Bell's palsy (BP), also known as idiopathic facial paralysis (IFP), is an acute lower motor paresis of the seventh cranial nerve (CN VII). BP has been established as the leading cause of facial paralysis (1, 2). Damage to the motor nerve fibers of CN VII might result in complete or partial dysfunction of the facial musculature (2). House-Brackmann Grading (HBG) scale is a subjective grading system that involves observing muscle movement to assess the degree of facial muscle functionality and recovery (3). Currently, there are no established etiologies accounting for BP development; however, inflammatory processes or hypercoagulability could lead to facial nerve injury (4, 5). Acute oral herpes simplex virus type 1 (HSV-1) has been shown to cause facial paralysis in lab rats when the virus invades subcutaneous tissue, with facial nerve demyelination as the underlying mechanism (6). It has also been identified that reactivation of latent varicella-zoster virus (VZV) resulted in extensive viral replication, triggering an inflammatory response leading to necrosis within the affected ganglion (7, 8). BP typically presents with unilateral facial weakness, rarely bilateral, and may be accompanied by paresthesia, postauricular pain, hyperacusis, ageusia, or epiphora (9–11). The yearly incidence of BP was estimated to be 24.5 patients per 100,000 population from 2007 to 2022 (12). An article published in 2010 in Italy reported that the cumulative incidence was 53.3/100,000 per year (13). Additionally, researchers have reported an increased incidence during the third and fourth decades of life and inconsistent distribution between males and females (10, 14–17). More recently, a large-scale genetic meta-analysis has found evidence supporting a correlation between BP and a sequence variant, rs9357446-A (18).

One current significant discussion regarding BP is the influence of temperature on its onset. Research findings have indicated a notable link between low temperatures and BP development, suggesting that colder months may correlate with a greater BP incidence (19–22). Conversely, two studies have demonstrated differing results, with BP cases increasing during warmer months, particularly in spring and summer (23, 24). This discrepancy highlights the complexity of seasonal changes in BP incidence. Understanding the seasonal effect on BP occurrence is crucial for guiding preventive strategies and patient education, potentially reducing the overall burden of this unpredictable disease. To date, there are limited comprehensive systematic reviews or meta-analyses that have investigated the effects of seasonal changes on BP frequency. The present study aimed to systematically review and analyze the current literature examining the impact of seasonal variability (winter, spring, summer, and autumn) on the incidence of BP.

The present study was designed to determine the effect of seasonal variation on the incidence of idiopathic BP across several articles. Researchers in this field have hypothesized that cold weather increases the risk of idiopathic BP development. Thus, multiple studies have been conducted worldwide to test this hypothesis; however, inconsistencies have been reported. Therefore, this systematic review and meta-analysis aimed to consolidate these findings to provide well-supported evidence. This analysis revealed that the highest proportions of BP were encountered during winter (0.27) and the lowest during summer (0.22), reflecting an absolute difference of 5%. The subgroup analysis was found to be statistically insignificant. However, the analysis indicated that there is borderline trend toward higher incidence in cold-weather months.

The pattern observed in colder periods could be explained by vascular and inflammatory factors. Chen and Sun (33) reported that cold exposure increases vasoconstrictor endothelin 1 (ET-1) levels in cardiovascular tissues. Moreover, another study revealed that ET-1 could contribute to BP development by promoting secondary ischemia to the facial nerve (34). Another possible mechanism involves an inflammatory response triggered by upper respiratory tract infections (URTIs), which are commonly encountered during cold months, such as infections with influenza A, respiratory syncytial virus (RSV), or rhinovirus (35). These infections induce the release of inflammatory mediators, which may impair the vascular supply and function of the facial nerve (4).

A similar study employed Google Trends to investigate the seasonal variations of facial paralysis from data across 19 countries, revealing significant peaks in search activity during spring in the northern hemisphere and winter in the southern hemisphere (36). Their different approach offers valuable insight from different perspectives by reflecting public interest in BP. However, this paper is limited in that it solely relies on infodemiological data, unlike our study, which is based on clinical cohort studies of diagnosed BP cases. Our findings contrast with those of previously published studies, which have suggested a significant influence of climate on BP frequency. Zhang et al. (37) published an article in 2024 demonstrating a reduction in daily BP cases of 0.026 (β = −0.026, p = 0.003) with each 1 °C increase in temperature in China. Similarly, a study in Greece revealed a strong positive correlation between a composite metric combining temperature and wind speed with BP incidence (Spearman's ρ = 0.406, p < 0.001), with cases peaking in the winter months and declining at higher temperatures (38). Additionally, Franzke et al. (39) concluded that the air temperature coupled with the atmospheric pressure are risk factors for BP development. Our findings also align with those of several studies (19, 20, 22–24, 30, 38) showing a peak during winter and others (19–22, 24, 30, 31) demonstrating a decrease in summer. In contrast, three studies (28, 29, 40) demonstrated higher BP cases during the summer and lower proportions during the winter. Five studies (20, 21, 23, 24, 31) demonstrated that, compared with warmer seasons, colder seasons, including autumn and winter, are associated with a greater incidence of BP. Notably, some of the included studies presented inconsistent results compared with other studies conducted in the same region with similar climates according to the Köppen–Geiger climate classification system (41). Moreover, two studies (28, 31) conducted in Greece and two (19, 23) conducted in Turkey revealed inconsistent results despite the almost identical population characteristics and climates. These observations and the pooled effect analysis support our finding of no significant seasonal effect on BP development.

The importance of these findings lies in understanding the patterns of BP. Per our findings, we now know that there is no association between seasonal shifts and the incidence of BP. However, the increased number of cases in winter months may still carry clinical relevance when applied to larger populations, where even small percentage shifts translate into a considerable number of additional cases. This information could be helpful to healthcare providers in alerting at-risk populations and encouraging awareness regarding the disease. On the basis of our results and those of previously published papers, general weather conditions do not affect BP; however, specific climatic parameters, such as atmospheric pressure, wind speed, and acute dramatic temperature changes, might play a role.

This study employed a comprehensive search strategy across multiple databases, adhered to the PRISMA (25) guidelines to ensure high quality and a proven reporting process, and used a unified seasonal definition to minimize categorization bias. In this review (8 studies, n = 3,363), we address a notable gap in the literature by systematically evaluating the potential seasonal variation in BP, offering evidence that enhances understanding of its potential epidemiological patterns. Furthermore, the geographic proximity of the included studies strengthens the validity of the findings. Additionally, the funnel plots and Egger's test revealed no publication bias among the included studies, further strengthening our findings' validity. Despite these strengths, this study has several limitations. First, pooled estimates were calculated using raw proportions without variance-stabilizing transformation, which may introduce minor distributional bias. Second, significant heterogeneity was observed across studies (I² = 74.4%). This heterogeneity could negatively impact the results if it is due to variability in population characteristics or geographical location, potentially affecting the accuracy and generalizability of our findings. Conversely, the heterogeneity could have arisen from variability in the reported proportions, which would be expected if the seasonal changes had no influence. Nonetheless, heterogeneity was accounted for using a random effects model and subgroup analysis, as well as testing for publication bias. Third, the limited number of included studies (n = 8) may have reduced the statistical power to detect true seasonal variations. Moreover, the results of Egger's regression test should be interpreted with caution, as the test has limited statistical power in meta-analyses that include fewer studies. With only eight studies in this analysis, the ability to detect small-study effects or publication bias may be reduced. Fourth, most of the included studies were conducted in regions that are part of or close to the Middle East, the Mediterranean Sea, and with limited representation from European regions. This restricted geographical coverage limits the generalizability of our results to global seasonal patterns and biases them toward no global seasonality. Fifth, meta-regression analyses did not identify any study-level characteristics (such as region, publication year, NOS quality rating, mean participants' age, or sample size) that significantly contributed to the observed heterogeneity. This suggests that unmeasured factors, such as methodological differences, population characteristics, or contextual factors (e.g., healthcare systems, climate, or diagnostic criteria), may account for the variability between studies. Sixth, the NOS evaluation revealed that the included studies had a high-to-moderate risk of bias and low-to-fair quality but to a degree that was insufficient to warrant their exclusion, and no inter-rater agreement metrics were assessed, which may have introduced subjectivity in the quality assessment. Another limitation is that the exclusion of studies published before 2000 and non-English publications may have introduced selection bias by omitting otherwise eligible studies. Additionally, none of the studies provided a detailed breakdown of HBG severity scores for each season, limiting our ability to assess potential seasonal influences on BP severity. Finally, risk factors and clinical status, such as the affected side, previous infections, and comorbidities, were not considered when the seasonal impact on BP was analyzed, as these factors were also not provided by the included studies for each season.

Future studies, preferably longitudinal studies, should be undertaken to confirm whether there is a link between specific meteorological parameters and BP and to clarify whether seasonal patterns differ by age, sex, or comorbidities, such as hypertension, diabetes mellitus, and immunity status, which could reveal additional information regarding the underlying etiology of this disease. Furthermore, future meta-analyses including a larger number of studies are warranted to confirm these findings.

This systematic review and meta-analysis investigated the association between the four seasons and BP occurrence. No statistically significant difference across seasons on the risk of developing BP has been found. However, the highest proportion of BP cases occurred during winter, followed by autumn, spring, and summer. This observation promotes targeted patient education during colder months, emphasizing recognition of BP symptoms to encourage earlier presentation and timely management. Because the studies included in this review were conducted in limited number of geographical locations, further epidemiological research is needed across diverse climatic regions using specific climatic parameters.