Changes in Temperature, Dew Point Linked to Stroke

Susan Jeffrey

February 13, 2014

SAN DIEGO, California — Low temperatures, large changes in temperature within a day, and a higher average dew point are all associated with increased stroke hospitalizations, a new study suggests.

While previous studies have shown higher stroke rates during winter months vs other seasons, this study finds that regardless of these seasonal effects, colder temperatures seem to drive increased stroke hospitalizations.

"I think what this indicates for us is that meterologic factors could be triggers for acute stroke events, but clearly additional research is needed to understand what are the underlying mechanisms," said lead author Judith Lichtman, PhD, MPH, associate professor of epidemiology at Yale School of Public Health in New Haven, Connecticut. "I think if we can understand the reasons for the associations, then this could potentially lead to targeted preventive interventions for those at risk."

Dr. Lichtman presented the results here at the American Stroke Association International Stroke Conference (ISC) 2014.

Seasonal Variation

Stroke incidence is known to vary seasonally, with most studies showing higher stroke rates in colder winter months, the authors note, although some report higher rates in other seasons as well. Few studies, Dr. Lichtman said, have looked at the association of temperature changes on stroke incidence that are irrespective of seasonal effects.

"Many of the studies that have been conducted have been in small geographic regions, so you're looking at 1 particular area. They often combined stroke types when we know that they have different underlying reasons for causing stroke, and they really did not adjust for patient-level factors, things like did they have hypertension or diabetes," she said. "So we felt that these were some important gaps in the field."

The aim of this study, then, was to look for any associations between temperature and ischemic stroke hospitalizations and in-hospital mortality, adjusting for patient demographic characteristics and comorbid conditions. They used data from the Healthcare Cost and Utilization Project (HCUP) Nationwide Inpatient Sample, the largest all-payer database in the United States, for a stratified sample of all hospitals in the country.

Included were discharges for patients 18 years of age and older from the last 2 years available, 2010 and 2011. Data on temperature and dew point were obtained from the National Climatic Data Center, looking specifically at average daily temperature, the daily temperature difference (that is, the difference between maximum and minimum temperatures during the day), and the dew point, "which we see as a proxy for humidity," Dr. Lichtman noted.

"We actually linked the data and the weather down to the county level for the hospital where they were treated, so we really could take a look at very localized temperature patterns for people who had been hospitalized for a stroke," she said.

To identify stroke discharges, the investigators used International Classification of Diseases, Ninth Revision, Clinical Modification, codes 433, 434, and 436, and in-hospital mortality. Using hierarchical logistic regression, they estimated the odds of hospitalization for ischemic stroke and in-hospital mortality, adjusting for patient demographic characteristics, season, census regions, and comorbid conditions.

They identified 157,130 hospitalizations for acute ischemic stroke. The average patient age was 71 years; about half of patients were women, and 68.4% were white, with high rates of comorbid conditions, including hypertension (80.4%), diabetes (33.3%), history of stroke (13.8%), and heart failure (12.1%). The average length of stay was 4.3 days.

Looking at increments of 5°F, the researchers found that as average temperatures went up, hospitalizations for stroke went down; "in other words, we found that there were higher stroke rates in the colder temperatures."

Looking at fluctuating temperatures within a day, they found that for each 5-degree fluctuation in temperature, stroke hospitalization rate increased 6%. "Similarly, we found that as the dew point, or proxy for humidity went up, we also saw an increased risk in stroke hospitalization," Dr. Lichtman said.

In-hospital mortality did not show as dramatic an effect, she noted. "We saw a little bit of an inclination that in the colder months the in-hospital mortality might be a little bit higher but it was not statistically significant."

Table. Stroke Hospitalizations and In-Hospital Mortality by 5°F Temperature Increments

Measure Stroke Hospitalizations OR (95% CI) In-Hospital Mortality OR (95% CI)
Average temperature (5°F) 0.97 (0.97 - 0.98) 0.97 (0.94 - 1.00)
Daily temperature difference (5°F) 1.06 (1.05 - 1.07) 1.02 (0.98 - 1.05)
Dew point temperature (5°F) 1.02 (1.01 - 1.03) 1.00 (0.97 - 1.03)

CI = confidence interval; OR = odds ratio.

Limitations of the findings include use of diagnostic codes for stroke events; linking of weather data to the treating hospital, not necessarily the county of residence; and exclusion of nonhospitalized strokes, she noted, "and we couldn't really follow people over time. I think future research in this area would be interesting to look at the long-term effects of large weather changes."

Although the mechanisms linking colder temperatures to stroke are not clear, some that have been suggested are vasoconstriction, increased blood pressure, or serum lipid concentrations. "I think the idea is that it challenges your body; that for those who might already have high blood pressure, if you are then put in this additional factor or stressor, that could then perhaps be a trigger," she said.

Asked for comment on these findings, Philip B. Gorelick, MD, MPH, neuroscience medical director, Mercy Health Hauenstein Neurosciences, Grand Rapids, Michigan, said in his 30 years of practice in the Midwest, "in the winter, and in the summer, when we see extremes of temperature, we see a lot more stroke patients and our service in the hospital is a lot busier taking care of these patients. And in the spring and in the fall, there's a lull; people have 'gone fishing' for some reason. So I've come to believe there is a real association here."

While the possible effects of temperature extreme in the winter is easy to understand, he noted, winter is also influenza season, and acute infections are now being linked to stroke.

"I think there's a lot of stress when these temperatures are swinging, your blood pressure can change, you can have vasoconstriction," Dr Gorelick told Medscape Medical News. "They've probably been going along for a while at a steady clip of vascular tone, and then this sudden swing or change [in temperature] could make things a lot different in terms of your physiology and turn it into pathophysiology."

The study was funded by the Yale School of Public Health. Dr. Lichtman has disclosed no relevant financial relationships.

International Stroke Conference (ISC) 2014. Abstract WP123. Presented February 12, 2014.

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