We define optic nerve sheath radius (rONS) as rONS = optic nerve sheath diameter (ONSD) / 2 as a function of cerebrospinal fluid pressure (pCSF). We assume that rONS depends on pCSF, while optic nerve radius (rON) and dura thickness (tONS) do not. The introduction of the deformation (ε) facilitates the estimation of pCSF 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).
Customize your JAMA Network experience by selecting one or more topics from the list below.
Shinojima A, Kakeya I, Tada S. Association of Space Flight With Problems of the Brain and Eyes. JAMA Ophthalmol. 2018;136(9):1075–1076. doi:10.1001/jamaophthalmol.2018.2635
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 (pCSF) during space flight based on published reports2 and found that SANS was not caused mainly by increased pCSF 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.
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 rONS rise due to increased pCSF (p0+Δp) is expressed as: ΔrONS(Δp) = rONS(p0+Δp) − rONS(p0), in which p0 and rONS (p0) are taken as standard values of pCSF and rONS, respectively. Thus, ONS deformation (ε) is defined as: ε (Δp) = ΔrONS (Δp) / rONS (p0). This procedure enables us to estimate pCSF from rONS, 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.
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 pCSF 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.
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
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 pCSF 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.
Our model enables us to estimate pCSF from ONSD. However, the estimated pCSF 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.
Corresponding Author: Ari Shinojima, MD, PhD, Department of Ophthalmology, Lariboisière Hospital, Assistance Publique-Hôpitaux de Paris, University Sorbonne Paris Cité, 2 Rue Ambroise Paré, Paris 75010, France (firstname.lastname@example.org).
Correction: This article was correct online August 23, 2018, for typos in the Additional Contributions and the caption for Figure 2.
Published Online: July 5, 2018. doi:10.1001/jamaophthalmol.2018.2635
Author Contributions: Dr Shinojima had full access to all of the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis. All authors contributed equally to this report.
Concept and design: Shinojima, Kakeya.
Acquisition, analysis, or interpretation of data: All authors.
Drafting of the manuscript: All authors.
Critical revision of the manuscript for important intellectual content: Shinojima, Kakeya.
Supervision: Kakeya, Tada.
Conflict of Interest Disclosures: All authors have completed and submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest and none were reported.
Additional Contributions: We thank Ramin Tadayoni, MD, PhD, Lariboisière Hospital, for allowing this article to be written. He was not compensated for his contribution.
Create a personal account or sign in to: