By Dieter Heiss
Aphasia, the most disabling functional defect after ischaemic stroke, affects more than a third of all stroke victims and recovery in most cases is slow and incomplete. Early and intensive speech and language therapy (SLT) is the only effective treatment to date but is usually limited in duration and intensity. Therefore, additional treatment strategies are required to improve recovery of language functions.
The purpose of non-invasive brain stimulation (NIBS) in the neurorehabilitation of aphasic patients is to act on disturbed networks involved in language processing and to promote adaptative cortical reorganisation after stroke.
The rehabilitation of post-stroke aphasia is based on two different strategies: the recruitment of perilesional cortical regions in the dominant (left) hemisphere on the one hand and the development of language ability in the nondominant (right) hemisphere on the other hand using either repetitive transcranial magnetic stimulation (rTMS) or transcranial direct current stimulation (tDCS). The compensatory potential of the non-dominant hemisphere is probably limited and the recovery from post-stroke aphasia seems to be more effective in patients who reactivate left hemisphere networks. Therefore, the majority of NIBS trials in post-stroke 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 or anodal tDCS) to reactivate areas in the lesioned hemisphere 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 inferior frontal gyrus (pars triangularis, BA 45) aiming to reduce right hemisphere hyperactivity and transcallosal inhibition exerted on the left Broca’s area. However, most studies are concerned with isolated clinical cases without any control condition and controlled studies including sham stimulation in the subacute stage after stroke are scarce.
In a proof-of-principle study, 30 patients with subacute post-stroke aphasia were randomised to a 10-day protocol of 20 minutes inhibitory 1Hz rTMS over the right triangular part of the posterior inferior frontal gyrus (pIFG) or sham stimulation over the vertex followed by 45 minutes 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 primary outcome measure, global AAT score change, was significantly higher in the rTMS group (t-test, p=0.003). Increases were largest for subtest naming (p=0.002) and tended to be higher for comprehension, token-test and writing (p<0.1). Patients in the rTMS group activated proportionally more voxels in the left-hemisphere after treatment than before (difference in activation volume index, AVI) compared to sham treated patients (t-test, p=0.002). There was a moderate but significant linear relationship between AVI change and global AAT score change (r2=0.25, p=0.015).
The results of this study indicate that inhibitory 1Hz rTMS over the right pIFG in combination with SLT improves recovery from post-stroke aphasia and favours recruitment of left hemisphere language networks. The proposed protocol sets the stage for larger multicentre trials to further confirm the effectiveness of NIBS and to specifically address the influence of lesion location, stimulation site, activation pattern and possibly timing of NIBS therapies. Finally, studies directly comparing different NIBS modalities are required to determine the most effective and economical treatment strategy under clinical conditions.
Dieter Heiss is professor of Neurology, Max Planck Institute for Neurological Research, Cologne, Germany