Intra-observer reliability of the Harms tangent screen test.

Purpose: The objective of this study is to evaluate the intra-observer reliability of the Harms Tangent Screen Test (HTS) for ocular motility analysis in eyes with diplopia due to concomitant, paralytic, or restrictive strabismus.

Methods: This prospective clinical case study was conducted at the Department of Ophthalmology, Goethe University, Frankfurt am Main, Germany. Squint angles were assessed subjectively in patients aged ≥18 years, presenting with diplopia due to concomitant, paralytic, or restrictive strabismus, using HTS across nine diagnostic gaze positions (0°; 25°). Repeatability was assessed by performing two consecutive measurements per patient by the same examiner. Measurement consistency was evaluated using the intraclass correlation coefficient (ICC) and the 95% limits of agreement (LOA).

Results: A total of 58 patients (46.6% women) with a mean age of 54.0 ± 17.12 years were included. ICCs were highest for the horizontal squint angles of the nine gaze positions (≥0.99), followed by the vertical (0.96-0.99) and the torsional squint angles (0.93-0.98). Mean difference between the two measurements was ≤0.5° for the primary gaze position across all directions. LOA ranged from -3.1°; 2.9° (horizontal), -2.0°; 2.2° (vertical), and -3.6°; 3.1° (torsional). Maximum differences between the two measurements in the primary gaze position were ±8.0° (horizontal), ±5.0° (vertical), and ±6.0° (torsional).

Conclusion: The HTS demonstrates good repeatability across all nine gaze positions in the horizontal, vertical, or torsional squint angles, indicating consistent measurement reliability regardless of gaze position.