Pairing Vagus Nerve and Tactile Stimulation Improves Somatosensory Function Following Nerve Injury
Post by Elisa Guma
What's the science?
Sensory dysfunction is a common consequence of injury to the nervous system due to nerve damage or stroke. Sensory training using tactile stimulation in affected areas is the most common form of rehabilitation for these patients, but most are left with sensory loss. Promising clinical data has identified a potential novel therapy involving pairing stimulation of the vagus nerve with tactile rehabilitation; this is thought to enhance synaptic plasticity and facilitate recovery of sensory function. This week in Annals of Neurology, Darrow and colleagues rigorously test this hypothesis in a rodent model of nerve damage.
How did they do it?
To create a model of chronic sensory loss in rodents, rats underwent transection followed by repair of the median and ulnar nerves in the forelimb, which produces lasting deficits in somatosensation in spite of reinnervation. This injury results in a denervation of mechanoreceptors on the ventral (but not dorsal) surface of the forepaw. Typically, reinnervation does occur over time, however animals still experience long lasting impairments in somatosensation and have disruptions in nerve morphology. The rats were also implanted with stimulating cuff electrode on the left cervical vagus nerve.
Sixteen weeks after the injury, animals were randomized to receive either 1) a tactile rehabilitation paradigm consisting of the presentation of various mechanical stimuli to the surface of the paw, or 2) a tactile rehabilitation paradigm with 0.5 s bursts of vagus nerve stimulation paired with the presentation of each tactile stimulus, daily for 6 weeks. The tactile stimuli included a paintbrush, a 10g filament, a copper rod, and a puff of air to the affected ventral forepaw. Mechanosensory thresholds were measured at baseline, weekly throughout the therapy, and every two weeks for 8 weeks after the cessation of therapy to see if benefits were long-lasting. Given that sensory and motor function are highly related, additional measures of forelimb sensorimotor function were recorded including the spontaneous use of the injured forelimb during exploration, grip strength, placement of the forelimb in a horizontal ladder rung task, and toe spread analysis.
What did they find?
The authors found that pairing tactile rehabilitation with vagus nerve stimulation improved recovery of somatosensation in the forelimbs of animals with chronic sensory deficits compared to tactile rehabilitation alone. Improvements were already detectable in the first week of therapy and were maintained up to 2 months after the cessation of therapy. Furthermore, the animals that received paired vagus nerve and tactile stimulation therapy also had improved motor function in the injured forelimb, observable in exploratory behaviour, as well as reducing the length of toe spread during normal walking, and decreased missed placements and slips in the horizontal ladder task. The only motor behaviour that the paired therapy did not improve was for grip strength, where no difference between treatment groups were observed.
What's the impact?
This study, motivated by recent clinical findings, provides compelling evidence for the efficacy of pairing tactile stimulation with vagus nerve stimulation for restoring somatosensation and motor function in a rodent model of sensory loss. This paired therapy could be a promising new approach for recovery from neurological injury. Future studies are needed to validate this strategy in other clinical populations, as well as to uncover the precise mechanisms supporting recovery.
Michael J Darrow et al. Restoration of somatosensory function by pairing nerve stimulation with tactile rehabilitation. Annals of Neurology (2019). Access the original scientific publication here.