A-exotropia is also known as exotropia A sign, exotropia Aphenomenon, A-exotropia, and divergent strabismus A syndrome, in which the exotropia becomes smaller or even disappears when looking directly upward, while the exotropia increases when looking directly downward. In this case, the exotropia becomes smaller or even disappears when looking straight up, while the exotropia increases when looking straight down. There are many reasons for the formation of the A-V sign, which are summarized in the following eight aspects: 1. The reason for the horizontal muscles Urist believes that the formation of the A-V sign is related to the difference in the strength of the internal and external rectus muscles when looking upward and downward. In the physiological state, when the eyes gaze upward, there is a mild increase in separation (<15△), and when gaze downward, there is a mild increase in convergence (vergence) (<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, V-sign is caused by over-strong horizontal muscles, while 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, when the strength of lower oblique muscle is too strong, it can cause V phenomenon; while when the strength of lower oblique muscle is not strong enough, it causes A phenomenon; when the strength of upper oblique muscle is too strong, it causes A phenomenon; when the strength of upper oblique muscle is not strong enough, it causes V phenomenon. In conclusion, the increase in horizontal obliquity when gazing directly upward is caused by the inferior oblique muscle, while the increase in horizontal obliquity when gazing downward is caused by the superior oblique muscle. von Noorden 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. In A-V syndrome with abnormal function of the strabismus muscle, rotational strabismus cannot be corrected after tilt correction with the horizontal muscle stop. This rotational strabismus is confirmed by fundus photography. Brown believes that the strength of the upper and lower rectus muscles is one of the reasons for the formation of A-V syndrome, because both muscles have a secondary role in making the eye turn inward, so when the upper rectus muscle is too strong, it can cause the A phenomenon; while when the upper rectus muscle is weak, it can cause the V phenomenon; when the lower rectus muscle is strong, it can cause the V phenomenon; and when the lower rectus muscle is weak, it can 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. Those who hold this opinion believe that the A-V sign is caused by abnormalities of both horizontal and vertical muscles, not by abnormalities of any 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 the functions of both horizontal and vertical muscles. (1) The A-V sign is related to the shape of the face: for example, a Mongolian-like face (with an upward shift of the external canthus) may cause A-internal and V-external obliquity; while an anti-Mongolian-like (Caucasian) face (with no shift or a slight downward shift of the external canthus) may cause A-external and V-internal obliquity. (2) Fascial abnormalities: For example, Brown's superior oblique sheath syndrome is often combined with V-exotropia, which is due to the lack of elasticity of the superior oblique sheath, resulting in forced abduction during upward rotation. In Johnson's adhesion syndrome, mechanical scattering during vertical rotation can also occur. (3) Abnormal muscle attachment points: It is believed that some patients with V phenomenon have a higher than normal attachment position of the medial rectus tendon and a lower than normal attachment position of the lateral rectus tendon. In addition, the forward or backward shift of the attachment point can also cause the A-V sign. A-V sign is rare due to anatomical factors alone, but it is more common due to paralysis. This is because the definition of A-V sign shows that it is a subtype of horizontal strabismus accompanied by vertical non-common strabismus, which means that whether horizontal or vertical strabismus is predominant, there is a combination of muscle imbalance that is too strong or too weak. There is no definite conclusion as to which of the vertical rectus and oblique muscles is predominant, both horizontal and vertical muscles have a role, but it is not possible to explain who is the most important of the two, the vertical muscles have both nerve impulse and mechanical roles, while the horizontal muscles may be more prominent when the muscles are too strong or too weak. In addition, there is a physiological V phenomenon, that is, in the original eye position without strabismus, when gazing upward, it can produce exotropia (up to 17△), and when gazing downward, it can produce A phenomenon (up to 5△), which may be related to the innervation factors. 7, convergence and fusion function abnormal causes When gaze downward, can not maintain fusion, can produce A phenomenon; when gaze upward can not maintain fusion, can produce V phenomenon, this situation, in intermittent exotropia common. In China, 11 cases of V-exotropia in 5 generations of a family were reported, which are autosomal dominant. Only one case was operated on in a preexisting patient, and no abnormalities of extraocular muscle attachment were seen intraoperatively. In conclusion, among the many factors mentioned above, the pathogenesis of all cases cannot be explained by one simple etiology, but it is mainly caused by the extraocular muscles.