We define optic nerve sheath radius (r_ONS) as r_ONS = optic nerve sheath diameter (ONSD) / 2 as a function of cerebrospinal fluid pressure (p_CSF). We assume that r_ONS depends on p_CSF, while optic nerve radius (r_ON) and dura thickness (t_ONS) do not. The introduction of the deformation (ε) facilitates the estimation of p_CSF because ε is a ratio of an increased ONSD to a standard ONSD. CSF indicates cerebrospinal fluid.

A, Intracranial views from the right side. The cerebellum is caught in part of the tentorium cerebelli because of brain upward shift during space flight, and the cerebrum thereby rotates counter-clockwise with a narrowing of the cerebrospinal fluid (CSF) spaces at the vertex. The gray shading indicates the original position in the 1g terrestrial environment. B and C, The hypothesis for the increased optic nerve sheath diameter (ONSD) and the globe flattening is illustrated. The optic nerve (ON) shifts rearward as the brain shifts upward, resulting in globe flattening and the deformation of the dura. In addition, the Bruch membrane (BM) is deflected downward. The yellow arrowheads indicate the (1) uplifting of the optic chiasm accompanied by brain upward shift, that (2) the portion of the ON from the optic chiasma to the optic canal is pulled rearward and upward, and that (3) the other portion of the ON from the optic canal to the eyeball shifts rearward, while the red arrowheads indicate that (4) the restoration force of the dura on the eyeball results in globe flattening (C).