Theme 8 Clinical imaging and electrophysiology

Background: We have commonly observed involuntary jerks and tremor in patients with motor neuron disease (MND), even though these features are not considered typical for the disease.Objectives: We conducted prospective clinical and electrophysiological study to explore the prevalence, phenomenology and pathophysiology of involuntary movements in MND.Methods: Seventy-four consecutive patients were clinically examined and video-recorded. Based on regularity and distribution, movements observed at rest position were classified as minipolymyoclonus (MPMC) or rest thumb tremor (RTT) and movements present during action as action MPMC or action tremor. In 11 patients with tremor, accelerometry was recorded at (a) rest position, (b) with arms outstretched (postural condition) and (c) at postural condition with 500 g mass attached to the hand.Results: Involuntary movements were present in 54 patients (73%). Rest MPMC was present in 26 patients (35%), RTT in 22 patients (31%), action MPMC in 22 patients (30%) and action tremor in 20 patients (27%), with some overlap. Sixteen patients (22%) reported negative impact of involuntary movements on their ability to use hands. Regression model showed that lower distal muscle power and less prominent upper motor neuron involvement significantly increased the odds of MND patient having involuntary movements. Sex, age and disease duration did not significantly predict the occurrence of involuntary movements. At rest, tremor frequency ranged from 5.2 to 8.2 Hz, at postural position from 4.9 Hz to 7.6 Hz and during postural position with mass attached from 3.6 Hz to 7.6 Hz. On the group level, tremor peak frequency statistically significantly decreased from 6.1 Hz to 5 Hz without versus with loading.Discussion and conclusions: Involuntary movements are very common yet largely overlooked feature of MND that may also have negative impact on patient's functional abilities. Lower distal muscle power increases and the presence of upper motor neuron signs decreases the probability of involuntary movements. Together with finding of decrease in tremor frequency with mass loading, these results suggest that generation of involuntary movements is of peripheral origin.