Intermittent exotropia (IXT) accounts for 44.9–72% of patients with primary horizontal strabismus in Asia [
Motor fusional vergence is the ability to maintain binocular vision through a range of induced vergences (often prism-induced) until a point is reached at which the binocular vision is interrupted and the patient experiences diplopia. The normal range of fusional vergence has been described since 1948 [
In Western populations, it has been found that esotropia is twice as common as exotropia; however, recent studies have shown that exotropia is far more common in Asian populations [
This was a prospective observational study. The research protocol and informed consent form were approved by the Institutional Review Board of the Beijing Tongren Hospital. Written informed consent was obtained from a parent or legal guardian for each participant. The study was registered with the Chinese Clinical Trial Registry (
Ninety-two consecutive patients with IXT at distance and near, aged between 8 and 15, were evaluated at the outpatient clinic of the Beijing Tongren Hospital between July 2013 and July 2014 and were included in the study. Eighty-six asymptomatic age- and gender-matched normal subjects were also recruited during the same period as controls.
Inclusion criteria were (1) IXT with exodeviation of at least 10 prism diopters (pd) at distance and near and (2) best corrected visual acuity of 6/6 or better in each eye. Exclusion criteria were (1) amblyopia; (2) >2 diopters of anisometropia; (3) incomitance of the horizontal or vertical deviation; (4) vertical deviation of ≥5 pd; or (5) significant oblique muscle overaction. Cases with convergence insufficiency-type IXT (near angle >10 pd greater than distance) or those with a previous history of strabismus surgery were also excluded. A patient was excluded if he was manifest at distance, in which condition the vergence could not be determined by using prism bar.
The revised Newcastle Control Score.
Score | |
---|---|
Homing control | |
XT or monocular eye closure seen | |
Never | 0 |
<50% of time fixing at distance | 1 |
>50% of time fixing at distance | 2 |
>50% of time fixing at distance + seen at near | 3 |
Clinical control | |
Near | |
Immediate realignment after dissociation | 0 |
Realignment with aid of blink or refixation | 1 |
Remains manifest after dissociation or prolonged fixation | 2 |
Manifest spontaneously | 3 |
Distance | |
Immediate realignment after dissociation | 0 |
Realignment with aid of blink or refixation | 1 |
Remains manifest after dissociation or prolonged fixation | 2 |
Manifest spontaneously | 3 |
Total: NCS = home + near + distance |
XT: exotropia.
Fusional convergence and divergence amplitudes were detected using a synoptophore L-2510B/L-2510HB (Inami & Co., Ltd., Japan) approximately 1 hour after the prism bar examination to allow sufficient time for recovery of fusion. Horizontal fusional vergence was measured with fusion slides subtending a visual angle of 6 degrees horizontally and 8 degrees vertically. The break points and recovery points were measured using the synoptophore and were recorded in prism diopters.
For the present study, we used the following definitions in order to simplify the description of the parameters detected by prism bar and synoptophore. The values of break points measured by prism bar were defined as the amplitude of the vergence, which was defined as convergence reserve in the study by Hatt et al. [
Statistical analysis was performed using SPSS 13.0 for Windows (SPSS Inc., Chicago, IL, USA). Mean vergence amplitudes and the distance between fusional recovery and break points were calculated for children with IXT and normal subjects. Independent sample Student’s
Ninety-two consecutive cases of IXT were included in our study. Eighty-six age- and gender-matched controls were evaluated. The mean age of the patients was 10.29 ± 1.53 (range 8 to 14) years and that of controls was 10.55 ± 2.30 (range 8 to 15) years (
Characteristics of patients with intermittent XT and normal subjects.
IXT | Normal control |
|
|
|
---|---|---|---|---|
Number of cases | 92 | 86 | ||
Mean age (years) | 10.29 ± 1.53 | 10.55 ± 2.30 | −0.45 | 0.67 |
Gender | ||||
Male | 49 | 48 | 0.09 ( |
0.79 |
Female | 43 | 38 | ||
Refraction | ||||
Right eye | −1.95 ± 1.63 | −1.68 ± 1.79 | −0.52 | 0.61 |
Left eye | −2.01 ± 1.73 | −1.57 ± 1.51 | −0.89 | 0.38 |
We detected fusional convergence and divergence amplitudes using a prism bar and synoptophore. Using the prism bar, the mean convergence amplitudes were significantly lower for children with IXT compared with normal children at both distance and near (distance: 18.65 ± 1.50 versus 26.46 ± 1.53 pd,
Comparison of the fusional vergence detected using a prism bar in patients with IXT and normal subjects. The mean convergence amplitude was significantly lower for children with IXT compared with normal children at both distance and near (distance: 18.65 ± 1.50 versus 26.46 ± 1.53 pd,
Using synoptophore, the convergence amplitudes were significantly lower for children with IXT compared with normal children (22.62 ± 2.15 versus 30.19 ± 1.95 pd,
Comparison of fusional vergence detected using a synoptophore between patients with IXT and normal subjects. The mean convergence amplitudes were significantly lower for children with IXT compared with normal children (22.62 ± 2.15 versus 30.19 ± 1.95 pd,
When measured using the prism bar, there was a moderate inverse correlation between convergence amplitudes and control score at distance (
Correlation between control of deviation and fusional vergence detected using a prism bar.
Control score | ||
---|---|---|
|
|
|
Convergence breakpoints for near | −0.63 | <0.001 |
Convergence breakpoints for distance | −0.57 | 0.002 |
Divergence breakpoints for near | 0.08 | 0.71 |
Divergence breakpoints for distance | 0.28 | 0.12 |
Distance between convergence recovery/break points for near | −0.05 | 0.76 |
Distance between convergence recovery/break points for distance | −0.07 | 0.71 |
Distance between divergence recovery/break points for near | 0.08 | 0.62 |
Distance between divergence recovery/break points for distance | 0.22 | 0.21 |
In the present study, we measured fusional vergence both by using the standard clinical method of progressively increasing prismatic power from a state of spontaneous binocular fusion and by using a synoptophore to neutralize the strabismic deviation. At both distance and near, the mean convergence amplitudes were significantly lower for children with IXT than those for normal children using both methods. When measured with a prism bar, the mean divergence amplitudes were significantly greater for children with IXT than for normal children. There was no significant difference in mean divergence amplitudes between the two groups when tested using a synoptophore. A moderate inverse correlation was found between convergence amplitudes and control score at both distance and near. However, no significant correlation was found between the control score and the divergence amplitudes for distance or near.
Hatt et al. have recently reported that the mean convergence break points (defined as convergence reserve) were significantly lower at distance fixation for children with IXT compared with orthophoric children [
Various studies have found variability in reporting divergence break points. Sharma et al. reported reduced divergence amplitudes in IXT subjects that differed from the results of our present study [
In the present study, in addition to the traditional prism bar method, we detected fusional vergence using a synoptophore. In contrast to the prism bar method in which the subject begins in a state of spontaneous binocular fusion, the strabismic deviation is first neutralized when assessing fusional vergence by synoptophore. When detected by synoptophore, the mean convergence amplitudes were significantly lower for children with IXT compared with normal children, which is similar to the results observed by the prism bar. However, we did not observe a significant difference in mean divergence amplitudes between patients with IXT and normal subjects, which is different from the findings detected by prism bar. The difference of the smooth (synoptophore) and step (prism bar) vergence testing might be a reason for the different results between the two methods [
The distance measured between the recovery and break points using both the prism bar and synoptophore was significantly larger in IXT patients compared with normal controls. Our data indicated that the patients with intermittent deviations often have relatively poor ability to recover fusion.
There was a strong correlation between the convergence break points detected using the prism bar and the control score at distance and near, similar to the study by Hatt et al. [
In children with IXT, some factors may influence the fusional convergence ability such as fixation distance, the type of visual stimulus, the size of the target, the type of visual environment, and long-term adaptation [
In conclusion, we observed that in Chinese children with IXT, the mean convergence break points were lower and that the fusional recovery was poorer compared with normal controls, as detected using both prism bar and synoptophore. Of interest, the mean divergence break points of patients with IXT were comparable with those of normal subjects when detected with synoptophore but significantly larger when measured with the prism bar. This may support a “deconvergence” mechanism of IXT and indicates that convergence is involved in the control of IXT.
The authors declare that there is no conflict of interests regarding the publication of this paper.
This research was supported by the Beijing Natural Science Foundation (7132057, to Tao Fu) and Excellent Talent Training Aid from the Organization Department of Beijing Government (2010D003034000007, to Tao Fu).