Encephalitis hospitalization rates and inpatient mortality in the United States, 2000-2010

Abstract

Background: Encephalitis rates by etiology and acute-phase outcomes for encephalitis in the 21st century are largely unknown. We sought to evaluate cause-specific rates of encephalitis hospitalizations and predictors of inpatient mortality in the United States.

Methods: Using the Nationwide Inpatient Sample (NIS) from 2000 to 2010, a retrospective observational study of 238,567 patients (mean [SD] age, 44.8 [24.0] years) hospitalized within non-federal, acute care hospitals in the U.S. with a diagnosis of encephalitis was conducted. Hospitalization rates were calculated using population-level estimates of disease from the NIS and population estimates from the United States Census Bureau. Adjusted odds of mortality were calculated for patients included in the study.

Results: In the U.S. from 2000-2010, there were 7.3±0.2 encephalitis hospitalizations per 100,000 population (95% CI: 7.1-7.6). Encephalitis hospitalization rates were highest among females (7.6±0.2 per 100,000) and those <1 year and >65 years of age with rates of 13.5±0.9 and 14.1±0.4 per 100,000, respectively. Etiology was unknown for approximately 50% of cases. Among patients with identified etiology, viral causes were most common (48.2%), followed by Other Specified causes (32.5%), which included predominantly autoimmune conditions. The most common infectious agents were herpes simplex virus, toxoplasma, and West Nile virus. Comorbid HIV infection was present in 7.7% of hospitalizations. Average length of stay was 11.2 days with mortality of 5.6%. In regression analysis, patients with comorbid HIV/AIDS or cancer had increased odds of mortality (odds ratio [OR] = 1.70; 95% CI: 1.30-2.22 and OR = 2.26; 95% CI: 1.88-2.71, respectively). Enteroviral, postinfectious, toxic, and Other Specified causes were associated with lower odds vs. herpes simplex encephalitis.

Conclusions: While encephalitis and encephalitis-related mortality impose a considerable burden in the U.S. in the 21st Century, the reported demographics of hospitalized encephalitis patients may be changing.

Figure 1. Encephalitis hospitalization rates^a, 2000–2010: (A) by sex, age, and region-specific rates and (B) disease category.

Abbreviations: NOS  =  not otherwise specified; inf.  =  infectious; Postimmun.  =  Postimmunization; Spec.  =  Specified. a) Population rates are calculated using hospitalization estimates from the Nationwide Inpatient Sample and population-specific estimates from the United States Census Bureau.

Figure 2. Encephalitis hospitalization rates by five-year age groups stratified by disease category^a.

Abbreviations: HSV  =  Herpes Simplex Virus; VZV  =  Varicella Zoster Virus; HHV  =  Human Herpesvirus; NOS  =  not otherwise specified. a) Population rates are calculated using hospitalization estimates from the Nationwide Inpatient Sample and population-specific estimates from the United States Census Bureau. Toxic encephalitis not shown here.

Figure 3. Seasonal variation of hospital admissions for viral encephalitis (A) by month^a and (B) by season^b (n = 83,225).

Abbreviations: NOS  =  not otherwise specified; VZV  =  Varicella Zoster Virus; HHV  =  Human Herpesvirus; HSV  =  Herpes Simplex Virus. a) Includes a stacked representation of all known viral and viral NOS hospital admissions for encephalitis from 2000 to 2010. For example, there was an estimated total of 9,827 viral and viral NOS admissions in the month of August over the entire 11-year period from 2000 to 2010, of which, 1,085 (11%) were due to West Nile Virus infection. Data on admission month was missing for 8.7% of encephalitis admissions. b) Significance: *P<.01 in both base case (2000–2010) and sensitivity analysis (2005–2010) using adjusted Wald test comparing each interval to January-March. Under the base case analysis, July-September for other viral causes and October-December for viral NOS causes were significant (P<.05); however, these were insignificant in the sensitivity analysis (P = .12 and P = .75, respectively).