint (P less than 0.007). Several factors such as age, parental understanding, initial visual acuity and improvement rate seem to be involved in compliance.

Mechanism of anomalous retinal correspondence: Maintenance of binocularity with alteration of receptive-field position in the lateral suprasylvian (LS) visual area of strabismic cats

We have examined the effects of rearing kittens with a unilateral convergent strabismus, induced surgically at 3 weeks of age, on the binocularity (ocular dominance) and receptive-field position of neurons in the motion-sensitive lateral suprasylvian (LS) area of cat extrastriate cortex. Data were compared to those obtained from area 17 in the same animals, and from the two areas of cortex in normal adult cats. Interocular alignment of the operated cats was assessed in alert adults using corneal reflex photography and during recording from the positions of retinal landmarks under paralysis. The strabismus magnitude in each operated cat was calculated by comparison with equivalent data from the normal animals. Strabismus always caused a major loss of binocularity in area 17. The remaining binocular neurons had receptive-field (RF) pairs arising from positions of normal correspondence in the two retinae and would thus have been responsive to different regions of visual space through the misaligned eyes in the alert animal. In area LS, the effects were dependent on the strabismus magnitude. In the group of four cats with pronounced strabismus (18-30 deg crossed), a loss of binocularity occurred in area LS equivalent in severity to that in area 17. The majority of the remaining binocular LS neurons possessed RF pairs in normal retinal correspondence and would thus, in the alert animal, have received spatially disparate visual input through the two eyes. This also occurred in three other cats with more moderate strabismus (11-15 deg crossed), although only a small breakdown in the binocularity of area LS was apparent. The group of cats with mild strabismus (less than or equal to 10 deg crossed) had normal proportions of binocular neurons in area LS. In three of these cats, the maintenance of binocularity was accompanied by shifts in RF position, with visual inputs arising from anomalous retinal locations. These shifts compensated, in part, for the strabismus angle present in each cat, so that most of the binocular LS neurons would have received inputs from regions of visual correspondence through the misaligned eyes when the animal was alert. Similar mechanisms could afford a basis for the binocular visual compensations that occur in humans with small-angle strabismus of early onset. If so, anomalous retinal correspondence in such individuals would have as a locus areas of extrastriate cortex with a role in motion perception, and would involve alterations to the neural substrate underlying normal binocular vision.

Clinical Characteristics and Surgical Treatment of Intermittent Esotropia/Clinical Characteristics and Surgical Treatment of Intermittent Esotropia: Discussion

This study describes the age of onset, symptomatology, and clinical characteristics of 25 patients with intermittent esotropia. This subgroup of esotropic patients typically has onset prior to 10 years of age and shows excellent bifoveal fixation ability, reflected by a high degree of stereopsis. In contrast to patients with other forms of esotropia, there is minimal to no hyperopia, an average esodeviation of approximately 20 prism diopters, and a low incidence of amblyopia, DVD, and oblique overaction. Symptomatic diplopia or cosmetic concern necessitated surgical intervention in 17 (68%) patients. Fifteen (88%) patients had good or excellent surgical results of either orthophoria, esophoria, or residual esotropia of 10 delta or less. One patient (6%) had residual esophoria greater than 10 delta. One patient had 4 delta of exophoria postoperatively. Intermittent esotropia should be differentiated from the various entities of microtropia, fixation disparity, and monofixation syndrome.

Stereoacuity Testing in the Monofixation Syndrome

A group of patients with the monofixation syndrome were assessed prospectively to compare performance on Titmus and Randot stereograms under monocular and binocular conditions. These patients achieved monofixation either as a result of surgery for infantile esotropia, or because of a partially accommodative esotropia or significant anisometropia. Their ability to detect elevation of the Titmus circles was not significantly better with two eyes as compared to that level achieved using only their monofixating eye. However, performance on the Titmus animals and on Randot circles or animals showed better discrimination. These data would suggest that the monocular clues available in determining elevation of the Titmus circles would invalidate establishing a diagnosis of monofixation.

Long-term stability of alignment in the monofixation syndrome.

From a collection of 328 patients with congenital esotropia, 80 patients were identified who had a minimum 4-year follow-up after surgical alignment within 8 prism diopters (pd) of orthophoria. These patients were subdivided into two groups: the M.S. group (38 patients) demonstrated the monofixation syndrome; the NO M.S. group (42 patients) did not have monofixation. The alignment of patients in each group was followed over time. After 17.5 years, 74% of patients in the M.S. group maintained alignment within 8 pd of orthophoria. In the NO M.S. group, only 45% were well aligned after 14 years. The mean time to loss of stability was significantly shorter for the NO M.S. group (P less than 0.005). Our findings suggest achievement of the monofixation syndrome improves but does not guarantee stability of ocular alignment.

Acquired monofixation syndrome.

Monofixation syndrome is a well documented sensory abnormality occurring in patients with childhood-onset strabismus. We report the development of monofixation syndrome in six adults with strabismus. It is proposed that constant strabismus, which precludes and prevents bifixation, can result in the development of monofixation syndrome if the strabismic deviation is not corrected promptly.

Interocular transfer. An assessment of binocularity in strabismus.

The interocular transfer of the motion aftereffect (MAE) was used to probe for the existence of binocularity in subjects with strabismus. The interocular transfer was measured with coarse and fine gratings subtending 2° and 8° of visual angle. Strabismic subjects were categorized by the extent of their binocular vision, particularly with respect to extrafoveal fusion and retinal correspondence. Subjects classified as having the monofixation syndrome consistently showed a substantial, though less than normal, amount of transfer with the larger 8° gratings. This is consistent with the peripheral binocular cooperation usually found in these subjects, and establishes interocular transfer of the MAE as a suitable test for binocularity in strabismus. The test is especially suitable in patients with suppression in which an existing binocularity may be masked by suppression on clinical testing. The tests were found to be applicable in children as young as eight years. MAE tansfer for large coarse gratings is believed to imply the existence of an early cortical binocularity, which provides the potential for sufficient binocular cooperation to permit stable ocular alignment.

Suppression and the pattern visual evoked potential.

We assessed the effect of amblyopic suppression on the pattern visual evoked potential (P-VEP). Since anisometropic amblyopia is associated with the monofixation syndrome and a central suppression scotoma in the amblyopic eye, patients with this functional defect were used for the study. Five anisometropic amblyopes and four normal subjects were studied using the P-VEP test. A modified haploscope was used to present a pattern stimulus to the area of suppression. By comparing responses generated under monocular conditions (non-suppression) with monocular responses under binocular viewing conditions (suppression), we measured the effect of suppression on the P-VEP. Our results show that amblyopic suppression completely abolishes P-1 amplitudes in patients with large suppression scotomas; in patients with small suppression scotomas the P-1 amplitudes were greatly reduced. Rivalry in normals also caused a reduction in P-1 amplitudes, but to a much lesser degree than that associated with anisometropic suppression. Patients with significant amblyopia did not show a rivalry effect on the P-VEP. Because the P-1 wave of the transient P-VEP is thought to represent activity at the level of the striate cortex, our results indicate that suppression probably occurs at the striate cortex or lateral geniculate nucleus.

Photorefraction: a useful tool to detect small angle strabismus.

Forty patients with small angle strabismus and 30 straight eyed controls were examined by photorefraction in order to establish the validity of this method to detect strabismus. The sensitivity of the method was based on detection of asymmetry of the corneal light and/or fundus reflexes. All cases with a deviation of 2 degrees or more were detected, and 2 controls were falsely considered to be strabismic patients. The sensitivity of the method decreased when the patient wore optimal correction. Photorefraction therefore is considered to be a reliable, simple, and cheap tool with high sensitivity and specificity in screening for strabismus as well as refractive errors.

Sensory and sensori-motor adaptations in strabismus: Their role in space perception

In comitant strabismus many perceptual adaptive processes take place, which involve perception of space. Suppression of the image of the deviated eye and anomalous retinal correspondence (ARC) are the two main antidiplopic mechanisms. ARC may be present without suppression in small-angle strabismus (up to 10 degrees), supporting an anomalous binocular cooperation in spite of the deviation. Both psychophysical and electrophysiological evidence for anomalous binocular vision in strabismus are provided. Sensori-motor adaptations in strabismus develop as well. They are represented by vergence eye movements which, although not identical to, have similar characteristics as normal fusional vergences. These anomalous fusional eye movements tend to return the eyes to their original deviation when elements are introduced to change the position of the eyes, e.g., prisms or surgery. In conjunction with ARC, these movements serve to maintain binocular visual perception despite the strabismus.

Suppression in strabismus--an update.

Previous reports have described suppression scotomas, suppression varying with the type of strabismus and suppression confined to one half of the retina (hemiretinal suppression). Our findings show that suppression in all varieties of strabismus, with the exception of the monofixation syndrome, involves the whole of the visual field of the deviating eye except for its monocular temporal crescent. In the monofixation syndrome our findings show a small central suppression scotoma involving the fovea but leaving the rest of the visual field of the deviating eye unsuppressed. We could find no evidence to support the concept of hemiretinal suppression but found evidence to support the presence of a trigger mechanism for suppression which operates on a hemiretinal basis.

Binocularity in comitant strabismus: II. Objective evaluation with visual evoked responses.

Campos (1982a) reported on his psychophysical studies on the binocularity of patients with comitant strabismus. With binocular visual field techniques, it was possible to show that patients with small-angle eso- and exotropia exhibit a binocular vision, without suppression of the deviated eye. In the present paper visual electrophysiology is used objectively these findings and to provide more insight in the problem. First, the studies on binocularity in normals and strabismics, done by using visual evoked responses (VER) are reviewed. This type of investigation is relatively new and the results of the literature are still conflicting. Then personal results of the authors are reported. It is shown that with VER it is possible to objectively assess the presence of anomalous binocular vision (ABV) sustained by anomalous retinal correspondence (ARC) in small-angle strabismus. In patients with large angle deviations this type of binocularity is absent. A correlation between psychophysical and electrophysiological data is provided. Lastly a simple method is described for differentiating the binocularity of normals from that of Strabismics with ARC. This method is based on the recording of binocular VER with the anteposition in front of the fixing eye of neutral filters of increasing density.

Hemiretinal Trigger Mechanism-Reply

In Reply. —We would like to thank Dr Campos for his interesting remarks. It is difficult, to be sure, with any test in patients with a monofixation syndrome and peripheral fusion, whether this fusion is on normally corresponding points or abnormally corresponding points (ARC). The reason for this is that the error of the tests is often greater than the few prism diopters of deviation present in these patients. The two patients with ARC mentioned in our article had convergent angles of 18 diopters and 20 prism diopters, respectively, at the test distance and, hence, we could be more certain that fusion, in the periphery, was occurring on noncorresponding points in the deviating eye. We think that the other points raised by Dr Campos have been covered in our article.

Suppression in strabismus and the hemiretinal trigger mechanism.

A perimeter and a modified Lees screen were used to determine the area of visual field that is suppressed in strabismus. Strabismic patients without fusion suppress the whole area of the visual field of the deviating eye that could have correspondence with the dominant eye. In the monofixation syndrome, suppression only involves the fovea and the diplopic point of the deviating eye. Diplopia, if present, is appreciated throughout the visual field shared by the two eyes, except for the blind spots. No evidence of hemiretinal suppression was found in any type of horizontal strabismus, including exotropia. However, there is a hemiretinal trigger mechanism for suppression. Once suppression has been triggered by this mechanism, it involves the whole visual field of the deviating eye, except for the monocular temporal crescent.

Binocularity in comitant strabismus: binocular visual fields studies.

A critical review is made of the literature on binocular perimetry in strabismus. A broad range of results was obtained by various authors, because different testing techniques were used. The concept of sensory testing of strabismic patients in casual seeing condition should be introduced also into binocular perimetry. Therefore a series of experiments is presented in which patients with small-angle comitant strabismus were tested. It was shown that: a) in small-angle esotropia the areas of single vision previously thought to be due to suppression are, instead, areas of binocular vision sustained by anomalous retinal correspondence (ARC); b) this can be seen only when using fusable stimuli as test targets; c) no suppression scotomas were found in patients with small-angle strabismus in the whole visual field; d) appropriate control-marks for binocularity are necessary, i.e. monocularly presented items. They influence the results, as well e) the area of binocular single vision in strabismus (called pseudo-Panum's area) is wider than the Panum's area of normals. Moreover it is easier to disrupt binocular cooperation sustained by ARC causing diplopia than binocular cooperation in normals; f) ARC seems to be more deeply rooted in the center than in the periphery of the visual field; g) superimposable findings were obtained in small-angle constant exotropia; h) in large-angle exotropia wide suppression scotomas were found, which often override the midline. Hemianopic suppression scotomas in exotropia can be found only when 'dissociating' testing techniques are used; i) the same group of patients with small-angle esotropia was examined with the author's technique and with the classical method proposed by Harms. The results were strongly dependent on the method used and the same patient responded differently to the two tests. It is concluded that in small-angle strabismus there is an anomalous type of binocular single vision, which can be tested with binocular visual field techniques. This binocularity can only be found when non-artificial testing conditions are used. The usefulness of this anomalous binocular vision in the every day seeing condition of the patient is discussed.

Motion aftereffect transfer in the monofixation syndrome.

If one adapts to a moving repetitive stimulus of stripes that is suddenly stopped, the stripes will appear to move backward. This apparent backward motion is the motion aftereffect (MAE), and its duration is a measure of its magnitude. If one eye adapts to the moving stimulus and the other eye experiences the aftereffect to the stationary stimulus, the aftereffect has been transferred from one eye to the other and is termed the interocular transfer of the MAE. Experimental evidence indicates that the degree of MAE transfer correlates with clinical binocularity. This study compares the MAE transfer in six subjects with the monofixation syndrome to five normal subjects. The stimuli used are sinusoidal stripes generated on two cathode-ray tubes, subtending either 8 degrees or 2 degrees of visual angle with a periodicity of either 0.5 or 3 cycles/degree presented haploscopically. Subjects with the monofixation syndrome differed significantly from normal subjects in the amount of MAE transferred, implying a lack of central neuronal connections in addition to those mediating conscious central fusion in clinical sensory testing.

Monofixational Intermittent Exotropia

<h3>To the Editor.</h3> —We read with interest the report on "Monofixational Intermittent Exotropia" in the JanuaryArchives(97:93-95, 1979) by Baker and Davies and can confirm their observations. We, too, have cared for a number of patients with intermittent exotropia who had, in fact, an ultra-small-angle monofixational exotropia when their eyes were "straight," and who maintained the same level of sensory binocular cooperation after surgical correction of the exotropia. We question, however, the prevalence of monofixation syndrome that Baker and Davies found in one third of their patients. They noted this frequency to be higher than expected according to Parks.¹ Diagnosing monofixation syndrome in children solely on the basis of Titmus stereo test stereoacuity is difficult, since performance on this test is age related and variable.²Using this test, a stereoacuity of 60 s of arc is found in only 50% of normal children at the age 4½

Zero retinal image disparity: a stimulus for suppression in small angle strabismus.

Suppression was examined with binocular perimetry in patients with small angle strabismus. Suppression stimuli were presented to the fovea of the fixating eye or 5 degrees nasal or temporal from the fovea. Suppression was found in the deviating eye of patients with normal or anomalous correspondence at retinal points corresponding to the retinal locus of the suppression stimulus. Suppression was demonstrated on both the nasal and temporal hemiretina of the deviating eye. Results indicate that zero retinal disparity is a strong stimulus for suppression in the central and peripheral visual field of patients with small angle strabismus.

Primary Monofixation Syndrome with Good Stereopsis

Primary Monofixation Syndrome with Good Stereopsis