Posture‐dependent collapse of the optic nerve subarachnoid space: a combined MRI and modeling study

Abstract

AbstractPurposeLarge trans‐lamina cribrosa pressure differences (TLCPD) have been suggested to play a role in the development of normal‐tension glaucoma. To avoid substantial increases in the TLCPD in upright postures, we hypothesize that a collapse of the optic nerve subarachnoid space (ONSAS) protects the eyes from large TLCPDs when we stand up, i.e., by preventing low cerebrospinal fluid pressures around the optic nerve. In this study, we combined magnetic resonance imaging (MRI) and mathematical modeling to investigate the suggested ONSAS collapse and its effects on the TLCPD.MethodsFirst, MRI was performed on six healthy volunteers in 6° head‐down tilt (HDT) and 13° head‐up tilt (HUT) to assess changes in ONSAS volume with changes in posture. The volume change represents optic nerve sheath distensibility. Second, we used the MRI data and mathematical modeling to simulate the ONSAS pressure and the potential ONSAS collapse in a fully upright posture (90°).ResultsThe MRI showed a decrease of 33% in the ONSAS volume when going from the HDT to the HUT position (p < 0.001). In the upright posture, the simulations predicted an ONSAS collapse in the orbital section of the optic nerve, disrupting the pressure communication between the ONSAS and the intracranial subarachnoid space. The collapse reduced the simulated postural increase in TLCPD by roughly 1 mmHg, although this reduction was highly sensitive to the optic nerve sheath distensibility, varying between 0–4.8 mmHg when varying the distensibility by ±1 SD.ConclusionsThe ONSAS volume along the optic nerve is posture dependent during acute posture changes. The simulations supported the hypothesized ONSAS collapse in the upright posture and showed that even small changes in optic nerve sheath stiffness/distensibility may substantially affect the TLCPD, and thereby bear relevance for suggested TLCPD‐related disorders such as normal‐tension glaucoma.Ref‐erenceHolmlund, P. et al. Posture‐Dependent Collapse of the Optic Nerve Subarachnoid Space: A Combined MRI and Modeling Study. IOVS 62.4 (2021).