Reader Response: Healthy subject bias may explain observed effect of sauna bathing on stroke risk
Diederik W.Dippel, Neurologist, Erasmus MC University Medical Center Rotterdam
Puck S.Fransen, Neurologist, Erasmus MC University Medical Center Rotterdam and Isala Ziekenhuis Zwolle
M. KamranIkram, Neurologist, Epidemiologist, Erasmus MC University Medical Center
Submitted June 11, 2018
We read with interest the paper by Kunutsor et al., which describes the association of sauna bathing with the risk of stroke in a large cohort of Finnish men and women. [1] Cohort studies comparing life style factors in relation to cardiovascular risk can suffer from a specific form of selection bias, for which different terms are used throughout the literature: healthy volunteer bias or healthy user bias. [2] Individuals who volunteer for a study may possess different characteristics than those in the target population. This implies that unobserved factors may explain the difference in stroke risk and not the exposure to, in this case, sauna bathing. The authors adjusted for baseline risk factors, but during the mean 14 years of follow up large differences in risk factors and exposure may have occurred which could account for the observed effect. Moreover, the lack of an adequate explanation of the biological mechanism and the similar effects on conditions such as Alzheimer disease and respiratory conditions suggest healthy subject bias rather than a specific effect of sauna bathing. [3, 4] In view of the healthy volunteer bias and an inadequate reference group, any inferences from this study cannot be generalized to the general population.
1. Kunutsor SK, Khan H, Zaccardi F, Laukkanen T, Willeit P, Laukkanen JA. Sauna bathing reduces the risk of stroke in finnish men and women: A prospective cohort study. Neurology 2018;90:e1937-e1944.
2. Shrank WH, Patrick AR, Brookhart MA. Healthy user and related biases in observational. studies of preventive interventions: A primer for physicians. J Gen Intern Med 2011;26:546-550.
3. Laukkanen T, Kunutsor S, Kauhanen J, Laukkanen JA. Sauna bathing is inversely associated with dementia and Alzheimer's disease in middle-aged Finnish men. Age Ageing 2017;46:245-249.
4. Kunutsor SK, Laukkanen T, Laukkanen JA. Sauna bathing reduces the risk of respiratory diseases: A long-term prospective cohort study. Eur J Epidemiol 2017;32:1107-1111.
We read with interest the paper by Kunutsor et al., which describes the association of sauna bathing with the risk of stroke in a large cohort of Finnish men and women. [1] Cohort studies comparing life style factors in relation to cardiovascular risk can suffer from a specific form of selection bias, for which different terms are used throughout the literature: healthy volunteer bias or healthy user bias. [2] Individuals who volunteer for a study may possess different characteristics than those in the target population. This implies that unobserved factors may explain the difference in stroke risk and not the exposure to, in this case, sauna bathing. The authors adjusted for baseline risk factors, but during the mean 14 years of follow up large differences in risk factors and exposure may have occurred which could account for the observed effect. Moreover, the lack of an adequate explanation of the biological mechanism and the similar effects on conditions such as Alzheimer disease and respiratory conditions suggest healthy subject bias rather than a specific effect of sauna bathing. [3, 4] In view of the healthy volunteer bias and an inadequate reference group, any inferences from this study cannot be generalized to the general population.
1. Kunutsor SK, Khan H, Zaccardi F, Laukkanen T, Willeit P, Laukkanen JA. Sauna bathing reduces the risk of stroke in finnish men and women: A prospective cohort study. Neurology 2018;90:e1937-e1944.
2. Shrank WH, Patrick AR, Brookhart MA. Healthy user and related biases in observational. studies of preventive interventions: A primer for physicians. J Gen Intern Med 2011;26:546-550.
3. Laukkanen T, Kunutsor S, Kauhanen J, Laukkanen JA. Sauna bathing is inversely associated with dementia and Alzheimer's disease in middle-aged Finnish men. Age Ageing 2017;46:245-249.
4. Kunutsor SK, Laukkanen T, Laukkanen JA. Sauna bathing reduces the risk of respiratory diseases: A long-term prospective cohort study. Eur J Epidemiol 2017;32:1107-1111.
For disclosures, please contact the editorial office at journal@neurology.org.