Fusional Amplitudes: Developing Testing Standards

The stability of binocular vision depends upon good fusional amplitudes, but the clinical assessment of fusional amplitudes varies around the world. The purpose of this study was to determine whether or not there is variation in the assessment of fusional amplitudes in normal subjects. The author looked at the testing distance, the order of testing, the role of examiner encouragement, and the subject's level of alertness.

In a prospective study using a modified crossover design, the author assessed fusional amplitudes in 99 subjects with normal eye exams. The measurements were done in two separate sessions on different days with each subject being randomized as to the order of fusional vergence testing. All subjects were assessed without and with encouragement in the first session. In the second session, all were assessed at different testing distances.

The author previously presented data on 50 subjects. In this expanded cohort, statistical significance was reached confirming the previous findings that convergence is significantly affected by encouragement, divergence is significantly reduced if assessed after convergence, and near amplitudes are significantly higher than distance amplitudes. Finally, there is a negative correlation between age and convergence break point.

The results of this study demonstrate that divergence is significantly reduced if assessed after convergence in the subject with normal binocular function. Next, convergence is significantly affected by the use of encouragement. Measurements at near produced significantly higher results for all of the convergence and divergence tests. Finally, there is a significant negative correlation between age and convergence break point. We need to develop a standard of testing fusional amplitudes so there is consistency in the clinical assessment.