There are many reasons for the formation of the rhomboid-phenomenon, including factors of the extraocular muscles themselves, anatomical factors, innervation factors, and genetic factors. In summary, there are several aspects as follows: 1. Reasons of horizontal muscles: Urist believes that the formation of the A-V sign is related to the difference in the strength of the inner and outer rectus muscles when gazing upward and downward. In the physiological state, when the eyes gaze upward, there is a mild increase in the action of separation (<15△), and when gaze downward, there is a mild increase in the action of assembly (convergence) (<10△), but neither exceeds the normal limit. The above physiological differences are the result of the different action of the external and internal rectus muscles, and it is generally believed that the V phenomenon is due to excessive physiological differences, while the A phenomenon is due to small physiological differences. In other words, V-external strabismus is due to excessive action of external rectus muscle, V-internal strabismus is due to excessive action of internal rectus muscle, A-external strabismus is due to insufficient strength of internal rectus muscle, and A-internal strabismus is due to insufficient strength of external rectus muscle. In short, the V-sign is caused by over-strong horizontal muscles, while the A-sign is caused by insufficient strength of horizontal muscles. 2. Causes of oblique muscles: Jampolsky believes that the strength of the upper and lower oblique muscles is an important reason for the formation of the A-V sign. Since the secondary role of oblique muscle makes the eye turn outward, so when the strength of lower oblique muscle is too strong, it can cause V phenomenon; and when the strength of lower oblique muscle is not strong enough, it causes A phenomenon; the strength of upper oblique muscle is too strong and causes A phenomenon; the strength of upper oblique muscle is not strong enough and causes V phenomenon. In conclusion, the increase in horizontal obliquity when looking upward is caused by the lower oblique muscle, while the increase in horizontal obliquity when looking downward is caused by the upper oblique muscle. vonNoorden believes that the abnormal function of the oblique muscle is the more common cause of A-V syndrome, and A-V syndrome with abnormal function of the oblique muscle often causes rotational strabismus. The rotational strabismus produced by A-V syndrome with abnormal function of the oblique muscle cannot be corrected after tilt correction with the horizontal muscle stop. This rotational strabismus is confirmed by fundus photography. 3, the cause of the upper and lower rectus muscle: Brown believes that the strength of the upper and lower rectus muscles is a reason for the formation of A-V syndrome, because both muscles have a secondary role in making the eye turn inward, so when the strength of the upper rectus muscle is too strong, it can cause the A phenomenon; while the strength of the upper rectus muscle is weak, it can cause the V phenomenon; when the strength of the lower rectus muscle is strong, it can cause the V phenomenon; the strength of the lower rectus muscle is weak to cause the A phenomenon. In short, the difference in horizontal obliquity when looking upward is due to the upper rectus muscle, while the difference in horizontal obliquity when looking downward is due to the lower rectus muscle. 4.The cause of horizontal-vertical rectus muscle: People who hold this opinion believe that the A-V sign is caused by abnormalities of both horizontal and vertical muscles, not by abnormalities of one muscle alone. In some patients, the A-V sign may be caused by secondary changes in the vertical muscles due to excessive strength or weakness of the horizontal muscles, while in other patients, the A-V sign may be caused by secondary changes in the horizontal muscles due to excessive strength or weakness of the vertical muscles, or by changes in both horizontal and vertical muscle functions.