Association of Space Flight With Problems of the Brain and Eyes

Abstract

This study explores the association of cerebrospinal fluid pressure and brain upward shift with space flight–associated neuroocular syndrome in postflight astronauts.

Space flight–associated neuroocular syndrome (SANS), characterized by increased optic nerve sheath diameter (ONSD) and globe flattening, is detected in some astronauts.¹ Because inflight cerebrospinal fluid (CSF) pressure measurement is excessively invasive, it is not realistic to conduct. We estimated CSF pressure (p_CSF) during space flight based on published reports² and found that SANS was not caused mainly by increased p_CSF but rather by brain upward shift (BUS), recently demonstrated in postflight astronauts.³ Our findings suggest that eyes are portals into effects on the brain during space flight.

Methods

We created a model of the optic nerve sheath (ONS) as a tube including a cylinder (Figure 1). This modeling allowed us to apply the material mechanics theory of thin-walled tubes. This study used published data and institutional review board approval was not required. The r_ONS rise due to increased p_CSF (p₀+Δp) is expressed as: Δr_ONS(Δp) = r_ONS(p₀+Δp) − r_ONS(p₀), in which p₀ and r_ONS (p₀) are taken as standard values of p_CSF and r_ONS, respectively. Thus, ONS deformation (ε) is defined as: ε (Δp) = Δr_ONS (Δp) / r_ONS (p₀). This procedure enables us to estimate p_CSF from r_ONS, which is measurable by ultrasonography¹ during space flight. To derive parameters that represent the mechanical strength of ONS tissues, the anatomical data of Hansen et al² were used. We also used the inflight ONSD¹ value of 12 mm and the human standard ONSD value¹ of 5.9 mm for these calculations.

Figure 1. Cross-sectional View of the Modeled Optic Nerve Sheath.

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.

Results

The ε in our ONS model was 0.15 by a pressurization (Δp) of 5 mm Hg, and increased proportionally with further pressurization. Assuming a linear relation between ε and Δp according to the material mechanics, the formula ε(Δp) = 4.0 × 10⁻³Δp + 0.16 was obtained by linear fitting with the data of Hansen et al² for Δp > 10 mm Hg. Hence we obtained Δp = 210 mm Hg from the calculation of ε = (12-5.9) / 5.9 ≅ 1.0 for an inflight astronaut.¹ This p_CSF value, which exceeds the human standard value, suggests a substantial deterioration of the elasticity of the ONS, and the origin of this deterioration is discussed in the following section.

Discussion

Because postflight sagittal magnetic resonance images of astronauts show an uplifting of the optic chiasm,³ it is assumed that the optic nerve (ON) is pulled rearward according to the BUS along with brain rotation around the edge of the cerebellar tentorium during space flight (Figure 2A). This rearward shift of the ON may result in an expansion and bending of the ONS (ie, increased ONSD) because the periosteum is connected to the dura of the ONS at the orbit (Figure 2B). Furthermore, this rearward force on the ON yields a deformation of the eyeball (ie, globe flattening) because of the restoration force of the dura on the eyeball (Figure 2C). This is because the dura of the ONS is known to be as hard as the ocular sclera. Our hypotheses are consistent with the globe flattening that typically affects both eyes¹ and the downward deflection of the Bruch membrane opening.⁴

Figure 2. Theoretical Inflight and Postflight Conditions of Optic Nerve.

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).

Barratt⁵ reported that his standing height during space flight reverted to a preflight baseline within 3 hours in response to him wearing a penguin suit or in combination with heavy resistive exercise. This height reversion is attributed to the recovery of the thoracic curve⁵ that is induced by these compressions. This may raise p_CSF because of the associated reduction of total subarachnoid volume. The redundant CSF accumulates in the extracranial portion of the ONS, where the volume is more easily changed than in the intracranial portion. Therefore, one might reconsider wearing a penguin suit repeatedly and performing resistive exercise during space-flight.

Conclusions

Our model enables us to estimate p_CSF from ONSD. However, the estimated p_CSF suggests that the model is invalid for some astronauts because their ONS tissues may be changed by BUS. Furthermore, compression forces that reduce subarachnoid volume exacerbate SANS.