WCN 2001 - Day 3 - Wednesday 20 June
Investigator: Adolfo Bronstein
Wednesday Jun 20th, 2001
by Laura Spinney
Some patients who recover from infection or traumatic insult to the vestibular system continue to suffer recurring dizziness and vertigo long afterwards. The cause is not known, and so far these patients have tended to be untreatable, with some being redefined as psychiatric cases. In the UK, at least, they constitute between 30 and 40 per cent of those who end up at tertiary or last resort balance-disorder clinics. But new British research suggests that the problem is visual, and may be treatable by psychological means.
Adolfo Bronstein, a consultant neurologist at the department of neuro-otology at Imperial College School of Medicine at Charing Cross Hospital, London, saw so many patients who complained of what he calls "visual vertigo" that he decided to get to the bottom of it. Typically, these patients complain of dizziness that is triggered by an unstable visual environment - crowds, moving traffic, a busy wall or floor pattern. They also have a history of vestibular disease of which no residual trace can be detected.
Bronstein followed a hunch based on some work carried out in the 1940s by an American psychologist named Witkin. Witkin wanted to know how much people relied on vision, as opposed to their other senses, to orientate themselves. He built an ingenious machine in which a person sat in a chair, in a mock-up room, and the experimenter was then able to tilt the chair to change the person's orientation with respect to the room, or the real world, or both. The question was, when the room and the real world were out of synch, and subjects were free to choose their own orientation, would they choose to align themselves with the room that they could see, or with the gravity they could feel?
Witkin identified a group of healthy individuals whom he described as "visually dependent," who consistently aligned themselves with the pictures on the walls and the plane of the room rather than with gravity. According to Bronstein, other "visually independent" healthy people will always opt for gravity as their cue of choice, and most of us fall somewhere between these two extremes. To find out where in the spectrum the visual vertigo patients fell, he designed a variation on Witkin's machine.
In one set of experiments, people were asked to stand on a force plate that detected minute changes in their posture while watching a revolving disk or a rod that turned within a frame, they wore a belt around their head connected to movement sensors. He compared three groups: one of visual vertigo patients, one of patients diagnosed with a vestibular disease, and one of healthy controls.
The idea was that those who depended heavily on their eyes would have more trouble keeping their balance when the visual stimulus was rotated than those who depended more on proprioceptive, positional, cues. And that turned out to be the case. The vestibular disease patients, who were forced to rely on their eyes because their balance was impaired, showed more than twice the amount of movement than the healthy controls. But so did the visual vertigo patients, even though their vestibular systems were intact.
It's not clear, says Bronstein, whether these people were already visually dependent before their vestibular insult, or whether they learned to become so while ill and just retained that dependency. But, he says, the unpublished findings indicate that desensitizing them to visual stimuli could offer a potential cure. "You want people to cope with an unstable visual surrounding which is, after all, rife in an urban environment," he concluded.
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