Slave to the rhythm: Seasonal differences in non-motor symptoms in Parkinson's disease

Introduction: Although circadian variation of (motor) symptoms in Parkinson disease (PD) patients has been described, it remains unclear what effect seasonal variation may have on non-motor symptoms (NMS). Methods: Cross-sectional retrospective study on 372 consecutive PD patients taking part in the Non-motor Longitudinal International Study at King's College Hospital London between November 2011 and July 2018. Patients were divided into three groups based on their date of assessment, using a simplified seasonal model: group 1: November-February (n = 107); group 2: March-15 June (n = 107); and group 3: 16 June-October (n = 158). Primary outcome was a seasonal difference in NMS scale (NMSS) total scores (higher scores reflecting greater disability). We hypothesized that PD patients exhibit circannual NMS burden patterns. Results: All groups were identical concerning disease onset and duration, HY stage, Levodopa equivalent dose, and gender. There was a seasonal difference in NMSS total scores (p = 0.040), with the highest scores (57.1 +/- 42.5) in season 1 (winter months) and the lowest (45.1 +/- 34.4) in season 3 (summer months) (p = 0.037). Seasonal differences were observed in NMSS domain 1 (cardiovascular symptoms) (p = 0.011), domain 4 (perceptual problems) (p = 0.017) and domain 9 (miscellaneous symptoms) (p = 0.009). A trend was observed for domain 2 (sleep) (p = 0.057). Conclusion: NMS in PD fluctuate throughout the year, with worsening of symptoms in the winter months compared to the summer months suggestive of dysfunction of the body's master clock, the suprachiasmatic nuclei. Such variations must be accommodated in daily care to ascertain appropriate changes in medication regimes and in clinical trials for the interpretation of outcomes.