Clinical Investigation On Relationship Between Types Of Intermittent Exotropia In Children And AC/A Ratio

Background:Intermittent exotropia is the most common type of concomitant exotropia and it’s a type of strabismus between exophoria and constant extropia.Intermittent exotropia is more commom in children.Duane first classified intermittent exotropia as falling into one of three types:basic type, divergence excess type, and convergence insufficiency type.Subsequently,Burian added a fourth category gone by the name of "simulated divergence excess type"to Duane’s classification.The simulated divergence excess type can be separated from true divergence excess type by 1-hour diagnostic occlusion test.Patients with intermittent exotropia were divided into 4 types based on the Burian classification methods which is according to the difference between the distance deviation and the near deviation: basic type, simulated divergence excess type, true divergence excess type and convergence insufficiency type.In order to get a clear object when the two eyes are staring at the target,accommodation and convergence must play a role at the same time.Accompanied by accommodation,eyes adduce and cause convergence.The greater the accommodation, the greater the convergence will be.This is a linkage process keeping a synergetic relationship.The AC/A ratio is the rotia of accommodative convergence and accommodation.At present,the methods of measuring the AC/A ratio commonly used include lens gradient method,heterophoria method,synoptophore method and clinical distance-near relationship.There are larger difference between different methods to measure the AC/A ratio.The method of clinical distance-near relationship cannot calculate the AC/A ratio directly,but through the near deviation minusing the distance deviation to evaluate the AC/A ratio.Both synoptophore method and heterophoria method will be affected by proximal convergence.When useing lens gradient method,the distance between eyes and the target is fixed,only by increasing or decreasing the spherical dioper to strengthen or relax accommodation.Lens gradient method can eliminate the influence of proximal convergence.Most scholars believe that intermittent exotropia is a progressive disease,which result from a combination of innervation and anatomic factors and mostly be related to dysequilibrium betweem divergence and convergence,fusion weakness as well as convergence insufficiency. Cooper and Medow concluded that the AC/A ratio is either normal or just slightly higher than normal in patients who have intermittent exotropia.Parks considered that intermittent exotropia can be explained solely on the basis of the AC/A ratio.A patient with a divergence excess exotropia would have a high AC/A ratio,and a patient with a convergence insufficiency exotropia would then be said to suffer from a low AC/A ratio.Research carried out by Wright and Juan found that the distance-near disparity of true divergence excess intermittent exotropia was caused by a high AC/A ratio or increased proximal convergence or both.In addition, Wright considered that increased tonic near fusional convergence caused the distance-near disparity of simulated divergence excess intermittent exotropia.Kushner has attributed disparity between distance and near deviation in intermittent exotropia to tenacious proximal fusion and to alterations in AC/A ratio.Objective:To investigate the relationship between accommodative convergence/accomodation ratio (AC/A) and four types of intermittent exotropia including true divergence-excess type, simulated divergence-excess type, basic type and convergence insufficiency type.Methods:Prospective clinical series study. Eighty cases of children with intermittent exotropia who met the inclusion criteria were recruited from February 2014 to September 2014 in the Eye Center of Provincial Hospital affiliated to Shandong University in this study. The exodeviations with cycloplegic refraction correction were measured by the alternate prism cover test at both distance and near with fixation on accommodative targets before and after 1-hour diagnostic occlusion test. The lens gradient method at distance was employed to assess the AC/A ratios of all patients. After 1-hour diagnostic occlusion test, all patients were divided into four groups according to the classification system proposed by Burian:the basic type, the true divergence-excess type, the simulated divergence-excess type and convergence insufficiency type. The average of AC/A ratios in these four groups were calculatd separately. Kruskal-Wallis test and Nemenyi test were applied to compare the differences of AC/A ratios among the four groups and between each both groups, respectively. Ap value of 0.05 was considered to be significant statistically.Results:Of all the 80 cases,8 were divergence-excess type,12 of the simulated divergence-excess type,36 of basic type and 24 of convergence insufficiency type. The average AC/A ratios of four groups were 4.37±5.58,2.03±1.02,1.91±2.07, 0.71±0.74△/D, respectively. There were significant differences between 4 groups (P =0.006). The difference was significant in group of convergence insufficiency type compared with the other three groups(P< 0.01). There was no significant difference between the other groups (P> 0.05).Conclusions:Measured by lens gradient method at distance, the AC/A ratios were small in general and great in individual difference. There are some differences in AC/A ratios among these four groups of the basic type, the true divergence-excess type, the simulated divergence-excess type and convergence insufficiency type. Compared with true divergence excess type, simulated divergence excess type and basic type, the AC/A ratio of convergence insufficiency type is lower.The AC/A ratio of true divergence-excess type is the highest and the AC/A ratio of convergence insufficiency type is the lowest.