[Estimating squint angle according to the degree of refixation saccades. Model trial fixation saccades]

Objective, linear strabismometric methods are used preferentially to assess the eye alignment in patients with bad eyesight or with a deeply amblyopic eye, and also in uncooperative subjects, for instance children. The common point of these methods is that a distance must be measured or estimated, from which the eye rotation, or alignment, is computed through a conversion factor, the angular ratio. Such a ratio was introduced by Hirschberg in the 19th century. The purpose of the study was to examine the influence of the size of angular ratios and examiners influence upon the accuracy of estimation of eye alignment from the limbus position.

As an experimental test, 9 trained examiners estimated the amount of refixation movements between 0.5 degree and 26 degrees (1 cm/m and 48.8 cm/m). These refixation movements were simulated by a test person who executed fixation saccades in front of a tangent scale designed for a distance of 2.5 m.

The magnitude of a refixation movement was estimated best by looking at the limbus shift, with a ratio of about 5 degrees/mm (8.7 cm/m/mm), and an accuracy of +/- 2 degrees (3.5 cm/m) (95% confidence interval) was reached for refixation movements up to +/- 5 degrees (8.7 cm/m). The threshold for detection of microtropia reached about 0.5 degree (1 cm/m). Accuracy was limited by two factors: the difference between the individual and average angular ratio, and estimating errors.

Experienced examiners estimate the angle of strabismus from the magnitude of refixation movements up to +/- 5 degrees (8.7 cm/m) almost as accurately as with the prism and cover test.