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The total Klockgether score was only very weakly related to vestibular-neck interaction deficits, while the trunk-specific ataxia symptoms “gait and stance ataxia” were not. As a consequence, vestibular- proprioceptive interaction may possibly be independent from the entities of “gait ataxia” and “stance ataxia”, as tested in clinical routine by having patients stand and walk under visual observation. So the question is, whether this “vestibular-neck ataxia” might be a fully independent sub-type of trunk ataxia.

Studies defining “multi-joint asynergia” Mori et al 2004, Thach et al 2004, Ilg et al 2007

primarily referred this term to deficits in joint movements. The cerebellar deficit approached by our study, in comparison, refers to mal- adaptive re-alignment of sensory reference frames.

Thus, we propose the following theory:

patient. Once the head is turned away from the centre, however, mal-adaptive coordinate re-alignment can come into play with increased dystasia: the “basic” gait and stance ataxia results in stance full of unplanned small excursions. Additionally, natural small body excursions due to breathing and short- loop spinal reflexes constantly disturb balance.

During a head turn away from the neutral position, unplanned movements of the body cannot be sufficiently compensated for by postural reflex arcs, as would be normally. When the compensation to these atactic unplanned movements is wrongly directed, due to the supposed coordinate mal- alignment, additional unplanned movements are added to the basic stance ataxia, multiplying its effect. Analogous mechanisms would also apply for gait ataxia.

This theory would form a probable explanation to the clinically observable increase in gait and stance ataxia during lateral head excursions, as seen in our patients with vestibular-neck-interaction deficiency.

We suggest there may be “vestibular-neck ataxia” next to – and possibly also independent from – the two types of classic joint asynergia called “gait ataxia” and “stance ataxia”. The possible independence from the other ataxia entities might relate to the missing statistical correlation between the clinical vestibular neck testing and the clinical assessment of gait and stance ataxia, as well as the weak correlation with the overall Klockgether score.

Figure 33 illustrates the proposed concept of gait-, stance- and vestibular-neck ataxia resulting from cerebellar disease: vestibular-neck ataxia can appear in severe cerebellar deficits, additional to gait- and stance ataxia. It was not observed without considerable other deficits in palaeo-cerebellar function in this study, possibly because vestibular-neck interaction is a more robust function, or its circuits are more redundantly (and possibly also bilaterally) imprinted. Its primary manifestation is mal- compensatory postural control during lateral head excursions.

Figure 33

Figure 33:

This figure illustrates the proposed concept of different anterior lobe syndrome entities. Vestibular-proprioceptive interaction mechanisms may exist independently from those manifesting with gait- and stance ataxia upon lesion. Deficits in inter- sensory interaction however can aggravate symptoms of the two forms of multi-joint coordination “gait ataxia” and “stance ataxia”. The illustration of cerebellar circuitry is based on figures 8 (p. 21) and 10 (p. 29).

Figure created with Microsoft Office©

Whether the Purkinje cells of the anterior lobe vermis Manzoni et al 1998, 1999 and 2004 _{or the vestibular-only-cells of} the rostral fastigial nucleus Kleine et al 2004

are responsible for vestibular-neck interaction individually or as a whole functional unit, could not be distinguished by the mode of this study, due to the predominance of general cerebellar atrophy.

Anatomical and physiological data suggest that the circuitry including the cerebellar cortex, its nuclei, the inferior olive and the brainstem vestibular nuclei form such a functionally tightly-knit unit, that damage to one part would inadvertably influence or take out the mechanism as a whole. Computational Neuroscience with its neural modelling capabilities, incorporating data from clinical and neurophysiological trials, could provide striking answers. This topic will be subject to future research.

We conclude that in humans vestibular-neck interaction is probably a paramount feature of the cerebellar functional complex Mori et al 2004

. In analogy to animal single cell recordings, structures like the anterior lobe vermis and the rostral fastigial nuclei in man may be decisively involved.

response in patients with severe bilateral cerebellar disease.

However, it could not yet account for findings in the lateralized cerebellar syndrome. In patients with left-predominant ataxia, a change from linear vestibular-neck interaction to non-linear interaction was found in the more affected left side, but not in the less affected right side. This specific non-linear asymmetry has not been reported before.

The dissociation in non-linear vestibular-neck interaction towards the more affected side and the constant linear interaction towards the other might speculatively indicate two cerebellar circuits, computing vestibular-neck interaction for each side. However, why lateralization manifests in an exponential relation between head-on-trunk position and sway direction, instead of a unilaterally attenuated linear relation across all head positions of one side, remains entirely unknown.

Further research in lateralized cerebellar syndromes in coordination with Computational Neuroscience approaches might possibly construct mathematical models of unilaterally defect cerebellar circuitry, possibly identifying the computational source and functional nature of this novel finding.