Objective: To investigate whether postural/action tremor in motorneuron disease (MND) has a central origin. 

Background: MND refers to the whole spectrum of progressive degenerative diseases which differentially affect the upper and lower motor neurons. Clinical and postmortem observations demonstrate that there is considerable variability in the phenotypic expression of MND, indicating that it should be considered a multisystem neurodegenerative disease where the extrapyramidal, cerebellar, somatosensory and autonomic systems can also be affected (1). Postural/action tremor, which is often determined by cerebellar dysfunction, is present in MND patients (nearly 5% in our experience), but its pathophysiology is currently unknown. 

Methods: We recruited 6 MND patients with postural/action tremor without other extrapyramidal signs. In all of them, tremor was recorded bilaterally from flexor and extensor carpi radialis using surface EMG and a triaxial accelerometer placed on the dorsal surface of the most affected hand. Tremor was recorded without weight and with a variable weight load based on patients’ muscle power (50-500 g). Power spectra of rectified EMG and accelerometric signal was calculated. Eye blink classic conditioning (EBCC) was also studied in 4 out of the 6 patients and compared with results obtained in a group of age-matched healthy subjects.

Results: Among the 6 patients, the analysis of accelerometric signal demonstrated that the frequency varied between 4.9Hz and 10.1Hz in the “no weight” condition, and between 4.6Hz and 10.1Hz in the “weight” condition. Furthermore, the EMG frequency spectrum exhibited that the frequency ranged from 4.3Hz to 9.5Hz and from 4.0Hz to 10.4Hz, respectively in the “no weight” and “weight” condition. Within each patient there was no effect of mass loading on peak frequency (all p values > 0.05) in both the accelerometer and EMG analysis (the mean absolute value of the difference in frequency were respectively: 0.2±0.3Hz and 0.6±0.6Hz). With regard to the EBCC, the number of conditioned responses in MND patients was significantly lower than healthy subjects in blocks 4-5-6.

Conclusions: Our results suggest that postural/action tremor in MND has a central origin, possibly resulting from a cerebellar dysfunction. 

References:

1. Swinnen B, Robberecht W. The phenotypic variability of amyotrophic lateral sclerosis. Nat Rev Neurol. 2014 Nov;10(11):661-70.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/tremor-in-motorneuron-disease-central-or-peripheral-origin/