The term synkinesis may be used in different ways. It may refer to involuntary movements which accompany or are associated with certain voluntary movements (mitbewegungen, motor overflow). These may be physiological, for example the swinging of the arms when walking. Alternatively, such associated phenomena may be pathological, e.g., the involuntary contraction of orbicularis oculi when opening the mouth (the Marin-Amat syndrome: inverse Marcus Gunn phenomenon), acquired after lower motor neurone facial (VII) nerve palsies and presumed to reflect aberrant reinnervation. Aberrant nerve regeneration is common to a number of synkinetic phenomena, such as elevation of a ptotic eyelid on swallowing (Ewart phenomenon) and upper eyelid elevation or retraction on attempted downgaze (pseudovon Graefe’s sign). Crocodile tears, lacrimation when salivating, due to reinnervation following a lower motor neurone facial nerve palsy, may also fall under this rubric, although there is no movement per se (autonomic synkinesis).
Abnormal synkinesis may be useful in assessing whether weakness is organic or functional (cf. Hoover’s sign).
Synkinesis may also refer to the aggravation of limb rigidity detected when performing movements in the opposite limb (e.g., clenching and relaxing the fist), also known as activated rigidity or Froment’s sign.