Is there an ideal speed for the prism fusion range?

Aim: To evaluate the effect of speed of prism increase on prism fusion range (PFR) and to determine a recommended speed for performing PFR. Methods: Twenty-six participants (18–32 years) with binocular single vision (BSV) and minimum TNO stereo-acuity of 60 seconds of arc underwent PFR assessment at 1 3 m and 6 m. Ocular dominance was assessed. Three rates of prism strength increase were uniformly employed: prism increased every one second (1 s), every two seconds (2 s) and every three seconds (3 s) (in random order). Base in (BI) was assessed before base out (BO). A 10-minute period of binocular viewing was given to participants between each assessment speed. Break point of fusion was recorded. The participant’s preferred assessment speed was recorded post testing. Results: The total PFR was significantly extended by increasing the viewing time through each prism (F 2,50 = 15.977, p < 0.0001). Near PFR was extended significantly more than distance PFR with increased viewing time (F 2,50 = 4.074, p = 0.023). The BO range was significantly more affected by testing speed than the BI range (F 2,50 = 9.900, p = 0.0002). Ocular dominance did not have a significant effect on PFR ( p =0.75). 69% of participants favoured the two second per prism assessment speed. Conclusions: In participants with normal BSV, the PFR can be significantly extended when a longer target viewing time is given through each prism. This highlights the need for a uniform assessment speed. For reasons of participant comfort and clinical time efficiency, increasing the prism strength after 2 seconds fixation per prism is recommended for the clinical assessment of the PFR.