Objective: (1) We aimed to characterize, in primary orthostatic tremor (OT), the structural and functional defects in the cerebellar motor pathways; (2) We correlated the clinical and electrophysiological data with the imagery data to assess how the structural and/or functional defects contribute to the primary disorder or to compensatory phenomena.

Background: Primary orthostatic tremor (OT), a rare condition distinct from other types of tremor, is defined as a high frequency 13-18 Hz tremor during the standing position. The tremor affects primarily the legs and trunk and is associated with a sensation of unsteadiness. OT might be associated with cerebellar defects, but there is no direct evidence.

Methods: We used voxel-based morphometry and resting-state fMRI at 3 Tesla to compare 17 OT patients with homogeneous symptoms in the lower limbs and 17 age- and gender-matched healthy volunteers. Degree of postural instability as assessed by the scores at the Fullerton Advanced Balance scale and duration of the possible standing position were measured off line. Tremor was quantified from electromyographic recordings done in the standing position.

Results: Relative to the healthy controls, OT patients’ cerebellum exhibited (i) a decrease of grey matter (GM) volume in the cerebellar lobule VI that correlated with disease duration; (ii) an increase of GM volume in the cerebellar vermis associated to an increase in the amplitude of low-frequency fluctuations (ALFF) of the BOLD signal; (iii) an increase of ALFF in the lobule IX that contains the lower limb representation. Both GM volume and ALFF in the cerebellar vermis correlated positively with the duration of the possible standing position. Also, patient’s SMA exhibited an increase of GM volume and ALFF, both correlated positively with the duration of the possible standing position. No structural or functional difference was observed in the primary motor regions and the brainstem locomotor regions.

Conclusions: Localized cerebellar atrophy and hypertrophy of the SMA were found in OT patients in the present study and in patients with essential tremor in a previous study (Gallea et al. 2015). This pattern could represent a “generic” signature of tremor. Hypertrophy of the cerebellar vermis is specific to the OT patients and likely represents a compensatory response, opposing the postural instability caused by the lower limb tremor.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/structural-and-functional-signature-of-primary-orthostatic-tremor/