The Study of Perceptual Eye Position Examination and Visual Perceptual Training in Postoperative Intermittent Exotropes

Introduction

Intermittent exotropia [X(T)] is the most common form of exotropia.^1–3 Binocular visual function is often disturbed by X(T).^4,5 Surgery is a commonly used treatment especially for patients with a large angle of deviation.^6,7 But the relapse is common in postoperative X(T), even with a successful surgical alignment. ⁸ According to previous study, only about 30–75 percent of cases regained normal binocular visual function after a successful surgical realignment.^9–13 And the abnormal postoperative binocular conditions may increase the danger of relapse. So, the assessment and reconstruction of the binocular visual function in postoperative X(T) is important for best long-term results.

Perceptual eye position (PEP), used to describe sensory binocular alignment, is a standard psychophysical approach and one of the indicators evaluating fixation disparity and binocular visual function. PEP is measured under a dichoptic vision condition. What's more, the PEP pixels are recorded by a computer so that binocular misalignment can be quantified more perceptively and precisely.

In this study, we use PEP to detect the binocular visual function of postoperative X(T) with normal eye position. Furthermore, we conduct a study on the effect of dichoptic visual perceptual training based on a virtual reality (VR) platform in postoperative X(T) patients to evaluate the potential success of this therapy.

Methods

Measurement of PEP

The system automatically records vertical and horizontal bias by the 360° test object to observe any ocular misalignment. The devices, used to measure PEP, include Windows XP system PC host, LG2342p polarized three-dimensional (3D) monitor with a resolution power of 1920 × 1080 and refreshing frequency of 120 Hz, and 3D polarized glass. A visual and perceptual examination evaluation system, invented by the National Engineering Research Center for Healthcare Devices, was used. PEP was measured by the cross-into-circle test, which allowed the left eye to see a cross and the right eye to see a circle (Fig. 1). The midpoint of the monitor was held 80 cm away and as high as the position of patients' eyes, with the average light source of 80 cd/m² in white, attenuating to 50 cd/m² when the patients wear 3D polarized glasses, and 30 cd/m² in black, attenuating to 3 cd/m² when the patients wear 3D polarized glasses. The stimulating template was 51 circles was 0.4° × 0.4°, whereas the size of the cross was 0.33° × 0.33° (1° fixation test object). Patients used a computer mouse to place within what they perceived to be the circle's center, and were then instructed to click the mouse. The system automatically recorded vertical and horizontal bias by the 360° test object to observe any ocular misalignment. The minimum unit of ocular misalignment observed by this computer-controlled ocular misalignment system was 1 pixel, which equals 0.04 prism. To distinguish from conventional eye position, we defined this bias pixel as PEP.

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FIG. 1.

Measurement of PEP. By wearing 3D polarized glasses, patients should see a cross in their left eye and a circle in their right eye (A). They were asked to use a computer mouse to place the cross into their subjective view of the circle's center and then click the mouse (B). The system automatically recorded horizontal and vertical bias pixels. 3D, three-dimensional; PEP, perceptual eye position.

Results

A total of 84 subjects were recruited in the study and divided into three groups: 33 postoperative X(T) children in TG, 21 postoperative X(T) children in non-TG, and 30 non-strabismus children in control group. There were no statistically significant differences in gender (p = 0.059) or age (p = 0.156) of the patients among the three groups (Table 1). Details are shown in Tables 2 and 3 and Figures 2 and 3.

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FIG. 2.

Comparisons of the PEP pixels in controls and P-X(T) patients. *P-X(T) versus control, p < 0.05. P-X(T), postoperative intermittent exotrope.

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FIG. 3.

Comparisons of the PEP pixels in training group and non-training group in postoperative 3 days and postoperative 3 months. *Non-TG in postoperative 3 months versus TG in the same time, p < 0.05; **TG in postoperative 3 days versus in 3 months, p < 0.05. Non-TG, non-training group; TG, training group.

Discussion

In clinical practice, the traditional definition of eye position is described and measured by the prism plus cover test, which is convenient for clinical assessments, but not sufficient in providing a quantitative measurement of the sensory eye balance. PEP reflects the integration of a variety of substances in the human brain after they act upon the sensory organs directly.^14–16 PEP pixels can be measured by the computer-controlled perceptual examination evaluation system under a dichoptic vision condition.^14–16 The previous studies have shown that the sensory eye balance is abnormal in patients with amblyopia, anisometropia, and strabismus. ¹⁴

In our study, we quantitatively assessed the sensory eye dominance of surgically corrected X(T) with normal apparent eye position. Using measurement of PEP, we show clear evidence that the two eyes are still unbalanced even though they have a normal eye position after the surgery. We found that most of X(T) after a successful strabismus surgery with normal clinical eye position also had an obvious abnormal PEP, compared with the non-strabismus children with a normal PEP. This may be attributed to anomalous retinal correspondence, which remapped the usual relationship between the two eyes.^17–19 Compared with the eye position measured by traditional way, the PEP shows a more precise result.

More importantly, we divided the postoperative X(T) children into two groups, one group with some dichoptic visual perceptive training based on a unique VR platform (TG) and the other without any training (non-TG). After 3 months, we measured PEP pixels in all 54 postoperative X(T) children again. We found that, interestingly, the PEP pixels in TG dramatically decreased, whereas the PEP pixels in non-TG almost did not change any more. The training based on computer-controlled perceptual examination could rebuild sensory eye balance to obtain satisfying binocular visual function. Maybe the computer-controlled perceptual examination can reduce the relapse of X(T), by reconstructing the binocular retina corresponding points after strabismus surgery.

In summary, this preliminary study reveals that a functional eye imbalance still remains in surgically corrected cases that have clinically normal eye position with normal stereopsis. Our results demonstrate that the successful postoperative X(T) children still had an abnormal PEP when comparing with the non-strabismus children with a normal PEP, but the PEP pixels after visual perceptual training dramatically decreased. Our study provides the possibility that a further measurement of the PEP might reveal the sensory eye dominance, and a perceptive treatment, aiming at the rebalance of the ocular dominance, might be necessary for binocular stability and reconstruction of the binocular visual function in postoperative X(T).