Aim: To investigate the relationship between prism
fusion range (PFR) and vergence facility (VF)
measurements in subjects with normal binocular
vision.
Methods: Twenty-eight subjects (mean age 19 ± 1
years) with normal binocular single vision (BSV)
underwent measurement of the PFR and VF in a
varied order, at a test distance of 1/3 m. The PFR
measurements recorded were the base out (BO) range
to blur and break point and base in (BI) range to
break point. The total PFR was calculated. The VF
was assessed over a 1 min time period using a 12(Δ)BO/
3(Δ)BI flip prism and recorded in cycles per minute
(cpm).
Results: No correlation was demonstrable between
any of the single measures of the PFR and the VF
results. The BO PFR to break point and the BI PFR
results obtained (means 46(Δ) BO and 14(Δ) BI) were not
significantly different from quoted ‘normal’ values.
The VF results obtained (mean 12 ± 4.2 cpm) were
found to be significantly different from the reported
mean value.
Conclusion: In a group of young adults with normal
BSV, no correlation between PFR and VF was found.
The two tests may quantify different aspects of
vergence or, alternatively, results of one or both tests
in this study may be unreliable
