The subjective visual vertical
It is defined in relation to the physical vertical which corresponds to the force of gravity. Otolithic, visual and somaesthetic information complement each other to compare the position of the head in relation to an ideal of balance parallel to the force of gravity. The subjective visual vertical proposes, by eliminating all visual reference to this physical vertical, to study the otolith function and more precisely the utricular function (6).
Technique for measuring the VSL
patient sitting head up in the dark
with light bar placed at 1 m
Mono or binocular measurement with correction of a possible visual problem
restricted field of vision (to avoid visual cues of verticality)
the patient must indicate when he thinks the bar is vertical (or horizontal)
average over 10 measurements
If the average deviation is less than 2.8°, the test is considered normal.
Beyond that it becomes pathological and indicates either a peripheral otolith disorder, a central impairment or a visual disorder (astigmatism, oculomotor paralysis).
In acute vestibular disorders this test is positive in the first 8 days in 33 to 42% of cases. However, it does not confirm whether the condition is peripheral or central (8)