[1] Hacettepe Univ, Ankara, Turkey
[2] Univ Verona, I-37100 Verona, Italy
[3] Univ Navarra, E-31080 Pamplona, Spain
[4] Univ Freiburg, Freiburg, Germany
[5] Cent Manchester Univ, Manchester, Lancs, England
[6] Univ Antwerp, St Augustinus Hosp, B-2020 Antwerp, Belgium
[7] Klinikum Braunschweig, Neurosurg Dept Staedt, Braunschweig, Germany
[8] Royal Natl Throat Nose & Ear Hosp, London WC1X 8DA, England
[9] Royal Free Hosp, London NW3 2QG, England
[10] Univ Uppsala Hosp, Uppsala, Sweden