The Correlation Study Between Classification And Fusional Control Ability Of Intermittent Exotropia

Background:Intermittent exotropia is one of the most common forms of eye diseases in children, and it also is the most common forms of divergent strabismus. There is no clear conclusion about the natural history of intermittent exotropia so far. However, many scholars believe that the deviation occurs at distance first, then at near. Jampolsky[16] mentioned that intermittent exotropia may be developed from exophoria, and would deteriorate to constant extropia. He thought that[16] suppression was the key to destroy fusion function. Patient’s binocular fusion function is determined by the degree of suppression and disease period. Therefore, binocular fusion function is one of indicators to assess the severity of intermittent exotropia. Binocular fusion function is more likely to be damaged when patient’s control of ocular position becomes weaker and the deviation occurs more frequently. This kind of situation is the sign of deterioration. Kushner[17] mentioned that it was necessary to measure the deviation by the prism and alternate cover test, however, it was eaqually important to assess control ability of intermittent exotropia patients. Many scholars came up with different methods to evaluate control ability of intermittent exotropia patients. In 1987, Calhoun[18] raised four phases of intermittent exotropia. In 2005, Chia el at[19] proposed that patients’control ability can be scored as three levels (good, moderate, poor). In 2004, Haggerty el at[20]proposed the method of Newcastle Control Score, which combined scores of patients’ control ability both in daily life and in clinic. However, none of them can provide a quantitative measurement of the severity and duration of the manifest component of the exotropia until in 2006, Mohney and Holmes came up with the office-based 6-point control scale. This method can assign objective score to each patient with intermittent exotropia during the examination. It is free, easy to be handled, and it is independent of patient’s age. Based on the difference between prism and alternate cover test at distance and at near before and after unilateral occlusion, in 1966, Burian devided intermittent exotropia into four types:the basic type, the true divergence excess type, the simulated divergence excess type, and the convergence insufficiency type. Since the difference between the deviation at distance and at near is the key point classifying intermittent exotropia, it is questioned that whether certain types of intermittent exotropia have special characteristics in the control ability.Objective:To explore correlation between classification and fusional control ability of intermittent exotropia.Methods:A prospective case series.164 patients with intermittent exotropia presenting to the Provincial Hospital affiliated to Shandong University from November,2012 to August,2014 were involved in our study. The control ability of all patients in our study were evaluated at distance and at near fixation using Mohney and Holmes’ method[21] (the office-based 6-point control scale). Deviations of patients at distance and at near were measured by prism and alternate cover test before and after 1 hour monocular occlusion. All patients were devided into four groups according to Burian’s classification for intermittent exotropia. Patients’ near stereopsis acuity was evaluated by the Randot Stereotest. The difference between the scale at distance and at near was defined as D-value. D-values were analysed with the SPSS statistical software by rank-sum test. A p-value of<0.05 was considered statistically significant. The results of near stereopsis acuity were analysed with the SPSS statistical software by rank-sum test. A p-value of <0.05 was considered statistically significant.Results:In 164 patients involved in our study, the level of control at distance was worse than the level of control at near for 127 (77.4%) patients, the level of control at distance was equal to the level of control at near for 37 (22.6%) patients. D-values in four groups were analysed by Mann-Whitney U test. D-value in basic type group was smaller than that in true divergence excess type group (p< 0.001). D-value in basic type group was smaller than that in simulated divergence excess type group (p=0.022). D-value had no statistical difference between basic type group and convergence insufficiency type group (p=0.173). D-value had no statistical difference between true divergence excess type group and simulated divergence excess type group (p=0.102). D-value in convergence insufficiency type group was smaller than that in true divergence excess type group (p<0.001). D-value in convergence insufficiency type group was smaller than that in simulated divergence excess type group (p=0.012).The results of near stereopsis acuity were analysed by Kruskal-Wallis test. There were no statistical differences between the four groups (p=0.515).Conclusions:Patients with intermittent exotropia had relatively weak control abilities at distance than at near fixation. This phenomenon was much more obvious in patients with true divergence excess type and simulated divergence excess type.