For twenty five years, we have been studying clinical syndromes, unknown to basic researchers, and known but misunderstood by physicians. They are unknown to basic researchers because they are beyond their scope. They are so misunderstood by physicians that they take contradictory decisions in this matter.

     These syndromes share a complex of symptoms in relationship with the upright position. Patients complain either of instability, dizziness, or of pains radiating from body axis when standing upright, or of circulation disorders when they remain upright.

     Progressive analyses of the clinical and stabilometric tests of these patients enabled us to distinguish between the pathology of dynamic balance and that of upright posture. Today this distinction is very clear and leads us to think that the mechanisms controlling orthostatic posture are a little bit different from those controlling balance.

     These syndromes, that with Da Cunha we describe as «postural deficiency syndromes», have a mysterious origin. They do not correspond to any macroscopic lesion of an anatomically defined system. Moreover, we have the proof that they can be set off by very minor causes, with no relationship to the severity the symptoms they provoke. Basic researchers on our team propose a reply to this disturbing question: Mechanisms of control of orthostatic posture function as a dynamic non-linear system.

The post-concussional syndrome

     The first syndrome we studied was the post-concussional syndrome that is a part of the sequelae of benign closed head injury. An epidemiological survey on eight hundred head injuries attracted our attention to the strong relationship that exists between the severity of the trauma, evaluated by the duration of the initial loss of consciousness, and the severity of the syndrome, evaluated by the number of symptoms presented by the subject. Factor analysis shows that the number of symptoms increases with the duration of the loss of consciousness. (fig.1)

FIG.1 Loss of consciousness and number of symptoms

     Furthermore, the stabilometric recordings of eight hundred patients suffering from post-concussional syndrome show that all these patients, with few exceptions, present an abnormal control of their orthostatic posture (fig.2). Practically all individuals are outside the ninety-five percent limits of normality.

FIG. 2. Area of statokinesigrams among patients suffering from a post-concussional syndrome. The Gaussian curve to the left represents the normal theoretical distribution of the area of the statokinesigram; the bar graph represents the distribution of the same parameter in the population of post-concussional syndromes.

     Now, it is well known that functional vestibular examinations, which test only the status of semicircular canals, are inside normal limits in the course of post-concussional syndrome. Therefore, we obtained proof, that a pathology of the control of orthostatic posture exists with no impairement of canalicular function.

The instability syndrome in elderly

     Our study of the instability syndrome in the elderly confirms this conclusion more strongly. Studying, with Toupet, the effects of aging, from fourty to ninety years, on the control of orthostatic posture, we have been able to constitute two populations of five hundred subjects each, differentiated by a unique criterion: the results of vestibular functional tests. One population was constituted of only those patients whose vestibular function was abnormal, the other one of only those subjects whose vestibular function tests were normal. Between the two populations there is no statistically significant difference for any of the six stabilometric parameters studied (fig.3). The progressive degradation of the control of orthostatic posture with aging is identical, whatever be the state of semicircular canals.

FIG.3. Degradatation of the VFY parameter with aging The solid lines show the means and standard deviations of the parameter among vestibular patients, the dashed lines show those of non-vestibular patients.

The fine postural control system

     This upright posture independence from canalicular information is confirmed by different basic research, such as the studies by Fitzpatrick; they show that accelerations of normal postural sway are below to the threshold of the canals.

     According to these experimental arguments and according to our clinical experience, it is essential for clinicians that control mechanisms of orthostatic posture be individualized as a subsystem of the postural control. In memory of J-B. Baron, we propose giving them the name of «fine postural control system».

The postural clinic

     Thus, any syndrome associated with upright posture that cannot be linked to a precise diagnosis, has to be explored not only by conventional neurootological tests but more generally by a postural examination and specifically by an assessment of the fine postural control system. The infracanalicular pathology exists. It merits being looked for.
When a postural deficiency syndrome is confirmed by clinical examination and stabilometric recordings, manipulation of one or the other of the inputs of the fine postural control system (by a prism, insoles, a thrust, muscle stretching, a bite-plane) modifies clinical signs immediately and, in the long term, stabilometric abnormalities.

     Moreover these manipulations show to what extent the fine postural control system is adaptable and sensitive, dependent on the whole sensory context in which the subject is placed. This extreme sensitivity of the fine postural control system, envisaged before us by André Thomas, that we continually observe, is a reality so strange for clinicians that it pushes us into seeking explanations outside traditional medical concepts. It is possible that this sensibility arises from the fact that the fine postural control system functions as a dynamic non-linear system.