Millimetric cover test--a linear strabismometric technique

In ten normal subjects we measured the angular rotations of the globe, both in adduction and in abduction, corresponding to limbus shifts from 1 to 7 mm. Our study demonstrated that to each millimeter of temporal limbus shift corresponds an average adduction of about 4.6 degrees and to each millimeter of nasal limbus shift an average abduction of about 4.8 degrees. These experimental results give a sound foundation to the clinical evaluation of the strabismic angle by measuring, with a millimeter ruler, the limbus shift of the nonfixating eye by assuming fixation. This technique appears particularly useful as a substitute for the simultaneous prism and cover test, in the evaluation of a squint with a variable angle, and in measurements in other positions of gaze.