[Characteristics of Guyton's exaggerated forced duction test (FDT) and torsional FDT in congenital superior oblique palsy].

Objective: To investigate the characteristics of Guyton's exaggerated forced duction test (FDT) and torsional FDT in patients with congenital superior oblique palsy (CSOP) and their correlation with clinical features. Methods: This cross-sectional study included single-eye CSOP patients and intermittent exotropia (IXT) patients scheduled for strabismus correction surgery at Tianjin Eye Hospital from September 2021 to March 2022. Prior to surgery, measurements of fovea-disc angle (FDA) and maximum cross-sectional area of the superior oblique muscle (max-CSA) were obtained in both eyes of the patients. The Guyton's exaggerated FDT and torsional FDT were performed intraoperatively to assess the degree of superior oblique muscle relaxation. The characteristics of the two FDT tests and their correlation with vertical strabismus angle, FDA, and max-CSA were analyzed. Statistical analyses were conducted using t -test, ANOVA, Tukey's test, Mann-Whitney U test, and chi-square test. Results: A total of 42 patients (84 eyes) were included in the study, including 19 IXT patients (38 eyes) and 23 CSOP patients (46 eyes, 23 eyes with palsy and 23 eyes without palsy). There were no statistically significant differences in gender composition or age between the IXT and CSOP patients (all P >0.05). The degrees of superior oblique muscle relaxation measured by the Guyton's exaggerated FDT were (-2.52±1.20), (-0.35±0.71), and (-0.03±0.16) for the palsy eye, non-palsy eye, and IXT eyes, respectively, showing significant differences ( F =88.10, P <0.001). The torsional FDT measurements yielded external rotation angles of 48.70°±9.67°, 37.39°±5.40°, and 38.95°±2.88° for the palsy eye, non-palsy eye, and IXT eyes, respectively, showing significant differences ( F =16.67, P <0.001). There was no statistically significant difference in internal rotation angles ( F =2.36, P =0.100). The FDA values were-12.11°±7.42° for IXT patients and-19.02°±4.95° for CSOP patients, while the max-CSA values for the palsy eye and non-palsy eye of CSOP patients were (7.59±4.69) mm² and (11.63±3.64) mm², respectively, all showing significant differences (all P <0.001). The degree of superior oblique muscle tendon relaxation assessed by the Guyton's exaggerated FDT was negatively correlated with the external rotation angle measured by the torsional FDT ( r =-0.64, P =0.001). They were positively correlated with max-CSA ( r =0.45, P =0.030) and negatively correlated with max-CSA ( r =-0.52, P =0.011). However, there was no correlation with vertical and rotational strabismus angle ( r =-0.12, P =0.579; r =0.33, P =0.126) and FDA ( r =-0.02, P =0.921; r =-0.23, P =0.309). Conclusions: Guyton's exaggerated FDT and torsional FDT can both assess the degree of superior oblique muscle relaxation in patients with CSOP. Furthermore, these two tests are correlated with changes in superior oblique muscle morphology. However, FDT cannot reflect the degree of vertical and rotational strabismus in patients.