gms | German Medical Science

Introduction: BT-A is widely used in the aesthetic field of plastic surgery to treat facial wrinkles. A single treatment acts to paralyze muscles for around 3–5 months. Plastic interactions between face and hand cortical tactile circuits occur after severe injuries that affect the hand such as in amputation or spinal cord injury and are well known. However, whether loss of facial movements alters the cortical circuits involved in processing tactile inputs from the hand remains unknown. In this prospective observational study we used electroencephalography (EEG) to measure cortical activity evoked by tactile stimulation of the hands before and after botulinum toxin-A-induced facial paralysis.

Material and Methods: Twenty volunteers agreed to participate and they received BT-A injections (median age 32 years, range 25–44 years, 15 women, 19 right-handed). Another 20 volunteers were drawn from the general public to form the control group. None of the volunteers had previously received cosmetic BT-A injections or were under prescription drugs. Pretest measurements were conducted 1–3 weeks before the BT-A injections and posttest measurements were conducted 5–6 weeks after the injections. The volunteers in the control group were also measured twice, with a gap of 6-9 weeks between the measurements. Brief suprathreshold tactile pulses were applied on either thumb in a random order. Continuous electroencephalography (EEG) was recorded from 62 Ag-AgCl electrodes mounted to an equidistant elastic cap. The data from the left and right hand stimulations were statistically analyzed separately using the Matlab toolbox LIMO-EEG.11 Pre- and posttest repeated measures mixed-design analyses of variance were conducted using the general linear model framework and were corrected for multiple comparisons using two-dimensional spatio-temporal clustering based on 1000 bootstraps (significance threshold P=0.05).

Results: We analyzed the data across all electrodes and time points. Forehead paralysis significantly reduced the amplitude of the contralateral P200 wave evoked by the left thumb stimulations. Topographic maps of repeated measures statistics (F-statistics) revealed that the alterations were restricted to the electrodes above the contralateral somatosensory cortex. Forehead paralysis also reduced the responses to right thumb stimulation. ERPs to right thumb stimulations were significantly reduced in both hemispheres, although most significant reductions were found at left hemispheric sites above the somatosensory cortex.

Conclusion: We found a reduction in the tactile event-related potentials (ERPs) 6 weeks after the treatment. This suggests that the limited paralysis of facial muscles induced during cosmetic interventions designed to smooth lines and wrinkles on the face is sufficient to alter the cortical processing of tactile inputs from the hand.