Summary: People with functional dizziness do not appear to process sensory-motor impression correctly. Instead, they rely on a stored memory model which no longer matches immediate reality.
Many patients with functional dizziness look back on a long odyssey to numerous doctors because no organic causes could be found. Now for the first time, an experiment at the Technical University of Munich (TUM) has identified possible causes of the disorder: problems with the processing of sensory-motor signals in the brain that resemble those associated with dizziness due to organic causes.
The Munich researchers had already postulated several years ago that functional disorders may be caused by faulty processing of sensory stimuli. The team, headed by Prof. Nadine Lehnen, senior physician for psychosomatic medicine at the TUM University Hospital Rechts der Isar, was able to bolster this hypothesis with the results of an experimental pilot study.
Eight patients with functional dizziness and eleven healthy subjects who served as a comparison group participated in the study. The researchers also used data from dizziness patients with organic defects who had previously taken part in the same experiment. Those patients had either a cerebellar disorder or a complete loss of functioning vestibular (equilibrium) nerves.
Dizziness patients show marked deficits
During the experiment, the participants sat in a dark room in which points of light were flashed in rapid succession on the wall left and right of gaze straight ahead. They were asked to look in the direction of the light points. Their eye and head movements during the gaze shifts were recorded. They were then fitted with a weighted helmet to alter the inertia of their head. This resulted in significant head wobbling. The experiment was performed with and without the helmet.
Whereas the healthy subjects quickly adapted their movements to the new circumstances and managed to stop their head from wobbling, all the subjects with functional dizziness found the task difficult to perform. What surprised the research team was the fact that the latter behaved in exactly the same way as subjects with dizziness due to massive organic defects.
“Our results clearly show that functional dizziness is manifested exactly like severe physical disorders, for example after complete functional loss of the vestibular nerves. This reflects how severely impaired these people are,” Nadine Lehnen says.
Experiment provides possible explanation of functional dizziness
Based on previous experience, which is stored in the brain in the form of learned models, people have a certain expectation about the sensory impressions evoked by a movement. This expectation is compared with information from the vestibular organs. If the head behaves differently than normal, the two sets of information no longer match. This creates an imbalance between expectation and reality, a state known as prediction error.
“Healthy people can easily perceive this error, process it and adapt their movements accordingly. Patients with functional dizziness, by contrast, do not appear to process sensory-motor impressions correctly. They rely primarily on their stored model, but it no longer matches the new reality,” Nadine Lehnen explains, and adds: “We were excited to observe that they are still able to learn – albeit only to a limited degree.” It would therefore be important to treat such patients using therapeutic approaches that take into account this processing deficit. A large-scale study is planned to corroborate the recent findings.
Deficient head motor control in functional dizziness: Experimental evidence of central sensory-motor dysfunction in persistent physical symptoms
Understanding the mechanisms of symptoms that are insufficiently explained by organic dysfunction remains challenging. Recently, it has been proposed that such “functional symptoms” are based on erroneous sensory processing in the central nervous system (CNS), with internal expectations dominating sensory inputs.
In a pilot study, we used a head motor control set-up to assess the interplay between sensory input and expectation on the example of patients with functional dizziness. Eight patients and 11 age-matched healthy controls performed large active eye-head gaze shifts towards visual targets in the natural situation and with the head moment of inertia 3.3-fold increased. The latter induces head oscillations and the expected sensory outcome of the movement, estimated in the CNS, does not match the actual sensory input. Head oscillations were assessed in patients and in healthy subjects and compared to prior results from patients with organic disease (vestibular loss and cerebellar ataxia). Head oscillations in patients with functional dizziness were different from those of healthy subjects (F(1,17) = 27.26, P < 0.001, partial η2 = 0.62), and similar to those of patients with cerebellar ataxia, and with vestibular loss (F(2,19) = 0.56, P = 0.58). Even in the natural, unweighted, condition, head oscillations were higher in functional dizziness patients than in healthy subjects (P = 0.001). Since an extensive work-up failed to demonstrate any explanatory peripheral vestibular, motor, or cerebellar organic dysfunction, these motor control deficits are a first indication of erroneous interplay between expectations and sensory input in the CNS that could account for persistent physical symptoms.