Original ArticlesRegionalized sensorimotor plasticity after hemispherectomy fMRI evaluation
Introduction
The ability to visualize the brain’s capacity in vivo to reorganize and reallocate its physiologic functions after insult or injury will contribute to the fundamental knowledge of developmental brain plasticity and the generation of novel and efficient treatments for a large number of neurologic diseases, such as Sturge-Weber syndrome (SWS).
Hemispherectomies provide seizure control in almost 85% of children [1], [2]. The clinical recovery experienced by some patients supports the incredible resiliency of the child’s central nervous system [3]. Patients seem to transfer some functions from the resected areas into new cortical regions [4]. Remaining tissues seem capable of taking over specialized functions [5], [6].
Neuroimaging techniques, such as functional magnetic resonance imaging (fMRI), enable the clinician to view the structural and physiologic processes involved in the plasticity and development of a child’s brain in vivo [7], [8]. fMRI exploits the blood–oxygen-dependent level to calculate signal-intensity changes and produce individual cortical-activation maps [9], [10]. fMRI has been able to detect changes in cerebral blood flow in response to somatosensory stimuli, such as tactile stimulation [11]. Somatotopic mapping of the primary motor cortex in humans has been demonstrated with this imaging method [12], [13].
Developmental studies of motor recovery with fMRI, however, are very few [14]. Positron emission tomography studies [7], [15], [16] conducted in patients with hemispherectomies have provided insight into the relocalization of motor and speech function. Pawlik et al. [15], in their study of cortical-motor activation generated by the hemiparetic arm, demonstrated the recruitment of cortical-association areas during motor and speech activation in patients with hemispherectomies but provided little insight into the role of the normal arm activation and the participation of the somatosensory cortex. Chugani and Jacobs [7] demonstrated partial-to-complete recovery of glucose metabolic activity in the ipsilateral caudate nucleus after hemispherectomy in three children (age range = 1 year to 2 years, 6 months), suggesting the resiliency of the remaining basal ganglia regions. Recently, Muller et al. [16] suggested that earlier lesions in the primary motor cortex may influence the interhemispheric reorganization of the brain of patients with focal lesions. In the present study the authors investigated the potential of fMRI as a noninvasive neuroimaging technique to visualize the reorganization of both motor and sensory functions in the cortical sensorimotor areas of two patients who had undergone a hemispherectomy. The authors hypothesized that the residual function in the affected upper extremity would be reallocated to the remaining tissues of the contrasurgical hemisphere.
Section snippets
Patients
Patient 1 is a 20-year-old woman diagnosed with SWS at birth, who underwent left hemispherectomy at 9 years of age. Postoperatively, she has been seizure-free and medication-free since 12 years of age.
Preoperative computed tomography scans revealed gross atrophy of the left hemisphere with marked calcification throughout the cortex and atrophy of the left frontal region with marked thickening of the skull. Because single photon-emission computed tomography revealed no signs of SWS in the right
Results
Figure 1, Figure 2 present the statistically significant (P < 0.0001) physiologic activation sites, as measured by fMRI, for both the hemiparetic and nonhemiparetic upper extremities during the motor-activation and sensory-activation experiments. Axial slices at the same level for all four images were used to compare the cortical-activation maps. Slices superior and inferior to the chosen slice were not representative of central slices activation in the nonhemiparetic paradigm and are not
Discussion
The study of the human recovery capacity from a major cerebral insult has fascinated neuroscientists. The neural mechanisms involved in the stunning recoveries and development of functional compensatory behaviors recounted in anecdotal stories have been the object of intensive speculation [19].
The authors’ work focuses on the ability of fMRI to reveal how sensorimotor reallocation occurs after damage to healthy tissues early in life. The authors’ ability to detect physiologically significant
Conclusions
The brain is capable of significant clinical recovery and plastic changes after extensive surgery or disease. The remaining sensorimotor functions of the hemiparetic side can be transferred from the tissues resected to the opposite hemisphere. The associative sensorimotor areas can express transferred functions independently of the primary motor and somatosensory cortical regions. These transferred functions can be expressed by different combinations of associative sensorimotor areas for each
Acknowledgements
The authors thank Dr. MacIntyre Burnham, Department of Pharmacology, University of Toronto, Dr. O. Carter Snead III, Department of Neurology, Hospital for Sick Children, Professor Molly Verrier, Department of Physical Therapy, University of Toronto, and Dr. William MacKay, Department of Physiology, University of Toronto for their critical comments. This work was supported by the Bloorview Epilepsy Program from the Bloorview Children’s Hospital Foundation, Toronto, Ontario. This article was
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