Frontiers in Physiology

Intervertebral Disc Swelling Demonstrated by 3D and Water Content Magnetic Resonance Analyses after a 3-Day Dry Immersion Simulating Microgravity

Abstract

After space flight, astronauts frequently experience a vertebral deconditioning that is characterized by spine lengthening, herniated discs, muscle atrophy, and back pain (Hutchinson et al., 1995; Cao et al., 2005; Belavy et al., 2016). The underlying pathophysiology of intervertebral disc (IVD) herniation in astronauts is, therefore, a research priority for numerous space agencies. Other related research priorities include identification of predisposing factors and development of countermeasures to reduce or prevent this phenomenon after space flight (Belavy et al., 2016).

Intervertebral disc (IVD) herniation appears at recovery from space flight and is more frequent in returning astronauts when compared to the frequency in the general population on Earth, with astronauts experiencing 21 times more IVD herniation in the cervical region and 3 times more herniation in the lumbar region (Belavy et al., 2016). Despite such health issue with the high incidence rate of IVD herniation in astronauts, it is surprisingly that a single study on whole the rachis, particularly on the cervical region during space flight or simulation exists.

The main hypothesis found in the literature on the cause of IVD herniation is IVD swelling. Indeed, studies have demonstrated (Johnstone et al., 1992; Campana, 2004) that the fluid content of the disc, which governs its mechanical response and biological behavior, varies with external load. When the load decreases, liquid is reabsorbed into the IVD to reach a new osmotic equilibrium (Johnstone et al., 1992). Thus far, an overhydrated disc has not been thoroughly evaluated after exposure to microgravity (Adams and Hutton, 1982; Young and Rajulu, 2011; Belavy et al., 2016; Hargens and Vico, 2016).

The aim of the study was to analyze spinal changes during 3 days of simulated microgravity using dry immersion (DI) as the analog (Navasiolava et al., 2011). We hypothesized that (1) a 3-day DI could lead to morphological changes on the spine, which could explain the vertebral deconditioning described by astronauts after space flight; (2) the disc swelling induced by simulated microgravity is explained by an increase in IVD water content; and (3) this phenomenon is likely associated with deep muscle atrophy and could lead to postural balance disturbances, as demonstrated by another team that experienced the same conditions (Treffel et al., 2016).

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