Considerable evidence suggests that people with depressive disorder are at higher risk for somatic symptoms than normal people, but the mechanisms of susceptibility are unclear. Some studies suggest that the levels of precursor of brain-derived neurotrophic factor (ProBDNF) and inflammatory cytokines in depressive disorder patients are higher, but it is still unclear whether this is related to the accompanying somatic symptoms.

To explore the characteristics of somatic symptoms, and their correlations with BDNF and inflammatory cytokines in patients with major depressive disorder (MDD) .

A total of 59 MDD outpatients and inpatients from Mental Health Department, First Hospital of Shanxi Medical University and 32 healthy volunteers from the community were recruited from February 2019 to December 2020. Somatic symptoms were diagnosed using the Somatic Self-rating Scale (SSS, SSS score >36 and SSS score ≤36 were defined as with somatic symptoms or without, respectively) . Depression was assessed using the Hamilton Depression Rating Scale (HAMD-17) . Clinical data were collected, including gender, age, years of education, HAMD-17 score, SSS score, absorbance of ProBDNF and BDNF, and inflammatory cytokines (CRP, IL-4, IL-10, IL-18, IL-23a, HMGB1, IL-6, TNF-α, IFN-α) . Spearman's rank correlation analysis was used to investigate the correlation of somatic symptoms (including dizziness and headache, frequent and urgency of urination, cardiovascular symptoms, muscle soreness, gastrointestinal symptoms, tingling and twitching of hands and feet, choking and sighing, blurred vision, and discomfort of the throat) , SSS-S factor score and HAMD-17 score with ProBDNF and BDNF, and inflammatory cytokines.

MDD patients with somatic symptoms had higher total HAMD-17 score than those without (P<0.001) . Both MDD patients with and without somatic symptoms had higher total HAMD-17 score than healthy volunteers (P<0.001) . MDD patients with somatic symptoms had higher total SSS score and SSS-S factor score than those without (P<0.001) . And they also had higher total SSS score and SSS-S factor score than healthy volunteers (P<0.001) . MDD patients with somatic symptoms had higher BDNF than those without (P<0.05) . The IFN-α in healthy volunteers was lower than of MDD patients with or without somatic symptoms (P<0.001) . ProBDNF was negatively correlated with muscle soreness (r_s=-0.262, P<0.05) . CRP was negatively correlated with tingling and twitching of hands and feet (r_s=-0.386, P<0.01) . IL-4 had a negative correlation with gastrointestinal symptoms (r_s=-0.336, P<0.01) . IL-10 had a positivecorrelation with blurred vision (r_s=0.286, P<0.05) . BDNF was positively correlated with dizziness and headache (r=0.339, P<0.01) , cardiovascular symptoms (r_s=0.309, P<0.05) , gastrointestinal symptoms (r_s=0.278, P<0.05) , muscle soreness (r_s=0.419, P<0.01) , tingling and twitching of hands and feet (r_s=0.286, P<0.05) , choking and sighing (r_s=0.372, P<0.01) , discomfort of the throat (r_s=0.392, P<0.01) and SSS-S factor score (r_s=0.418, P<0.01) . IL-6 was positively correlated with cardiovascular symptoms (r_s=0.283, P<0.05) and choking and sighing (r_s=0.374, P<0.01) . TNF-α was negatively correlated with muscle soreness (r_s=-0.299, P<0.05) . IFN-α showed a negative correlation with blurred vision (r_s=-0.267, P<0.05) .

The peripheral blood IFN-α of MDD patients was lower than that of normal people. The severity of depression and BDNF level were higher in MDD patients with somatic symptoms than those without. The elevation of multiple inflammatory cytokines was correlated with the increase in the risk of somatic symptoms, suggesting that patients with MDD may be prone to somatic symptoms and adverse outcomes, requiring early intervention.