The Uninjured Hemisphere in Hemiplegia: Friend or Foe?
Organized by Jason Carmel, MD, PhD
.jpeg) Lara Boyd, PT, PhD |
The Uninjured Hemisphere in Hemiplegia: Friend for some and Foe for others This talk will discuss recent data from work illustrating the role of the contralesional hemisphere in motor learning and recovery after stroke. Dr. Boyd will provide evidence to support the idea that for some individuals with stroke the contralesional hemisphere interferes with motor learning and recovery while in others it appears to aid motor function. Data illustrating showing individuals fall into these categories will be presented. |
|
Is the contralesional hemisphere a suitable target for noninvasive brain stimulation after stroke? Stroke is the leading cause of adult disability and there are no treatments that can repair neural damage that results from stroke. Functional recovery for many patients is modest and therefore adjuvants to traditional therapies are urgently required. This talk will revisit the use of non-invasive brain stimulation (NIBS) as a potential adjuvant for stroke rehabilitation, which is well documented in clinical neurophysiological research, but not routinely used in clinical practice. One tenet is that functional restoration may be enhanced using techniques that increase excitability in the hemisphere in which the lesion has occurred, through LTP-like mechanisms, and this has led to some promising outcomes. Perhaps more contentious is the idea that the non-stroke (contralesional) hemisphere is also a target for NIBS that aim to suppress neuronal excitability. This idea is fostered by a model of interhemispheric competition that espouses an elevation in neural excitability within the contralesional hemisphere. The majority of neurophysiological studies investigating interhemispheric imbalance have been conducted with patients at the chronic stage after stroke, with only a few exceptions. The heightened contralesional excitability may reflect neurophysiological processes that exacerbate functional deficits through interhemispheric competition (via transcallosal inhibition). However, at the chronic stage these changes might also reflect a pattern of use (i.e., learned non-use of the weak or paretic side) and activity-dependent plastic reorganisation in the non-stroke hemisphere. From recent studies of motor neurophysiology, I will describe how the contralesional hemisphere may be a viable target for excitability suppression at the chronic stage, but that this is not ‘one-size-fits-all’. Finally, I will present neurophysiological and clinical data from a large sample of patients spanning the initial days, weeks and months at the sub-acute stage after stroke. These data indicate interhemispheric imbalance is driven primarily, if not exclusively through diminished excitability in the lesioned hemisphere. |
Winston Byblow, PhD |
 Jason Carmel, MD, PhD |
The Uninjured Hemisphere in Hemiplegia: Friend or Foe? The most common cause of paralysis is injury to one cerebral hemisphere, leading to hemiparesis of the opposite half of the body. The pattern of paralysis is largely attributable to injury of the corticospinal tract, a crossed connection that is the principal pathway for voluntary movement in people. To restore motor control to the impaired half of the body, the primary strategy has been to restore motor control from the injured hemisphere. One reason that the injured hemisphere is limited in its ability to regain motor control is that it receives inhibitory signals from the uninjured hemisphere. These inhabitory connections, which are transmitted via the corpus callosum, allow independence of movement of the two sides of the body in health. After injury, however these circuits can be deranged, causing the uninjured hemisphere to "bully" the injured hemisphere with excessive transcallosal inhibition. To reduce this bullying, many groups have sought to reduce activity of the uninjured hemisphere. An alternative approach is to drive control of both halves of the body from the uninjured hemisphere. This pattern of innervation can allow substantial hand function of the more affected hand, although it may come at a loss of independence of the two hands. Which hemisphere to support is a major question for systems neuroscience and especially for investigators who use focal therapy for brain repair. |
| Contributions of the healthy and lesioned hemisphere to recovery of motor function Previous work demonstrated that depending on the circumstances, the healthy and lesioned hemisphere may contribute to recovery of motor function after chronic stroke. The magnitude of the adaptive/maladaptive role of each hemisphere depends on a variety of factors, one of which appears to be the degree of motor impairment. The presentation will discuss this evidence and the implications for the choice of rehabilitative interventions for motor function after chronic stroke. |
Leonardo Cohen, MD |
.jpg)
