[Hirschberg erred here: the correct angle factor is 12 degrees pro mm corneal reflex decentration. Geometric optical analysis of various methods in strabismometry].
BACKGROUND
The use of prisms and cover is the established clinical method to measure eye alignment, the accuracy of which may reach 0.5 degree (1 cm/m). By contrast, there are contradictory recommendations in the literature regarding the measurement with corneal reflections. The purpose of the study was to present a synopsis of the foundations on which such measurements are based, and to derive parameters for the measurement of eye alignment.


METHODS
Simple eye models for an adult average eye were derived. From these the dependence of the shift of the corneal reflex and of the limbus upon eye rotation was derived and expressed as rotation versus shift ratio WF, in degrees per millimeter or prism diopters per mm.


RESULTS
This study confirmed the findings of a number of studies in that the frequently quoted Hirschberg ratio of 7-8 degrees/mm (13 cm/m/mm) is much too low. The correct ratio is 12 degrees/mm (21 cm/m/mm).


CONCLUSIONS
Without covering, a misalignment is estimated best by judging the difference between the positions of the corneal reflexes in both eyes. Due to the limited accuracy of this test of at best +/- 5 degrees (8.7 cm/m), pseudotropia may not be distinguished from microtropia. The magnitude of a refixation movement may be estimated best by looking at the limbus shift, with a ratio of about 5 degrees/mm (8.7 cm/m/mm).
Barry
J C
JC
Universitäts-Augenklinik, Abt. II, Tübingen. jc.barry@med.uni-tuebingen.de
D000328:Adult; D001767:Blinking; D003290:Convergence, Ocular; D006801:Humans; D008962:Models, Theoretical; D055095:Optics and Photonics; D012016:Reference Values; D013285:Strabismus / Q000175:diagnosis*; D015348:Vision, Binocular