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Sources: Thulborn KR, et al. Plasticity of language-related brain function during recovery from stroke. Stroke 1999;30: 749-754; Heiss W-D, et al. Differential capacity of left and right hemispheric areas for compensation of poststroke aphasia. Ann Neurol 1999;45:430-438.
Functional brain imaging techniques are helping to advance our knowledge of the brain’s language networks and are providing new insights into the surprising degree of plasticity the brain sometimes demonstrates after damage to its eloquent areas.
Thulborn and colleagues used anatomic, metabolic, and functional magnetic resonance imaging (fMRI) techniques to study two patients with aphasia who ultimately recovered a significant degree of language function. One 45-year-old right-handed man suffered a dense expressive aphasia secondary to a left middle cerebral artery infarct. The other, a 34-year-old right-hander, awoke from epilepsy surgery with a severe receptive aphasia despite intraoperative electrocorticographic localization of Wernicke’s area 2 cm from the site of resection. These patients were compared to six healthy right-handed control subjects who underwent fMRI activation during a sentence reading and comprehension task. The recovery of language in the patient with expressive aphasia was paralleled by an increasing degree of activation of the right-sided homolog of Broca’s area, with continued left-sided dominance of Wernicke’s area. The partial resolution of the other patient’s receptive aphasia was associated with right-sided dominance of activation in Wernicke’s area. The apparent redistribution of function to the right hemisphere in these two cases began within days of the injurious events and continued over the subsequent several months. In light of these findings, Thulborn et al assert that functional neuroimaging may be essential in testing the efficacy of stroke interventions, in order to distinguish apparent therapeutic efficacy from spontaneous mechanisms of recovery.
Results similar to the above were obtained by a team of German researchers, Heiss and colleagues, who carried out cerebral blood flow activation studies using positron emission tomography (PET) in 23 right-handed aphasic stroke patients and 11 normal controls. The infarct site was frontal in seven cases, temporal in seven, and subcortical in nine patients. Subjects were asked to perform a word repetition task during PET scans carried out two and eight weeks after the stroke in the aphasic group. Aphasics who could not comply with the task demands were excluded. Analysis of task-related blood flow changes was performed, focusing on 14 regions in the left and right hemispheres with speech-related functions. Aphasics with frontal and subcortical strokes improved substantially, while patients with temporal infarcts improved less. As in the case of the expressive aphasic studied by Thulborn et al, recovery in the frontal and subcortical groups associated with language-related activation of the right inferior frontal gyrus and left superior temporal gyrus. Patients with temporal lobe strokes improved only in their word comprehension ability and showed activation of the right superior temporal gyrus and left-sided Broca’s area. Heiss et al conclude that right hemisphere language structures play an important role in recovering language after stroke, and preserving and/or reintegrating Wernicke’s area is essential to recover language after left temporal lobe infarction.
These studies by Thulborn et al and Heiss et al used quite different functional brain imaging methods, but yielded remarkably similar results concerning the role of the right hemisphere in recovery from aphasia. The serial nature of the observations and the temporal association between recovery of language and recruitment-specific subregions of the right hemisphere provide some of the most convincing evidence to date that reorganization of functional language networks begins within days of a stroke and continues for months following the acute brain injury.
The inference by Heiss et al that the integrity of Wernicke’s area is the critical determinant of the degree of recovery of language after stroke is not proven by their data. The inclusion criteria used by Heiss et al preclude drawing such conclusions, since they excluded patients who were initially unable to perform the word repetition task satisfactorily, thereby creating a selection bias relating to severity of the aphasia. It would be more prudent to conclude that location of stroke, not simply infarct volume, influences the extent of recovery of language function after stroke.
The assertion by Thulborn et al that functional brain imaging is essential to testing putative stroke interventions is debatable. Although brain imaging is likely to play an important role in assessing the efficacy of future neuroprotective and thrombolytic therapies, good study design should permit medication effects to be distinguished from spontaneous recovery without necessarily resorting to the expense and technical difficulty of obtaining functional brain images. There are many other potential applications for the paradigms used in these studies, however, particularly in the context of neurosurgical planning for epilepsy and brain tumor surgery.
a. is always greater in the case of frontal rather than temporal infarcts.
b. requires that both Wernicke’s and Broca’s areas be preserved.
c. has been linked to recruitment of right hemisphere language areas.
d. has been shown to depend on nonlanguage-related areas of the left hemisphere.
a. Not all aphasics developed increased functional activity in the undamaged right hemisphere.
b. Patients suffering from expressive aphasia improved more than those with receptive aphasia.
c. Increased functional activity in the right temporal lobe was consistently associated with improved sensory aphasia.
d. PET scanning is essential for judging rehabilitation efforts following acute stroke.