Comparison of Measured Fusional Vergence Amplitudes using Prism Bar and Synoptophore in Sudanese Patients with Near Exophoria

Saif Hassan Alrasheed1, 2, *, Sulaiman Aldakhil1
1 Department of Optometry, College of Applied Medical Sciences, Qassim University, Buraydah, Saudi Arabia
2 Department of Binocular Vision, Faculty of Optometry and Visual Sciences, Al-Neelain University, Khartoum, Sudan

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© 2022 Alrasheed and Aldakhil

open-access license: This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 International Public License (CC-BY 4.0), a copy of which is available at: This license permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

* Address correspondence to this author at the National AIDS Control Program – Ministry of Public Health and Population, Yemen;



Assessment of the fusional vergence amplitudes constitutes one of the most important diagnostic tools to obtain information about the ability to maintain binocular vision. Several techniques can be used to assess this function. However, those methods are not interchangeable, and the measurement repeatability has been questioned.


This study aimed to compare fusional vergence range measurements using prism bars and synoptophore in Sudanese patients with near exophoria


The study was a comparative cross-sectional hospital-based, performed in the binocular vision clinic at the Al-Neelain Eye Hospital. Fusional vergence amplitudes (positive and negative) were measured on 122 patients (67 females and 55 males), and the mean age and standard deviation were 16.79 ± 5.22 years old using prism bar and synoptophore methods.


The findings showed that the higher positive fusional vergence was obtained using the synoptophore method (24.7 ± 7.2 Δ base-out), whereas the prism bar method provided the lower finding (22.6± 7.6 Δ base-out). Conversely, the prism bar method revealed a higher measurement for negative fusional vergence (13.9± 3.9 Δ base-in) than the synoptophore method (12.7± 3.7 Δ base-in). Using the t-test, significant differences were found between all measurements with the two techniques, P<0.05. Measurements of positive and negative fusional vergence amplitudes by the two methods showed no relationship between age and fusional vergence, P>0.05.


Given the significant difference in the results obtained between the two methods for measuring the positive and negative fusional vergence amplitudes, caution should be taken when making decisions regarding fusional vergence assessment in patients with latent and manifest strabismus.

Keywords: Fusional vergence amplitude, Positive fusional vergence, Negative fusional vergence, Prism bar, Synoptophore, Latent strabismus.