Binocular Single Vision may be defined as the state of simultaneous vision, which is achieved by the coordinated use of both eyes, so that separate and slightly dissimilar images arising in each eye are appreciated as a single image by the process of fusion.(1) Eye teaming or binocular vision is a visual efficiency skill that allows both eyes to work together in a precise and coordinated way. Two eyes must work together as a team to give us one image. Eye focusing is a visual efficiency skill that allows the images of objects at sharp focus regardless of the distance.(3)
Eye teaming disorders or Vergence dysfunction involves disjunctive eye movements in which the visual axes move toward each other(convergence) or away from each other (divergence), resulting in the inability of the eyes to accurately fixate and stabilise a retinal image(3).
An accommodative and vergence dysfunction can have negative effect on a child's school performance, especially after third grade when he must read smaller print and reading demand increase. The child may not be able to complete reading or home assignments and may be easily distracted in presence of eye teaming and eye focusing disorders(5).
The most widely accepted classification is the Duke-Elder classification of accommodative dysfunction. These include accommodative insufficency, ill-sustained accommodation, accommodative infacility, paralysis of accommodation and spasm of accommodation. (6)
Vergence Dysfunction
Vergence eye movements (both convergence and divergence) are prerequisite of normal binocular vision. Vergence eye movements minimise retinal disparity and place the two retinal images of a single object on corresponding retinal points. The term initial convergence is used to describe the movement of the eyes from the physiological position of rest to the position of single binocular fixation of a distance object.(7)
The vergence dysfunction is classified into different categories. It was first developed by Duane for strabismus which was then modified for the classification of heterophoria and intermittent strabismus. These categories include convergence insufficiency, divergence excess, basic exophoria, convergence excess, divergence insufficiency and basic esophoria.
It is diagnosed as a condition based on the finding of a remote near point of convergence and decreased fusional convergence at near fixation.(4) It is the most common vergence dysfunction.
Divergence excess can be described clinically as exophoria or exotropia at far greater than the near deviation by at least 10 prism diopters (ΔD). Divergence excess can be further divided into true or simulated DE on the basis of responses to occlusion. In simulated DE, occlusion dramatically affects slow vergence, increasing the angle of deviation slightly at distance and significantly at near. Occlusion does not affect true DE.
The patient with basic exophoria has a deviation of similar magnitude at both distance and near.
The patient with convergence excess has a near deviation at least 3 ΔD more esophoric than the distance deviation. The etiology of the higher esodeviation at near most commonly is indicated by a high accommodative convergence/accommodation (AC/A) ratio.
In a patient with divergence insufficiency, tonic esophoria is high when measured at distance but less at near. Symptomatic patients usually have low fusional divergence amplitudes at distance and low AC/A ratios.
The patient with basic esophoria has high tonic esophoria at distance, a similar degree of esophoria at near, and a normal AC/A ratio.
Convergence Insufficiency is reported to be prevalent in 17.3% of school children in a study conducted by Borsting et al.(10) The CIRS group reported 13% of fifth and sixth graders to have CI.(11) In elementary school children the prevalence of CI was 2.25% as reported by Letourneau and Duci.(12) The prevalence of accommodative insufficiency in school children is reported to be 17.3% by Borsting et al and 9.9% by CIRS group.(8,9)
There is less number of studies concerning accommodation and vergence disorder among school children. Prevalence of other binocular vision disorders is reported only in adults or in optometry clinic. The prevalence from those studies varies from 1.5-15% for Convergence Excess, 0.1-0.7% for Divergence Insufficiency, 0.8% for Divergence Excess, 0.3-3.1% for Basic Exophoria and 0.6- 0.9% for Basic Esophoria.(13, 14)
1.3 Rationale
If only vision is tested, a significant number of children with binocular vision anomalies will go undiagnosed. Good visual acuity alone will not ensure optimal functioning in school children. Performing visual screening tests that only evaluate VA is inadequate and often misleading, in that passing the test leads both parents and teachers into believing that the child's visual system is functioning normally and should not be contributing to any visual difficulties experienced in the classroom. The visual system is considered normal even if the child displays symptoms like blur, loosing place while reading, headache, etc.
Though there are several eye health programs being run, emphasis is not being laid on the need of ruling out eye focusing and eye teaming problems. Certain behavioral problems affecting school performance such as inattention, avoiding reading and studying, and difficulty finishing assignments are observed by parents and teacher. A comprehensive visual examination often reveals that the child has a visual anomaly even in the presence of normal VA. So, this study will help to rule out those sources of problems.
General Objective
• To determine the prevalence of eye focusing and eye teaming disorders among school children between age 6- 16 years.
Specific Objective
• To determine the distribution of convergence insufficiency in school children.
• To determine the prevalence of fusional vergence deficits in school children.
• To determine the prevalence of accommodative lag, insufficiency, infacility and abnormal relative accommodation.
• To find out the correlation in vergence dysfunctions.
• To find out the correlation in accommodative dysfunctions.
• To determine the interrelation between eye teaming and eye focusing disorders.