[Analysis of distribution characteristics and influencing factors of chord μ in young myopia]

Objective: To explore and analyze the distribution characteristics of chord μ related parameters, as well as the pupil center's relative position to the coaxial corneal light reflex on the corneal surface, and the influencing factors in young myopia. Methods: This was a cross-sectional study. A total of 761 myopic patients (761 eyes) were collected from March 2021 to December 2021 in the Refractive Surgery Center of Tianjin Eye Hospital, including 388 males and 373 females, with an average age of (24±6) years. The relationship between age, sex, diopter, anterior and posterior corneal surface parameters, and chord μ related parameters was analyzed, including the x and y absolute values of the pupil center, chord μ length, and angle. The normality of the data was tested using the Kolmogorov-Smirnov test, and the influencing factors of chord μ were analyzed through Pearson and Spearman correlation analysis. Results: The equivalent spherical degree and chord μ length were (-5.47±1.66) D and (0.178±0.095) mm, respectively. The chord μ length followed an approximately normal distribution. The chord μ length of 266 eyes (35%) was distributed in the range of 0.120 to 0.200 mm, while the chord μ length of 479 eyes (63%) was<0.200 mm, and the chord μ length of 620 eyes (81%) was<0.260 mm. The chord μ angle distribution accounted for the largest proportion in the superior nasal quadrant (45.6%), followed by the superior temporal quadrant (34.3%), the inferior temporal quadrant (10.1%), and the inferior nasal quadrant (10.0%). High myopia (r=0.11, P=0.002) and high astigmatism (r=0.08, P=0.023) were associated with an increase in chord μ length. The higher the degree of myopia, the smaller the chord μ angle (r=-0.09, P=0.019). The larger the ISV (r=0.09, P=0.017), IVA (r=0.08, P=0.025), and IHD (r=0.08, P=0.039) on the anterior surface of the cornea, the longer the chord μ length. The higher the astigmatism of the posterior corneal surface, the greater the absolute value of the Y coordinate of the pupil center (r=0.07, P=0.044), and the longer the chord μ length (r=0.08, P=0.035), and the smaller the chord μ angle (r=-0.08, P=0.032). Conclusions: The chord μ length of young myopic individuals in China followed an approximately normal distribution, with the majority located in the superior nasal and superior temporal quadrants. High myopia, high astigmatism, and irregular corneal shape are the main factors related to an increase in chord μ length.