Aetiology, clinical presentation, and outcome of encephalitis in Hong Kong

Introduction: Autoimmune encephalitis is now recognised as an important cause of encephalitis. On the other hand, encephalitis of unknown aetiology remained a diagnostic and management challenge. We aimed to study the clinical presentation and outcome of encephalitis of different aetiologies. Methods: Immunocompetent patients aged 18 years or above with encephalitis, who were admitted from January 2010 to September 2016, were recruited in this single-centre cohort study. Systemic laboratory testing was utilised to identify the aetiology of the encephalitis. The 6-month outcome was assessed using the Modified Rankin Scale (mRS). Good outcome was defined as mRS 0-2 and poor outcome was defined as mRS 3-6. Results: We identified 26 patients with encephalitis. Herpes simplex encephalitis was the most common identified aetiology (n=6; 23%), followed by anti-NMDA receptor encephalitis (n=3; 12%). There were six patients with encephalitis of unknown aetiology. Patients with autoimmune encephalitis had a lower frequency of fever at presentation (25.0% vs 88.9%; P=0.001) and cerebrospinal fluid protein level elevation (25.0% vs 70.6%; P=0.032), and a longer duration of symptoms before presentation (median 19.5 days, interquartile range 8.5-30 vs 1 day, interquartile range 1-3; P=0.030). The aetiology of the encephalitis was the major determination factor for neurological outcome. Autoimmune encephalitis was associated with good neurological outcome (hazard ratio=8.8; P=0.064). Encephalitis of unknown aetiology was associated with poor neurological outcome (hazard ratio=11.7; P=0.040) with the 6-month mortality rate of 50%. Conclusion: Autoimmune encephalitis has a distinct clinical characteristic and was associated with a good neurological outcome. Further research is required for the management of encephalitis of unknown aetiology, which has a much guarded prognosis