Background: Fibromyalgia syndrome (FMS), is one of the most common causes of widespread pain and diffuse tenderness.
It is characterized by reduced pain threshold (hyperallgesia) & pain with normally innocuous stimuli (allodynia).This diffuse
pain is often disease associated with wide range of other symptoms including fatigue, sleep disturbance, stiffness & more.
FMS often occur concomitantly with other rheumatologic diseases such as rheumatoid arthritis (RA), systemic lupus
erthymatosus (SLE) and many other systemic connective tissue diseases. The pineal hormone melatonin (MT) exerts a
variety of effects on the immune system. MT activates immune cells and enhances inflammatory cytokine and nitric oxide
production. Cytokines that are strongly involved in the synovial immune and inflammatory response in rheumatoid arthritis,
reach the peak concentration in the early morning, when MT serum level is at its highest concentration. Objective: The aim
of this study was to assess serum melatonin level & investigate it's correlation – if any to clinical features of fibromyalgia
syndrome (FMS). Methods: This is a cross sectional case control study in which we've studied 50 FMS patients defined
according to the American Colleague of Rheumatology (ACR 2010) classification criteria (Wolfe et al., 2010)[24]. They were
classified into two groups: Group I: They included 25 primary FMS patients . Group II: They included 25 secondary FMS
patients, they were fulfilling the classification criteria of other rheumatologic disease such as RA, SLE. Additionally, twenty
age and sex matched healthy individuals were included in the study as a control group. Results: Mean Melatonin titers were
significantly reduced (p<0.0001) in primary FMs patients compared to the controls (21.32 vs. 30.9 pg/ml), but they were
significantly elevated (p<0.0001) in secondary FMS compared to controls (140.8 vs.30.9 pg/ml). Our data imposed that, in
1ry FMS there were negative correlations of MT titers with tender points (r=-0,967**,p<0.0001), sleep disturbance (r=-
0.963**, p<0.0001**), Fatigue (r=-0.972**, p<0.001**), WPI (r=-0,933**, p<0.0001) and SS (r=-0.934**, p<0.0001). There
were positive correlations of MT titers with cognitive symptoms (r=0.36, p<0.061). Conclusions: In primary FMS patients
melatonin level is lower than melatonin level in controls, meanwhile MT level is higher in secondary FMS patients than that
of controls. There was a significant negative correlation between MT titers with tender points, sleep disturbance, fatigue,
symptoms severity (SS) & widespread pain index (WPI) in primary fibromyalgia syndrome. However, there was a positive
correlation between MT level & cognitive symptoms.