The amplitude of postural sway in the 0.2 Hz frequency band has been studied, thanks to the parameter of normalised amplitude, in two groups of postural patients whose vestibular functional tests were normal or not.

     In those two groups, that normalised amplitude is abnormal.

     Therefore the vestibular functional tests cannot detect all the functioning anomalies of postural control.

     As stabilometry can record the sway when its accelerations are inferior to the threshold of perception of the semi-circular canals, it is possible to affirm that stabilometry, and for the time being stabilometry only, can measure the effects of an «infracanalicular» postural pathology. However, it is not possible, on the basis of that study, to state positively the existence of that pathology because there remains a doubt on the capacity of the vestibular functional tests to evaluate the state of the vertical canals.

     The possibility of such a pathology is under discussion.

     It is admitted that stabilometry is a valid method of exploration of the control of the orthostatic posture (A.A.N, 1992; Cambier, 1993), but nobody ever explained how exactly it could help clinical diagnosis. And indeed, it is not used in neurology. It even proved, very early, totally useless to otoneurologists, who had not waited stabilometry to make their diagnostic criteria (Aubry et al., 1968). That last observation is surprising: how can the control of the orthostatic posture explored by stabilometry be so completely foreign to the balance disorders studied by otoneurologists?

     A possible explanation for that apparent contradiction consists in assuming the existence of a pathology of postural control in a field that escapes the control of the semi-circular canals. An «infracanalicular» pathology would exist; to use the excellent expression proposed by the Académie de Médecine.

     That hypothesis is based on the sensitivity of the static posturography instruments that can record the body sway when its accelerations are inferior to the threshold of perception of the semi-circular canals. That great sensitivity of posturography had already been noted with Miles' ataxiameter (Birren, 1945), and is now to be found in stabilometers (Gagey & Toupet, 1988).

     To test that hypothesis, we have measured the stabilometric recordings of postural patients whose semi-circular canals were or were not functioning normally. It appears that the patients whose vestibular function is normal behave on a stabilometry platform just as the patients whose vestibular function is abnormal. A postural pathology would then exist, that the conventional otoneurologic examinations do not reveal because they only test the state of the semi-circular canals.

     The existence of that particular postural pathology, described by stabilometry, is coherent with the notion of a subsystem of postural control which works without canalicular information, the «Fine Postural Control System» (Gagey, 1994).

The groups of patients

     From ten thousand files of «postural» patients who had all undergone a stabilometric recording and a vestibular functional exploration, it was possible to draw two groups of about a hundred patients in each of the following five decades: 40/49, 50/59, 60/69, 70/79, 80 years old and over (table 1).

     In each decade, one group is composed only of the patients whose vestibular function was abnormal in the functional tests, the other group being composed only of the patients whose vestibular function was normal.

     The expression «postural patients» refers to all the patients who complain of symptoms related to a dysfunction of the orthostatic posture control.

     The patients will be called «NonVestibular» or «Vestibular» depending on whether their functional tests were normal or not.

TABLE 1 - Number of patients in each age group

     The groups of patients whose vestibular function was abnormal have been equally composed of vestibular neuritis, of pathologies of Ménière, of benign positional paroxystic vertigo - in critical phase and after treatment -, of central syndromes.

     The normality criteria of the vestibular function were based on a search for the nystagmus, spontaneous and provoked by positioning movements, caloric tests, a rotation test, and a pendular pursuit test.

     We have excluded from the forming of those groups all the patients whose recordings showed either a statokinesigram area superior to 2,000 mm2 in an open eyes situation, or a sinusoidal intercorrelation.

Stabilometric recordings

     The recordings have all been performed on a stabilometry platform normalised by the Association Française de Posturologie (Bizzo et al., 1985), computerised, validated by the works of that same Association (AFP, 1985), commercially available in southern Europe.
     The visual environment is strictly normalised: target at 90 cm in front of the subject, lit at 2,000 lux, lateral walls at 50 cm. The position of the feet on the platform is normalised: feet at 30°, heels spaced out 2 cm apart, barycenter of the support basis always set on the same spot, whatever the subject's shoe size. The recordings have been realised in an open eyes situation, if necessary correcting the subject's eyesight according to his habit, then in a closed eyes situation in darkness.
Sampling at 5 Hz of the position of the centre of pressure. Recording time 51.2 s. The analogic signal coming from each of the three pressure gauges was screened by an antiwithdrawal filter, running band 0/2 Hz, structure of the fourth order.

Analysis of the stabilometric signal

     After normalisation relatively to its mean value and application of the window of Hamming, the signal was submitted to the FFT algorithm of Cooley-Turkey, which gave the value of the spectrum of amplitude for each of the 125 bands of elementary frequency of 0.02 Hz between 0 and 2.5 Hz.

     From that spectrum of amplitude, we have calculated the «ANØ2» parameter, value of the normalised amplitude in the 0.2 Hz frequency band (Gagey & Toupet, 1998).

Statistical analysis

     Statistical analysis compares the distribution of parameter ANØ2 in the normal population and the groups of patients by Student's t-test.

     The results are presented for the right-left sway (XAD) and the forward-backward sway (YAD), in open eyes (OE) and closed eyes (CE) situations, as a comparison of the patients and the normal subjects and between themselves.

Right-left sway

Open eyes situation

     In the Vestibular, in XAD, OE, the mean of parameter ANØ2 is of 23.39 ± 15.32 (fig. 1).

Closed eyes situation

     In the Vestibular, in XAD, CE, the mean of parameter ANØ2 is of 22.05 ± 14.31 (fig. 2).

Forward-backward sway

Open eyes situation

     In the Vestibular, in YAD, OE, the mean of parameter ANØ2 is of 17.50 ± 11.97 (fig. 3).

Closed eyes situation

     In the Vestibular, in YAD, CE, the mean of parameter ANØ2 is of 16.98 ± 10.77 (fig. 4).

Right-left sway

Open eyes situation

     In the NonVestibular, in XAD, OE, the mean of parameter ANØ2 is of 19.26 ± 12.55 (fig. 5).

Closed eyes situation

     In the NonVestibular, in XAD, CE, the mean of parameter ANØ2 is of 21.69 ± 13.14 (fig. 6).

Forward-backward sway

Open eyes situation

     In the NonVestibular, in YAD, OE, the mean of parameter ANØ2 is of 15.66 ± 10.45 (fig. 7).

Closed eyes situation

     In the NonVestibular, in YAD, CE, the mean of parameter ANØ2 is of 18.16 ± 11.41 (fig. 8).

     The results of the comparison of means between the groups of Vestibular and NonVestibular patients are presented table 2.

TABLE 2 - Student's t-test of the comparisons of means between the distributions of parameter ANØ2 in the Vestibular and the NonVestibular patients

     The effect of age on parameter ANØ2 has been studied separately in the Vestibular and the NonVestibular patients.

The effect of age in the Vestibular patients

     The results of the comparisons of means of the distributions of parameter ANØ2 between the group of Vestibular patients aged 40/49 and the groups of Vestibular patients of the other age brackets are presented table 3.

TABLE 3 - Student's t of the comparisons of means of the distributions of parameter ANØ2 in the groups of Vestibular patients aged 40/49 and the other age brackets.

The effect of age in the NonVestibular patients

The results of the comparisons of means of the distributions of parameter ANØ2 between the group of NonVestibular patients aged 40/49 and the groups of NonVestibular patients of the other age brackets are presented table 4.

TABLE 4 - Student's t of the comparisons of means of the distributions of parameter ANØ2 in the groups of NonVestibular patients aged 40/49 and the other age brackets.

The hypothesis of an «infracanalicular» pathology

    From that analysis, it is possible to assert that the studied population of NonVestibular «postural» patients shows, as the Vestibular patients, an abnormal distribution of parameter ANØ2 - even if the distribution of that parameter, in an open eyes situation, is more abnormal in the Vestibular than in the NonVestibular (table 2). The very strict dichotomy established by the vestibular functional tests between the Vestibular and the NonVestibular disappears when those patients are submitted to a stabilometric recording. That last examination reveals the anomalies of the control of the orthostatic posture that escape the classical vestibular functional tests used during that study.

      It is admitted that the classical vestibular tests only evaluate the state of the semi-circular canals via the vestibulo-ocular loop. Therefore those tests only allow saying that the patients' canalicular function was normal on the day of their stabilometric recording, yet one must remain cautious on the state of the vertical canals. Because of such caution, the results of that study do not allow to assert positively that there exists an «infracanalicular pathology», but they make the hypothesis more probable.

      Indeed, that hypothesis has already been implicitly sustained, as early as 1945, by Birren, who clearly dissociated the control of the static balance and the canalicular function. And ten years earlier, Rademaker (1935) had noticed that the patients affected by a bilateral destruction of the eighth pair, did not have any problem standing still whereas they were totally unable to walk on a rail.

      At present, the works of the International Society for Postural and Gait Research broadly underline the distinction between the control of the static and the dynamic balance (Igarashi & Black, 1985; Amblard et al., 1989; Brandt et al., 1990; Woollacott & Horak, 1992; Taguchi et al., 1994). And Fitzpatrick & McCloskey have published some physiology works confirming that the holding of the orthostatic posture does not use canalicular information (1994).

The fine postural control system

      Therefore there is, very likely, a subsystem of postural control that does not use the canalicular function. Its existence was suspected in the 1960's by Baron and it was named the «fine postural control System» (Gagey, 1994).

      As that system controls the holding of the orthostatic posture, it is logical to check whether or not the functional disorders of the standing position are related to a dysfunction of that system (Gagey & Weber, 1995). And the only method we know at the time being to measure the general functioning of that system, is stabilometry.

      As a great number of disorders of the standing position have never been related to a lesion in the seat of the central nervous system, we may wonder what those dysfunctions of the fine postural control system supposed to explain them would correspond to. That question is not new, Babinski and Froment (1918) already raised it at the beginning of the century when they examined those «nervous» pathologies that did not correspond to lesions in the seat of the central nervous system or to psychiatric disorders. Concerning the control of the orthostatic posture, the notion of fine postural control system brings a possible answer to that question. The chaotic analyses of the stabilometric signal showed, indeed, that the fine postural control system works as a nonlinear dynamic system (Martinerie & Gagey, 1992; Myklebust et al., 1995; Thomasson, 1995, personal communication) and the extreme sensitivity of those systems is now perfectly known: «The flap of a butterfly's wings in Brazil set off a tornado in Texas», to quote but Lorenz (1993). The «mysterious» dysfunctions of the fine postural control system at the origin of some disorders of the standing position would be but an expression of the strange way those metastable systems work.

The patients

      The patients' selection has not been carried out according to the usual nosologic categories as, precisely, that analysis tests the hypothesis of a new nosologic category. In order not to introduce a statistical distortion, the groups of Vestibular patients have been made of an equal number of patients representing the various pathologies that modify the vestibular functional tests. As for the groups of NonVestibular patients, they have been made of patients who showed disorders of the control of the orthostatic posture, whatever their conventional diagnosis. Those patients' «postural» complaints were therefore mostly related to instability and/or vertigo sensations, but it is not excluded that the body axis pains showed by some of them could also have modified their ANØ2 parameter (Gagey, 1986; Guillemot & Duplan, 1995).

      According to some otoneurologists, a single normal vestibular functional check-up would be insufficient to assert that those patients' canalicular function would always be satisfying. It could be possible, but what is sure is that the day those patients have been recorded on the stabilometer, that day their semi-circular canals showed a normal functioning.

      The patients whose statokinesigram area, in open eyes or closed eyes situations, was superior to 2,000 mm2 have been excluded from that study because the theoretical calculations showed that such areas correspond to a postural sway with accelerations likely to be perceived by the semi-circular canals (Gagey & Toupet, 1988). Indeed, the margins of error of that 2,000 mm2 limit are unknown, but given our present knowledge it was indispensable to pay attention, even if imperfectly, to that intervention of the semi-circular canals in postural control.

      The patients with a sinusoidal intercorrelation of the stabilometric signal have been excluded from that study because such a line proves that the right-left and the antero-posterior sway share the same frequency - therefore they lose all the criteria that would ensure their normal independence (Kapteyn, 1973). So it is possible that they are under the control of a single, probably superior, centre - that they are «overcontrolled» (Ferrey, 1995).

The effect of age

      Five of the 32 combinations of the means comparisons between the various age groups, in tables 3 and 4, show statistically significant differences.

      In the Vestibular we observe a significant reduction of parameter ANØ2 in the eldest (in YAD, CE, in the seventh decade its mean is of 14.74 ± 9.28 and in the eighth decade it is of 15.10 ± 8.65 instead of 18.25 ± 11.99 in the fourth decade).

      In the NonVestibular, on the contrary, we observe an increase of parameter ANØ2 in the eldest (in XAD, CE, in the sixth decade its mean is of 24.75 ± 13.63, instead of 21.03 ± 12.45 in the fourth decade; in XAD, OE, in the seventh decade its mean is of 20.03 ± 12.92 and in the eighth decade it is of 23.22 ± 14.08, instead of 16.58 ± 10 in the fourth decade).

      The evolution of parameter ANØ2 with age does not have the same characteristics as the evolution of the other stabilometric parameters (Toupet et al., 1992).

      Those various significant differences between age groups does not prevent us from joining them for the statistical analysis, as it is well known that ageing implies difficulties of the control of the orthostatic posture (Woollacott et al., 1986; Horak et al., 1989; Toupet et al., 1992).

The choice of parameter ANØ2

      There are many parameters in stabilometry: Area and Length of the statokinesigram, mean position in X and in Y, standard deviation of speed, etc. Most of those parameters are modified during disorders of the standing position (Gagey & Weber, 1995). But we can never be completely sure that such parameters have not been modified more or less consciously by certain subjects. There are indeed some criteria, during a clinical examination, that allow to suspect - not to prove, however - malingering. But during studies bearing on a great number of patients, those mediocre criteria cannot be used. However, it seems hard to suspect the least malingering aimed at modifying the percentage of the amplitude of postural sway in the 0.2 Hz band, even more so by subjects who are completely ignorant of the techniques of analysis of the stabilometric signal.

The role of the ventilation rhythm

      During the stabilometric recordings of that study, the ventilation rhythm has never been recorded at the same time as the movements of the centre of pressure. Therefore it is not possible to study the role of the ventilation rhythm in the appearance of the reported anomalies of parameter ANØ2. But it is also impossible to avoid wondering about that subject. What relation could there be between a lesion of the semi-circular canals and abnormal amplitude of the postural sway synchronous with breathing? That question is new enough to deserve our special attention.

      Let us first remind that many physiology works have studied the relation there is between ventilation rhythm and postural sway and nobody ever proved that this relation, being actual in the normal man, disappears in the patient. On the contrary, Gurfinkel showed that postural sway in the 0.2 Hz frequency band became much broader in some patients, so as to be directly visible on the stabilograms without Fourier analysis (Gurfinkel & Elner, 1973). To explain that increase of 0.2 Hz sway in certain patients, Gurfinkel suggests that in the normal man there is a synergy between ventilation and the movements of the coxo-femoral joint that reduces the impact of the ventilation movements on postural sway. That synergy disappears during certain pathologies (Gurfinkel et al., 1971). Without denying the possible existence of such synergy, Hunter and Kearny (1981), confirmed by Bouisset and Duchêne (1994), contest that it is realised at the level of the coxo-femoral joint. Tardy (1997) situates it at the level of the thorax, it could be modified by an asymmetry of the activity of the breathing and postural muscles of that region. That last proposition could account for the link between the lesions of the semi-circular canals and an increase of postural sway related to ventilation - indeed, the postural tonic asymmetries of the vestibulars are well known.

     That study on the normalised amplitude of postural sway in the 0.2 Hz frequency band shows that the vestibular functional tests limited only to the semi-circular canals are unable to detect certain anomalies of the control of the orthostatic posture revealed by stabilometry. As that last technique can record the sway when its accelerations are inferior to the threshold of perception of the canals, it is possible to sustain the hypothesis that stabilometry explores an «infracanalicular» pathology, without pretending, however, that the present study proves it.

     Within such context, various fundamental works have been evoked, that give coherence and credibility to the notion of a subsystem of postural control proper to the orthostatic posture, the «fine postural control system» towards which those clinical and fundamental approaches converge.

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