The Hirschberg test: correlation with corneal radius and axial length.

The results of this study indicate that the axial length is not strongly associated with the Hirschberg formula, and therefore the axial length does not have to be considered when measuring an ocular deviation with the Hirschberg test. This study does demonstrate an association between the radius of curvature of the cornea and the Hirschberg formula. For patients with keratometer readings below 46 D, the use of the standard Hirschberg formula, 1 mm = 22 delta, is appropriate. For patients with keratometer readings greater than 46 D, and with deviations greater than about 30 delta, the use of the Hirschberg formula, 1 mm = 27 delta, is more appropriate. When performing the Hirschberg test on infants up to about 2 months of age, the use of the higher Hirschberg formula would probably be more appropriate because this age group often has higher keratometer readings.