Beyond posterior putamen lesions in post-stroke spasticity: widespread structural and functional breakdown across brain networks

Background: Post-stroke spasticity (PSS) is a common and disabling motor complication whose neuroanatomical underpinnings remain incompletely understood. Beyond focal lesion localization, PSS likely arises from large-scale network disruptions involving both cortical and subcortical systems. Here, we combined functional and structural lesion network mapping (LNM) to identify the brain networks and white-matter tracts whose dysconnection best explains the presence and severity of PSS. Methods: We analyzed 281 patients with hemorrhagic stroke, including 88 with PSS and 193 without PSS (NPSS). After matching for age, sex, and lesion size (N=81 PSS; N=60 NPSS), we computed functional LNM using normative connectomes from 1000 healthy participants of the Human Connectome Project and structural LNM using the HCP1065 tractography template. Group-level voxel-wise and tract-wise analyses identified regions and tracts with significant dysconnectivity differences. Finally, a ridge regression model with leave-one-out cross-validation assessed the relationship between dysconnectivity (both functional and structural) and spasticity severity, quantified by the Modified Ashworth Scale (MAS). Results: Lesion-symptom mapping revealed that PSS was primarily associated with lesions in the posterior putamen. Structural LNM showed significantly higher tract dysconnectivity in PSS, particularly within the corticospinal, corticopontine, corticobulbar, corticostriatal, medial lemniscus, and superior thalamic radiation pathways (all p<0.001). Functional LNM analyses demonstrated widespread cortical dysconnectivity, with two complementary patterns: a Positive Dysconnected Network (PSS>NPSS) and a Negative Dysconnected Network (PSS<NPSS). Despite these bidirectional effects, overall dysconnection was consistently higher in the PSS group. The most affected networks included the cerebellum, sensorimotor, auditory, dorsal attention, default mode, and medial visual resting-state networks. Ridge regression analysis confirmed a strong association between MAS scores and the degree of dysconnectivity across these networks (R2=0.70). Conclusions: Post-stroke spasticity is not the result of damage to a single locus but rather reflects the dysconnection of a distributed structural and functional network encompassing, not only motor, but also attentional and high order cognitive networks. These findings provide a network-based framework for understanding spasticity and support the use of lesion network mapping to predict and potentially guide targeted neuromodulation in stroke rehabilitation.