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VOR gain is most helpful for diagnosing ototoxicity and other bilateral vestibulopathies
The Dynamic illegible 'E' test or DIE is a very helpful procedure (Longridge, 1987). One has a patient view a eye chart similar to that shown below with the head still. This measures visual acuity with the head still. Then one moves the patient's head horizontally at about 2 hz, +- 30 deg excursion and again obtains visual acuity. This measures visual acuity with the head in motion.
Persons with substantial function of their vestibular system have no loss in visual acuity with this procedure. Even persons with unilateral vestibular loss -- i.e. having lost 50% of their vestibular system -- will generally score normally. Rigorous psychophysical techniques can detect and lateralize unilateral loss (Tian et al, 2001), but practially there are easier ways to detect unilateral loss (such as caloric testing).
Patients with bilateral vestibular loss, especially acutely, often lose 6-8 lines of visual acuity. Vertical movement is not as sensitive or specific as the horizontal test (Schubert et al, 2002).
As patients recover from bilateral loss, they perform better on this test. This is probably related to a combination of adaptive changes in the vestibuloocular reflex as well as predictive pre-programming of eyemovements (Herdman et al, 2001).
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Figure 1: Eye chart made for use at 10 feet, suitable for use in the Dynamic Visual Acuity test. |
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