Stereopsis in normal infants and infants with congenital esotropia

The onset of sensitivity to horizontal disparity in infancy: a short-term longitudinal study

In this short-term longitudinal study, infants were examined for their natural preference of a square defined by crossed horizontal disparity (either 1° or 0.5°) over a square defined by a vertical disparity (either 1° or 0.5°). The square targets were embedded in a dynamic random dot stereogram. The stimuli were presented on an autostereoscopic monitor equipped with a face-tracking device. The infants were tested weekly between 6 and 16 weeks of age. Four experiments were conducted. In two experiments, the infants were examined with the forced-choice preferential looking (FPL) method for their ability to perceive either 1° or 0.5° horizontal disparity. In the remaining two experiments, the classical natural preference (CNP) method (measurement of looking times) was applied. According to the results of the FPL experiments, mean relative preference for the horizontal disparity square became significant at 8 weeks of age. The CNP data indicated an onset of stereopsis at 12-15 weeks. The mean relative preferences for horizontal disparity indicated by the CNP method were smaller than those found in the FPL experiments. Thus, the FPL method was more sensitive than the CNP method in the measurement of infant responsiveness to crossed horizontal disparity.

Stereoacuity outcomes after treatment of infantile and accommodative esotropia

PURPOSE

To review what is known about the normal maturation of stereoacuity, the stereoacuity deficits associated with infantile and accommodative esotropia, the rationale for making improved stereoacuity a goal of treatment, and strategies for improving stereoacuity outcomes.

METHODS

Studies of stereoacuity maturation during normal development, studies of stereoacuity outcomes after treatment for infantile and accommodative esotropia, and studies of primate models of esotropia are reviewed.

RESULTS

Stereoacuity maturation normally proceeds rapidly during the first year of life. Infantile and accommodative esotropia are associated with profound and permanent disruption of stereopsis. Although rehabilitation of stereoacuity after treatment of esotropia remains a challenge, even the achievement of subnormal stereoacuity may have real benefits to the child.

CONCLUSIONS

Some abnormalities in stereoacuity may exist before the onset of esotropia, but others may result directly from abnormal binocular experience. Several strategies for improving stereoacuity outcomes in esotropia are currently under active investigation. Improved stereoacuity outcomes are associated with better long-term stability of alignment, reduced risk for and severity of amblyopia, improved achievement of sensorimotor developmental milestones, better reading ability, and improved long-term quality of life.

Update on strabismus and amblyopia

This review analyzes progress in the field of infantile strabismus and amblyopia. Only works which are of clinical interest have been reviewed. A brief survey is provided also of subjects matter of international debate, as surgery for accommodative esotropia, the prism adaptation test and the use of sectors for amblyopia treatment. Mention is made of new developments in the field which are not yet applicable to clinical practice. Finally, progress in the approach towards ocular nystagmus is considered as well.

Why do the eyes cross? A review and discussion of the nature and origin of essential infantile esotropia, microstrabismus, accommodative esotropia, and acute comitant esotropia

PURPOSE

To try to explain the long-term stability of bilateral medial rectus botulinum toxin (botox) chemo-denervation in essential infantile esotropia; to evaluate divergent fusion amplitude in accommodative esotropia and acute comitant esotropia of emmetropes; to look for accommodation anomalies in high AC/A ratio accommodative esotropia and acute comitant esotropia of myopes; and to discuss characteristics of microstrabismus.

METHODS

Retrospective analysis of 61 essential infantile esotropia patients with early treatment with one botox injection in both medial rectus; measurement of divergent fusion amplitude in accommodative esotropia and acute comitant esotropia; measurement of Near point of accommodation in high AC/A ratio accommodative esotropia and acute comitant esotropia of myopes.

RESULTS

Stable results were found in 85.24% of essential infantile esotropia treated patients; reduced divergent fusion amplitude was detected in accommodative esotropia and acute comitant esotropia; hypo-accommodation was found in some patients with high AC/A ratio accommodative esotropia and a convergence spasm in acute comitant esotropia of myopes.

CONCLUSIONS

Very early botox treatment probably eradicates the effect of an excessive convergence tonus in essential infantile esotropia. A prevention of accommodative esotropia with full retinoscopic correction is only mandatory with a significantly reduced amplitude of fusional divergence. A deficit in accommodation should be looked for in high AC/A ratio accommodative esotropia, before bifocal lenses prescription. Early diagnosed acute comitant esotropia of myopic patients can be treated as a convergence spasm. Only surgery treats acute comitant esotropia, in patients with emmetropia or moderate hypermetropia.

Decision analysis with Markov processes supports early surgery for large-angle infantile esotropia

PURPOSE

To assess tradeoffs in time spent with aligned visual axes or stereopsis and risked reoperations between three strategies for the treatment of large-angle infantile esotropia: first surgery at 6, 24, or 48 months of age.

DESIGN

Decision analysis.

METHODS

We simulated three hypothetic cohorts of children with this condition until their eighth year. Potential achievement of stereopsis, the diagnosis of dissociated vertical divergence or inferior oblique overaction, and various complications were modeled. Probability estimates were obtained from a comprehensive literature review. Analyses aimed to determine the strategy that was associated with longer time spent with aligned eyes and/or stereopsis and fewer reoperations.

RESULTS

Eight-year-old children are expected to spend 72.6 vs 58.0 vs 37.8 months with aligned eyes and 26.8 vs 9.0 vs 1.5 months with stereopsis and to receive on average 1.76 vs 1.67 vs 1.46 surgeries, if they first underwent operation at 6 vs 24 vs 48 months, respectively. Children at 8 years are expected to have stereopsis at the rate of 36.1% vs 17.2% vs 5.1%, respectively. Operation at 6 months instead of 48 months is expected to yield an additional 9% of total follow-up time in health states with eye alignment and 11% in health states with stereopsis, per 5% increase in risk for extra operations (5% and 7%, respectively, per 5% increase in risk for operation at 6 vs 24 months). Wide-range sensitivity analyses and long-term projections point to the same direction.

CONCLUSION

Given the benefit/risk tradeoffs, it is probably warranted to attempt to correct large-angle infantile esotropia as early as possible.

Amblyopia characterization, treatment, and prophylaxis

Amblyopia has a 1.6-3.6% prevalence, higher in the medically underserved. It is more complex than simply visual acuity loss and the better eye has sub-clinical deficits. Functional limitations appear more extensive and loss of vision in the better eye of amblyopes more prevalent than previously thought. Amblyopia screening and treatment are efficacious, but cost-effectiveness concerns remain. Refractive correction alone may successfully treat anisometropic amblyopia and it, minimal occlusion, and/or catecholamine treatment can provide initial vision improvement that may improve compliance with subsequent long-duration treatment. Atropine penalization appears as effective as occlusion for moderate amblyopia, with limited-day penalization as effective as full-time. Cytidin-5'-diphosphocholine may hold promise as a medical treatment. Interpretation of much of the amblyopia literature is made difficult by: inaccurate visual acuity measurement at initial visit, lack of adequate refractive correction prior to and during treatment, and lack of long-term follow-up results. Successful treatment can be achieved in at most 63-83% of patients. Treatment outcome is a function of initial visual acuity and type of amblyopia, and a reciprocal product of treatment efficacy, duration, and compliance. Age at treatment onset is not predictive of outcome in many studies but detection under versus over 2-3 years of age may be. Multiple screenings prior to that age, and prompt treatment, reduce prevalence. Would a single early cycloplegic photoscreening be as, or more, successful at detection or prediction than the multiple screenings, and more cost-effective? Penalization and occlusion have minimal incidence of reverse amblyopia and/or side-effects, no significant influence on emmetropization, and no consistent effect on sign or size of post-treatment changes in strabismic deviation. There may be a physiologic basis for better age-indifferent outcome than tapped by current treatment methodologies. Infant refractive correction substantially reduces accommodative esotropia and amblyopia incidence without interference with emmetropization. Compensatory prism, alone or post-operatively, and/or minus lens treatment, and/or wide-field fusional amplitude training, may reduce risk of early onset esotropia. Multivariate screening using continuous-scale measurements may be more effective than traditional single-test dichotomous pass/fail measures. Pigmentation may be one parameter because Caucasians are at higher risk for esotropia than non-whites.

A clinical evaluation of random dot stereoacuity cards in infants

PURPOSE

To clinically evaluate the random dot stereoacuity cards developed by Birch and Salomao on a population of infants within the first 12 months of life and to generate baseline normative data.

METHODS

Random dot stereoacuity cards were constructed using vectographic vertical bar stimuli with cross disparities ranging from 45 to 1735 seconds of arc (1.65 to 3.2 log seconds of arc). Wearing polarising goggles, infants were assessed using a two-alternative forced choice preferential looking staircase procedure. Success rate and stereoacuity values for performance as a function of age were evaluated.

RESULTS

One hundred and seventy-seven tests were completed on 41 infants with ages ranging from 5 to 56 weeks. The random dot stereoacuity procedure was successful in 128 (72.3%) infants, success being dependent upon age. At 0-8 weeks of age, none of the infants produced a fixation preferance towards even the coarsest stereogram. The success rate increased to 50% at 9-16 weeks, 97% at 17-24 weeks, 93% at 25-36 weeks, dropping to 69% at 37-56 weeks due to increasing intolerance to the goggles. Stereopsis was not demonstrable in infants under 8 weeks of age. By 9-16 weeks the mean stereoacuity level was 2.91 log seconds of arc, further increasing to 2.53 at 17-24 weeks, reducing slightly to 2.65 at 25-36 weeks. At 37-56 weeks the mean stereoacuity value was recorded as 2.53 log seconds of arc.

CONCLUSION

Our results indicate that the infant random dot stereoacuity cards provide a simple, quick and portable test for the clinical assessment of stereopsis in infants aged between 17 and 36 weeks, and should prove to be a useful clinical tool to monitor infants at risk of binocular vision disorders.

Visual capacity and Prader-Willi syndrome

PURPOSE

Prader-Willi syndrome (PWS) refers to a genetic disorder induced by an anomaly on chromosome 15 occurring with a frequency of one in 10,000 to 20,000. It is characterized by a unique set of features including infantile hypotonia, obesity in childhood, small hands and feet, hypogonadism, and mental retardation. Reported here are the results of ophthalmic examinations of persons with PWS, together with results from controls comparable in age, percentage of body fat, and intelligence. These data bear on the hypothesis that the ocular anomalies in PWS are unique to this syndrome.

METHOD

A comprehensive investigation of PWS brought children and adults to Vanderbilt University for extended testing, which included an ophthalmic examination. Genetic analysis determined unequivocally the PWS diagnosis and identified subgroups-deletion and maternal disomy. A group of persons without PWS but generally comparable in age, body composition, and intelligence served as controls.

RESULTS

Significant differences between the deletion and disomy subgroups were not found for the clinical ophthalmic measures. The incidence of anomalies in the combined PWS was similar to those reported in previous studies. A similar pattern was present in the control group except for myopia and stereopsis. An effect of genetic subgroup, however, was observed for random element stereopsis with the maternal disomy group having a greater degree of impairment.

CONCLUSION

The overall similarity between the PWS and control groups on all measures except myopia and stereopsis suggest that many of the anomalies in PWS found in prior studies are due to factors inherent in a general dysfunctional population, rather than reflective of an ocular signature unique to PWS.

Results of early alignment of congenital esotropia

OBJECTIVE

To determine the long-term motor and sensory results after early surgical correction of patients with congenital esotropia.

DESIGN

Noncomparative interventional case series.

PARTICIPANTS

Ten infants with congenital esotropia.

INTERVENTION

Patients had bimedial rectus recession between 83 and 159 days of age; were re-examined in a regular follow-up program; and were retreated when required for strabismus, amblyopia, and refractive errors.

MAIN OUTCOME MEASURES

Final alignment, stereo acuity, variations in vision, alignment, refraction, and number and types of retreatments required during the period of observation.

RESULTS

All patients were aligned initially with bimedial rectus recession of 8.0 to 10.0 mm measured from the limbus. A total of 11 additional surgical procedures were performed on 7 patients to maintain alignment. Four patients required hyperopic spectacle correction to maintain alignment, and two patients required short periods of patching. Visual acuity was 20/40 or better in 19 eyes at the most recent examination, which was between 8.3 and 11.8 years after initial surgery. All patients had final alignment to within 10 prism diopters (PD) of orthotropia at either distance or near. Nine of ten patients had dissociated vertical deviation (DVD), and four of ten patients had latent nystagmus. Four patients had measurable stereo acuity at their last visit, with two achieving a stereo acuity of 3000 seconds (the Titmus fly), one 400 seconds, and one 140 seconds.

CONCLUSION

Surgical alignment of congenital esotropia can be achieved in the 4-month-old with bimedial rectus recession, but this does not ensure continued alignment. At least one additional surgical procedure is required on average to maintain alignment in the first 10 years after initial successful surgery. These patients can also be expected to have one or more of the following: DVD, latent nystagmus, refractive component, or latent strabismus. Regardless of outcome, patients with congenital esotropia have optokinetic asymmetry. Attainment of stereo acuity, including high-grade stereo acuity, may be enhanced by attainment of orthotropia or small-angle esotropia but is likely to be ultimately dependent on constitutional factors rather than age of alignment.

Infant random dot stereoacuity cards

PURPOSE

The overall goal was to develop a simple test of random dot stereoacuity that can be used during the first 24 months of life to measure sensory outcomes following treatment of ophthalmopediatric disorders, both in the context of clinical trials and in the context of clinical management.

METHODS

A series of random dot stereoacuity test cards were constructed using vectographic vertical bar stimuli with crossed disparities ranging from 1735 to 45 sec. A two-alternative, forced-choice, preferential-looking test protocol was used to measure stereoacuity in 95 healthy term infants (173 tests) over the 1.5-to-24-month age range. Success rate, validity, and tolerance limits for normal performance as a function of age were evaluated.

RESULTS

The success rate was high (158 of 173 tests; 91.3%). The maturation of random dot stereoacuity was similar to that measured in earlier lab-based preferential-looking and visual evoked potential protocols; few infants younger than 2 months of age demonstrated stereopsis although by 12 months, mean stereoacuity was 2.1 log sec (120 sec). A further improvement to 1.7 log sec (56 sec) was seen during months 18 to 24, consistent with data obtained in an earlier operant random dot protocol. Tolerance limits for normal ranged from approximately 1000 sec at 6 months to 100 sec after 12 months.

CONCLUSIONS

The infant random dot stereoacuity cards provide a simple, quick, and portable preferential-looking test of random dot stereoacuity for infants that has a high success rate and excellent concordance with stereoacuity outcomes from laboratory-based protocols.

Stereopsis and disparity vergence in monkeys with subnormal binocular vision

The surgical treatment for strabismus in infants generally results in microtropia or subnormal binocular vision. Although the clinical characteristics of these conditions are well established, there are important questions about the mechanisms of binocular vision in these patients that can best be investigated in an appropriate animal model. In the present psychophysical investigations, spatial frequency response functions for disparity-induced fusional vergence and for local stereopsis were studied in macaque monkeys, who demonstrated many of the major visual characteristics of patients whose eyes were surgically aligned during infancy. In six rhesus monkeys, unilateral esotropia was surgically induced at various ages (30-184 days of age). However, over the next 12 months, all of the monkeys recovered normal eye alignment. Behavioral measurements at 4-6 years of age showed that the monkeys' prism-induced fusional vergence responses were indistinguishable from those of control monkeys or humans with normal binocular vision. Investigations of stereo-depth discrimination demonstrated that each of the experimental monkeys also had stereoscopic vision, but their stereoacuities varied from being essentially normal to severely stereo-deficient. The degree of stereo-deficiency was not related to the age at which surgical esotropia was induced, or to the presence or absence of amblyopia, and was not dependent on the spatial frequency of the test stimulus. Altogether, these experiments demonstrate that a temporary, early esotropia can affect the binocular disparity responses of motor and sensory components of binocular vision differently, probably because of different sensitive periods of development for the two components.

The modified frisby stereotest

BACKGROUND

The Frisby stereotest commonly is used in clinical practice to estimate stereoacuity. Assessment of the presence or absence of stereopsis is valuable particularly in toddlers because of the difficulties encountered in this age group with assessment of other aspects of visual function, such as monocular visual acuities.

METHODS

The present study describes two modifications to the Frisby stereotest: 1) the introduction of a nonstereo practice plate; and 2) the use of an auditory "reward" for correct identification of the target. These modifications aim to increase the success rate of the test and provide a means to discriminate between testable and untestable children. Subjects were 165 children aged between 0.5 and 47 months.

RESULTS

The modifications improved the age range over which results could be obtained with the Frisby test, allowing infants as young as 7 months to complete testing. By 12 months of age, more than 60% of children were able to complete testing. The modifications also allowed the examiner to distinguish untestable children from those without stereopsis.

CONCLUSIONS

By simple modification of the Frisby stereotest, the authors have increased the ease with which the Frisby stereotest can be used to assess stereoacuity in infants and children and provided a means by which children unable to cooperate with testing can be distinguished from those without stereopsis.

Abnormal motion processing and binocularity: infantile esotropia as a model system for effects of early interruptions of binocularity

Infantile esotropia, a common form of strabismus with onset prior to 6 months of age, occurs at a time of rapid visual development. While monocular visual acuity is relatively unaffected in these patients, the majority of them fail to achieve fully normal stereopsis. In addition, these patients show a spectrum of abnormalities in their ocular following responses, visual perception and visual evoked potentials (VEPs) that suggest a failure to develop a normal complement of motion processing mechanisms. While abnormalities of of stereopsis have been studied for many years, motion processing in strabismus is a rapidly evolving area of current research. Motion mechanisms are normally binocular and may form a distinct binocular sub-system. This review summarises which is known about sensory and motor abnormalities in infantile esotropia, with special emphasis on recent motion VEP recordings. The monocular motion VEP shows directional biases early in infancy that are consistent with a nasalward/temporalward response bias. Patients with infantile esotropia maintain their neonatal biases beyond the age at which they normally disappear. The motion VEP biases persist into visual maturity in patients whose strabismus is treated after about 2 years of age. Treatment prior to age 2 can lessen the magnitude of the motion VEP asymmetry and these improvements can be maintained into visual maturity. A recording from the striate cortex of a visually deprived macaque monkey indicates that the motion VEP asymmetry arises early in the visual pathway.

Surgical alignment prior to six months of age for congenital esotropia

PURPOSE

To complete the first independent and largest multicenter outcome investigation to analyze the motor and functional results of a series of patients surgically aligned prior to age 6 months and followed for a minimum of 4 years.

METHODS

Sixteen patients, surgically aligned at an average age of 4.2 months, were examined at an average of 7.1 years to assess their motor and functional outcomes.

RESULTS

Motor and sensory tests showed 11 patients to have a small or negligible motor misalignment at near point with both binocular fusion and gross stereopsis ability. A single patient aligned by 3 months of age demonstrated reproducible refined stereoacuity on sensory testing. However, the patients who achieved alignment by 4 or 5 months did not demonstrate any better quality of binocularity than that found in a previously studied group of patients aligned at 6 months.

CONCLUSION

Binocularity that includes refined stereoacuity remains an elusive target and a rare outcome for an ophthalmologist treating congenital esotropia, despite very early surgical alignment.

Early versus late alignment of infantile esotropia

Fifty-two children with infantile esotropia had surgery for their esotropia between two and seven years of age. Another 49 had similar surgery before the age of two years. Of these 101 operated children, the esotropia in 70 patients was aligned to within 10 prism diopters. The patients with successful alignment were reviewed to study the effect of early versus late surgical intervention of their deviation, i.e. before and after two years of age. The motor and sensory states of the selected patients were analysed before and after surgical correction. The choice and number of surgical procedures employed are discussed. It appears that patients two years and older when strabismus was surgically corrected have a good chance of alignment, but less chance of attaining binocularity. In many cases there was a V-pat-tern esotropia, requiring surgery on the overacting inferior oblique muscles. In comparison, the younger group with earlier surgery appeared to show a better chance of attaining binocularity and fewer patients had significant V-pattern requiring surgery on the inferior oblique muscles.

Dichoptic luminance beat visual evoked potentials in the assessment of binocularity in children

Direct evidence of a distinct cortical binocular pathway has been provided by the production of nonlinear (difference) beats from dichoptic luminance stimulation in stereonormal adults and the absence or diminution of these beats in stereoblind subjects. We have investigated a clinically useful application of this technique in a pediatric population with potentially abnormal binocular vision. We recorded dichoptic luminance beat visual evoked potentials (VEPs) from 20 children (ages 7 months to 8 years) with abnormal binocular ability secondary to strabismus and/or amblyopia and compared this to a control group of 20 children with normal binocularity. Stereoblind children generated significantly lower dichoptic signal-to-noise ratios than stereonormal children (P < .001). Responses to monoptic multifrequency flicker were not significantly different between the two groups (P = .936). This dichoptic VEP can be performed quickly and easily on young children and gives a quantitative assessment of cortical binocularity that may not be determinable by standard clinical methods. This technique may also prove useful for the preoperative gradation of binocular potential and prediction of postoperative binocular fusion.

Artifacts in fusion and stereopsis testing based on red/green dichoptic image separation

A red/green anaglyph stereotest (TNO) was administered to a group of patients who had anisometropic amblyopia, and the Worth 4-Dot test for fusion to two groups of strabismus patients. Both tests were administered twice, with the red/green lens position of the glasses reversed between eyes for the second administration. Twelve of 15 patients with anisometropic amblyopia exhibited a stereo acuity difference of 2:1 or more, and 18 of 89 strabismic patients changed Worth 4-Dot fusion or suppression status, between the two positions of the glasses. A group of seven strabismic patients who showed the reversal effect on the Worth 4-Dot test had the results of one glass' position discrepant with those of a geometrically identical achromatic test. It appears that the red/green format can introduce artifacts in binocular vision testing. Testing twice, with the red/green glasses reversed between trials, will prevent misinterpretation of binocular status and may provide diagnostically useful information.

VECP evidence for binocular function in infantile esotropia

Since commonly used clinical methods of measuring binocular function require subjective responses, this testing has been limited to more cooperative children, usually older than 2 years of age. Recently, we have begun using a clinically practical, visually evoked cortical potential (VECP) method to detect the presence of binocular neurons in infants and young children. We studied 14 children, ages 4 to 44 months, with infantile esotropia. Nine had surgical correction for esotropia by the age of 2 years. Twenty-five normal infants ages 6 weeks to 22 months served as controls. Most normal infants showed the development of the "beat" by 2 months of age. The "sum" VECP was not consistently present until age 6 months. Four of the five esotropic infants less than 1 year of age, demonstrated neither a "sum" nor "beat" response. All nine patients with corrective surgery performed before 2 years of age developed a significant sum response and three developed a beat. Five patients had not had surgery until after 2 years of age. Two developed a sum and one a beat, but none had both beat and sum responses. The results suggest that there is a loss of nonlinear binocular response in esotropic children not corrected before the age of 2 years and that these responses can be restored after early treatment even if not present at the time of surgery.

19th annual Frank Costenbader Lecture--the origins of congenital esotropia

Congenital esotropia develops in the first 4 months of life in an infant who lacks the inborn mechanism for motor fusion. It manifests as an esotropia which is not eliminated by correction of hyperopia and occurs in an otherwise neurologically normal infant. The earliest practical time for surgery is 4 months of age. The eye is anatomically suited for surgery at this age and also, this is the earliest age that congenital esotropia can be diagnosed with confidence. The best attainable result of treatment of congenital esotropia is subnormal binocular vision. This result is more likely to be attained if infants are aligned by 18 months of age. Satisfactory alignment is produced in 80% to 85% of infants in one procedure with an appropriate bimedial rectus recession. An array of motor defects including DVD, latent nystagmus, oblique dysfunction, and A- and V-pattern appear at varying times after successful alignment. These associated findings are commonly found with, but are not unique to, congenital esotropia. The onset and clinical picture of congenital esotropia is satisfactorily explained by a theory first suggested by Worth that the strabismus is caused by an inborn defect in the motor fusion mechanism and aggravated by esotropital factors as suggested by Chavasse. In contrast to congenital esotropia, all other strabismus can be thought of as occurring on a secondary basis in a person with the inborn capacity for motor fusion, but who failed to maintain it because of conatal insurmountable strabismus (congenital third nerve palsy), who lost it because of acquired (postnatal) strabismus, who uses a strategy such as head posture alteration to retain fusion under favorable circumstances (Duane syndrome), who has intermittent strabismus with part-time suppression (X(T)), or who is maintaining alignment with nonsurgical means (refractive esotropia). For the future, I believe that advances in the management of congenital esotropia will depend on a better understanding of etiology leading to design and use of innovative nonsurgical techniques to discourage convergence and stimulate bifoveal fusion.

A new VEP system for studying binocular single vision in human infants

Visual evoked potentials (VEPs), that provide unequivocal objective evidence of cortical binocularity have been recorded from adults and young infants using a new VEP system developed for this purpose. The system uses alternating field stereoscopy (AFS) to present separate visual stimuli to each eye. With this system, the binocular image pairs to the right and left eyes alternate at a high rate on a single video monitor. The subject wears spectacles incorporating light-scattering liquid crystal lenses which alternate electronically between opaque and clear modes in synchrony with the video monitor. To detect cortical binocularity, the system analyzes VEP activity mathematically and identifies significant responses at test frequencies reflecting binocular cortical interactions exclusively. Three types of binocular stimuli were presented: (1) dynamic random dot correlograms (correlograms); (2) dynamic random dot stereograms (stereograms); and (3) dichoptic checkerboard stimuli. The correlograms are generated when moving random dot patterns presented to each eye alternate between two phases, correlated and anticorrelated. With the stereograms, portions of random dot patterns presented to each eye are shifted horizontally relative to each other at a fixed rate, alternately producing crossed and uncrossed binocular disparities. Subjectively, these patterns appear to shift in depth. Dichoptic checkerboard stimuli are regular checkerboard patterns which reverse at different rates (frequencies) for each eye. Binocular VEPs are generated due to cortical interactions at the difference (beat) frequency. Using this VEP system, we have recorded binocular VEPs from 10 normal adults and more than 40 infant subjects.(ABSTRACT TRUNCATED AT 250 WORDS)

Maternal observations of ocular alignment in infants

Seventy-five infants of state-registered orthoptists were observed by their mothers from birth to at least 6 months of age, with particular attention paid to the nature of any deviations noticed in the first few weeks. Most of the infants showed brief periods of inaccurate vergence during the first 2 months, with a wide variation in the amount of deviation seen, despite going on to develop normal binocular single vision. Most deviations were transient, unilateral, alternating esodeviations. There was a statistically significant relationship between the time that deviations were noticed and the development of demonstrable binocular convergence. Few exodeviations were found, contrasting with previous studies, and it is suggested that neonates are more likely to achieve binocular single vision when interacting with their mothers.

Postoperative binocularity in adults with longstanding strabismus

The authors tested preoperatively and postoperatively for binocularity with the Bagolini lenses in a series of 359 adults who underwent surgery for long-standing constant strabismus. Eighty-six percent of patients showed a binocular response with the Bagolini lens test almost immediately after surgery. Regardless of the type of deviation present preoperatively, the duration of strabismus, or the depth of amblyopia in the deviating eye (if present), the vast majority of patients developed binocularity. The development of binocularity with the Bagolini lenses after surgery appears to be related to the stability of the postoperative ocular alignment.

18th annual Frank Costenbader Lecture. Fusion and suppression: development and loss

A retrospective study (coauthored with Geraldine Tillson) of 118 patients with the congenital esotropia syndrome is described. All patients were aligned with surgery and, in some cases, glasses as well, within 10 prism diopters of orthotropia before the age of 2 years and had remained so aligned for at least a year. A standardized final examination was performed on patients over the age of 6 years, after at least 5 years' follow up, to study the effect of significant refractive error, amblyopia, nystagmus, dissociated vertical divergence, and inferior oblique overaction on the sensory outcome. None of the patients obtained central fusion. Fifty-three of 118 obtained peripheral fusion with at least 5 delta of fusional amplitude. Twenty-eight of the 53 fusers had stereopsis. The findings did not show any factors to explain why 65 did not obtain fusion. A congenital lack of the potential to develop fusion is postulated. The development of fusion and suppression is discussed with relevant references. Evidence is presented that the anatomical location of the lesion in central fusion disruption is in the midbrain. Fusion may be lost, in visual adults, without developing suppression from prolonged sensory deprivation. Examples include unilateral cataract, uncorrected unilateral aphakia, and unilateral severe herpetic keratitis.

Strabismus surgery in children: the prospects for binocular single vision

The restoration or maintenance of useful binocular single vision (BSV) represents the ideal outcome in the management of a squinting child. However, in planning such management it is essential to have a clear appreciation of the likelihood of attaining that goal, and what factors will help in its attainment. Using both a literature review and some preliminary information from our own patient database we examine the prospects for BSV in different strabismus groups. We also discuss the techniques available to allow prediction of which children are candidates for the development of BSV following squint correction.

Binocular function in human infants: correlation of stereoptic and fusion-rivalry discriminations

Studies of stereopsis in infants have shown that the average age of onset is 3.5 months. This is the same age at which infants first show evidence of another binocular function, namely, preference for binocularly fusible patterns over rivalrous ones. We tracked the development, using two-alternative forced-choice preferential looking, of the two forms of binocular function in 17 infants, 11 male and six female. They were tested at regular intervals until they showed preferences for 1) a fusible pattern (vertical stripes presented to each eye) over a rivalrous one (vertical stripes presented to one eye, horizontal stripes to the other), and 2) a line stereogram of 32 min crossed disparity over a comparable stereogram with zero disparity. The correlation between the age of onset of the fusion preference (mean 12.4 weeks) and the age of onset of stereopsis (mean 11.0 weeks) was r = 0.79. Female infants showed a preference for the fusible stimulus at a mean age of 9.9 weeks, significantly earlier than the males at a mean age of 13.8 weeks. Similarly, females also showed evidence of stereopsis at an earlier age (9.1 weeks compared with 12.1 weeks for males).

Strabismus in infancy

In order to prospectively study the development of strabismus in infancy, the ocular alignment of a large population of normal neonates was studied and follow-up examinations were obtained in a subset of these infants. The characteristic findings of congenital esotropia subsequently developed in three infants who were either orthotropic or exotropic at birth. Pathologic exotropia developed in two infants; both were exotropic at birth, but no more so than most normal neonates. In infants with congenital esotropia or pathologic exotropia, the characteristic deviation appears to develop between 2 and 4 months of age, a period during which normal infants are becoming increasingly orthotropic.

Developmental aspects of the Brückner test

Normal infants ranging from newborn to 10 months of age were examined in order to study age-related variation in the behavior of the fundus reflexes seen in the course of performing the Brückner test. Most infants 8 months of age and older show the characteristic symmetric dimming of the fundus reflexes in both eyes occurring with central fixation. Neonates and most infants younger than 2 months of age do not show dimming of the fundus reflex with fixation, probably due to an inability to accommodate accurately. Between 2 and 8 months of age, up to 28% of infants have asymmetric dimming of the fundus reflexes in the two eyes. In contrast to older children in which this is a pathologic finding, asymmetric fundus reflexes occurring in this age range may represent a normal stage of development.

Bowman lecture. Current concepts of infantile esotropia

Several forms of esotropia with a different pathophysiology that meet the criterion of an onset early in life must be distinguished from essential infantile esotropia. A hypothesis is presented, according to which a delayed development or a congenital defect of retinal disparity sensitivity (motor fusion) in an otherwise normal infant with immature sensory functions causes esotropia under the influence of various strabismogenic factors. Some of these factors are genetically determined, hence the familial occurrence of essential infantile esotropia. The absence or marked decrease of stereopsis and the asymmetry of optokinetic nystagmus are interpreted as the consequence of ocular misalignment early in life rather than of structural anomalies in the afferent visual pathways of esotropic patients. The therapeutic results after surgery are classified into four groups: subnormal binocular vision, microtropia, small angle eso- or exotropia and large angle residual or consecutive eso- or exodeviations. Analysis of data from 358 operated patients with a documented onset of esotropia prior to the sixth month of life has shown that the probability of obtaining an optimal functional result is increased when surgical alignment is completed before completion of the second year of life. However, surgery after the age of two or even four years of life does not preclude the development of binocular vision on a subnormal or anomalous basis.

Why should surgery for early-onset strabismus be postponed?

The author presents a continued study of 82 cases of pseudoparalysis of the bilateral rectus muscles in early-onset convergent strabismus following early surgery. Up to 10 years after surgery motor results show that 72% of cases remain within +/- 10 prism dioptres after a single surgical procedure. Sensorial findings corroborate the results of other authors' studies in that binocular association is only achieved in those patients where surgery was carried out at a very young age, not older than 1 year. Surgical procedures consisting of bimedial rectus recession with retroequatorial myopexy are described and the development of alphabetical syndromes and dissociated vertical divergence discussed. The evolution of refraction in these cases is also shown.

A reassessment of infantile esotropia. XLIV Edward Jackson memorial lecture

Essential infantile esotropia is an early acquired, not a congenital, condition, although congenital factors may favor its development between the ages of 3 and 6 months. It must be distinguished from other forms of esotropia with an onset between birth and the first six months of life. The cause of essential infantile esotropia remains unknown, but advances in our knowledge can be expected from the rapidly emerging discipline of infant psychophysics. In analyzing treatment results, a clear distinction must be made between normal, subnormal, and anomalous forms of binocular cooperation. While complete restoration of normal binocular function is rarely, if ever, achieved, anomalous binocular cooperation has many functional advantages over suppression or diplopia and should not be disturbed by overzealous treatment. Subnormal binocular vision is considered to be an optimal, microtropia a desirable, and a residual small angle heterotropia an acceptable end stage of surgical therapy. In a study of 358 surgically treated patients with a documented onset of essential infantile esotropia before age 6 months, subnormal binocular vision was present in 71 (20%), a microtropia in 25 (7%), and a small angle esotropia or exotropia in 140 (39%) of the patients. Surgical alignment before completion of the second year of life improved the chances for an optimal treatment result.(ABSTRACT TRUNCATED AT 250 WORDS)

Stereoscopic contours and optokinetic nystagmus in normal and stereoblind subjects

Moving stereoscopic contours in a dynamic random-dot stereogram have been previously shown to induce optokinetic nystagmus in subjects with normal stereopsis. For this to be validated as an objective test of stereopsis, stereoblind subjects must also be shown not to develop OKN, especially since it has been shown that the optomotor system of stereoblind individuals retains sensitivity to some cyclopean stimuli. In this report we verify that stereoblind subjects do not have an optomotor response to stereoscopic contours--regardless of the alignment angle at which the stereo image pair is presented.