# Neurophysiological Assessment of F-Wave, M-Wave, and Cutaneous Silent Period in Patients with Caput-Pattern Cervical Dystonia at Waning and Peak Response Phases of Botulinum Toxin Therapy

## TL;DR

This study examines how botulinum toxin affects spinal nerve activity in patients with a specific type of neck dystonia.

## Contribution

The study is the first to investigate the neurophysiological effects of botulinum toxin in caput-pattern cervical dystonia.

## Key findings

- BoNT-A significantly prolonged Fmin latency and increased F–M interlatency in caput-pattern CD patients.
- CSP duration and onset latencies increased, suggesting enhanced spinal inhibition and central sensorimotor modulation.
- Findings suggest CSP could serve as a biomarker for BoNT-A effects in dystonia.

## Abstract

While distinguishing between collis and caput patterns in cervical dystonia (CD) has clear clinical and therapeutic relevance, the effects of botulinum toxin type A (BoNT-A) on segmental spinal excitability and inhibitory function in caput-pattern CD have not been previously investigated. This study aimed to advance understanding of the effects of BoNT-A and its broader neurophysiological impact in cervical dystonia, particularly in the caput subtype. The study utilised non-invasive neurophysiological methods to assess F-wave and cutaneous silent period (CSP or CuSP) parameters in 21 CD patients with caput motor patterns at waning and peak response phases of BoNT-A therapy. Significant prolongation of Fmin latency, increased F–M interlatency, reduced F-wave amplitude, and a marked increase in CSP duration and onset latencies were observed following BoNT-A administration, indicating that BoNT-A not only reduces spinal motoneuron excitability and strengthens spinal inhibitory processes, but also highlights its capacity to modulate central sensorimotor pathways beyond local chemodenervation. Together, the observed changes in CSP support its use as a potential biomarker for nervous system effects of BoNT-A in dystonia; however, further validation in controlled studies is warranted.