Demographics of patients
In the present study, 104 IXT patients were recruited along with 128 age- and gender-matched case controls. The mean age of the IXT patients and controls were 8.40 ± 3.89 years (range from 3 to 28 years) and 9.20 ± 2.80 years (range from 4 to 26 years), respectively (𝑃 = 0.072). There were 48 males (46.15%) and 56 females (53.85%) in the IXT group and 63 males (49.22%) and 65 females (50.78%) in the control group (𝑃 = 0.642).
The mean horizontal deviation (prism dioptres, PD) in patients with IXT was -32.93 ± 14.70 (range -105 to -10) at near and -33.02 ± 13.50 (range -105 to -15) at distance by using prism and alternate cover test.
Eye position deviation as measured by synoptophore and the PEP test in normal
We first measured the horizontal and vertical eye positions of normal controls using both synoptophore and the PEP tests. The mean horizontal eye deviation was -0.34±1.95° in synoptophore and 28.09±32.81 pixels in PEP. The mean vertical eye deviation was 0.03±0.15° in synoptophore and 4.29±4.28 pixels in PEP (Fig. 1A). The normal ranges were calculated using these values by mean±1.96 SD. Thus, the normal horizontal eye position ranges were -4.16~+3.48° versus 0~92.40 pixels; the vertical eye position ranges were 0~0.32° versus 0~12.68 pixels, as measured by synoptophore and PEP tests, respectively.
Subtle vertical misalignment measured by prism and alternate cover test, synaptophore and the PEP test in patients with intermittent exotropia
Among 104 IXT patients, there were two patients (1.92%) who had accompanied vertical misalignment (3△ and 4△, respectively) measured by prism and alternate cover test. And synoptophore detected only three patients (2.88%) with vertical deviation larger than 0.5°. However, there were 44 cases with concurrent vertical misalignment greater than 13 pixels and 5 patients who showed interocular suppression as measured by the PEP test (Fig. 1B). The abnormality ratio of subtle vertical misalignment was significantly greater as measured by the PEP test than by prism and alternate cover test, synoptophore (PEP: 47.12% versus prism and alternate cover test: 1.92%, synoptophore: 2.88%, 𝑃<0.001). The mean vertical misalignment for the 44 cases with abnormal vertical deviation and 55 cases with normal vertical deviation were 46.05±38.36 pixels (14~179 pixels) and 5.20±3.21 pixels (0~13 pixels), respectively.
Difference of clinical profiles between patients with normal and abnormal vertical PEP
Of 104 IXT patients, PEP detected 49 cases with abnormal vertical deviation and 55 cases with normal vertical deviation. Both groups underwent additional examination to determine whether vertical deviation affected stereoacuity. The mean near stereoacuity in patients with normal and abnormal vertical PEP test were 2.21±0.34 and 2.40±0.44, respectively (Fig. 2A). The mean medium distance stereoacuity in patients with normal and abnormal vertical PEP test were 2.09±0.23 and 2.25±0.31, respectively (Fig. 2B). Patients with abnormal vertical PEP test had significant worse near and medium distance stereoacuity than those of normal vertical PEP test (P=0.017, P=0.004).
The mean age in patients with normal and abnormal vertical PEP test were 8.87±4.03 and 7.85±3.68, respectively. There was no significant difference between two groups (P=0.186) (Fig. 3). The mean horizontal deviation (prism dioptres, PD) in patients with normal and abnormal vertical PEP test were -32.06±15.91 vs -34.02±13.14 for near deviation (Fig. 4A), -31.90±14.53 vs -34.39±12.16 for far deviation (Fig. 4B), respectively. No statistic differences were found between two groups both in near and far deviation (P=0.527, P=0.376).
The mean value of bilateral differences in spherical equivalent (SE) was 0.44±0.50 D for patients with normal vertical PEP and 0.31±0.32 D for those with abnormal vertical PEP. No significant differences were found between the two groups (P = 0.117) (Fig. 5). These results indicated that the difference in stereoacuity was caused by subtle vertical deviation and rather than other factors such as age, horizontal deviation and anisometropia. Taken together, it suggests that abnormal subtle vertical PEP can affect stereopsis in IXT patients.
Difference of stereoacuity between IXT patients and normal controls
The mean near stereoacuity in IXT patients and normal controls were 2.30±0.40 and 2.02±0.22, respectively (Fig. 6A). The mean medium distance stereoacuity in patients and normal controls were 2.16±0.28 and 2.04±0.10, respectively (Fig. 6B). IXT patients had significant worse near and medium distance stereoacuity than those of normal controls (P<0.001, P<0.001). In order to conduct further analysis, IXT patients with normal and abnormal vertical PEP test were compared with normal controls separately. The near stereoacuity of patients with normal vertical PEP test was significantly worse than those of normal controls (2.21±0.34, 2.02±0.22, P<0.001) (Fig. 2A). Although the medium distance stereoacuity of patients with normal vertical PEP test was worse than those of normal controls, the difference was not statistically significance (2.09±0.23, 2.04±0.10, P=0.067) (Fig. 2B). Both the near and medium distance stereoacuity of patients with abnormal vertical PEP test were significantly worse than those of normal controls (near: 2.40±0.44, 2.02±0.22, P<0.001; medium: 2.25±0.31, 2.04±0.10, P<0.001) (Fig. 2).