Positive and negative stroke signs revisited: dissociations between synergies, weakness, and impaired reaching dexterity

Stroke affects more than ten million individuals in the world each year, with most victims experiencing motor deficits usually referred to collectively as hemiparesis. While hemiparesis is one of the most common and clinically recognizable abnormalities, it remains under-characterized in terms of its behavioral subcomponents and how they combine to produce the overall post-stroke motor deficit. Hemiparesis is comprised of both negative and positive motor signs. Negative signs consist of weakness and loss of motor control (dexterity), whereas positive signs consist of spasticity, abnormal resting posture, and intrusive movement synergies whereby multiple muscles or joints become co-activated during voluntary movement. How positive and negative signs interact, and whether a common mechanism generates them, remains poorly understood. Here we employed a planar reaching task to assess post-stroke arm dexterity loss, which we compared to the Fugl-Meyer stroke scale; a measure primarily reflecting abnormal synergies. We examined 53 patients with hemiparesis after a first-time ischemic stroke. Reaching kinematics were markedly more impaired in patients with subacute (<3 months) compared to chronic (>6 months) stroke even when matched for Fugl-Meyer score. While our reaching dexterity task mitigated the effect of weakness on synergies through the use of arm-weight support, we found that the Fugl-Meyer scale was a poor predictor of arm dexterity. This suggests a dissociation between abnormal synergies (reflected in the Fugl-Meyer scale) and loss of dexterity, which in turns suggests that different mechanisms may generate these particular positive and negative signs. At the same time, dynamometry suggested that Fugl-Meyer scores may capture weakness as well as abnormal synergies, in line with these two deficits sharing a neural substrate. These findings have two important implications: First, clinical studies that test for efficacy of rehabilitation interventions should specify which component of hemiparesis they are targeting and how they propose to measure it. Second, there may be an opportunity to design specific rehabilitation interventions to address specific subcomponents of the hemiparesis syndrome.