Two Toxins, One Disorder: Neurophysiological and Clinical Effects in Caput-Type of Cervical Dystonia at Waning and Peak Response Phases of BoNT-A Therapy

Botulinum toxin type A (BoNT-A) is an established treatment for cervical dystonia (CD), but objective neurophysiological markers across the injection cycle and between preparations are not well defined. We assessed afferent conduction (captured by CSP parameters), efferent conduction (captured by F-wave parameters) and clinical severity (TWSTRS) in 28 patients with caput-type cervical dystonia during waning (>14 weeks post-injection) and peak (4–6 weeks) phases, comparing onabotulinumtoxinA (ONA) and abobotulinumtoxinA (ABO). F-wave measures changed only modestly: F–M latency increased with ONA, while F-wave persistence decreased with ABO. In contrast, CSP measures consistently increased at peak in both groups (CSP end latency and CSP duration; both p ≤ 0.001). Overall, BoNT-A treatment phase is better reflected by CSP-derived inhibitory measures than by F-wave indices. TWSTRS improved at peak for both toxins, with no difference between ONA and ABO in clinical change (ΔTWSTRS p = 0.5514).