To investigate whether neuronal activity within the supraoculomotor area (SOA—monosynaptically connected to medial rectus motoneurons and encode vergence angle) of strabismic monkeys was correlated with the angle of horizontal misalignment and therefore helps to define the state of strabismus.
Single-cell neural activity was recorded from SOA neurons in two monkeys with exotropia as they performed eye movement tasks during monocular viewing.
Horizontal strabismus angle varied depending on eye of fixation (dissociated horizontal deviation) and the activity of SOA cells (n = 35) varied in correlation with the angle of strabismus. Both near-response (cells that showed larger firing rates for smaller angles of exotropia) and far-response (cells that showed lower firing rates for smaller angles of exotropia) cells were identified. SOA cells showed no modulation of activity with changes in conjugate eye position as tested during smooth-pursuit, thereby verifying that the responses were related to binocular misalignment. SOA cell activity was also not correlated with change in horizontal misalignment due to A-patterns of strabismus. Comparison of SOA population activity in strabismic animals and normal monkeys (described in the literature) show that both neural thresholds and neural sensitivities are altered in the strabismic animals compared with the normal animals.
SOA cell activity is important in determining the state of horizontal strabismus, possibly by altering vergence tone in extraocular muscle. The lack of correlated SOA activity with changes in misalignment due to A/V patterns suggest that circuits mediating horizontal strabismus angle and those that mediate A/V patterns are different.
The author shows that neurons in the midbrain near-response area adjacent to the oculomotor nucleus, that are known to encode vergence in normal monkeys show activity related to strabismus angle in monkeys with a sensory-induced strabismus.
Rhesus monkeys reared with restricted visual environment during their first few months of life develop large ocular misalignment (strabismus). The purpose of this study was to describe ‘A and V’ patterns and DVD in these animals during fixation and eye movements and suggest that this form of rearing produces animals that are a suitable model to study mechanisms that might cause ‘A/V’ pattern incomitant strabismus and dissociated vertical deviation (DVD) in humans.
Eye movements were recorded during fixation, smooth-pursuit and saccades using binocular search coils in one monkey with esotropia, three monkeys with exotropia and one normal monkey.
1) Monkeys reared with Alternating Monocular Occlusion or Binocular deprivation (tarsal plates intact) showed both horizontal and vertical misalignment during monocular and binocular viewing.
2) Large ‘A’ patterns were evident in 2 out of 3 exotropes while a ‘V’ pattern was observed in the esotrope.
3) Similar ‘A/V’ patterns were observed with either eye viewing and during fixation or eye movements.
4) The vertical misalignment, which consisted of the non-viewing eye being higher than the fixating eye, appeared to constitute a DVD.
Visual sensory deprivation methods that induce large strabismus also induce ‘A/V’ patterns and DVD similar to certain types of human strabismus. The source of the pattern strabismus could be central, i.e., altered innervation to extraocular muscles from motor nuclei, or peripheral, i.e., altered location of extraocular muscle pulleys.
A/V patterns; dissociated vertical deviation; eye movement; monkey; incomitant strabismus; esotropia; exotropia
The authors show that central innervation to extraocular muscle is responsible for setting the state of horizontal misalignment and also generating abnormal cross-axis eye movements that result in A or V patterns of strabismus.
Monkeys reared under conditions of alternating monocular occlusion during their first few months of life show large horizontal strabismus, “A” patterns, and dissociated vertical deviation. “A” patterns manifest as an inappropriate horizontal component in the deviated eye during vertical eye movements (cross-axis movement). The objective of this study was to investigate response properties of medial rectus motoneurons (MRMNs) in relation to strabismus properties.
Burst-tonic activity of 21 MRMNs in the oculomotor nucleus were recorded from two monkeys with exotropia as they performed horizontal and vertical smooth pursuit (0.2 Hz, ±10°) under monocular viewing conditions. Neuronal responses and horizontal component of eye movements were used to identify regression coefficients in a first-order model for each tracking condition.
Comparison of position, velocity, and constant parameter coefficients, estimated from horizontal tracking data with either eye viewing, showed no significant differences (P > 0.07), indicating that neuronal activity could account for the horizontal misalignment. Comparison of the position, velocity, and constant parameter coefficients estimated from horizontal tracking and the cross-axis condition showed no significant differences (P > 0.07), suggesting that motoneuron activity could account for most of the inappropriate horizontal cross-axis movement observed in the covered eye during vertical smooth pursuit.
These data suggest that, in animals with sensory-induced strabismus, central innervation to extraocular muscles is responsible for setting the state of strabismus. Mechanical factors such as muscle length adaptation (for horizontal misalignment) and pulley heterotopy or static torsion (for “A” patterns) likely do not play a major role in determining properties in a sensory-induced strabismus.
The authors showed earlier that animals reared with certain types of visual sensory deprivation during their first few months of life develop large horizontal strabismus, A/V patterns, and dissociated vertical deviation (DVD). Cross-axis eye movements were observed in the nonfixating eye that reflected pattern strabismus and DVD. The purpose of this study was to investigate whether neuronal activity within the oculomotor nucleus could be driving the abnormal cross-axis eye movements observed in the nonfixating eye.
Burst-tonic activity was recorded from oculomotor nucleus neurons in three animals with A-pattern exotropia as they performed horizontal or vertical smooth pursuit during monocular viewing. Two animals were reared by alternate monocular occlusion for 4 months, and one animal was reared by binocular deprivation for 3 weeks.
In this study, efforts were focused on neurons modulated for vertical eye movements. Vertical burst-tonic motoneurons were strongly correlated with vertical eye movements regardless of whether the movement was purposeful, as in vertical smooth pursuit, or whether it was inappropriate, as in a vertical component observed in the nonfixating eye during horizontal smooth pursuit. Quantitative analysis of position and velocity sensitivities of the cells measured during the different tracking conditions suggested that motoneuron activity was sufficient to account for most of the inappropriate vertical cross-axis component.
Results suggest that, in animals with sensory-induced strabismus, innervation to extraocular muscles from motor nuclei produce the inappropriate cross-axis eye movements, resulting in change in ocular misalignment with gaze position associated with pattern strabismus and DVD.
Nonhuman primates reared with daily alternating monocular occlusion (AMO) during their first few months of life develop large horizontal strabismus, A/V patterns and dissociated vertical deviation (DVD). In addition, these animals often alternate or switch the fixating eye during binocular viewing. The purpose of this study was to characterize the alternating fixation behavior of these animals during visually guided saccade tasks.
Binocular eye movements were measured in two monkeys with AMO-induced exotropia as they performed a visually guided saccade task (random target presentation over a ±15° grid horizontally and vertically) during either monocular or binocular viewing.
During binocular viewing, large target steps into the temporal hemifield of the nonfixating eye (nasal retina of the nonfixating eye) produced fixation switches. Target steps into the nasal hemifield of the nonfixating eye (temporal retina of the nonfixating eye) tended not to produce a fixation switch. There were no significant differences in the amplitude–peak velocity or amplitude– duration main sequence relationships between alternating (binocular viewing) and nonalternating saccades (monocular or binocular viewing). Saccade latency tended to be greater during binocular viewing than during monocular viewing.
This study shows that the AMO model for strabismus may be used for studying neural circuits involved in generating alternating fixation and alternating saccade behavior. Since patterns of alternating fixation are likely to be influenced by patterns of visual suppression, alternating saccade behavior may also be used as a probe to study mechanisms of visual suppression in strabismus.
Previous studies have shown that binocular coordination during saccadic eye movement is affected in humans with large strabismus. The purpose of this study was to examine the conjugacy of saccadic eye movements in monkeys with sensory strabismus.
The authors recorded binocular eye movements in four strabismic monkeys and one unaffected monkey. Strabismus was induced by first occluding one eye for 24 hours, switching the occluder to the fellow eye for the next 24 hours, and repeating this pattern of daily alternating monocular occlusion for the first 4 to 6 months of life. Horizontal saccades were measured during monocular viewing when the animals were 2 to 3 years of age.
Horizontal saccade testing during monocular viewing showed that the amplitude of saccades in the nonviewing eye was usually different from that in the viewing eye (saccade disconjugacy). The amount of saccade disconjugacy varied among animals as a function of the degree of ocular misalignment as measured in primary gaze. Saccade disconjugacy also increased with eccentric orbital positions of the nonviewing eye. If the saccade disconjugacy was large, there was an immediate postsaccadic drift for less than 200 ms. The control animal showed none of these effects.
As do humans with large strabismus, strabismic monkey display disconjugate saccadic eye movements. Saccade disconjugacy varies with orbital position and increases as a function of ocular misalignment as measured in primary gaze. This type of sensory-induced strabismus serves as a useful animal model to investigate the neural or mechanical factors responsible for saccade disconjugacy observed in humans with strabismus.
In this study, we have used the double-step paradigm to test saccadic gain adaptation during monocular viewing in one normal monkey, two monkeys with exotropia, and one monkey with esotropia. In this paradigm, the target for the saccade is displaced during the saccade, resulting in a consistent visual error. Studies in normal humans and monkeys have shown that the brain responds to this consistent visual error by gradually changing saccade gain. Using this technique, we were able to elicit adaptation in both the viewing eye and the nonviewing eye in the normal monkey and in monkeys with strabismus. The rate of adaptation was not significantly different in the viewing and nonviewing eyes in the normal and strabismic monkeys. The magnitude of adaptation as calculated by a percentage change in gain was also not significantly different in the viewing and the nonviewing eyes in the normal and strabismic monkeys. Our data show that animals with strabismus retain the ability to elicit a conjugate adaptation of saccades using this mechanism. We also suggest that the double-step paradigm elicits a conjugate adaptation of saccades whether the animal is viewing monocularly (our studies) or binocularly (data published in literature).
In this study, stereopsis was preserved in optically strabismic infant monkeys by brief daily periods of normal binocular vision. This result indicates that the temporal integration properties of mechanisms responsible for vision development ensure that binocular vision development proceeds normally despite episodes of abnormal vision, but also implies that stereopsis may be preserved in human strabismic infants by providing brief daily periods of fusion prior to alignment surgery.
This study examines whether brief periods of binocular vision could preserve stereopsis in monkeys reared with optical strabismus.
Starting at 4 weeks of age, six infant monkeys were reared with a total of 30 prism diopters base-in split between the eyes. Two of the six monkeys wore prisms continuously, one for 4 weeks and one for 6 weeks. Four of the six monkeys wore prisms but had 2 hours of binocular vision daily, one for 4, one for 6, and two for 16 weeks. Five normally reared monkeys provided control data. Behavioral methods were used to measure spatial contrast sensitivity, eye alignment, and stereopsis with Gabor and random dot targets.
The same pattern of results was evident for both local and global stereopsis. For monkeys treated for 4 weeks, daily periods of binocular vision rescued stereopsis from the 10-fold reduction observed with continuous optical strabismus. Six weeks of continuous strabismus resulted in stereo blindness, whereas daily periods of binocular vision limited the reduction to a twofold loss from normal. Daily periods of binocular vision preserved stereopsis over 16 weeks of optical strabismus for one of the two monkeys.
Two hours of daily binocular vision largely preserves local and global stereopsis in monkeys reared with optical strabismus. During early development, the effects of normal vision are weighed more heavily than those of abnormal vision. The manner in which the effects of visual experience are integrated over time reduces the likelihood that brief episodes of abnormal vision will cause abnormal binocular vision development.
To evaluate the association between fixation preference (FP) and amblyopia in strabismic patients.
This study includes 50 patients with horizontal, vertical or mixed strabismus of at least 10 prism diopters. Best-corrected monocular visual acuity (VA) was measured using Snellen E-chart and the presence of amblyopia was determined accordingly; FP was evaluated and graded from 0 to 3.
Of 50 patients, including 27 female and 23 male subjects, 29 (58%) patients had FP but 18 (36%) subjects were truly amblyopic. Overall, the sensitivity and specificity of FP for detection of amblyopia was 88.9% and 59.4% respectively. The positive predictive value (PPV) and negative predictive value (NPV) were 55.2% and 90.5% respectively. Sensitivity, PPV and NPV were significantly higher in esotropic as compared to exotropic patients. Strong monocular FP was correlated with more than 3 lines of interocular difference (IOD) in visual acuity (P=0.001).
Although FP is not an ideal method for diagnosis of strabismic amblyopia, it has high sensitivity, PPV and NPV in esotropic patients and in subjects with more than 3 lines of IOD in VA.
Binocular Fixation Pattern; Fixation Preference; Strabismic Amblyopia
We determined the distribution of cells containing synthetic enzymes for the unconventional neurotransmitter, nitric oxide, with respect to the known populations within the oculomotor complex.
The oculomotor complex was investigated in monkeys and cats by use of histochemistry to demonstrate nicotinamide adenine dinucleotide phosphate diaphorase positive (NADPHd+) cells and antibodies to localize neuronal nitric oxide synthase positive (NOS+) cells. In some cases, wheat germ agglutinin conjugated horseradish peroxidase (WGA-HRP) was injected into extraocular muscles to allow comparison of retrogradely labeled and NADPHd+ cell distributions.
The distribution of the NADPHd+ and NOS+ neurons did not coincide with that of preganglionic and extraocular motoneurons in the oculomotor complex. However, labeled perioculomotor neurons were observed. Specifically, in monkeys, they lay in an arc that extended from between the oculomotor nuclei into the supraoculomotor area (SOA). Comparison of WGA-HRP–labeled medial and superior rectus motoneurons with NADPHd staining confirmed that the distributions overlapped, but showed that the C- and S-group cells were not NADPHd+. This suggested that NADPHd+ cells are part of the centrally projecting Edinger-Westphal population (EWcp). Examination of the NADPHd+ cell distribution in the cat showed that these cells were indeed found primarily within its well-defined EWcp.
Based on their similar distributions, it appears that the peptidergic EWcp neurons, which project widely in the brain, also may be nitridergic. While the preganglionic and C- and S-group motoneuron populations do not use this nonsynaptic neurotransmitter, nitric oxide produced by surrounding NADPHd+ cells may modulate the activity of these motoneurons.
The centrally projecting Edinger-Westphal nucleus (EWcp) in both the macaque monkey and cat contains a nitridergic cell population. The motoneurons in the preganglionic Edinger Westphal nucleus (EWpg) and the oculomotor nucleus are not nitridergic.
This study was designed to use infrared photorefraction to measure accommodation in awake-behaving normal and strabismic monkeys and describe properties of photorefraction calibrations in these monkeys.
Ophthalmic trial lenses were used to calibrate the slope of pupil vertical pixel intensity profile measurements that were made with a custom-built infrared photorefractor. Day to day variability in photorefraction calibration curves, variability in calibration coefficients due to misalignment of the photorefractor Purkinje image and the center of the pupil, and variability in refractive error due to off-axis measurements were evaluated.
The linear range of calibration of the photorefractor was found for ophthalmic lenses ranging from –1 D to +4 D. Calibration coefficients were different across monkeys tested (two strabismic, one normal) but were similar for each monkey over different experimental days. In both normal and strabismic monkeys, small misalignment of the photorefractor Purkinje image with the center of pupil resulted in only small changes in calibration coefficients, that were not statistically significant (P > 0.05). Off-axis measurement of refractive error was also small in the normal and strabismic monkeys (~1 D to 2 D) as long as the magnitude of misalignment was <10°.
Remote infrared photorefraction is suitable for measuring accommodation in awake, behaving normal, and strabismic monkeys. Specific challenges posed by the strabismic monkeys, such as possible misalignment of the photorefractor Purkinje image and the center of the pupil during either calibration or measurement of accommodation, that may arise due to unsteady fixation or small eye movements including nystagmus, results in small changes in measured refractive error.
Strabismic extraocular muscles (EOMs) differ from normal EOMs in structural and functional properties, but the gene expression profile of these two types of EOM has not been examined. Differences in gene expression may inform about causes and effects of the strabismic condition in humans.
EOM samples were obtained during corrective surgery from patients with horizontal strabismus and from deceased organ donors with normal EOMs. Microarrays and quantitative PCR identified significantly up- and down-regulated genes in EOM samples. Analysis was performed on probe sets with more than 3-fold differential expression between normal and strabismic samples, with an adjusted P value of ≤ 0.05.
Microarray analysis showed that 604 genes in these samples had significantly different expression. Expression predominantly was upregulated in genes involved in extracellular matrix structure, and down-regulated in genes related to contractility. Expression of genes associated with signaling, calcium handling, mitochondria function and biogenesis, and energy homeostasis also was significantly different between normal and strabismic EOM. Skeletal muscle PCR array identified 22 (25%) of 87 muscle-specific genes that were significantly down-regulated in strabismic EOMs; none was significantly upregulated.
Differences in gene expression between strabismic and normal human EOMs point to a relevant contribution of the peripheral oculomotor system to the strabismic condition. Decreases in expression of contractility genes and increases of extracellular matrix-associated genes indicate imbalances in EOM structure. We conclude that gene regulation of proteins fundamental to contractile mechanics and extracellular matrix structure is involved in pathogenesis and/or consequences of strabismus, suggesting potential novel therapeutic targets.
Strabismic human extraocular muscles show significant changes in gene expression with upregulation of genes related to extracellular matrix and downregulation of myosin genes. The strabismic transcriptome suggests a major contribution of the muscle to strabismus, as well as new therapeutic targets.
The previously reported nomenclature and clinical characteristics of dissociated vertical deviation have been recorded. The incidence and characteristics of DVD have been determined by evaluation of 1,000 consecutive strabismus or nystagmus patients, and with selected chart study carried out on the 111 DVD patients found in this series. Electro-oculographic studies of selected patients with DVD provided objective evidence of the speed and amplitude of the ocular movements in DVD. Bell phenomenon, strabismus sursoadductorius and the Bielschowsky phenomenon were recorded and compared to clinical findings of strabismus patients with DVD. The technique for the results of surgery for DVD were described. Dissociated vertical deviation was characterized as a component of the overall strabismus picture.
Vertically incomitant pattern strabismus comprises 50% of infantile horizontal strabismus. The oblique muscle dysfunction has been associated with pattern strabismus. High-resolution orbit imaging and contemporary neurophysiology studies in non-human primate models of strabismus have shed light into the mechanisms of pattern strabismus. In this review, we will examine our current understanding of etiologies of pattern strabismus. Speculated pathophysiology includes oblique muscle dysfunction, loss of fusion with altered recti muscle pull, displacements and instability in connective tissue pulleys of the recti muscles, vestibular hypofunction, and abnormal neural connections. Orbital mechanical factors, such as abnormal pulleys, were reported as a cause of pattern strabismus in patients with craniofacial anomalies, connective tissue disorders, and late-onset strabismus. In contrast, abnormal neural connections could be responsible for the development of a pattern in infantile-onset strabismus. Pattern strabismus is likely multifactorial. Understanding the mechanisms of pattern strabismus is pivotal to determine an appropriate surgical treatment strategy for these patients.
To assess whether outcomes of strabismus surgery are improved by using the adjustable suture technique and to determine which subgroups of strabismus patients benefit most from the adjustable suture technique.
A retrospective chart review.
Five hundred thirty-five adults who had strabismus surgery between 1989–2010.
Success was defined as ≤10 prism diopters (PD) for horizontal deviations and ≤2 PD for vertical deviations. Differences in the proportion of successful strabismus surgery were analyzed using a chi-square test with an alpha of 0.05.
Main outcome measures
Ocular alignment in primary position at a 7-day to 12-week follow-up examination.
491 patients met the inclusion criteria (adjustable suture, n=305; non-adjustable, n=186). The success rates for non-adjustable and adjustable groups were 61.3% and 74.8% respectively (χ2=9.91, p=0.0016). Adjustable suture use was particularly beneficial for patients undergoing a reoperation for childhood strabismus (success rate: non-adjustable, 42.4%; adjustable, 65.7% p=0.0268; n=100). The differences in outcomes were not statistically significant for patients with childhood strabismus undergoing a primary surgery (non-adjustable, 65.0%; adjustable, 81.4% p=0.1354; n=90) or with thyroid orbitopathy (non-adjustable, 76.7%; adjustable, 74.1% p=0.8204; n=57).
Strabismus surgery using adjustable sutures was associated with improved short-term ocular alignment compared to strabismus surgery without the use of adjustable sutures. Adjustable sutures were most beneficial for patients undergoing reoperations for childhood strabismus.
Background: Strabismus surgery for congenital esotropia can be complicated by the development of a postoperative head tilt.
Purpose: To determine the pathophysiology of acquired head tilting following horizontal realignment of the eyes in children with congenital esotropia.
Materials and methods: Retrospective analysis of nine children with congenital esotropia who developed unexplained head tilts following horizontal realignment of the eyes.
Results: Shortly after strabismus surgery, each child developed a head tilt in association with asymmetrical dissociated vertical divergence (DVD). Five children maintained a head tilt toward the side of the fixing eye (group 1), which did not serve to control the DVD. Four children maintained a head tilt toward the side of the hyperdeviating eye, which served to control the DVD (group 2). Children in group 2 had earlier horizontal muscle surgery and developed better stereopsis than those in group 1, suggesting that the higher degree of single binocular vision and stereopsis in these children may have led to a compensatory torticollis to control an asymmetrical DVD.
Conclusions: The onset of an unexpected head tilt after congenital esotropia surgery is usually a postural manifestation of asymmetrical DVD. In this setting, a head tilt toward the side of the fixing eye corresponds with a postural manifestation of the underlying central vestibular imbalance that produces DVD, while a head tilt toward the side of the hyperdeviating eye serves to counteract the hyperdeviation and stabilise binocular vision.
congenital esotropia; dorsal light reflex; torticollis
Purpose: Human infants at greatest risk for esotropia are those who suffer cerebral insults that could decorrelate signals from the 2 eyes during an early critical period of binocular, visuomotor development. The author reared normal infant monkeys, under conditions of binocular decorrelation, to determine if this alone was sufficient to cause esotropia and associated behavioral as well as neuroanatomic deficits.
Methods: Binocular decorrelation was imposed using prism-goggles for durations of 3 to 24 weeks (in 6 experimental, 2 control monkeys). Behavioral recordings were obtained, followed by neuroanatomic analysis of ocular dominance columns and binocular, horizontal connections in the striate visual cortex (area V1).
Results: Concomitant, constant esotropia developed in each monkey exposed to decorrelation for a duration of 12 to 24 weeks. The severity of ocular motor signs (esotropia-angle; dissociated vertical deviation; latent nystagmus; pursuit/optokinetic tracking asymmetry; fusional vergence deficits), and the loss of V1 binocular connections, increased as a function of decorrelation duration. Stereopsis was deficient and motion visual evoked potentials were asymmetric. Monkeys exposed to decorrelation for 3 weeks showed transient esotropia but regained normal visuomotor behaviors and binocular V1 connections.
Conclusions: Binocular decorrelation is a sufficient cause of infantile esotropia when imposed during a critical period of visuomotor development. The systematic relationship between severity of visuomotor sign, and severity of V1 connectivity deficit, provides a neuroanatomic mechanism for several of these signs. Restoration of binocular fusion and V1 connections, after short durations of decorrelation, helps explain the benefits of early repair in human strabismus.
Strabismus in human infants is linked strongly to nasotemporal asymmetries of smooth pursuit, but many features of this co-morbidity are unknown. The purpose of this study was to determine how the duration of early-onset strabismus (or timeliness of repair) affects the severity of pursuit asymmetries in a primate model.
Binocular image decorrelation was imposed on infant macaques by fitting them with prism goggles on day 1 of life. The goggles were removed after 3 weeks (n=2), 12 weeks (n=2) or 24 weeks (n=3), emulating surgical repair of strabismus in humans at 3, 12, and 24 months of age, respectively. Two control monkeys wore plano lenses. Several months after the goggles were removed, horizontal smooth pursuit was recorded using binocular search coils and a nasal-bias index (NBI) was calculated.
Each animal in the 12- and 24-week groups developed a constant, alternating esotropic strabismus and a nasotemporal asymmetry of pursuit when viewing with either eye. Spatial vision was normal (no amblyopia). The 3-week duration monkeys were indistinguishable from control animals; they had normal eye alignment and symmetric pursuit. In the 12- and 24-week monkeys, the longer the duration of binocular decorrelation, the greater the pursuit asymmetry: for 15 deg/s target motion, the NBI in the 12-week and 24-week animals was 16 X and 22 X greater respectively, than that in the 3 week animals (ANOVA, p = 0.03).
Binocular decorrelation in primates during an early period of fusion development causes permanent smooth pursuit asymmetries when the duration exceeds the equivalent of 3 months in human. These findings support the conclusion that early correction of infantile strabismus promotes normal development of cerebral gaze pathways.
visual development; ocular motor system; fusion; esotropia; visual cortex; brain repair
To evaluate the results of monocular surgery under peribulbar anesthesia for large-angle horizontal strabismus.
Monocular surgery may preserve some muscles if a repeat operation is required, may help to avoid the exposure of the dominant eye to the inherent risks of a surgical procedure and may reduce surgical time.
We evaluated ninety-two consecutive patients who underwent monocular surgery under peribulbar anesthesia for large-angle horizontal strabismus (angle of 40 prism diopters or greater). Patients were divided into group 1- esotropia and group 2 –exotropia. The postoperative follow-up was at 6 months, when the residual deviation was evaluated. In cases of residual deviations of over 15 PD (prism diopter), a second procedure was indicated.
In all patients with preoperative deviations up to 60 PD, residual deviations were under 15 PD. Some patients with preoperative deviations of 65 PD (two in group 1 and four in group 2) and all patients with deviations over 65 PD had residual deviations over 15 PD. The 13 patients who underwent a second procedure experienced successful outcomes. Our ROC curve analysis showed that the cutoff point for obtaining a successful surgical result was 62.5 PD. No patient presented with a major limitation in respect of ocular movement.
Monocular surgery under peribulbar anesthesia can be an alternative for horizontal large-angle strabismus given deviations of up to 60 PD. Monocular surgery did not result in successful outcomes for deviations of over 65 PD.
Local Anesthesia; Strabismus/surgery; Ophthalmology; Surgical procedures; Operative; Ocular motility disorders
PURPOSE: Dissociated vertical deviation (DVD) has eluded explanation for more than a century. The purpose of this study has been to elucidate the etiology and mechanism of DVD. METHODS: Eye movement recordings of six young adults with DVD were made with dual-coil scleral search coils under various conditions of fixation, illumination, and head tilt. Horizontal, vertical, and torsional eye movements were recorded for both eyes simultaneously. Analyses of the simultaneous vertical and torsional movements occurring during the DVD response were used to separate and identify the component vergence and version eye movements involved. RESULTS: Typically, both horizontal and cyclovertical latent nystagmus developed upon occlusion of either eye. A cycloversion then occurred, with the fixing eye intorting and tending to depress, the covered eye extorting and elevating. Simultaneously, upward versions occurred for the maintenance of fixation, consisting variously of saccades and smooth eye movements, leading to further elevation of the eye behind the cover. The cyclovertical component of the latent nystagmus became partially damped as the DVD developed. CONCLUSIONS: In patients with an early-onset defect of binocular function, the occlusion of one eye, or even concentration on fixing with one eye, produces unbalanced input to the vestibular system. This results in latent nystagmus, sometimes seen only with magnification. The cyclovertical component of the latent nystagmus, when present, is similar to normal vestibular nystagmus induced by dynamic head tilting about an oblique axis. Such vestibular nystagmus characteristically produces a hyperdeviation of the eyes. In the case of cyclovertical latent nystagmus, the analogous hyperdeviation will persist unless corrected by a vertical vergence. A normal, oblique-muscle-mediated, cycloversion/vertical vergence is called into play. This occurs in the proper direction to correct the hyperdeviation, but it occurs in an exaggerated form in the absence of binocular vision, probably as a learned response. The cycloversion/vertical vergence helps damp the cyclovertical nystagmus (a cyclovertical "nystagmus block-age" phenomenon), aiding vision in the fixing eye. But this mechanism also produces unavoidable and undesirable elevation and extortion of the fellow eye, which we call DVD.
To compare the psychosocial consequences of horizontal comitant strabismus in children between the families of urban and rural India.
Materials and Methods:
In this cohort study, an eight-question quality-of-life instrument was administered by trained staff to the guardians of strabismic children from rural and urban areas by a live interview.
This study included 93 strabismic-children aged 4-16 years of which 52 were females. Forty-one had esodeviation and 52 had exodeviation. Seventy per cent parents were extremely distressed due to squint, 65% were extremely distressed due to people's remarks, 65% were extremely worried, 55% children were extremely distressed due to people's remarks, 57% children were severely ostracized, 38% had severe difficulty in communication and 50% had difficulty to cope; 64% parents were not advised a corrective surgery. The difference between families from rural and urban areas, or whether a male child was affected or a female child or for an esodeviation or an exodeviation was statistically not significant. The questionnaire had a good internal consistency (Cronbach's Alpha = 0.71).
There was a significant negative psychosocial and emotional impact of childhood strabismus that was not affected by the rural or urban location of the family or the gender of the strabismic child or type of the deviation. The quality-of-life instrument can be used as part of the clinical examination for strabismic children.
Children; quality of life; squint
Reports are conflicting on the presence of increased drift in amblyopic eyes. Furthermore, the individual effects of either amblyopia or strabismus alone on ocular drift have not been systematically investigated. We therefore used a photoelectric method to record horizontal eye position during monocular and binocular fixation in patients having amblyopia without strabismus, intermittent strabismus, or constant strabismus amblyopia. Our principal finding was increased drift amplitude (up to 3.5 degree) and velocity (up to 3.0 degrees per second) in amblyopic eyes during monocular fixation. While increased drift was found 75% of the time in amblyopia without strabismus and 50% of the time in constant strabismus amblyopia, it was found only 20% of the time in intermittent strabismus. Amblyopic drift could be either error-producing or error-correcting in nature. Increased drift was not present during monocular fixation with the dominant eye or during binocular fixation in any of our 16 patients. We therefore conclude that amblyopia and not strabismus is a necessary condition for the presence of markedly increased fixational drift. Increased drift amplitude but not velocity may adversely affect visual acuity in the amblyopic eye.
To evaluate the correlation of the distance Krimsky test and the alternate prism cover test (APCT) for the distance deviation in patients with horizontal strabismus.
Forty patients with horizontal strabismus (20 esotropia and 20 exotropia) were included in this study. Patients with a variable angle of deviation, vertical angle over 5 prism diopters, impaired binocular vision, or poor cooperation were excluded. We instructed the patient to look a target 6 meters away, and applied a prism over the patient's dominant eye while flashing a light source 33 centimeters from the middle of both eyebrows. When the corneal light reflexes were located on the center of each cornea, we measured the angle of deviation. We defined this method as 'distance Krimsky test,' and the angle measured by this method was compared with the conventional Krimsky test and APCT at distance. We analyzed the accuracy and intra- and inter-observer reliability.
The angle of strabismus measured by the distance Krimsky test showed a significant agreement and correlation with the deviation angle measured by the APCT. We elicited the correlation gradient between the angle measured by the distance Krimsky test and the APCT. In addition, the distance Krimsky test showed significant intra- and inter-observer reliabilities.
The distance Krimsky test is expected to be more useful than the Krimsky test in measuring the distance angle of deviation for patients with strabismus in whom it is difficult to measure the angle of deviation using the APCT. The distance Krimsky test can be an accurate and useful test through the improvement of proficiency of examiners and the establishment of individualized normative data.
Distance Krimsky test; Esotropia; Exotropia; Strabismus
Diagnosis of amblyopia in preverbal strabismic patients is frequently made by binocular fixation preference (BFP) testing. The reports on reliability of BFP are equivocal. This study evaluated the reliability of BFP testing in patients with horizontal strabismus.
Materials and Methods:
This prospective observational study included patients with manifest, horizontal, comitant deviation >10 prism diopter (PD). Inter-eye acuity difference (IEAD) was calculated by converting Snellen visual acuity to logMAR and was compared with BFP testing. The fixation behavior of the non-preferred eye was evaluated by a single investigator as central or uncentral, steady or unsteady and maintained or unmaintained. Amblyopia was defined as the IEAD of >0.2 logMAR.
Of total 61 patients 36 were females and 36 had convergent squint, mean age 9.8 years. The correlation of BFP testing with IEAD was good for esotropia and exotropia. The sensitivity, specificity, positive and negative predictive value of central, steady, maintained (CSM) grading was 93%, 78%, 79%, and 93% respectively. Sensitivity and negative predictive values were higher in children aged four to nine years and anisometropia >1 diopter. The correlation between IEAD and lower grades of BFP testing was poor.
CSM grading for BFP testing is useful for the detection of strabismic amblyopia but not useful to differentiate the depth of the amblyopia.
Amblyopia; fixation preference testing; strabismus
Small disparity stimuli applied to large random-dot patterns elicit machine-like vergence eye movements at short latency. We have examined the sensitivity of these eye movements to simulated orthogonal tropias in three normal subjects by recording 1) the effects of vertical disparities on the initial horizontal vergence responses elicited by 2° crossed and uncrossed (horizontal) disparity stimuli, and 2) the effects of horizontal disparities on the initial vertical vergence responses elicited by 1.2° left-hyper and 0.8° right-hyper (vertical) disparity stimuli. Initial vergence responses were strongest when the orthogonal disparity was close to zero, and decreased to zero as the orthogonal disparity increased to 3–5°, i.e., there was only a limited tolerance for orthogonal disparity. Tuning curves describing the dependence of the initial change in the vergence angle on the orthogonal disparity were well fit by a Gaussian function. An additional subject, who had an esotropia of ~10° in our experimental setup, showed almost no horizontal vergence responses but did show vertical vergence responses to vertical disparity stimuli at short latency (albeit slightly longer than normal) despite the fact that her esotropia resulted in uncrossed disparities that would have totally disabled the vertical vergence mechanism of a normal subject, cf., anomalous retinal correspondence.
disparity vergence; tropia; anomalous retinal correspondence