GDF15 and multiple system atrophy: In patients with MSA, serum GDF-15 levels showed a significant positive correlation with age (r = 0.708; p = 0.010) and a significant negative correlation with disease duration (r = −0.638; p = 0.026), but not with the mRS score (r = −0.287; p = 0.365; Figure 4A–C).