Table 5.

Summary of key evidence for the etiological theory about acute cold exposure

Key references	Summary of evidence
[77]	Some authors estimated rates and trends of Bell’s palsy using a centralized surveillance system. They found both season and climate (adjusted ratio of cold to warm months = 1.31) were independent predictors of risk of Bell’s palsy
[78–80]	There is a clear relationship between the cold season and the number of cases observed. However, some researchers have found that BP is more frequent in warm seasons (spring and summer), with its incidence peaking in September
[81]	More deeply, one study evaluated the influences of meteorological factors on the incidence and onset of BP. Evidence suggests that stronger wind speed of preceding day may be related to the occurrence of Bell’s palsy
[82]	One study retrospectively reviewed 568 files of Bell’s palsy patients and concomitant data of meteorological factors. The result showed the number of cases per month was significantly and negatively was significantly and negatively correlated with the summer months and mean monthly temperatures (p = 0.002 and < 0.000, respectively) and strong positive correlation with monthly wind chill factor (p < 0.000). Wind chill factor is a novel, reliable estimator of the overall meteorological factors-derived risk
[83]	A community-based research in Qena Governorate, Egypt confirmed the most frequent precipitating factors for an episode of Bell’s palsy were exposure to air draft in 40%. This could be related to variations between day and night temperatures in their community. Sharp temperature changes may be one of the risk factors for facial nerve palsy, and especially the susceptibility to air draft exposure during the night