Analysis of tremor in multiple sclerosis using Hilbert-Huang Transform

Tremor is frequently described in patients with multiple sclerosis (MS) but remains poorly characterized using neurophysiological techniques. Accelerometric (ACC) and electromyographic (EMG) recordings were performed in 26 MS patients complaining of clumsiness, associated (n = 16) or not associated (n = 10) with visible tremor. Seventeen healthy subjects with physiological tremor (PT) and eight patients with essential tremor (ET) served as controls. Signals were analyzed using non-linear Empirical Mode Decomposition (EMD) and related Hilbert-Huang Transform (HHT), compared to the standard linear spectral analysis using Fast Fourier Transform (FFT). The presence of cerebellar signs and motor deficit was assessed on clinical examination. Using FFT, tremor was found in all patients with ET and 12% of subjects with PT, but in none of the MS patients, even in the presence of visible tremor. In contrast, EMD-HHT analysis of ACC-EMG coupling showed common frequency peaks characterizing tremor related to a central generator in 62.5% of MS patients with visible tremor, 40% of MS patients without visible tremor, 29% of subjects with PT, and all patients with ET. In EMD-HHT analysis, tremor characteristics were similar in subjects with PT and MS patients, regardless of the presence of a visible tremor, but these characteristics clearly differed in patients with ET. A visible tremor in MS patients was associated with more frequent cerebellar signs and less motor deficit at the upper limb. The low-frequency tremor observed in MS patients could therefore originate in lesions of the brainstem (midbrain) or cerebellothalamic circuits, or may correspond to an enhanced PT, partly favored by cerebellar dysfunction and being more visible during movement execution in the absence of concomitant motor deficit.