1. Department of Anaesthesiology , Medical Research Center Oulu , Oulu University Hospital , Oulu , Finland
2. Department of Primary Health Care , Helsinki University Central Hospital , Helsinki , Finland
3. Department of General Practice and Primary Health Care , University of Helsinki , Helsinki , Finland
4. Unit of Primary Health Care , Kuopio University Hospital , Kuopio , Finland
5. Department of Neurosurgery , Helsinki University Central Hospital , Helsinki , Finland
6. Mutual Insurance Company Etera , Helsinki , Finland