In this study, we investigated whether seasonal climatic conditions had an impact on the onset of RVO. There is significant seasonal variation in climatic conditions such as temperature, atmospheric pressure, humidity, sunshine, rainfall and wind velocity. The impact of weather conditions on the onset of stroke or myocardial infarction has been examined, with temperature variation found to be a risk factor. The highest mortality rate is found in regions where the mean temperature is approximately 0 °C and the diurnal variance of the temperature is the biggest, ranging approximately 8–10 °C. In northeastern Japan, such climatic conditions in winter seem to be risk factors for stroke or myocardial infarction . In contrast, there is limited evidence indicating whether climatic conditions represent risk factors for RVO onset. A Swedish study found a significant association between the onset of CRVO and the winter–spring period . In London, the onset of CRVO showed significant cyclic variation, being most frequent in the months of September through February . In Taiwan, the incidence of RVO is significantly associated with the seasons, with a peak in January . All of these studies were conducted in the Northern Hemisphere, meaning that the seasonal variation in temperature would be similar to that experienced in Japan.
The results in those previous studies [13–15] were similar to those in this study, with a higher incidence of RVO in winter (January/February) and a lower incidence in summer (July/August). However, studies conducted in Iowa City, Iowa, USA and (countrywide) in Armenia did not find any seasonal variation in the onset of CRVO [16,17]. The average monthly temperatures in each of the previous study locations are shown in Figure 5. A study investigating seasonal variation in stroke onset found that there was only an association in locations where the annual temperature differential was greater than 10 °C . In the aforementioned studies that found significant seasonal variation in the onset of RVO, the annual temperature differential was more than 10 °C. However, the studies conducted in Iowa City and Armenia, which did not find any seasonal variation in the onset of RVO, were also in regions that have an annual temperature differential of more than 10 °C. Therefore, the conclusion regarding the impact of the temperature differential is not universally applicable. It is anticipated that factors such as patient race and access to heating equipment could influence these findings, and future studies will need to elucidate the impact of those factors, as well as explore additional geographical regions, to clarify the influence of weather conditions on RVO.
In this study, RVO tended to develop more in January/February and May/June and less in July/August. The RVO rates may be related to low temperatures in January and February and high humidity in May and June (the rainy season in Japan). In another vascular occlusive disease, cerebral infarction, some studies showed that its occurrence was highest in summer [7,8], some in winter [9,10]. As possible mechanisms contributing to vascular occlusion, it has been suggested that arteriosclerosis and venous obstruction are more prominent during winter, with dehydration prevalent in the summer . As we found the incidence of RVO to be lowest during the summer, dehydration may not be a risk factor for the onset of this condition. In addition, the onset of cerebral hemorrhage and myocardial infarction have been found to be lower in summer and higher in winter , which is in accordance with the findings of this study.
Hypertension has been reported to be a major risk factor for cerebral hemorrhage and myocardial infarction, and is believed to also be important for the development of RVO . Therefore, in the pathophysiology of RVO, it is expected that high blood pressure would be a more important factor than is dehydration. However, because there was no dependence of RVO onset on hypertension status in the present study, it is possible that fluctuation in blood pressure within an individual could be a risk factor. Regarding the increased risk of arteriosclerosis, vein occlusion and hypertension during winter, several studies have investigated these elements. Low temperature is associated with an increase in blood viscosity. Low temperature may also cause an increase in platelets, erythrocytes and fibrinogen, and a decrease in antithrombin III [4,5]. Catecholamines, cholesterol and vasopressin increase during winter . It has also been suggested that lower blood levels of vitamin D in winter may be related to an increased incidence of CRVO . A negative correlation between atmospheric pressure and systolic blood pressure has also been observed . Finally, respiratory and circulatory system parameters, such as ventilation, heart rate, blood pressure and red blood cell count, increase when the temperature or atmospheric pressure decreases .
This study had some limitations. There was a recruitment bias with our cohort, whereby patients were only eligible to be included in the study if they visited the hospital. This excluded patients who were asymptomatic or those who did not visit the hospital because of a lack of concern about their condition. Of the patients who did visit the hospital, those for whom the date of RVO onset could not be established were excluded from the study; which may also introduce some bias. Finally, as this study was only conducted over a short period (36 months) and at a single facility, future studies should focus on longer-term recruitment and multiple sites.