Central Nervous System Infections

Central nervous system (CNS) infections—i.e., infections involving the brain (cerebrum and cerebellum), spinal cord, optic nerves, and their covering membranes—are medical emergencies that are associated with substantial morbidity, mortality, and/or long-term sequelae that may have catastrophic quality of life implications for affected individuals. Acute CNS infections fall broadly into three categories: meningitis, encephalitis, and abscesses. These infections generally result from blood-borne spread of the respective microorganisms, but there are several mechanisms by which CNS infections may occur. Head trauma resulting in fractures at the base of the skull or the cribriform plate can cause CNS infections via the creation of an opening between the CNS and the sinuses, mastoid, middle ear, or nasopharynx. Extrinsic contamination of the CNS can also occur intraoperatively during neurosurgical procedures and as a result of bacterial entry via congenital malformations (e.g., spina bifida or sinus tracts). Implanted medical devices or hardware (e.g., shunts, ventriculostomies, deep brain stimulators, or external drainage tubes) can become colonized and serve as foci of infection. Viruses, such as rabies, herpes simplex virus (HSV), or poliovirus, can spread to the CNS via intraneural pathways resulting in encephalitis. If infection occurs at sites contiguous with the CNS, such as the middle ear or mastoid, infection may spread directly into the CNS causing brain abscesses. Amoebae, like Naegleria fowleri, can infect people when contaminated water enters the body through the nose. Once the ameba enters the nose, it travels to the brain where it causes primary amebic meningoencephalitis, which is usually fatal. These infections may in turn spread directly to the subarachnoid space, causing meningitis. Alternatively, the organism may reach the CNS indirectly via venous drainage or via the sheaths of cranial and spinal nerves. An intact blood–brain barrier (BBB) is essential in preventing the development of CNS infections. Breaches in the BBB can be caused by damage to the surrounding tissue, overproduction of cytokines that degrade tight junction proteins, and microbe-specific interactions that facilitate transcellular passage of the microorganism. The microorganisms that cause CNS infections include a wide range of bacteria, mycobacteria, yeasts, fungi, viruses, ameba, spirochaetes (e.g., neurosyphilis), and parasites (e.g., cerebral malaria and strongyloidiasis). The clinical presentation of CNS infections can either be nonspecific or characterized by distinct, recognizable clinical syndromes. In severe cases, patients may require admission to the neuro-intensive care unit (NICU). This chapter addresses a wide range of severe CNS infections that often require admission to the NICU. Topics covered include the epidemiology, pathophysiology, and clinical symptoms of CNS infections as well as diagnostic techniques, therapeutics, and prognosis. In addition, this chapter will discuss the relevant neuroanatomy as it relates to the pathogenesis of these infections.