A perspective on spaceflight associated neuro-ocular syndrome causation secondary to elevated venous sinus pressure

Abstract

Spaceflight associated neuro-ocular syndrome (SANS) alters the vision of astronauts during long-duration spaceflights. There is controversy regarding SANS being similar to patients with idiopathic intracranial hypertension (IIH). IIH has been shown to be due to an elevation in venous sinus pressure. The literature suggests an increase in jugular vein pressure secondary to a headward shift of fluid occurs in SANS but this may not be enough to significantly alter the intracranial pressure (ICP). The literature regarding cardiac output and cerebral blood flow (CBF) in long-duration spaceflight is contradictory, however, more recent data suggests increased flow. Recent modelling has shown that an increase in CBF can significantly increase sinus pressure. The purpose of the present paper is to review the SANS vascular dynamics literature and through mathematical modelling suggest the possible underlying cause of SANS as an elevation in venous sinus pressure, secondary to the redistribution of fluids towards the head, together with a significant increase in pressure drop across the venous system related to the CBF.

Fig. 1. Venous pressure vs. blood flow.

A graph of the venous pressure response vs. blood flow for the five patients modelled in reference. The two vertical lines represent the normal flow rate at middle age on the left (792 mls/min) and a 40% increase in flow on the right (1110 mls/min). Note the pressure drop across the venous system is close to the normal figure of 2.5 mmHg at a normal flow rate for 3 of the 5 patients but elevated in 2. At a flow rate of 1110 mls/min there is a wide variation in response meaning there is a variable risk for the development of SANS if this condition is analogous to IIH. IVJ internal jugular vein, SSS superior sagittal sinus.