The rhombus-phenomenon, i.e., small or no internal obliquity in the original eye position and increased internal obliquity when gazing directly upward and downward, are both manifestations of A-V syndrome. Based on the patient’s clinical manifestations and eye examination results, a clear diagnosis can be made. The National Amblyopic Strabismus Prevention and Control Group of the Chinese Medical Association Ophthalmology Society (1987) stipulates the diagnostic criteria for A-V syndrome as follows: 1. Exotropia V syndrome: the obliquity is greater when gazing upward than when gazing downward (15△, 8deg;~9deg;). 3.Internal oblique V sign: the obliquity when gazing upward is smaller than when gazing downward (15△, 8deg;~9deg;). 3.Exotropia A sign: the obliquity when looking upward is smaller than when looking downward (10△, 5deg;~6deg;). 4.Internal oblique A sign: the obliquity when gazing upward is larger than when gazing downward (10△, 5deg;~6deg;). The difference between upward gaze and downward gaze must be 10△ to diagnose the A phenomenon; the difference between the two must be 15△ to diagnose the V phenomenon, because normal people also have mild pooling when they gaze downward. In order to further determine whether the A-V phenomenon is caused by purely horizontal muscular factors or vertical muscular factors, the strabismus of each diagnostic eye position should be measured by the trigeminal plus masking method or the same visual machine. In the case of normal retinal counterparts, a simultaneous rotational strabismus can also be detected with a synoptic examination (which can also be confirmed by fundus photography), which is helpful for the development of surgical treatment plans.