Could non-invasive brain stimulation help treat dysarthria? A single-case study

Dysarthria is a motor speech disorder that can occur following brain damage. It is characterized by impairments in producing the movements needed to articulate words but does not affect other language processing domains such as writing and comprehension.

Several rehabilitation approaches have been developed to improve dysarthria; they involve intensive exercise or alternative/augmentative communication devices but have shown no strong beneficial effects . Recent studies in neuro-modulation have effectively used non-invasive brain stimulation to treat post-stroke aphasia . Anodal transcranial direct current stimulation (tDCS) of the left inferior frontal gyrus (IFG) has been found to mitigate language articulation impairments in aphasia . Moreover, recent findings from healthy individuals showed that cathodal tDCS over the right cerebellum improved performance in a sequenced articulation task , which suggests promising perspectives for treating motor speech disorders.

To the best of our knowledge, tDCS has never been used in the context of dysarthria. However, the aforementioned studies suggest that modulating brain areas involved in speech articulation and motor control, such as the IFG and cerebellum, could contribute to dysarthria recovery. Moreover, tDCS has been found to induce long-lasting alterations of cortical excitability, by mimicking long-term potentiation and depression and by modulating gamma-aminobutyric acid and glutamate concentrations . Because both the IFG and the cerebellum are part of a complex network implicated in speech articulation , we hypothesized that inducing long-lasting alterations of cortical excitability in these regions could be effective in dysarthria recovery.

Here, we describe a single-case study of language rehabilitation in a young man with severe chronic dysarthria after severe traumatic brain injury (TBI), despite management by classical speech therapy twice a week. We assessed whether tDCS coupled with active training could help attenuate the speech disorder in this patient. Indeed, previous studies of TBI suggested that tDCS combined with active behavioral training can facilitate cortical reorganization and consolidation of learning in specific neural networks , as in stroke patients . Moreover, previous literature on post-stroke aphasia demonstrated an improvement with tDCS in oral production for some patients , which provides a possible rationale for applying tDCS in dysarthria.

To this purpose, we compared 2 tDCS protocols based on the literature, in which the right cerebellum or left IFG was stimulated by using cathodal or anodal polarities, respectively, coupled with speech therapy in one patient. Possible changes in brain integrity after tDCS were assessed by using diffusion tensor imaging (DTI), before and after the rehabilitation procedure.

At age 19, our right-handed, 21-year-old, native French-speaking patient experienced TBI. He spent 1 month in a coma, with an initial score of 5 on the Glasgow Coma Scale. The first cerebral MRI, performed at 2 weeks after the TBI, showed diffuse axonal injury in both hemispheres, predominantly in the right temporal lobe, corpus callosum, bilateral basal ganglia, bilateral internal capsule, and both peduncles. At 3 years after TBI, the patient remained severely impaired, with right hemiparesis, cerebellar syndrome, and pseudo-bulbar palsy and persistent severe chronic dysarthria (see Supplementary Table for the neuropsychological evaluation ). Naming, repetition, and reading were impaired, as was automatic language, with no further impairments in written and oral language and no signs of aphasic agraphia or oral facial apraxia. The lesional substrate of the articulatory impairments was attributed to a bi-genicular lesion ( Fig. 1 ).

The rehabilitation procedure lasted 8 weeks and is described in Fig. 2 . It was approved by the local ethics committee, and the study followed the ethical standards of the 1964 Declaration of Helsinki.

Language evaluation included tests for facial-apraxia, automatic language, repetition, naming, and reading. The tasks were selected from the Protocole d’examen linguistique de l’aphasie Montréal-Toulouse , a language assessment battery for evaluating language impairments in French-speaking adults, with the exception of the repetition task, which was chosen from the Batterie d’évaluation clinique de la dysarthrie . The score for automatic language, repetition, naming, and reading was calculated as a combination of words (1 point if the word was produced with no articulatory errors) and voice (1 point if the word was produced aloud/not whispered). The maximum possible score was 30 for automatic language, 66 for repetition, 31 for naming, and 33 for reading.

MRI with DTI sequences was performed before and after the rehabilitation procedure by using a Philips 3 T MRI Ingenia system (Philips Medical Systems, Best, The Netherlands; parameters: b-value 800 s/mm ² ; 32 directions; voxel size 2 mm isotropic; TR/TE 8495/100 ms; EPI factor 55). T2 FLAIR and T1 SWI sequences were added for anatomical reference and axonal damage assessment. The post-processing tractography steps were performed with a Philips extended workspace (Portal from Philips, Medical Imaging, Best, The Netherlands) with Mrtrix3 package software (J.-D. Tournier, Brain Research Institute, Melbourne, Australia). Regions of interest were placed to include the corpus callosum at midline and both the precentral gyri and the cerebral peduncles. A track density map was reconstructed on the basis of a full-brain probabilistic tracking of 1 million fibers with an interpolated resolution of 0.3 mm .

tDCS involved use of a battery-driven stimulator (NeuroConn, GmbH, Germany) with a pair of surface-soaked electrodes. Cerebellar stimulation consisted of 20 min of 2 mA direct current with the cathode electrode (5 × 5 cm) centered on the cerebellum (3 cm lateral to the inion on the right cerebellum) and the anode (5 × 5 cm) placed on the right deltoid muscle (Cb-tDCS). IFG stimulation (IFG-tDCS) consisted of 20 min of 1 mA direct current with the anodic electrode (5 × 7 cm) centered over the left IFG (BA 44/45, F5 of the 10–20 system for electroencephalography) and the cathode centered on the contralateral supraorbital region (FP2) .

Under both conditions, the patient received concurrent speech therapy on a daily basis for 2 weeks, excluding weekends (5 days/week). Thus, the patient received 10 cerebellar tDCS sessions and 10 IFG-tDCS sessions. During each rehabilitation session, the patient and speech therapist were seated face to face and the patient was asked to repeat simple and more complex sounds in a progressive manner starting from syllables, to disyllabic words, and finally short sentences. Two stimuli lists, of 12 words organized by sounds, were alternated on a daily basis. Each list contained from 3 to 8 words of 1 to 2 syllables; 18 sentences with up to 8 syllables were also used (e.g., “Il a lu le livre de Louis”). The speech therapy sessions lasted about 30 min each day and were combined with tDCS during the first 20 min. The stimulus lists used remained constant despite the change in stimulation protocols. The speech and language rehabilitation program performed during this study was more intensive (5 sessions per week) than the speech and language therapy the patient underwent before inclusion in this study (2 sessions per week), which on top of repetition, also comprised spontaneous speech, reading, and picture description following the principles of the Promoting Aphasia Communication Effectiveness (PACE) protocol.

After the treatment, the patient showed improved intelligibility, particularly after Cb-tDCS ( Fig. 3 ), for voice but not words. Performance concerning the reading task improved (for voice only) between the 2 pre-tests and after Cb-tDCS but not IFG-tDCS. Repetition for voice and words was improved after Cb-tDCS, with an additional improvement after IFG-tDCS for voice only. We observed no improvement in naming. For all language and articulatory evaluations, no long-term improvement was evidenced.