Measurement of fusional vergence: a systematic review

Background: Fusional amplitudes are important for clinical practice in diagnosing and managing binocular vision anomalies. Several measurement methods can be used to assess fusional amplitudes. However, those methods are not interchangeable, and measurement repeatability has been questioned. Objectives: To compare the normative values of tests for the measurement of fusional vergence and to investigate sources of heterogeneity of diagnostic accuracy including: age, variation in method of assessment, study design and size, type (convergent, divergent, vertical, cyclo) and severity of strabismus (constant/intermittent/latent). Data sources: Bibliographic databases were searched up to March 2018, including Cochrane registers, PubMed, Web of Science^TM, Google Scholar and Science Citation Index. Trial registers and conference proceedings were hand searched. Review methods: The review observed and reported according to the PRISMA guidelines and was registered with PROSPERO. The I² was used to show the percentage of observed total variation across studies that is due to real heterogeneity rather than chance. The results of the different studies and the overall effect (meta-analysis under the random effects model) are shown. Results: Eighty-one studies were included in the review. Heterogenous information about break vergence amplitudes is reported for the step vergence method (I² > 50%; p < 0.05) in children. Four parameters were reported consistently to affect measurements; age, method of assessment, order of testing and target size. For the smooth vergence technique break vergence values heterogeneity was not present in children and adults (I² = 0%; p > 0.05). Limitations: The results are based on cross-sectional studies that were performed independently of each other, with different examiners, methods of examination and different populations. Conclusions: The source of heterogeneity between studies for vergence break points measured with the step vergence method seems to be linked with age. Normal vergences reported in children had considerable heterogeneity compared with adults. In clinical practice, the population-based vergence ranges measured with the step vergence method in children should not be used as one single criterion. For the smooth vergence technique, normative population data can be used.