A Cervicogenic Mechanical–Glymphatic Shared Subtype Hypothesis of Bipolar Depression: A Theoretical Extension Based on the Investing Fascia–Carotid Sheath–Six-System Coupling Framework

Depressive episodes in bipolar disorder (BD) and major depressive disorder (MDD) overlap considerably in clinical presentation, making differential diagnosis challenging. Existing models have largely focused on central and molecular levels, with little systematic investigation of “cervicogenic mechanical constraint.” Within the framework of the “Investing Fascia–Carotid Sheath–Six-System Coupling Hypothesis,” this paper proposes the “Cervicogenic Mechanical–Glymphatic Shared Subtype Hypothesis of Bipolar Depression.” This hypothesis posits that chronic forward head posture, prolonged stress, aging, and chronic low-grade inflammation may synergistically drive deep cervical fascia fibrosis, leading to synchronized involvement of six systems via the carotid sheath and contributing to the maintenance and worsening of depressive episodes. This hypothesis proposes a two-layer framework of “glymphatic sharing + vagal difference”: impaired glymphatic clearance may be a shared feature of MDD and BD (Fu et al., 2026; Zhu JD et al., 2026); decreased diffusion tensor imaging analysis along the perivascular space (DTI-ALPS) is observed in high-risk offspring of BD and, relative to attention-deficit/hyperactivity disorder (ADHD) and borderline personality disorder (BPD), is not shared with other emotion dysregulation disorders (Pascucci et al., 2026); reduced vagal tone may be a specific dysregulation distinguishing bipolar depression from unipolar depression (Hage et al., 2019), although the evidence for heart rate variability (HRV) in BD is weaker than suggestive (BD vs. healthy controls; Wang Z et al., 2025). This hypothesis positions deep cervical fascia mechanical coupling as a downstream amplifying factor for the maintenance and worsening of depressive episodes, rather than a triggering factor for bipolar disorder; it may apply only to the subgroup of bipolar depression with clear cervicogenic mechanical abnormalities and glymphatic/vagal disturbances. This paper proposes six testable predictions and a three-stage validation framework.