The Cervical Mechanical Bottleneck of the Lymphatic–Glymphatic Clearance Chain: Conditional Coupling Between the Investing Cervical Fascia and the Carotid Sheath and the Intrasheath Lymph Node Hypothesis

Central clearance dysfunction in neurodegenerative diseases has long been regarded as an intracerebral event. However, the final outlet for cerebral metabolic waste—the deep cervical lymphatic system—shares the same mechanical space as the vascular and neural structures within the carotid sheath. This article proposes a hypothesis of conditional coupling between the investing layer of the deep cervical fascia (investing fascia) and the carotid sheath, centered on the lymph node within the carotid sheath (intrasheath lymph node). The central proposition is that chronic forward head posture, aging, and low-grade inflammation synergistically drive investing fascia fibrosis and reduce its elastic buffering capacity. Abnormal shear forces are then transmitted to the carotid sheath through composite pathways involving the deep cervical fascia, visceral fascia, or interstitial tissue pressure, thereby altering the conditional mechanical environment of lymphatic structures within the sheath. Six functional domains—arterial perfusion, venous return, deep cervical lymphatic drainage, lymphatic-glymphatic coupling, sympathetic tone, and the vagal anti-inflammatory pathway—are conditionally co-susceptible under shared mechanical boundaries, but they are not assumed to be equally affected, equally weighted, or involved in an identical sequence. The lymphatic and glymphatic systems constitute a serial clearance chain. Within the carotid sheath region, the intrasheath lymph node is the core structure and the conditional shared bottleneck at the cervical level. Functional stenosis, as defined here, refers to functional limitation of lymphatic structures within the sheath under a conditional mechanical environment; it does not presuppose homogeneous sheath-wall narrowing or constant direct fibrous continuity. Its downstream outcome should be described as altered clearance function (reduction or pathological redistribution) rather than a decline in a single clearance measure. Existing evidence is uneven across nodes of the chain. Meningeal lymphatic anatomy and functional necessity are independently validated; glymphatic driving and coupling frameworks are becoming clearer; human intrathecal tracer magnetic resonance imaging (MRI) shows drainage to deep cervical lymph nodes; external mechanical and intrinsic physiological regulation can alter cerebrospinal fluid (CSF) drainage; aging-related lymphatic decline is a systemic vulnerability axis; phosphorylated tau 181 (pTau181) enrichment in deep cervical lymph nodes supports the central-to-cervical transport direction; and fibrotic remodeling of deep cervical lymph nodes localizes the bottleneck to the lymph node level. The core empirical gap is whether, within the same individual, a decline in investing fascia elasticity precedes or accompanies changes in the mechanical state of lymph nodes within the carotid sheath and mediates altered clearance function, while remaining statistically independent of cervical bone-ligament structural parameters and global spinal alignment parameters.