Effect of suppression during tropia and phoria on phoria maintenance in intermittent exotropia

Treatment for intermittent exotropia X(T) aims to keep the eye in a phoric position and to maintain the phoria. However, maintenance of phoria is difficult even after treatment, and the cause is unclear. The aim of this study was to investigate the presence of suppression during tropia and/or phoria in X(T), and to determine how the suppression affected patient's ability to maintain phoria.

Medical records of 89 children with X(T) (mean age, 9.8 ± 2.7 years) were reviewed retrospectively. According to their previous treatment for X(T), the patients were divided into four groups and compared: untreated and under observation only (28 patients), surgical treatment (32 patients), orthoptic training (eight patients) and a combined treatment of surgery and orthoptic training (21 patients). Suppression during phoria was evaluated by a physiologic diplopia test, and suppression during tropia was evaluated by a convergence test or a cover test when fusion broke. Phoria maintenance was achieved if a phoric condition was maintained even when the fusion broke at both near and far. Furthermore, the Bagolini's red filter bar was used to quantitatively assess patient's ability to maintain phoria at near and far distances.

No subject only suppressed during phoria. Patients who suppressed under both conditions could not maintain phoria. Suppression under both conditions significantly correlated with phoria maintenance and the ability to maintain phoria (P < 0.01, Fisher's exact probability test). All the patients with a strong ability to maintain phoria did not suppress under either condition. As compared to the surgical treatment group, the combined treatment group had a higher percentage of patients who did not suppress under either condition and could maintain the phoria. Suppression under both conditions also significantly correlated the treatment methods (P < 0.01, Chi-square for the independence test).

Suppression under both tropic and phoric conditions significantly relates to the outcome of patients' phoria maintenance and their ability to maintain a phoric position. Suppression under both conditions is an important indication of whether X(T) shifts to constant exotropia.