[1] Univ Oslo, Sect Gen Practice, Inst Gen Practice & Community Med, N-0316 Oslo, Norway
[2] Akershus Univ Hosp, Head & Neck Res Grp, Res Ctr, Lorenskog, Norway
[3] Univ Oslo, Fac Div, Ulleval Univ Hosp, N-0316 Oslo, Norway
[4] Univ Oslo, Fac Div, Akershus Univ Hosp, Nordbyhagen, Norway
[5] Ullevaal Univ Hosp, Dept Neurol, Oslo, Norway
[6] Akershus Univ Hosp, Helse Ost Hlth Serv Res Ctr, Lorenskog, Norway