Comparison of near horizontal heterophoria tests in free space and with phoropter

Background: Measurements of phoria at near are important elements of the optometric assessment of binocular vision. It is important to know if different methods of measuring a phoria are interchangeable. This study was designed to compare phoria measures using in-phoropter (von Graefe) and in free space methods (Howell card). Methods: The study was conducted with 70 pre-presbyopic subjects, aged 23–35 years. Both near the von Graefe and near Howell card horizontal dissociated phorias along and with gradient +1.00 D and –1.00 D phorias were measured for each technique. Results: The mean values of the near dissociated phoria, for both test conditions (von Graefe and Howell card) showed a slight deviation in the two measures with von Graefe measures being generally more exophoric. The mean value of the phoropter phoria was 1.97 ∆ base-in and in free space, the value was 0.9 ∆ base-in. The phoropter phoria with the +1.00 D gradient was 5.24 ∆ base-in, and in free space, 2.73 ∆ base-in. Measurements of the phorometric phoria with a gradient of –1.00 D gave a mean of 0.42 ∆ base-out and 0.94 ∆ base-out in free space. The near phoria measures were significantly different for the basic near measure (p = 0.01) and for the +1.00 D gradient (p = 0.000002) but not for the –1.00 D measure (p = 0.36). Conclusion: The results of the differences in the near horizontal phoria in free space and in- phoropter were significantly different for the basic measure and for the +1.00 D gradient. The findings between the two methods of testing should therefore not be treated as equivalent or convertible. Each test method should have separate means and ranges established.