Neurophysiological assessment of F-, M-waves and cutaneous silent period in patients with caput patterns of cervical dystonia in the waning phase of botulinum toxin therapy

AIM OF THE STUDY: Evaluate and compare F‑wave and cutaneous silent period (CSP) parameters in caput‑pattern idiopathic cervical dystonia (CD) in the waning phase of botulinum toxin therapy vs. healthy controls to identify alterations in segmental excitability and spinal inhibition. CLINICAL RATIONALE FOR THE STUDY: Cervical dystonia likely reflects sensorimotor network dysfunction. Objective neurophysiological markers of spinal and supraspinal inhibition could clarify pathophysiology, improve diagnostic accuracy versus pseudodystonia and offer candidate biomarkers for monitoring and therapeutic studies. MATERIAL AND METHODS: A control study enrolled 21 patients with focal caput‑pattern CD (18 female, 3 male, mean age 53.5 ± ± 7.9 years) and 21 matched healthy volunteers (18 female, 3 male, mean age 51.6 ± 9.2 years). The standard F‑wave protocol, including F‑wave minimal latency (Fmin), chronodispersion (Fchronosp), persistence (Fpersistence), amplitude (Fampl), F/M amplitude ratio (F/Mampl), F‑wave maximal latency (Fmax), mean latency (Fmean), F/M latency ratio (F/Mlat ratio) and conduction velocities (CV1, CV2). Similarly, the CSP measurement protocol, which typically includes CSP onset latency (CSPo), CSP end latency (CSPe), CSP duration (CSPd), was expanded with CSP onset minimal latency (CSPom) and velocities CV3, CV4 and CV5. Because conduction velocity depends on anatomical distance and CSP latencies, incorporating precise distances enables a more reliable and accurate evaluation of both afferent and efferent overall neural pathway efficiency than latency measures alone. RESULTS: Compared with controls, patients with caput‑pattern CD demonstrated prolonged Fmax and Fmean (p = 0.012 and p = = 0.019 respectively), increased Fchronosp (p = 0.004) and Fampl (p = 0.043), increased F/Mlat ratio (p = 0.037), shortened CSPd and CSPe (p < 0.0001 and p = 0.0001), and altered (generally increased) CSP‑related conduction velocities: CV4 (p < 0.0001) and CV5 (p = 0.007). The most sensitive measures were CSPd, CSPe and CV4. CONCLUSIONS AND CLINICAL IMPLICATIONS: Cervical dystonia is associated with impaired inhibition and altered conduction velocities (CV4, CV5). CSPd, CSPe and CV4 are candidate markers to aid diagnosis, differentiate pseudodystonia, and monitor disease or treatment effects. These measures may also inform studies of other motor‑control disorders.