Space Flight-Associated Neuro-Ocular Syndrome Associated With Brain Upward Shift

Using the measured increase in the optic nerve sheath diameter (ONSD) and measured brain upward shift, a new research model can estimate the cerebrospinal fluid pressure (P_CSF) in space flight-associated neuro-ocular syndrome in post-flight astronauts, according to a research letter published in JAMA Ophthalmology.

Space flight-associated neuro-ocular syndrome with increased ONSD and globe flattening has been detected in some astronauts. Because measurement of P_CSF is excessively invasive, a non-invasive method of determining P_CSF was needed.

Ari Shinojima, MD, PhD, of the Department of Ophthalmology at Lariboisière Hospital, Assistance Publique-Hôpitaux de Paris, University Sorbonne in France, and colleagues, created a model of the optic nerve sheath using a tube and cylinder that allowed them to apply the material mechanics theory of thin-walled tubes.

The investigators found that space flight-associated neuro-ocular syndrome was not caused by increased P_CSF but rather by brain upward shift. They contend that their results demonstrate that the eyes are gateways into the effects of space flight on the brain.

The researchers noted that during space flight there is an uplifting of the optic chiasm in which the optic nerve is pulled back, which may result in an expansion and bending of the optic nerve sheath, increasing the ONSD. This rearward pressure on the optic nerve deforms the eyeball, resulting in globe flattening.

One astronaut reported that his in-flight standing height returned to its pre-flight baseline within 3 hours when he wore a penguin suit or in combination with heavy resistive exercise. The height reversion is ascribed to the return of the thoracic curve caused by these compressions. In turn, this may increase P_CSF because of an associated decrease in total subarachnoid volume. Cerebrospinal fluid then collects in the extracranial portion of the optic nerve sheath.

However, the investigators noted that their model for calculating P_CSF may be inaccurate in some astronauts because their optic nerve sheath may be changed by brain upward shift. They also suggested that compression forces that reduce subarachnoid volume exacerbate space-flight associated neuro-ocular syndrome.

Shinojima A, Kakeya I, Tada S. Association of space flight with problems of the brain and eyes [pubished online July 5, 2018]. JAMA Ophthalmology. doi:10.1001/jamaophthalmol.2018.2635