Chapter 6: Central Nervous System Infections

INTRODUCTION

CENTRAL NERVOUS SYSTEM INFECTIONS

Central nervous system (CNS) infections are fortunately rare, but they are extremely serious. The cerebral cortex and spinal cord are confined within the restricted boundaries of the skull and bony spinal canal. Inflammation and edema therefore have devastating consequences, often leading to tissue infarction that in turn results in permanent neurologic sequelae or death.

To understand the pathogenesis and clinical consequences of CNS infections, a working knowledge of basic neuroanatomy and neurophysiology is important.

The cerebral cortex and spinal cord are suspended in and bathed by cerebrospinal fluid (CSF), which is produced by the choroid plexus lining the walls of the cerebral ventricles and resorbed by the arachnoid villi that drain into a large midline vein, the superior sagittal sinus. The cortex and spinal cord are surrounded by three-tissue layers called the meninges. The two layers closest to the cortex are called the pia mater (directly overlying the cerebral cortex) and the arachnoid. These layers make up the leptomeninges. The third layer, the dura mater (pachymeninges), serves as the outer layer (Figure 6-1). The CSF flows between the pia mater and arachnoid in the subarachnoid space.

CNS infections are classified by the site of the infection. Infection of the cerebral cortex is called encephalitis, and infection of the meninges is called meningitis. Abscesses usually form in three locations within the CNS: the cerebral cortex, where they are termed brain abscesses; between the dura and arachnoid, where they are called subdural abscesses; or immediately outside the dura, where they form epidural abscesses.

The capillaries of the brain and spinal cord differ from those in other regions of the body. The tight junctions linking the endothelial cells of the vessels in this region are less permeable than they are in vessels elsewhere. The limited permeability of the CNS vessels forms a physiologic barrier that is commonly called the blood–brain barrier. This barrier protects the CNS from invading pathogens and toxic substances. However, the impermeability of the CNS capillaries not only confers a protective effect but also prevents the entry of immunoglobulins, complement, and antibiotics. Therefore, if a pathogen breaches the blood–brain barrier, the host’s initial defense mechanisms are impaired, which partly explain the rapid progression and serious consequences of CNS infections. Antibiotics used to treat CNS infections must be capable of penetrating ...