Increased Neck Circumference and Altered Cervical Fascial Mechanics: Rehabilitation Implications of a Six-System Coupling Framework for the Carotid Sheath—A Narrative Review

Background: Increased neck circumference is commonly viewed as a simple surrogate marker of upper-body adiposity and cardiometabolic risk. However, cervical fat accumulation simultaneously alters the mechanical load borne by the investing fascia, and reduced elastic buffering capacity may permit abnormal shear stress to be conducted through the deep cervical fascial network to the carotid sheath. The carotid sheath is an anatomical hub shared by the internal carotid artery, internal jugular vein, lymphatic vessels, glymphatic outflow pathways, cervical sympathetic trunk, and vagus nerve.Objective: To organize evidence from anatomical, epidemiological, imaging, and intervention studies within a testable integrative framework, to grade evidence maturity by node, and to propose a research agenda.Methods: This narrative review evaluates the “six-system coupling framework” as a testable integrative perspective rather than an established causal mechanism. Evidence maturity was graded by node using a custom five-tier scale. The core empirical gap was identified as the absence of simultaneous measurement of investing fascia elasticity, intra-sheath pressure, and intra-sheath functional markers in the same individual.Results: Anatomically, the investing fascia and the carotid sheath have a composite relationship that is partly loose and partly directly continuous. Focal absence and midline-crossing connections further increase the spatial heterogeneity of transmission, whereas the prevertebral fascia serves as an anatomical barrier and protective mechanical constraint around the sheath. Evidence is strongest at the arterial node, supported by longitudinal cohorts, vascular-bed specificity, and PIEZO1/2 molecular candidates, and is graded moderate-high. The venous node has human evidence of mechanical sensitivity but has not been replicated under fascial mechanical exposure, and is graded moderate-low. The autonomic node has human muscle sympathetic nerve activity (MSNA) associations and randomized controlled trial (RCT) evidence for heart rate variability (HRV), but the intervention was not neck-specific, and is graded moderate. The glymphatic node has only indirect human associations and animal mechanistic evidence, and is graded low.Conclusions: Neck circumference and fascial mechanical parameters should be incorporated into multisystem rehabilitation assessment and monitored as mediators linking superficial body load to deep structural function. For rehabilitation, entry-level weight reduction and effector-level autonomic modulation can serve as independent components; investing fascia-targeted interventions at the transmission-interface level are currently investigational only. Their status as an independent intervention target awaits clarification through longitudinal studies oriented toward mediation analysis and functional endpoints.