The primary curative treatment for intermittent exotropia is surgery. Surgery may carry a risk of complications, such as overcorrection, undercorrection, the formation of amblyopia and the loss of stereopsis.10-12 Non-surgical treatments for intermittent exotropia has been studied in many researches, yet controversy exists in regard to the optimum conservative therapy for IXT.13
Control ability is an important parameter reflecting the severity of IXT, which is more important than the angle of deviation. 14 Control usually refers to the frequency of manifest deviation and the ease of realignment. The six-point office control scale is used to quantify the control ability. A study performed by Buck et al. has shown that the distribution of the median NCS score at follow-up was unchanged, indicating that NCS was a better marker of progression of IXT. 14 And to reduce the influence of subjectivity on test results, the measurements of control were repeated three times and averaged.15 In 2004, Haggerty et al defined the threshold of surgical intervention for IXT as the NCS≥3. Conservative treatment could improve the control ability. 16-19
Occlusion is a well-known therapy to treat IXT by eliminating suppression and ensuring maximum reservations of binocular vision. It is suggested that occlusion interventions could decrease the size of suppression scotoma and turn exotropia into exophoria by reducing the squint.20-22 Suppression is an active, progressive phenomenon. To avoid diplopia, images from the exotropic eyes would be ignored by our brain during the exotropia period. Therefore, occlusion eradicates suppression by removing the stimulus of the retina from external environmental signals, which in turn will improve eye position control.3 Some researchers considered it to be a useful approach for younger children and patients who wish to delay surgery. The purpose of pencil push-up training is to mobilize the patient's autonomic convergence to overcome the deviation caused by fatigue or neurological instability. With no extra cost, both methodologies are easy to perform. A direct comparison of the occlusion therapy to push-up training was not performed.
According to the results of the office control 6-point scale, we found that far deviation control in patients with IXT who were between 5 and 7 years of age improved significantly over 12-week periods in both alternate occlusion group and pencil push-ups group. No improvement was observed in observation group. It is better to have observation combined with other interventions as an assessment method rather than applying it as a single treatment option.
Coffey et al. reported the pooled success rate of each non-surgical treatment modality based on the review containing 59 studies23, of which the success rate of occlusion treatment was 37% (N=170), 28% (N=201) for prism treatment , 28% (N=215) for over-minus lens therapy and 59% (N=740) for orthoptic vision treatment; the success rates of surgeries depending on function or appearance were 43% and 61%. But the pooled results are debatable as studies included had limitations, such as selection bias, small sample sizes, inappropriate statistical analysis and insufficient criteria for success. These problems indicate the need for a well-controlled study on the efficacy of different treatment modalities in controlling IXT.
Pediatric Eye Disease Investigator Group reported little deterioration over 6 months in all untreated childhood IXT, with or without patching therapy. Although patching group had a slightly lower deterioration rate.19A randomized clinical trial conducted by Mohammad Reza Akbari suggested that patients with IXT in the range of 3-8 years old may obtain a better treatment outcomes in comparison with observation.24
The Pediatric Eye Researchers Group (PEDIG) explored another application of part-time occlusion in the treatment of intermittent exotropia.17,19 They examined the effect of patching on preventing deterioration in patients with better control, but not the effect to improve control in poorly controlled patients. Having constant exotropia progressed from intermittent for 6 months is considered as the criterion for deterioration. Studies show that most IXT diagnosed patients do not need treatment within a year.25,26 Therefore, it is not surprising that the rate of deterioration in both groups was very low. Differences between groups were not statistically significant. PEDIG studies have not shown the effectiveness of occlusion therapy in improving control.
Pencil push-up is easy to learn and operate. It had a considerable effect on the treatment of undercorrection after intermittent exotropia surgery. Many investigators merely mentioned that pencil push-up was part of their routine for binocular vision training, without specifically reporting or investigating the effect of pencil push-up on control. The study found that both pencil push-up and occlusion could improve the control. They had similar therapeutic efficacy, and could be alternative conservative treatments for intermittent exotropia.
In addition to our primary outcome, we also evaluated the near stereoacuity in 12-week treatment. Aligned with the previous researches, no substantial improvement was found in sensory fusion after patching and pencil push-up. Distance stereoacuity function losses at the earliest stage of IXT progression. Meanwhile, the control level and stereoacuity at near is relatively stable, which may experience varying levels of stereoacuity loss in advanced stages. It is not difficult to understand that 12 weeks of patching and pencil push-up treatments did not promote the stereopsis at near significantly. Therefore, further research with longer follow-up periods is needed.
It is controversial to define deterioration of intermittent exotropia based on a decrease in stereoacuity at near. Holmes et al. reported that 6 out of 95 children (7%) for whom no treatment showed a reduction of 2 octaves in a single measurement of near stereoacuity, and 4 children in the future showed a regress to the baseline level of stereoacuity, emphasizing the need for retesting at the same or subsequent visits.27 Since our current study requires that the stereoacuity test be retaken on the same day, some patients classified as deteriorating may have poor results due to feeling uncomfortable or uncooperative that day or the inherent variability of IXT. Nonetheless, due to the small decrease in stereoacuity in the two treatment groups (3.6% and 0.6%), any overestimated deterioration of stereoacuity due to not requiring a retest on the next day will be minimal. In addition, any small overestimation of the deterioration is unlikely to affect the comparison of the treatment groups, considering that there is no difference in stereoacuity changes in the treatment groups. It is expected that the deterioration of the two groups will be overestimated by the same amount.
Based on the basic distance control, a secondary analysis was conducted to evaluate the effectiveness of occlusion and pencil push-up. It is noteworthy that the improvement of distance control is more evident for children with lower baseline distance control than those with better distance control. The larger response in children with poor baseline control may be partly due to a return to the average and having a lot of room for improvement. Yet, the same response level was not observed in the observation group, showing that this greater effect of occlusion and pencil push-up on children with poorer control may be real. However, due to the small sample size of the subgroup, this conclusion needs to be interpreted with care.