Effect of suppression during tropia and phoria on phoria maintenance in intermittent exotropia

Quantitative interocular suppression in children with intermittent exotropia

Purpose

We have demonstrated that the depth of unbalanced interocular suppression can be quantified by balancing the interocular luminance differences required when both eyes are viewing simultaneously. In this study, we aimed to investigate the applicability of this method in children with intermittent exotropia (IXT), offering a quantitative assessment of interocular suppression in individuals with binocular imbalance. Additionally, we evaluated its association with the clinical characteristics of IXT.

Methods

Interocular suppression in IXT was quantitatively measured using a polarizer and neutral-density (ND) filters. The density of the ND filter was adjusted incrementally from 0.3ND to 3ND, with a step size of 0.3ND (a total of 10 levels). Our prospective study involved 46 patients with IXT (mean age: 10.12 ± 4.89 years; mean ± SD) and 24 normal observers (mean age: 7.88 ± 1.83 years).

Results

The suppression test exhibited good test-retest reliability, supported by statistical analysis. We observed more pronounced interocular suppression in individuals with IXT compared to controls. Notably, the magnitude of suppression during distant and near viewing significantly differed in IXT (1.55 ± 0.93 vs. 0.57 ± 0.64; Z = 4.764, p < 0.001). Furthermore, we identified a positive correlation between interocular suppression and data obtained from the Worth-4-Dot test. Additionally, interocular suppression showed a significant association with distance control scores.

Conclusion

Our novel test offers a convenient and reliable means to quantify interocular suppression in patients with IXT. The quantitative assessment of interocular suppression provides a sensitive tool to evaluate the clinical characteristics of IXT.

Correlation of saccade amplitude during refusion with the fusional convergence amplitude in patients with intermittent exotropia

In patients with intermittent exotropia (IXT), to investigate the correlation between fusional convergence amplitude and saccade during refusion, as well as the differences in saccade caused by suppression.

We measured the fusional convergence amplitude using a synoptophore. Suppression was assessed in 15 patients with IXT (mean age 18.1 ± 11.0 yrs, range 8-54 yrs) having diplopia during exotropia. We performed the cover-uncover test and recorded the saccade during the shift from exotropia to binocular fixation using an eye-tracking system. We analyzed the correlation between the fusional convergence amplitude on the one hand and the saccade peak velocity (PV), the saccade amplitude, and the saccade amplitude of the fusion on the other. We also investigated the difference of those saccade variables between patients with and without suppression.

In 15 patients with IXT, the median fusional convergence amplitude was 14 (range 0-60) °, and suppression was confirmed in 11 patients. When the cover was removed from the dominant eye, the fusional convergence amplitude showed a positive correlation with the saccade PV and the amplitude in non-dominant eye (r = 0.570, p = 0.042 and r = 0.669, p = 0.012, respectively). The mean saccade PV, the mean saccade amplitude and the mean saccade amplitude of the fusion were not significantly different with the presence or the absence of the suppression.

The fusional convergence amplitude was correlated with saccade in patients with IXT. The saccade during refusion can thus be used to quantitatively evaluate sensory and/or motor fusion.

A Dichoptic Optokinetic Nystagmus Paradigm for Interocular Suppression Quantification in Intermittent Exotropia

Purposes: To investigate the effectiveness of a dichoptic optokinetic nystagmus (dOKN) test to objectively quantify interocular suppression in intermittent exotropia (IXT) patients during the states of orthotropia and exodeviation.

Methods: The OKN motion in subjects (15 controls and 59 IXT subjects) who viewed dichoptic oppositely moving gratings with different contrast ratios was monitored and recorded by an eye tracker. Interocular suppression in control subjects was induced using neutral density (ND) filters. The OKN direction ratios were fitted to examine the changes of interocular suppression in subjects under different viewing states. Two established interocular suppression tests (phase and motion) were conducted for a comparative study.

Results: The dOKN test, which requires a minimal response from subjects, could accurately quantify the interocular suppression in both IXT and control subjects, which is in line with the established interocular suppression tests. Overall, although comparative, the strength of interocular suppression detected by the dOKN test (0.171 ± 0.088) was stronger than those of the phase (0.293 ± 0.081) and the motion tests (0.212 ± 0.068) in the control subjects with 1.5 ND filters. In IXT patients, when their eyes kept aligned, the dOKN test (0.58 ± 0.09) measured deeper visual suppression compared with the phase (0.73 ± 0.17) or the motion test (0.65 ± 0.14). Interestingly, strong interocular suppression (dOKN: 0.15 ± 0.12) was observed in IXT subjects during the periods of exodeviation, irrespective of their binocular visual function as measured by synoptophore.

Conclusion: The dOKN test provides efficient and objective quantification of interocular suppression in IXT, and demonstrates how it fluctuates under different eye positions.

Maintenance of normal fusion in intermittent exotropia

PURPOSE

To evaluate the reliability of the Bagolini filter bar and striated lenses for measuring the fusion maintenance score, which is the ability of participants with intermittent exotropia to maintain normal sensorimotor fusion.

METHODS

Thirty-two Chinese participants aged 7-20 years with intermittent exotropia (excluding the convergence insufficiency type) were enrolled in this prospective study. At the eligibility screening, visual acuity, cover test and assessment of the office control score were performed. At study visit 1, eligible participants underwent negative and positive fusional vergence tests at far and near, eye dominance test and the fusion maintenance test. All eligible participants returned for study visit 2 on the same day (2-4 h later) and the testing was repeated. The primary outcome measure was the intra-class correlation coefficient of the fusion maintenance score between the two study visits.

RESULTS

The intra-class correlation coefficient of the fusion maintenance score was 0.84, indicating good reliability. There was no significant difference (mean difference = 0.05, p = 0.95) between the fusion maintenance scores for the first (5.62) and second study visits (5.57). The coefficient of repeatability and the smallest detectable change for the fusion maintenance scores were 7.6 and 6.3, respectively. The fusion maintenance score was significantly associated with the distance (Spearman correlation -0.57, p < 0.001) and near (Spearman correlation -0.4, p = 0.02) office control scores.

CONCLUSION

These data demonstrate that the fusion maintenance score is a reliable tool to evaluate sensorimotor fusion in intermittent exotropia. These results suggest that the fusion maintenance score may be a useful outcome measure in future clinical trials to evaluate the effectiveness of treatments for intermittent exotropia.

Quantifying Nasotemporal Asymmetry of Interocular Suppression in Alternating Strabismus After Correction

Purpose

This study identifies and characterizes the nasotemporal hemifield difference of interocular suppression in subjects who have been successfully treated for strabismus.

Methods

Interocular suppression in the nasal and temporal hemifields were measured using two methods, namely, binocular phase combination and dichoptic motion coherence, both entailed suprathreshold stimuli. We tested 29 clinical subjects, who had strabismus (19 with exotropia and 10 with esotropia) but regained good ocular alignment (within 10 prism diopters) after surgical or refractive correction, and 10 control subjects.

Results

Both the hemifield binocular phase combination and the hemifield dichoptic motion coherence tests revealed similar nasotemporal asymmetry of interocular suppression. Subjects with previous exotropia showed deeper suppression in the nasal hemifield, and those with previous esotropia showed deeper suppression in the temporal hemifield. This finding was consistent with the hemifield suppression theory. Furthermore, there was deeper suppression but less imbalance of nasotemporal asymmetry in the hemifield dichoptic motion coherence test. Finally, clinical stereopsis and the nasotemporal asymmetry of suppression (P < 0.05 in both tests) were negatively correlated in subjects with previous exotropia and measurable stereopsis.

Conclusions

Hemifield asymmetry of interocular suppression in corrected strabismus can be measured by using static and dynamic suprathreshold stimuli. Thus, the evaluation of binocular vision in strabismus should focus on both the magnitude and the pattern of interocular suppression.

Objective Evaluation of Visual Fatigue Using Binocular Fusion Maintenance

Purpose

In this study, we investigated whether an individual's visual fatigue can be evaluated objectively and quantitatively from their ability to maintain binocular fusion.

Methods

Binocular fusion maintenance (BFM) was measured using a custom-made binocular open-view Shack-Hartmann wavefront aberrometer equipped with liquid crystal shutters, wherein eye movements and wavefront aberrations were measured simultaneously. Transmittance in the liquid crystal shutter in front of the subject's nondominant eye was reduced linearly, and BFM was determined from the transmittance at the point when binocular fusion was broken and vergence eye movement was induced. In total, 40 healthy subjects underwent the BFM test and completed a questionnaire regarding subjective symptoms before and after a visual task lasting 30 minutes.

Results

BFM was significantly reduced after the visual task (P < 0.001) and was negatively correlated with the total subjective eye symptom score (adjusted R² = 0.752, P < 0.001). Furthermore, the diagnostic accuracy for visual fatigue was significantly higher in BFM than in the conventional test results (aggregated fusional vergence range, near point of convergence, and the high-frequency component of accommodative microfluctuations; P = 0.007).

Conclusions

These results suggest that BFM can be used as an indicator for evaluating visual fatigue.

Translational Relevance

BFM can be used to evaluate the visual fatigue caused by the new visual devices, such as head-mount display, objectively.

Role of fusional convergence amplitude in postoperative phoria maintenance in children with intermittent exotropia

PURPOSE

To examine the role of fusional convergence amplitude in postoperative phoria maintenance in childhood intermittent exotropia [X(T)].

METHODS

The medical records of 29 children aged 15 years or younger (mean age, 10.8 ± 2.4 years) and treated with monocular recession-resection for X(T) were reviewed retrospectively. The patients' fusional convergence amplitude (break point/total amplitudes), physiologic diplopia, and phoria maintenance (presence/absence of phoria maintenance and ability to maintain phoria) were assessed. The presence of phoria maintenance was confirmed by a cover test, and the ability to maintain phoria was quantified using the Bagolini red filter bar. Correlations of the amplitude size with the presence and ability of phoria maintenance were investigated.

RESULTS

A significant correlation was seen between fusional amplitude (break point/total) and ability to maintain phoria at near and at far (break point: P < .05 at near/P < .01 at far; total: P < .05 at near/far). Neither the break point amplitude nor the total amplitude significantly differed between the patients with phoria maintenance and those without it (break point: P = .71 at near, P = .29 at far; total: P = .98 at near, P = .85 at far). Phoria maintenance correlated with the suppression of physiologic diplopia during phoria (P < .01). The deviation angle did not significantly correlate with fusional amplitude either at near (P = .58) or at far (P = .27).

CONCLUSIONS

In childhood X(T), fusional amplitude plays a role in enforcing the patient's ability to maintain phoria. However, sufficient fusional amplitude does not guarantee fully functioning fusion if suppression is present during phoria.

Patient Characteristics and Surgical Approach Impacting Simultaneous to Alternate Prism Cover Test Disparity After Exotropia Surgery: A Quantitative Look at the Difference in Motor Outcomes

PURPOSE

To investigate the relationship between simultaneous prism and cover test (SPCT) and alternate prism and cover test (APCT) outcomes after exotropia surgery, and to identify characteristics associated with significant disparity between them.

METHODS

Review of sensorimotor outcomes 2 to 6 months after exotropia surgery identified patients with alignment documented by both SPCT and APCT at the same examination. Two hundred seventy-four and 319 patients had both measurements recorded at distance and near, respectively. Correlation between the SPCT and APCT and range of APCT when the SPCT measurement was zero were determined. Patient characteristics studied for association with a difference between the SPCT and APCT exceeding known APCT test-retest variability included age, visual acuity, fusion, intermittency, pattern, preoperative and postoperative angle, and treatment with or without medial rectus resection.

RESULTS

SPCT and APCT outcomes were strongly correlated (P < .001), significantly different (P < .001), and linearly related. The percentage of patients who were orthotropic (SPCT = 0) was 76% at distance and 80% at near. Misalignment of 10 prism diopters (PD) or less by the APCT was present in 92% of orthotropic patients at distance and 84% at near. Surgery without medial rectus resection (P = .015), larger preoperative angle (P = .003), intermittent exotropia (P = .028), and postoperative exotropia rather than esotropia (P < .001) were associated with a significant SPCT-APCT difference.

CONCLUSIONS

Although a greater postoperative SPCT-APCT disparity was confirmed for patients with intermittent exotropia, it also independently associated with a larger preoperative deviation and surgery without medial rectus resection. Performing medial rectus resection, a surgeon's prerogative, provides more apparently consistent postoperative alignment characterized by less SPCT-APCT disparity. [J Pediatr Ophthalmol Strabismus. 2017;54(4):222-230.].

Influence of Test Distance on Stereoacuity in Intermittent Exotropia

AIMS

To investigate influence of test distance on stereoacuity in intermittent exotropia (X[T]) using the same test conditions for both near and far distances.

METHODS

Subjects were 38 consecutive patients with X(T). All the patients were between ages 6 and 15 years and had decimal visual acuity of 1.0 or better. Another inclusion criterion was presence of phoric condition at near and far distances. Stereoacuity was measured at a near distance of 40 cm and at a far distance of 5 m. The following test conditions were used for both test distances: separation of the two eyes using polarized glasses, and a target with a random dot pattern. All the stereograms had the same subtended angle of 2.5º, and binocular disparity of 480, 240, 120, and 60 arcsec. We used two stereogram types with crossed and uncrossed disparities.

RESULTS

Far stereoacuity of 38 subjects measured with the crossed disparity was significantly worse than near stereoacuity (P<0.05, Wilcoxon signed-ranks test), although 30 (78.9%) of the 38 subjects showed no differences in stereopsis between the near and far distances. Far stereoacuity of 38 cases measured with the uncrossed disparity was significantly worse than at near (P<0.05, Wilcoxon signed-ranks test), although 20 (52.6%) of the 38 subjects showed no differences between stereoacuity at near and far. In comparison of stereoacuity with crossed disparity and uncrossed disparity, stereoacuity with crossed disparity was significantly better than that with uncrossed disparity both at near and far (P<0.05, Wilcoxon signed-ranks test).

CONCLUSIONS

Stereoacuity in X(T) was different according to test distance when measured controlling subtended angle of stereogram at both distances. Far stereoacuity was significantly worse than near stereoacuity when measured using test targets with both crossed and uncrossed disparities. Additionally, stereoacuity measured with crossed disparity was better than that with uncrossed disparity at both distances.