Binocular vision case studies

Further investigations

The additional tests should include:

? Cycloplegic refraction - to eliminate any uncorrected hypermetropia.

? AC/A ratio - the amount of accommodative convergence induced by every dioptre of accommodation.

Diagnosis

Convergence excess

This patient appears to have a 'convergence excess' esophoria, in which the magnitude of eso-deviation is greater at near compared to distance fixation. It is not an uncommon condition and in a symptomatic non-strabismic population, convergence excess was found to be the cause in 4.5 per cent of 265 patients.1 The common causes include the following.

Increased accommodation

This can occur if there is uncorrected hypermetropia, resulting in excessive accommodation to maintain clear vision, especially at near. As discussed in previous articles in this series, there is a close relationship between convergence and accommodation. For each dioptre of accommodation, there is induced convergence as described by the accommodative convergence/accommodation (AC/A) ratio. Therefore, increased accommodation will induce excessive convergence for near which can result in an esophoria or esotropia. The excessive accommodation can also be a result of spasms of accommodation, hysterical reaction, incipient presbyopia or prolonged work at very near working distances.2