Gut-brain axis mechanisms of remote brain dysfunction after traumatic spinal cord injury: immune inflammation, the vagus nerve, and neuroendocrine pathways

Traumatic spinal cord injury (SCI) has traditionally been regarded as a central nervous system injury mainly confined to the injured segment. However, increasing evidence indicates that SCI can also induce neuroinflammation, cognitive decline, and emotional disorders in remote brain regions, suggesting that its pathological impact is systemic rather than purely local. Remote brain dysfunction after SCI is unlikely to be driven by a single pathway, but may arise from the combined effects of systemic inflammation, autonomic imbalance, neuroendocrine dysregulation, and disruption of intestinal homeostasis. Unlike previous reviews that mainly discuss SCI-associated gut dysbiosis, neuroinflammation, or gut-brain communication separately, this review organizes SCI-related intestinal abnormalities around the concept of "gut-derived pathological signals" and further distinguishes direct SCI evidence from cross-disease mechanistic evidence and proposed mechanistic inference. Specifically, we summarize how intestinal dysmotility, microbial metabolic remodeling, abnormalities in short-chain fatty acids (SCFAs), barrier vulnerability, and mucosal immune imbalance after SCI may generate persistent pathological signals. We then analyze how these signals may affect the central nervous system through an immune-inflammatory main axis, a vagal neural relay branch, and a neuroendocrine modulatory branch, ultimately converging on a common downstream brain effector stage characterized by blood-brain barrier impairment, neuroinflammation, synaptic plasticity deficits, and dysfunction of key brain regions such as the hippocampus and medial prefrontal cortex (mPFC). Based on this cascade, we propose a stratified intervention framework involving upstream restoration of intestinal homeostasis, midstream regulation of interorgan transmission pathways, and downstream protection of brain effector mechanisms. Overall, this review provides an evidence-stratified gut-brain axis framework for understanding remote brain dysfunction after SCI and highlights the need for SCI-specific temporal mapping and pathway-selective causal validation.