Auditory startle reflex indicating reticulospinal excitability in post-stroke spastic dystonia

Spastic dystonia (SD), characterized by persistent involuntary muscle activity at rest, is a disabling manifestation of upper motor neuron syndrome following stroke. This study investigated whether reticulospinal excitability, as indexed by the auditory startle reflex (ASR), is more pronounced in patients with SD than in those with post-stroke spasticity alone. Seventy-six patients with post-stroke upper-limb spasticity were recruited. Seventy patients underwent surface electromyography to identify SD in the biceps brachii and flexor carpi radialis. SD was operationally defined as sustained spontaneous muscle activity at rest lasting at least 30 s. A subset of 59 patients completed ASR testing. SD was identified in 40.0% of affected biceps brachii and 25.7% of affected flexor carpi radialis. Both patients with SD (Z = 2.448, p = 0.014) and those without SD (Z = 4.229, p < 0.001) showed higher ASR elicitation frequency on the affected side, indicating that reticulospinal hyperexcitability is a shared background feature of post-stroke hypertonia in general. However, the lateralization index of ASR elicitation frequency in the biceps brachii was significantly lower in SD patients than in those without SD (U = 278.0, p = 0.027). These findings suggest that reticulospinal disinhibition, while shared across post-stroke hypertonia, is not sufficient to explain SD, and that additional mechanisms, such as spinal plastic reorganization or altered motoneuronal intrinsic properties, are likely required to produce the sustained resting discharge characteristic of SD.