title: Infectious encephalitis: a description of a Norwegian cohort.
abstract: BACKGROUND
  Prompt recognition and rapid initiation of adequate treatment are important for the outcome of encephalitis. Despite extensive diagnostic testing, the causative agent often remains unknown. The aim of this study was to investigate in how many patients the causative agent was found.
  
  
  METHODS
  Adults (≥ 18 y) diagnosed with ICD codes indicating encephalitis between 2000 and 2009 at Oslo University Hospital, Ullevål were retrospectively studied. Causative agents, clinical presentation, and demographic characteristics were registered. Those with an identified causative agent were compared to those for whom no agent could be found.
  
  
  RESULTS
  Of 136 registered patients, 70 were included in the study. Sixty-six did not fulfil our inclusion criteria or were diagnosed with other, more probable conditions. The causative agent was found in 30/70 (43%) patients; herpes simplex type 1 (10/70, 14%) and varicella zoster virus (6/70, 9%) were the most frequently identified agents. A bacterial cause was found in 6/70 (9%). Patients with an identified agent were more often men and had been ill longer than those for whom no agent could be found. Computed tomography and magnetic resonance imaging were more likely to be abnormal in those patients where a causative agent was found. Five of the 70 (7%) patients died of the infection. The identification rate did not increase during the study period.
  
  
  CONCLUSIONS
  The diagnosis of encephalitis remains a challenge, and in many patients no causative agent is found. Clinically, immune-mediated encephalitis cannot be differentiated from infectious encephalitis and represents an important differential diagnosis. More knowledge is needed to improve our diagnostic skills.
authors.affiliation: Department of Infectious Diseases, Oslo University Hospital, Ullevål, Ullevål, Norway. elpa1@ous-hf.no
authors.forename: Else
authors.initials: E
authors.lastname: Quist-Paulsen
authors.forename: Anne-Marte Bakken
authors.initials: AM
authors.lastname: Kran
authors.forename: Oona
authors.initials: O
authors.lastname: Dunlop
authors.forename: John
authors.initials: J
authors.lastname: Wilson
authors.forename: Vidar
authors.initials: V
authors.lastname: Ormaasen
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D004660:Encephalitis / Q000175:diagnosis* / Q000453:epidemiology / Q000382:microbiology / Q000821:virology; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D009664:Norway / Q000453:epidemiology; D012189:Retrospective Studies; D018709:Statistics, Nonparametric