Our investigation and statistical results indicated that the near exophoria was the most popular heterphoria in myopia juveniles. Sanker et al[5] found that 38 out of 49 college students were exophoria by near heterophoria tests. In 2007, Jin et al[6] tested 825 juveniles and found the percentage of esophoria, exophoria was 36.4%, 52.9% respectively in those students with normal refraction, and was 22.6% and 64.7% in the students with myopia. The distance exophoria, esophoria rate was 19.8% and 60.4% respectively in the students with normal refraction, while was 28.8% and 50% in the myopia students. Despite these studies had a discrepancy in their reported near and distance heterophoria tests, there was a pattern that near exophoria incidence was more than esophoria in the myopia population. Besides, some differences might exist in the distance and near heterophoria no matter in normal or myopia populations, which were consistent with our findings. We found the statistical difference (F=300.00, P=0.000) between the mean value of distance and near heterophoria . Meanwhile, the changes of convergence before and after glass wearing would affect heterphoria degree, leading to difference before and after visual correction. Therefore, we believed that heterophoria tests should include distance and near heterophoria before visual correction beyond of heterophoria after visual correction.
Heterphoria is believed to be associated with the development and progressive speed of myopia. Goss et al[7,8]found the association between near heterophoria and myopia. However, that association only occurred in those near heterophoria accompanying with accommodation lag [9]. In 2006, Adler et al[10] revealed that myopia increased faster in those near esophoria children than those orthophoria or exophoria ones, but with no significant difference [11,12]. Recently, myopia has become a social issue. In 2013, Fan et al[13]found that near exophoria juveniles accounted for 73.6% in those myopia ones, and these exophoria juveniles developed myopia faster than those esophoria and orthophoria ones, suggesting that the accommodation of eye position in juveniles is a risk factor of myopia, and reasonable optometry and accommodation practice might reduce the development of myopia. Therefore, each subject should have heterophoria tests to improve the comfort and success of optometry by excluding failure due to exophoria. Besides, practices could be administered according to the binocular vision test results to delay the myopia progress. The accurate measurement of heterophoria is important to myopia progress prediction and control selection.
There might be some differences of the heterphoria between normal and nearsighted populations. In 1980, Fang et al[14] investigated the heterophoria situations in 1156 normal visual acuity adolescents and the results showed that distance esophoria ones accounted for 44.63%, and exophoria ones accounted for 24.48%. In 2000, Ren et al[15] conducted heterophoria tests on 1027 navy pilots and found the ratio of orthophoria, distance esophoria, and exophoria was 13.1%, 48.4% and 23.8% respectively. In 1989, Xu et al[16] tested 650 eyes from 325 normal sighted subjects and they reported 295 cases of heterophoria (90.76%), 30 cases of orthophoria(9.24%). In those heterophoria ones, esophoria is the most common type (156 cases: 52.88%). In 2003, Chen et al[17] performed heterophoria tests on 36 subjects including 11 myopia and 25 normal ones, their findings discovered about 72% normal sighted eyes had near exophoria, and 68% nearsighted eyes had near exophoria, but with no statistical significance. Despite the heterophoria degrees varied at those different studies, it’s suggested that distance esophoria is more than exophoria, and near exophoria is more than esophoria in normal sighted healthy adolescents. As our research results disclosed, both distance and near exophoria was more than esophoria in nearsighted juveniles. The variance of our results from other publications might be due to the facts that included subjects were newly diagnosed myopia juveniles at their first optometry and they had no eyeglass wearing histories.
The approach of progressive multifocal eyeglass to control juvenile myopia is still in doubt, and most ophthalmologists and optometrists believe it should be selectively used. The progressive multifocal eyeglass wearing could add luminosity, decrease accommodation demand at near sight, enhance divergence, increase exophoria, reduce positive fusion, decrease accommodation, and increase accommodation lag. All these factors could increase myopia and therefore the progressive multifocal eyeglass should not be utilized in juveniles as routine. Furthermore, indications such as esophoria, high AC/A ratio should be strictly followed [18]. In 2012, a study showed that progressive multifocal eyeglass could control the development of myopia in those nearsighted children with high accommodation lag [19]. In 2011, Xu et al[20] reported their observations on 50 esophoria myopia subjects wearing eyeglass, and their results disclosed that the speed of degree increment was fastest in those subjects wearing normal eyeglass, then distance spectacles (-1.50DS) and multifocal eyeglass, and finally reading spectacles. The previous literature have reported [9,21,22]that bifocal or progressive multifocal eyeglass wearing could foster myopia in near exophoria children. Besides, most juveniles are not suitable to wear progressive eyeglass if they have normal visual functions. Although most myopia subjects wearing progressive multifocal eyeglass had no discomfort complaints, their optometry exams indicated a high percentage of exophoria and low AC/A ratio. Some subjects might loss convergence because of obvious intermittent exophoria problems. Progressive multifocal eyeglass might not cause serious outcomes in those juveniles with the normal position of the eye, however, it might turn to be serious in those ones with obvious exophoria without prism prescription. Only those juveniles having high AC/A ratio, esohoria, high positive relative accommodation (PRA) and negative relative accommodation(NRA) are suitable to wear progressive multifocal eyeglass. However, the juveniles meet above requirements account less than 20%, therefore, optometrists should deliver their best prescription and suggestion at their abilities and systematically record AC/A ratio, position of the eye, amplitude of accommodation (AMP), PRA and NRA before prescribing progressive multifocal eyeglass.
Near visual fatigue has been paid more and more attention as the developing of society and the increasing burden of near distance visual assignments. In the case of the low heterphoria degree, it could be compensated itself with no symptom only by divergence reserve. When the heterphoria degree is large or there is convergence insufficiency, long term over the use of divergence reserve will lead to muscular visual fatigue. In 2016, Veselý et al[23] evaluated the binocular visions in 68 cases of young adults (mean age: 26 years), of whom 26 cases were both near and distance orthophoria, and 42 cases were abnormal (12 cases of esophoria, 8 cases of exophoria, 13 cases of convergence insufficiency). Their results suggested that accurate spherocylindrical lenses optical correction could alleviate binocular visual disorders. Therefore, different refractive error and deviation of eye position should be taken into consideration for optical correction so as to improve the relationship of accommodation and convergence and to reduce the deviation degree which would increase otherwise. It’s necessary for optometrists to keep this knowledge in mind. In 2015, Vilela et al[24]reported visual fatigue accounted for 24.7% in 964 students aged between 6 to 16 years, reminding health care providers to pay attention to these visual fatigue students since the fatigue will affect those students’ lives and studies in schools. At the time of refractive correction, we could ask the examinees to have optic correction or training for heterophoria according to their ages. In order to find tropia or heterophoria patients earlier, we believed that eye position and near as well as distance heterophoria should be measured during optometry, the optic correction prescription based on which could protect juvenile vision and visual function mostly.