Performing Hirschberg testing, the left eye is fixating with an angle kappa of 0. You see that the corneal light reflex emitted from the right eye is halfway between the pupil center and the temporal limbus. Which of the following deviations does this patient have?
Ocular Motility
No
U
D
Hirschberg testing is arguably not very accurate for measuring the amplitude of ocular deviation since corneal size, corneal steepness, and angle kappa (i.e. the angle between the visual axis and pupillary axis) must be taken into account.
Procedure:
From less than 10 cm behind a light source (e.g. pen light), look at the patient's eyes.
With the patient 0.5 to 1 meter from the light, have them fixate on the light.
Compare the corneal light reflex of the fixating eye to that of the non-fixating eye.
The magnitude of the deviation can be estimated by where the light reflex falls:
pupillary margin: ~30 prism diopters
mid-iris: ~60 prism diopters
limbus: ~90 prism diopters
Another way to state this is that 1-mm of decentration of the pupillary light reflex corresponds to approximately 22 prism diopters (or 7 degrees).
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