A Reflection on Motor Overflow, Mirror Phenomena, Synkinesia and Entrainment

Abstract

In patients with movement disorders, voluntary movements can sometimes be accompanied by unintentional muscle contractions in other body regions. In this review, we discuss clinical and pathophysiological aspects of several motor phenomena including mirror movements, dystonic overflow, synkinesia, entrainment and mirror dystonia, focusing on their similarities and differences. These phenomena share some common clinical and pathophysiological features, which often leads to confusion in their definition. However, they differ in several aspects, such as the body part showing the undesired movement, the type of this movement (identical or not to the intentional movement), the underlying neurological condition, and the role of primary motor areas, descending pathways and inhibitory circuits involved, suggesting that these are distinct phenomena. We summarize the main features of these fascinating clinical signs aiming to improve the clinical recognition and standardize the terminology in research studies. We also suggest that the term "mirror dystonia" may be not appropriate to describe this peculiar phenomenon which may be closer to dystonic overflow rather than to the classical mirror movements.

Keywords: entrainment; mirror dystonia; mirror movements; overflow; synkinesia.

FIG. 1

(A) Unilateral manual task under normal conditions: the figure shows inhibitory mechanisms including surround inhibition (SI) and transcallosal inhibition. (B) Dystonic overflow/mirror dystonia, characterized by reduced SI, reduced PMd inhibitory control over the ipsilateral M1, abnormal interhemispheric connection and possible involvement of the reticulospinal tract. (C) Mirror movements, mechanism 1: abnormal ipsilateral corticospinal projections. (D) Mirror movements, mechanism 2: abnormal activation of both M1s and transmission of the command along both corticospinal tracts. M1, primary motor cortex; PMd, dorsal premotor cortex; PreM, pre‐motor areas; RF, reticular formation; SMA, supplementary motor area.