Basic principles of rTMS in aphasia treatment after stroke

Abstract

Aphasia, the most disabling functional defect after ischemic stroke, affects more than a third of all stroke victims. It improves during the first 4 weeks in one-third of patients and during the first 6 months in approximately half of them. Early and intensive speech and language therapy (SLT) is the only effective treatment to date but usually is limited in duration and intensity. Therefore, improved and additional treatment strategies are required to improve recovery of language functions Poststroke aphasia results from the lesion of cortical areas involved in the motor production of speech (Broca’s aphasia) or in the semantic aspects of language comprehension (Wernicke’s aphasia). Such lesions induce an important reorganization of speech/language-specific brain networks due to an imbalance between cortical facilitation and inhibition. In fact, functional recovery is associated with changes in the excitability of the damaged neural structures and their connections. Two main mechanisms are involved in poststroke recovery: the recruitment of perilesional regions of the left hemisphere in case of small lesions and the acquisition of language processing ability in homotopic areas of the nondominant right hemisphere when left hemispheric language abilities are severely impaired. The purpose of NIBS application in the neurorehabilitation of aphasic patients is to act on specific networks involved in the pathophysiology of language processing and to promote adaptative cortical reorganization after stroke. The rehabilitation of poststroke aphasia refers to two different strategies: the recruitment of perilesional cortical regions in the dominant (left) hemisphere on one hand and the development of language ability in the nondominant (right) hemisphere on the other hand using either rTMS or tDCS. The compensatory potential of the nondominant hemisphere is probably limited, and the recovery from poststroke aphasia seems to be more effective in patients who recover left hemisphere networks and left IFG function. Therefore, the majority of NIBS trials in poststroke aphasia aimed to reinforce the activity of brain regions in the left hemisphere. This goal can be achieved by using an excitatory NIBS protocol (either high frequency rTMS, intermittent TBS (iTBS) or anodal tDCS) to reactivate the lesioned area or an inhibitory NIBS protocol (either low-frequency rTMS or cathodal tDCS) to reduce activities in the contralesional homologous area. Most conventional rTMS studies employed an inhibitory paradigm (low-frequency stimulation) for the stimulation of the contralesional right IFG (pars triangularis, BA 45) aiming to reduce right hemisphere hyperactivity and transcallosal inhibition exerted on the left Broca’s area. In our controlled proof-of-principle study, 30 patients with subacute poststroke aphasia were randomized to a 10-day protocol of 20 min inhibitory 1 Hz rTMS over the right triangular part of the posterior inferior frontal gyrus (pIFG) or sham stimulation followed by 45 min of speech and language therapy (SLT). Activity in language networks was measured with O-15-water positron emission tomography during verb generation before and after treatment. Language performance was assessed using the Aachen Aphasia Test battery (AAT). The results of this study indicate that inhibitory 1Hz rTMS over the right pIFG in combination with SLT improves recovery from poststroke aphasia and favors recruitment of left hemisphere language networks.