Despite the availability of nucleic acid amplification tests (NAATs), most of aseptic acute meningitides, encephalitides, and meningoencephalitides (AAMEMs) in adults remain of unknown etiology so far. To shed light on such topic, we aimed to evaluate potential predictors for AAMEMs of unknown origin. We collected retrospectively data from all consecutive cases of AAMEMs in adults discharged from an Italian referral hospital, from January 2004 to December 2016. Laboratory analysis included common immunometric methods and NAATs. Potential predictors for unknown etiology (age, seasonality, serum C-reactive protein value, antibiotic use before lumbar puncture, immunodeficiency conditions, clinical symptoms and signs) were evaluated by a logistic regression analysis model. A p value ≤ 0.05 was considered to indicate statistical significance. The study included 92 patients (median age 39 years; 54.3% males) affected by meningitis (n = 57), encephalitis (n = 25), and meningoencephalitis (n = 10). The identified agents that cause AAMEMs were herpesviruses (20.7%), enteroviruses (5.4%), tick-borne encephalitis virus (3.3%), influenza virus A (2.2%), West Nile virus (1.1%), and parvovirus B19 (1.1%), while 66.3% of cases were of unknown etiology. No predictor was found to be significantly associated with AAMEMs of unknown etiology. We suggest that potential infectious agents causing undiagnosed cases should be investigated among non-bacterial, non-opportunistic, and non-seasonal organisms.
