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Proportional (fractional) displacement of the Hirschberg corneal light reflection (test): a new easily memorized aid for strabometry; photogrammetric standardization (calibration).

PURPOSE
To improve the usefulness of the Hirschberg Corneal Light Reflection Test (CLRT), using a photogrammetric technique independent of the size of the pupil, and to devise an easily remembered sequence of numbers corresponding to the fractional displacement of the reflection in various approximate angles of esotropia.


METHOD
Photographs (slides) were taken of the right eye of 25 young adults using a centered strobe light source to produce the corneal light reflection in 5 prism diopters (PD) increments of adduction from 0 to 90 PD to standardize the Hirschberg CLRT for esodeviations. On a large, dual-projection screen, the eye of the subject looking straight ahead (primary position) was projected on one side and the various slides of the same eye of that subject looking into various degrees of adduction were projected on the other side of the screen. Measurements of the distance between the lateral limbus and the corneal reflection were made with a centimeter rule on the screen. The displacement (shift) of the corneal light reflection was expressed as a fraction of the distance between the lateral limbus and the position of the reflection (in relation to the lateral margins of the pupil) with the eye fixing in the primary position.


RESULTS
The values found were 1/4 = 30 +/- 3 PD; 1/3 = 41 +/- 4 PD; 1/2 = 59 +/- 4 PD; 2/3 = 75 +/- 6 PD; and 3/4 = 80 +/- 6 PD.


CONCLUSIONS
An easily memorized similar series of values is 30, 40 60, 75 and 80 PD corresponding to 1/4, 1/3, 1/2, 2/3 and 3/4 PD. This is useful to know which prism to pick up to begin the prism alternate cover test efficiently in a small child.