under the weather with stroke; more data emerge
TRANSCRIPT
Leading opinion
Under the weather with stroke; more data emerge
Jesse Dawson�, Terry Quinn, and Matthew R. Walters
Key words: barometric pressure, epidemiology, risk factors,
stroke, temperature, weather
If the popular press is correct, we will spend the next 10 years
basking in the increasing temperatures brought to us by global
warming, only for the Gulf stream to cease and for those of us
in the North to become very cold indeed. The influence of
weather and climatic variables on health is at best poorly
understood but is important to consider. Much morbidity is
unexplained and, while traditional cardiovascular risk factors
explain a large proportion of cardiac and cerebrovascular
disease, the question of ‘why today’ and what precipitates
events in those at risk is less well understood. Could it be
changes in our external environment outwith our control?
We already know that the incidences of both myocardial
infarction (MI) and stroke (1) appear higher on Mondays, a
phenomenon that may largely be driven by a cluster of
incidence in the working population. Further, the onset of
stroke and MI is most common in the morning. This temporal
pattern could be explained by the well-documented circadian
rhythms of blood pressure, plasma catecholamines (and other
hormones) and, perhaps more obviously, levels of activity (2).
On the surface, it seems little can be done about this; if only we
could stop going for work. Such information could be used,
however, to inform blood pressure-lowering strategies to
ensure the morning rise in blood pressure is controlled. Could
other variables trigger or precipitate events in those who have
atherosclerosis?
We have recently shown, in a west of Scotland population,
that the risk of ischaemic stroke increased by 2�1% with each
11C increase in temperature in the preceding 24 h (3). Higher
daily maximum temperatures linked with increased rate of
lacunar infarction. We found the risk of intracerebral haemor-
rhage to be increased with each 10 hPa decline in atmospheric
pressure in the preceding 48 h. This is of potential interest and
plausible explanations exist but we must acknowledge that,
while others have found similar results (4, 5), considerable data
suggest the opposite (6) or that no such association exists (7).
A complex relationship
Any relationship between the weather and cardiovascular
morbidity and mortality is likely to be complex and difficult
to characterise. Data suggest that the impact of climactic
variables could differ between stroke subtype and the range
of climactic variables that may influence health is huge. They
include temperature, humidity, air quality, barometric pres-
sure, wind speed, precipitation and solar and geomagnetic
energy. Further, both absolute levels and changes in these
variables may be of importance and climates worldwide are
diverse and varied. The fact that in the West of Scotland, where
rain and cold are ubiquitous, that increasing temperature links
with increased ischaemic stroke risk seems particularly unfair.
The most important issue to consider is that the ambient
temperature outside may not reflect the temperature to which
we are exposed because of air conditioning and heating
systems. Also, changes due to temperature or other climactic
variables will be difficult to differentiate from seasonal differ-
ences in cardiovascular disease that likely reflect factors more
than just the weather. Further, hospital attendance is also
affected by the weather, which may bias data based on date of
hospital attendance or admission (8).
Inconsistent data
At best, the data on this topic are inconsistent, which is little
wonder, given the complexities outlined above. Multicentre
studies or registries will provide less readily interpretable in-
formation while single-centre data can only be generalised to
similar climactic regions. As an example, some reports show an
increased stroke risk with higher temperatures (3), while others
show no association or a link with lower temperatures (6).
Potential mechanisms
Temperature may affect blood pressure and sympathetic
activity and haematologic variables such as serum fibrinogen
Correspondence: Dr Jesse Dawson�, Acute Stroke Unit, Division of
Cardiovascular and Medical Sciences, University of Glasgow, Western
Infirmary, Glasgow, G11 6NT, UK. Tel: 144 141 211 6395; e-mail:
& 2009 The Authors.Journal compilation & 2009 World Stroke Organization International Journal of Stroke Vol 4, February 2009, 19–20 19
levels and blood viscosity. Most evidence suggests that blood
pressure increases with lower ambient temperature (9, 10),
although higher temperature may increase nocturnal levels.
Serum fibrinogen levels have been shown to be highest in the
winter months in the elderly and also link to lower ambient
temperature (11). Other data suggest changes in fibrinogen
levels that may be seasonal rather than truly related to the
ambient temperature (12). Increased temperature has also
been associated with worsening of endothelial function (13).
Systemic infection, whose rates are influenced by season and
ambient temperature, could influence all these variables and is
already known to increase stroke risk temporarily (14).
Air pollution with higher concentrations of small particu-
late matter has been associated with increased stroke risk, both
in studies that measure this directly (5, 15) and indirectly (in
areas with known high concentrations) (16). A proposed
mechanism is alveolar inflammation with a subsequent in-
creased release of proinflammatory and procoagulant factors
(17) and perhaps an increased risk of respiratory infection.
Interestingly, one report suggests that this is a phenomenon
restricted to warmer periods (18).
Changes in barometric pressure have been shown to increase
the risk of subarachnoid haemorrhage in a number of studies
(19, 20). The mechanism is unclear but changes in intra-
aneurysmal or transaneurysmal pressure would be expected to
influence the risk of rupture. This is more obvious in cases of
subarachnoid haemorrhage associated with vigorous activity
and it is interesting that data suggest this relationship may also
hold for intraparenchymal brain haemorrhage.
To further confound issues, it is important to note, however,
that the weather and ambient temperature may influence
changes in behaviour, such as alcohol consumption and
activity, which could themselves influence stroke risk.
Summary
Climatic variables may impact on stroke risk. To date, data are
conflicting and confusing, which is understandable, given the
diverse climates studied, the multiple risk factors for stroke and
the differing relative importance of these within the stroke
subtypes. A systematic review of the literature to date and
further research into the possible mechanisms of any associa-
tion would be timely and valuable. It is of course unclear
whether such knowledge could be translated into specific
public health advice or intervention but at the very least it
may help unlock some of the mechanisms by which acute
cerebrovascular events are precipitated.
References
1 Manfredini R, Casetta I, Paolino E et al. Monday preference in onset
of ischemic stroke. Am J Med 2001; 111:401–3.
2 De Maertelaer V, Jacquy J: Multivariate analysis of the effect of climatic
factors on the probability of cerebral infarction according to age. Acta
Neurol Scand 1987; 75:56–61.
3 Dawson J, Weir C, Wright F et al. Associations between meteorolo-
gical variables and acute stroke hospital admissions in the west of
Scotland. Acta Neurol Scand 2008; 117:85–9.
4 Berginer VM, Goldsmith J, Batz U, Vardi H, Shapiro Y: Clustering of
strokes in association with meteorologic factors in the Negev Desert of
Israel: 1981–1983. Stroke 1989; 20:65–9.
5 Low RB, Bielory L, Qureshi AI, Dunn V, Stuhlmiller DFE, Dickey DA: The
relation of stroke admissions to recent weather, airborne allergens, air
pollution, season, upper respiratory infections, and asthma incidence,
September 11, 2001, and day of the week. Stroke 2006; 37:951–7.
6 Feigin VL, Nikitin YP, Bots ML, Vinogradov TE, Grobbee DE: A popula-
tion-based study of the associations of stroke occurrence with weather
parameters in Siberia, Russia (1982–92). Eur J Neurol 2000; 7:171–5.
7 Field TS, Hill MD: Weather, Chinook and stroke occurrence. Stroke
2002; 33:1751–7.
8 Bachenheimer EA: The relationship between weather and hospital
emergency department volume. J Healthcare Manag 2007; 52:127–36.
9 Jehn M, Appel LJ, Sacks FM, Miller ER: The effect of ambient
temperature and barometric pressure on ambulatory blood pressure
variability. Am J Hypertens 2002; 15:941–5.
10 Woodhouse PR, Khaw KT, Plummer M: Seasonal variation of blood
pressure and its relationship to ambient temperature in an elderly
population. J Hypertens 1993; 11:1267–74.
11 Stout RW, Crawford V: Seasonal variations in fibrinogen concentra-
tions among elderly people. Lancet 1991; 338:9–14.
12 van der Bom JG, de Maat MP, Bots ML, Hofman A, Kluft C, Grobbee
DE: Seasonal variation in fibrinogen in the Rotterdam study. Thromb-
Haemostasis 1997; 78:1059–62.
13 Nawrot TS, Staessen JA, Fargard RH, Van Bortel LM, Struijker-Boudier
HA: Endothelail function and outdoor temperature. Eur J Epidemiol
2005; 20:407–10.
14 Smeeth L, Thomas SL, Hall AJ, Hubbard R, Farrington P, Vallance P:
Risk of myocardial infarction and stroke after acute infection or
vaccination. N Engl J Med 2004; 351:2611–8.
15 Maheswaran R, Elliott P: Stroke mortality associated with living near
main roads in England and Wales. Stroke 2003; 34:2776–80.
16 Miller KA, Siscovick DS, Sheppard L et al. Long-term exposure to air
pollution and incidence of cardiovascular events in women. N Engl
J Med 2007; 356:447–58.
17 Seaton A, MacNee W, Donaldson K, Godden D: Particulate air
pollution and acute health effects. Lancet 1995; 345:176–8.
18 Kettunen J, Lanki T, Tiittanen P et al. Associations with fine and
ultrafine particulate air pollution with stroke mortality in an area
of low air pollution levels. Stroke 2007; 38:918–22.
19 Buxton N, Liu C, Dasic D, Moody P, Hope DT: Relationship of
aneurysmal subarachnoid hemorrhage to changes in atmospheric
pressure: results of a prospective study. J Neurosurg 2001; 95:391–2.
20 Jehle D, Moscati R, Frye J: The incidence of spontaneous subarachnoid
hemorrhage with change in barometric pressure. Am J Emerg Med
1994; 12:90–1.
& 2009 The Authors.20 Journal compilation & 2009 World Stroke Organization International Journal of Stroke Vol 4, February 2009, 19–20
Leading opinion J. Dawson et al.