Encephalitis refers to a constellation of syndromes that present with brain inflammation and rapidly progressive neurologic symptoms. Although traditionally recognized as being caused by infection, a growing number of autoimmune encephalitis syndromes have been identified (see Chapter 115 ). Brain tumors, acute drug toxicities, metabolic encephalopathies, and vascular diseases can mimic encephalitis, making workup and management particularly challenging.
Suspect encephalitis in any patient with rapidly progressive (days to weeks) memory deficits, encephalopathy, behavioral symptoms, focal neurologic signs, or new unexplained seizures.
Obtain a detailed history including recent travel and possible exposures. Many causes of viral encephalitis have seasonal incidence patterns. Baseline laboratory testing helps identify systemic infections and toxic/metabolic encephalopathies that mimic encephalitis. Test human immunodeficiency virus (HIV) status, as acute HIV infection can cause encephalitis and immunocompromised patients are prone to opportunistic brain infections.
Start acyclovir empirically in patients with suspected encephalitis for coverage of herpes simplex virus (HSV) and varicella zoster virus (VZV) encephalitis. Patients with headaches, fevers, or stiff neck should also be started on empiric antibiotics for bacterial meningitis (see Chapter 106 ).
Head computed tomography (CT) is a useful initial screening test but is often normal in encephalitis, in which case lumbar puncture (LP) is necessary to evaluate for brain inflammation and assess for specific infectious causes. LP is contraindicated if CT shows evidence of increased intracranial pressure (i.e., mass effect, diffuse edema, or radiographic herniation); in this scenario, neurosurgical consultation for possible ventricular drainage and cerebrospinal fluid sampling should be obtained. If a unilateral focal brain lesion is seen on CT, this makes encephalitis less likely (although does not rule it out) and suggests an alternative etiology. Multifocal brain lesions seen on CT raise concern for a number of specific diagnoses which can present with symptoms similar to encephalitis. In both cases, magnetic resonance imaging (MRI) brain with contrast should be pursued to better characterize the visualized lesions. If this cannot be obtained promptly, or if there are signs of meningeal inflammation (stiff neck, fever), proceed directly to LP.
CSF pleocytosis (corrected white blood cell count > 5/mm 3 ) suggests encephalitis, although encephalitis can occur with normal WBC counts. Send cryptococcal antigen and HSV PCR for all patients with suspected encephalitis. Other testing depends on clinical presentation, risk factors, and geographical region (see Table 105.1 ).