Encephalitis is, by definition, inflammation of the brain. Because the blood brain barrier is so effective at protecting the central nervous system, encephalitis is extremely uncommon-each year there are only about 10,000-20,000 cases in the United States. However, when infectious or other pathophysiologic processes cause brain inflammation, the damage can be devastating. Some of these disorders can resolve without major long term sequelae if accurately diagnosed early enough to provide appropriate supportive management. Some are treatable; others occur in outbreaks, which can be abrogated once recognized. Consequently, understanding the major forms of encephalitis is of considerable importance.

Herpes infections were first recognized in ancient Greece. Hippocrates and other scholars described the skin lesions of the herpes simplex viruses (HSVs) as herpein meaning to creep or crawl. The herpes family of viruses is ubiquitous and the lifetime risk of acquiring a herpesvirus approaches 100%. Acute infections cause a myriad of clinical syndromes and vary depending on the type of virus, but rarely cause encephalitis. The herpesviruses establish latency in the nervous system and may reactivate causing various clinical syndromes, including encephalitis. The risk of reactivation and subsequent encephalitis increases with age and immunosuppression. The herpesviruses are the most commonly identified etiologic agents of acute viral encephalitis in the western hemisphere (1-4). Additionally, HSV type 1 is the most common fatal cause of acute viral encephalitis (5).

Acute disseminated encephalomyelitis (ADEM) is an acute, immunemediated, inflammatory demyelinating disorder of the central nervous system (CNS), characterized clinically by new onset neurological signs and symptoms, coupled with neuroimaging evidence of focal or multi-focal demyelinating lesions. ADEM (also referred to as post-or para-infectious encephalomyelitis) frequently follows a recognized prodromal infectious illness (most often viral) or antecedent event, such as vaccination or immunization (post-immunization/vaccinial), administration of serum, adverse reaction to drugs, or transplant. ADEM is classically considered a monophasic illness.

Although an important cause of central nervous system (CNS) infection, Listeria monocytogenes poses the paradox of frequent exposure with rare development of life threatening illness. This environmental microbe has a tropism for the CNS, which expresses itself in a variety of clinical syndromes, including pure meningitis, meningo-encephalitis, and rhomboencephalitis. Consideration of this pathogen is essential when providing empiric antimicrobial therapy for these clinical syndromes.

Cancer affects the nervous system in a myriad of ways. Most common by far is direct invasion-of either the central or peripheral nervous system-either by contiguous spread or by metastatic disease. However, it has long been recognized (1) that nervous system function can be affected in patients with cancer in the absence of direct nervous system invasion-even before the cancer is diagnosed. Such remote effects of cancer can be biochemical, such as confusion or somnolence associated with hyponatremia in a lung cancer patient with the syndrome of inappropriate antidiuretic hormone secretion. It may be more directly hormonal-such as the myopathy associated with a cortisol secreting adenoma. Most intriguing are those disorders that are now known to be immune mediated, in which the immune response elicited by the tumor also recognizes neural antigens and triggers immune-mediated damage to the nervous system. The term "paraneoplastic neurologic disorder" is used primarily to describe these immune-mediated, cancer-triggered, remote nervous system abnormalities.

Patients with encephalitis may require critical care management (1,2). Some have a mild course and can receive supportive care on the ward, but a good number of patients with rapidly progressive symptoms will need close monitoring in the intensive care unit. Reasons include altered consciousness causing respiratory difficulties, concerns about subclinical seizures, and presence of cerebral edema associated with increased intracranial pressure (ICP). Surgical management (brain biopsy), might be indicated for some patients, prompting monitoring in the intensive care unit (ICU), particularly when the cause is unknown and PCR’s are unrevealing. In other patients, emergency surgery is needed to reduce ICP (e.g., temporal lobectomy in herpes simplex encephalitis) (3,4). This chapter provides a template for the initial care and more specific system care of patients who have severe forms of encephalitis. The major issues to consider are summarized in Figure 1.