Magnetic Resonance Imaging Study of Extraocular Rectus Muscles in Intermittent Exotropia

Objective: To observe the pulley insertion location and extraocular rectus muscle changes in intermittent exotropia using the magnetic resonance imaging (MRI) technique. Methods: This was a case series study. Data were collected from January 2017 to June 2017 at Tianjin Eye Hospital. Thirty-two patients who were diagnosed with intermittent exotropia included 17 males and 15 females, whose average age was 15.4 ± 1.8 years. Thirty normal volunteers whose average age was 16.1 ± 1.6 years were used for comparison. MRI coronal scans were performed on the pulley insertion locations of the extraocular rectus muscles. Linear regression and correlation coefficients were calculated, as well as the maximum cross-sectional area and volume changes of the extraocular rectus muscles in the primary gaze, and were analyzed by an independent samples t test. Results: The difference in pulley insertion locations of the extraocular rectus muscles between intermittent exotropia patients and normal volunteers was not significant. However, the maximum cross-sectional area changes of the medial rectus muscle in intermittent exotropia was smaller compared to the normal volunteers and the volume changes of the medial rectus muscles were significantly weaker (t =10.47, P=0.006). The maximum cross-sectional area of the lateral rectus muscles was larger in intermittent exotropia patients than in the normal volunteers but the difference was not significant. The ratio of the lateral rectus/medial rectus muscle volume was significantly different (t=7.33, P=0.007), it was larger in intermittent exotropia patients (1.6 ± 0.4) than in normal volunteers (1.1 ± 0.1). Conclusions: In patients with intermittent exotropia, MRI scans show that the maximum cross-sectional area of the medial rectus muscle decreases and the volume is smaller, while the ratio of the volume of the lateral rectus/medial rectus muscle is much larger. This indicates that the mechanism for intermittent exotropia is related to a change in the horizontal rectus muscle, not the pulley insertion location. Key words: intermittent exotropia; magnetic resonance imaging; Pulley; horizontal rectus muscles