Spinal changes after 5-day dry immersion as shown by magnetic resonance imaging (DI-5-CUFFS)

Astronauts frequently report microgravity-induced back pain, which is generally more pronounced in the beginning of a spaceflight. The dry immersion (DI) model reproduces the early effects of microgravity in terms of global support unloading and fluid shift, both of which are involved in back pain pathogenesis. Here, we assessed spinal changes induced by exposure to 5 days of strict DI in 18 healthy men (25-43-yr old) with (n = 9) or without (n = 9) thigh cuffs countermeasure. Intervertebral disk (IVD) height, spinal cord position, and apparent diffusion coefficient (ADC; reflecting global water motion) were measured using magnetic resonance imaging before and after DI. After DI, IVD height increased in thoracic (+3.3 ± 0.8 mm; C₇-T₁₂) and lumbar (+4.5 ± 0.4 mm; T₁₂-L₅) regions but not in the cervical region (C₂-C₇) of the spine. An increase in ADC after DI was observed at the L₁ (∼6% increase, from 3.2 to 3.4 × 10^-3 mm²/s; P < 0.001) and L₂ (∼3% increase, from 3.4 to 3.5 × 10^-3 mm²/s; P = 0.005) levels. There was no effect of thigh cuffs on spinal parameters. This change in IVD after DI follows the same "gradient" pattern of height increase from the cervical to the lumbar region as observed after bed rest and spaceflight. The increase in ADC at L₁ level positively correlated with reported back pain. These findings emphasize the utility of the DI model for studying early spinal changes observed in microgravity.