Fusional amplitudes: exploring where fusion falters

The stability of binocular vision depends on good fusional amplitudes, but the assessment of fusional amplitudes varies around the world. The author reviewed the literature on fusional amplitudes and surveyed international orthoptists on their assessment of fusional amplitudes. The purpose of the study was to determine whether or not there is variation in fusional amplitudes in normal subjects when looking at the role of examiner encouragement; the order of testing; the method of testing; and the subject's level of alertness. The preliminary results from this ongoing, prospective study are presented.

Using a modified crossover design study, the author prospectively assessed fusional amplitudes in fifty participants with normal eye exams who met inclusion criteria. The measurements were done in two separate sessions with each participant being randomized as to the order of fusional vergence testing. All participants were assessed without and with encouragement in the first session. In the second session, all were assessed at different testing distances.

Convergence is significantly affected by encouragement and divergence is significantly reduced if assessed after convergence. Numbers were too small to get meaningful data on the effect of fatigue on final outcome measures.

We need to develop consistency in assessing fusional amplitudes and agree upon a standard of testing. Variables such as the order of testing, whether or not encouragement is given, and a person's level of alertness can affect the final outcome. Encouragement should be done especially when assessing convergence fusional amplitudes and divergence should be assessed prior to convergence. Finally, it is important to note a patient's level of alertness during vergence testing, especially if they are feeling foggy on one visit and are alert on the next. By implementing these strategies into your assessment of fusional amplitudes, the examiner will know if a change in amplitudes is due to treatment effect or just testing method.