OCTA is a relatively new non-invasive imaging technique that uses dynamic and static tissue contrast imaging to obtain high-resolution information and generate blood flow to angiographic images. OCTA not only can quantify blood flow VLD, PD, and FAZ areas and more parameters [4–6] but also can be used to quantify the eccentric distance based on OCT technology [7–10], and fixation conditions can be observed by the FastTrac tracking function. In fact, this kind of patient fixation condition is very poor, accounting for 29%. Most patients need a longer time for capturing images with stable fixation, and the size of the eccentric fixation distance has certain fluctuations. We will measure to ensure the accuracy of the results many times, although it conflicts with OCTA inspection requirements, which may have a certain influence on the results of the inspection; however, the side effects reflect the clinical features of this disease, so OCTA research is necessary.
Regarding the cause of eccentric fixation, previous studies have focused on two theories [11]: one is the "correspondence theory", which holds that abnormal retinal correspondence is the cause of eccentric fixation. The other is the " scotoma theory", which holds that eccentric fixation is caused by scotoma in the macular fovea of the strabismus eye. WYBAR believes that eccentric fixation is caused by inhibition of the macular fovea in the strabismus eye, which is a feature of monocular fixation behaviour, while the abnormal retina corresponds to binocular fixation behaviour, and there is no causal relationship between them [12]. Keiner believes that the type of eccentric fixation is related to age; the younger the age that eccentric fixation presents, the more likely it is caused by abnormal vision in the monocular eye, and the older the age that eccentric fixation presents, the more likely it is caused by abnormal retina correspondence [13]. After years of research, Von Noorden found that abnormal retinal correspondence could not explain all the phenomena of eccentric fixation, and eccentric fixation could also occur in the case of normal retinal correspondence [14]. He was more inclined to the theory of scotoma, believing that eccentric fixation was related to the size and degree of inhibition of the scotoma [15]. Wong confirmed that the existence of abnormal retinal tissue is inversely proportional to the deviation angle; the greater the deviation angle is, the smaller the possibility of abnormal retinal correspondence is, and the smaller the deviation angle is, the greater the possibility of abnormal retinal correspondence [16]. In our study, both small-angle strabismus and large-angle strabismus were present, but not all cases of small-angle strabismus had abnormal retinal correspondence. Eccentric fixation is proportional to the deviation angle, and the greater the deviation angle is, the greater the eccentric fixation distance is. At present, eccentric fixation is not associated with abnormal retinal correspondence, and the relationship between central scotoma and eccentric fixation needs further study.
In our study, the SE of the amblyopic eye was significantly better than that of the lateral eye and the control group eye. The time sequence of strabismus, anisometropia and amblyopia is not always obvious or not found in time. The causal relationship between these three cases is hard to assess. Helveston believes that anisometropia is the cause of small-angle strabismus [17], but Lepard believes that monocular strabismus amblyopia has a strong tendency to retain farsightedness and that anisometropia may be the result, not the cause [18]. Smith analysed non-human primate data of anisometropia, strabismus and amblyopia, concluding that early esotropia can cause hyperopic anisometropia and amblyopia. Early hyperopic anisometropia is an important risk factor for amblyopia, while isolated amblyopia does not result in a significant risk for hyperopia or anisometropia [19]. We prefer the view that anisometropia is a result rather than a cause of strabismus. Helveston found that 20 cases of small strabismus all had a certain degree of stereopsis, with an average of 67" and a best result of 20" [17]. Epstein studied 15 patients with a stereoscopic average of more than 40". He thought that stereopsis may be associated with the central scotoma size. The smaller the central scotoma was, the better the stereopsis [20]. Our study consisting of small-angle strabismus patients demonstrated rough stereopsis, and patients with large-angle strabismus had almost no stereopsis, with a median of approximately 800. There was no significant difference compared with the control group, which may be associated with the small sample size. Studies with a small sample size is another important bias factor affecting the outcome.
There are many OCTA studies in fundus diseases; however, in terms of amblyopia, there are fewer studies [21–23]. In addition, in terms of strabismus amblyopia [24], most of the research results show no significant difference between amblyopia eyes and healthy eyes in terms of the capillary plexus vascular density of the macular fovea. Our research conclusions coincide with these research results. As a new examination method, OCTA is expected to be more widely used in the study of strabismus amblyopia in the future.