Cortical changes in dental technicians exposed to vibrating tools

Loss of inhibition in ipsilateral somatosensory areas following altered afferent nerve signaling from the hand

Cutaneous stimulation of the hand results in increased neural activity in the contralateral primary somatosensory cortex (S1) in humans, whereas an inhibition of neurons is seen in the ipsilateral S1. The aim of this study was to assess changes in neural activity in the S1 bilaterally, with a focus on the ipsilateral hemisphere, following altered afferent nerve signaling from the hand. Three cohorts, all with altered afferent nerve signaling from the hand, participated in the study. There were: 18 patients with traumatic median nerve injury, 10 patients with vibration induced neuropathy and 11 healthy subjects who had their dominant hand and wrist immobilized for 72 h. In addition, 36 healthy subjects were included as controls. Each subject was examined using functional magnetic resonance imaging at 3 T. All three study cohorts showed enlarged activation in the contralateral S1 during tactile stimulation compared to healthy controls. Moreover, inhibition of the ipsilateral S1 was significantly decreased or completely lost. Thus, somatosensory areas of both hemispheres respond to changed afferent nerve signaling from the hand. The loss of inhibition of neurons in the ipsilateral S1 suggests an important role of the ipsilateral hemisphere in the cerebral adaptation following a change in afferent nerve signaling.

Neuromagnetic correlates of adaptive plasticity across the hand-face border in human primary somatosensory cortex

It is well established that permanent or transient reduction of somatosensory inputs, following hand deafferentation or anesthesia, induces plastic changes across the hand-face border, supposedly responsible for some altered perceptual phenomena such as tactile sensations being referred from the face to the phantom hand. It is also known that transient increase of hand somatosensory inputs, via repetitive somatosensory stimulation (RSS) at a fingertip, induces local somatosensory discriminative improvement accompanied by cortical representational changes in the primary somatosensory cortex (SI). We recently demonstrated that RSS at the tip of the right index finger induces similar training-independent perceptual learning across the hand-face border, improving somatosensory perception at the lips (Muret D, Dinse HR, Macchione S, Urquizar C, Farnè A, Reilly KT.Curr Biol24: R736-R737, 2014). Whether neural plastic changes across the hand-face border accompany such remote and adaptive perceptual plasticity remains unknown. Here we used magnetoencephalography to investigate the electrophysiological correlates underlying RSS-induced behavioral changes across the hand-face border. The results highlight significant changes in dipole location after RSS both for the stimulated finger and for the lips. These findings reveal plastic changes that cross the hand-face border after an increase, instead of a decrease, in somatosensory inputs.

Improving hand sensibility in vibration induced neuropathy: A case-series

OBJECTIVES

We report a long-term series of nine workers suffering from vibration-induced neuropathy, after many years of exposure to hand-held vibrating tools at high or low frequency. They were treated with temporary selective cutaneous anaesthesia (EMLA® cream) of the forearm repeatedly for a period up to one year (in two cases four years). The aim was to improve their capacity to perceive touch and thereby improve hand function and diminish disability. The treatment principle is based on current concepts of brain plasticity, where a deafferentation of a skin area results in improved sensory function in adjacent skin areas.

METHODS

All participants had sensory hand problems in terms of numbness (median touch thresholds > 70 mg) and impaired hand function influencing ADL (mean DASH score 22).After an initial identical self-administered treatment period of 8 weeks (12-15 treatments with increasing intervals) they did one treatment every 2-3 month.

RESULTS

After one year sensibility (touch thresholds and tactile discrimination) as well as hand function (mean DASH score 13) were improved in a majority of the cases. Seven of the participants choose to continue the treatment after the first year and two of them have continued at a regular basis for up to four years. A surprising, secondary finding was diminishing nocturnal numbness of the hand and arm in eight of the nine subjects from "frequently" to "hardly ever or never".

CONCLUSIONS

Our observations open new perspectives for treatment of impaired sensibility and hand function in a group of patients with vibration induced hand problems where we have no treatment to offer today.