An Optimal Measurement of Fixation Disparity Using Ogle's Apparatus

Fixation disparity (FD) is a small misalignment of the eyes within the normal alignment when viewing under binocular condition. Ogle's apparatus measures FD. Standards of procedures vary, which may lead to different outcomes.

Students with normal ocular alignment, stereopsis ≤60 seconds of arc and visual acuity <0.1 logMAR, were included in this prospective comparative study. Four procedures (P1-P4) of measuring FD with Ogle's apparatus were performed with divergent placement of the line (P1 and P3), or the line moving from subjective zero (P1 and P2: prisms of ascending strength; P3 and P4: prisms alternating base in base out; combined and P4). Differences in the FD curve were determined by looking at point zero, motor fusion amplitude, and the degree of FD.

Twenty-six participants were examined by these 4 procedures. Point zero showed a significant difference between P1-P2 (P=0.006) and P3-P4 (P=0.001). P1 and P3 indicated the highest point zero: median of -1 and -1.5 minutes of arc exodisparity. Motor fusion amplitude showed a significant difference between P1-P2 (P=0.037), P1-P3 (P=0.004), and P2-P4 (P=0.002). P1 revealed the highest motor fusion amplitude (median of 34Δ) and P4 the lowest amplitude (median of 28Δ). No significant differences were found in esodisparity. In exodisparity there was a significant difference comparing P1-P2 (P=0.000), P3-P4 (P=0.000), and P1-P3 (P=0.021). P1 gave the highest exodisparity (median 22 minutes of arc) and P4 the lowest (median 10 minutes of arc).

Clinically relevant differences were found in exodisparity, mainly caused by difference in line shifting. Exodisparity was significantly lower, moving the line from subjective zero. The most accurate procedure is using prisms of ascending strength combined with divergent placement of the line (P1). These findings standardize a reliable procedure of measuring the FD curve for clinical use. Patients will not be misdiagnosed with reduced FD.