Botulinum Toxin-A dose dependent perceptual loss on the hand after its cosmetic use on the face

Assessing Changes in Hand Tactile Sensitivity After Glabellar Botulinum Toxin Treatment

PURPOSE

We aimed to assess behavioral changes in tactile sensitivity in patients receiving cosmetic glabellar botulinum toxin-A injections.

METHODS

In this prospective cohort study, we conducted quantitative sensory testing on 20 patients receiving 15 to 35 units of glabellar botulinum toxin-A treatment between October 1, 2022 and March 8, 2023. We used modified Von Frey filaments to exert forces between 0.25 mN and 512 mN to the dorsal hand just prior to botulinum toxin-A injections. Filament tips were uniform, rounded, and 0.5 mm in diameter to prevent nociceptor activation. This process was repeated 4 to 6 weeks after injection to assess for any change in minimal mechanical detection thresholds.

RESULTS

Minimal mechanical detection thresholds decreased (patients detected smaller amounts of force) overall, in patients with prior botulinum toxin-A treatment, and in patients without prior botulinum toxin-A treatment: 5.34 mN to 4.33 mN (p = 0.22), 6.43 mN to 5.97 mN (p = 0.31), and 4.44 mN to 3.00 mN (p = 0.53), respectively.

CONCLUSIONS

Our results suggest that the plastic changes observed in previous studies do not necessarily result in clinically significant manifestations when utilizing small to moderate amounts of botulinum toxin-A for aesthetic correction of glabellar lines, thus highlighting the safety of botulinum toxin-A for this indication. Further research is required to gain a comprehensive understanding of whether hand-associated cortical activity is altered after aesthetic amounts of botulinum toxin are injected.

Transfer of tactile perceptual learning to untrained neighboring fingers reflects natural use relationships

Tactile learning transfers from trained to untrained fingers in a pattern that reflects overlap between the representations of fingers in the somatosensory system (e.g., neurons with multifinger receptive fields). While physical proximity on the body is known to determine the topography of somatosensory representations, tactile coactivation is also an established organizing principle of somatosensory topography. In this study we investigated whether tactile coactivation, induced by habitual inter-finger cooperative use (use pattern), shapes inter-finger overlap. To this end, we used psychophysics to compare the transfer of tactile learning from the middle finger to its adjacent fingers. This allowed us to compare transfer to two fingers that are both physically and cortically adjacent to the middle finger but have differing use patterns. Specifically, the middle finger is used more frequently with the ring than with the index finger. We predicted this should lead to greater representational overlap between the former than the latter pair. Furthermore, this difference in overlap should be reflected in differential learning transfer from the middle to index vs. ring fingers. Subsequently, we predicted temporary learning-related changes in the middle finger's representation (e.g., cortical magnification) would cause transient interference in perceptual thresholds of the ring, but not the index, finger. Supporting this, longitudinal analysis revealed a divergence where learning transfer was fast to the index finger but relatively delayed to the ring finger. Our results support the theory that tactile coactivation patterns between digits affect their topographic relationships. Our findings emphasize how action shapes perception and somatosensory organization.

Bilateral Symmetry of Distortions of Tactile Size Perception

The perceived distance between touches on the limbs is generally bigger for distances oriented across the width of the limb than for distances oriented along the length of the limb. The present study aimed to investigate the coherence of such distortions of tactile size perception across different skin surfaces. We investigated distortions of tactile size perception on the dorsal and palmar surfaces of both the left and right hands as well as the forehead. Participants judged which of two tactile distances felt larger. One distance was aligned with the proximodistal axis (along the body), the other with the mediolateral axis (across the body). Clear distortions were found on all five skin surfaces, with stimuli oriented across the width of the body being perceived as farther apart than those oriented along the length of the body. Consistent with previous results, distortions were smaller on the palmar than on the dorsal hand surface. Distortion on the forehead was intermediate between the dorsal and palmar surfaces. There were clear correlations between distortion on the left and right hands, for both the dorsal and palmar skin surfaces. In contrast, within each hand, there was no significant correlation between the two skin surfaces. Distortion on the forehead was not significantly correlated with that on any of the other skin surfaces. These results provide evidence for bilaterally symmetric representations underlying tactile size perception.