Serotonin and interleukin-6: Association with pruritus severity, sleep quality and depression severity in Prurigo Nodularis

• There was increased IL-6 and decreased serotonin in PN.
• Serotonin and IL-6 showed significant correlation with itching severity of PN.
• Immune mediated abnormality of serotonin system may underlie etiology of PN.
• These findings may have implications on the treatment of PN.

Prurigo Nodularis (PN) is a chronic skin condition of unknown etiology. It is said to be associated with psychological factors. However, studies to identify the same are few. Hence, we undertook to assess the association between quality of sleep, depression, pruritus severity in cases of PN and to assess association of serum serotonin, interleukin (IL)-6 with sleep, depression and pruritus severity. This cross-sectional study involves 39 patients with PN and 39 age and gender matched healthy controls. Subjective quality of sleep, depressive symptom severity and pruritus severity were collected through clinician based interview. Serum serotonin and serum IL-6 were estimated by ELISA. Spearman correlation was used to assess the strength of association between biochemical parameters and clinical parameters. Cases had significantly higher percentage of depression, a higher score of depression severity, higher serum IL-6 level and lower serum serotonin levels. Serum serotonin and serum IL-6 showed significant correlation with the severity of pruritus (r = −0.66, p < 0.01 and r = 0.60, p < 0.01). However, both these parameters showed a significantly negative correlation between them (r = −0.92, p < 0.01). Though cause and effect cannot be established in this type of study, there is a disturbance of the inflammatory–neuroendocrine axis (interleukins–serotonin). PN is characterized by higher IL-6 and lower serotonin and greater depression severity than healthy controls. Findings of this study may have an influence on the treatment of PN.