Optic nerve cerebrospinal fluid drainage via lymphatic vessels in its sheath.

Cerebrospinal fluid (CSF) surrounding the optic nerve is essential for nutrient transport, waste removal, and optic nerve function. Optic disc edema in astronauts during long-duration spaceflight-Spaceflight Associated Neuro-Ocular Syndrome (SANS)-may result from impaired optic nerve CSF drainage, but the drainage pathway remains unclear. Given that the optic nerve sheath is an extension of the cranial meninges, we hypothesized that its CSF drains via lymphatic vessels rather than refluxing into the cranium. We assessed the pressure relationship between intracranial and optic nerve CSF and traced its drainage pathway. Our results show that optic nerve sheath CSF pressure is consistently lower than intracranial pressure, with no reflux observed. Instead, optic nerve CSF drains into deep cervical lymph nodes via lymphatic vessels within the sheath. These findings provide new insights into optic nerve CSF dynamics and may help elucidate SANS pathogenesis.