The effect of testing distance on the bielschowsky head tilt test.

Aims: The Bielschowsky Head Tilt Test (BHTT) is a commonly used test in Orthoptic practice and is used mainly to differentiate between a long standing Superior Oblique and Superior Rectus palsy. No consistent test distance is recommended in the literature and therefore the aim of this study was to determine whether test distance had an effect upon the measurements obtained.

Method: Thirteen participants were recruited. Participants had either longstanding or recent onset unilateral Superior Oblique palsy. Prism Cover Test measurements of the vertical angle of deviation were taken whilst the participant tilted their head to either side whilst fixating on a target at 33 cm, 3 m and 6 m.

Results: The test distance does have a significant effect on the change in vertical angle measured with head tilted to either side (x2+7.747,DoF 2, p=0.021). When the median values are considered it appears that the significant effect occurs due to a smaller change in angle between head tilt to the affected side when fixing at 3 metres. This was confirmed using the Wilcoxon signed rank test (33cmv3m p=0.039, 3m v 6m p=0.013 and 33cm v 6m p=0.67).

Conclusions: The testing distance at which the BHTT is performed appears to have an effect upon the measurements obtained. The clinical importance of the difference in the change of angle with head tilt to either side is debatable due to the fact that the difference in the median measurement value between the three test distances is a maximum of 3 prism dioptres.