Abstract
Purpose :
Spaceflight associated neuro-ocular syndrome (SANS) is thought to result from the cephalad fluid shift that occurs during spaceflight, although the exact mechanism is unknown. Investigating changes in ONH position may provide insight into the relative contribution of intracranial pressure (ICP) to the development of optic disc edema (ODE) in this and other conditions. In patients with idiopathic intracranial hypertension (IIH), the optic nerve head (ONH) displaces anteriorly on average, while astronauts during long-duration spaceflight (LDSF) exhibit mixed to slightly posterior ONH displacement. This study aims to investigate the anterior-posterior ONH position changes during strict head-down tilt bed rest (HDT), a common terrestrial analog for spaceflight, and determine whether ONH displacement is associated with the development of ODE.
Methods :
Optical coherence tomography images were collected in 35 individuals before HDT (seated posture) and on day ~30 in the strict 6° HDT posture. Radial scans centered over the ONH were segmented at the inner limiting membrane, Bruch’s membrane (BM), and BM opening (BMO). BMO height (BMOh) was measured from each BMO point to a 4 mm BM reference line centered on the ONH. Total retinal thickness (TRT) was quantified within an annular region extending from BMO to 250µm. Associations between ocular structural changes were evaluated in eyes with signs of ODE, defined as a change in TRT greater than 19.4µm.
Results :
On average, BMO was displaced anteriorly during bedrest (mean +8.4µm, 95% CI: 4.4 to 12.4µm, P < .0001). TRT increased during bedrest (mean +29.5µm, 95% CI: 22.2 to 36.9µm, P < .0001) and 34 of 70 (49%) eyes demonstrated an increase in TRT greater than 19.4µm. There was no significant association between change in TRT and change in BMOh (R2=0.08, P=.06).
Conclusions :
The majority of HDT subjects with ODE displayed an anterior shift in BMOh, similar to previous findings in IIH patients. This suggests that ICP may represent a larger contributing factor to the development of ODE in HDT subjects than in astronauts; however, additional factors during LDSF, such as how choroidal engorgement and globe flattening may influence the location of the BM reference used to calculate BMOh, should be considered when interpreting these findings. The pathophysiology of SANS is likely multifactorial, and the relative role of ICP in the development of SANS during spaceflight remains to be determined.
This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.