Latency in the improvement course of fusional ability during orthoptic training for intermittent exotropia: a retrospective observational case series

Purpose: To clarify the existence and characteristics of the latency period in the improvement course of fusional ability in patients with intermittent exotropia (IXT) treated with orthoptic training, and to contribute to clinical decision-making regarding the optimal timing for training termination. Study design: Retrospective observational case series. Methods: Fusional ability was assessed in 13 patients with IXT using the ratio of absolute fusional range divided by ocular deviation (FR/D ratio) and graphed to analyze the shape of the improvement course. The latency period was defined as the duration from the initiation of training to the apparent attainment of training effect. The improvement course of the group accompanied by a latency period (latency group) was compared with the group unaccompanied by a latency period (non-latency group). The main outcome was defined as the progression of the FR/D ratio reaching 3.0, per Japanese standards relative to Sheard's criterion (see Methods). Results: FR/D ratio reached 3.0 in 12 out of 13 cases. The improvement course of the fusional ability represented three periods: latency, progression and sustained. The latency period included 61.5% of patients with a maximum duration of 9 weeks. Conclusions: The existence of the latency period in the improvement course of fusional ability was determined; that period can be considered as the period of learning and acquiring the necessary skills of performing the assignments throughout the training. Therefore, even if no improvement is evident at the early stages of training, it is advisable to continue the training for at least 9 weeks.