AALBORG, DENMARK. There is evidence that hospital admissions for certain cardiovascular diseases such as heart attack, sudden death, and heart failure are substantially higher during the winter than during the rest of the year. A similar trend has been reported for hospital admissions related to atrial fibrillation (AF), which seem to be inversely associated with outdoor temperatures. Now a team of Danish and New Zealand researchers reports a significant seasonal variation in hospitalizations for AF-related stroke.

Their study involved 243,000 Danish men and women (48% female) diagnosed with AF during the period 1980 to 2008, and 51,500 New Zealand men and women (48% female) diagnosed with AF during the period 1991 to 2008. The median age at diagnosis was 75 years for men and 78 years for women in the Danish cohort, and 76 years for men and 78 years for women in the NZ cohort. About 33% of Danish study participants and 53% of NZ participants had one or more comorbid conditions with congestive heart failure at 28% being the most common amongst New Zealanders and hypertension at 11.5% being most common amongst Danes.

Hospitalization rate for AF was significantly higher amongst study participants above 65 years of age than for younger ones. It is also clear that the incidence (new cases on an annual basis) of AF-related hospital admissions increased significantly during the study period � about 5% annually in Denmark and 2.6% in NZ for patients older than 65 years. Corresponding numbers for younger participants were 5.4% in Denmark and 0.2% in NZ.

During follow-up, 36,000 Danish study participants and 7518 NZ participants (54.6% females) were hospitalized with stroke. The risk of stroke was found to be 22% higher in winter than in summer in Denmark, and 27% higher in NZ. The risk of dying within 30 days of suffering a stroke was about 20% in both countries as measured during the period 2000 to 2008.

The researchers conclude that the incidence of AF is increasing significantly and that the incidence of AF-related stroke peaks during the winter season in both Denmark and New Zealand.
Christensen, AL, et al. Seasonality, incidence and prognosis in atrial fibrillation and stroke in Denmark and New Zealand. BMJ Open, Vol. 2, No. 4, August 24, 2012

Editor�s comment: The finding that the incidence of AF-related stroke peaks in winter in intriguing. The authors suggest that climate factors such as ambient temperature, humidity, and hours of sunshine may play a role. There is evidence that hypertension is more pronounced during winter and that the degree of blood pressure elevation is directly related to blood viscosity and fibrinogen level. Thus it would seem plausible that the observed increase in stroke rate during winter time is related to increased blood viscosity.