Reappraising traumatic brain injury and ventilator-associated pneumonia through the brain-lung-immune-microbiome axis

Ventilator-associated pneumonia (VAP) remains the most prevalent and lethal infectious complication among patients with severe traumatic brain injury (TBI) requiring invasive mechanical ventilation. Historically, the pathogenesis of VAP within neurocritical care settings has been attributed to mechanical and physical factors. This traditional paradigm posits that the endotracheal tube bypasses natural upper airway defenses, impairs glottic reflexes, and allows for the continuous micro-aspiration of pathogen-laden oropharyngeal secretions into the lower respiratory tract. However, this biomechanical model fails to adequately explain the disproportionately high incidence of VAP in TBI patients compared to other critically ill populations. A profound conceptual paradigm shift is transitioning the focus from isolated airway mechanics toward multidimensional biological construct: the Brain-Lung-Immune-Microbiome Axis. The fundamental catalyst for respiratory vulnerability following neurotrauma is central nervous system injury-induced immunodepression syndrome (CIDS), a profound systemic immune remodeling driven by acute neuroendocrine and autonomic dysregulation, as well as the translocation of pathogenic gut microbiota to the lungs. By synthesizing evidence from neuroimmunology, microbiology, and critical care medicine, this narrative review details the tripartite interplay of neurological trauma, immune exhaustion, and microbiome evolution. Ultimately, this review evaluates emerging host-directed immunomodulatory therapies and microbiome-targeted interventions, advocating for a critical transition from simple mechanical airway hygiene to precision immuno-microbiome therapeutics in the neuro-intensive care unit.