Walsh & Hoyt: Barriers to Infection

The structures of the CNS that limit the pathogenesis of infection include bone and fibrous membranes that cover the brain and spinal cord. These protective coverings may be overcome by transport of organisms from the scalp or face, by emissary veins, or by diploic veins that anastomose within the skull and have external as well as internal connections. The dura mater, its two layers fused over the skull but separated within the vertebral canal, also is an important barrier and can prevent the spread of external infection into the CNS regardless of its severity. The external cellular layer of the arachnoid provides a barrier to infection in both directions, and the pia mater, delicate though it is, inhibits the spread of infection from the subarachnoid space to the cerebral cortex (and vice versa). Separate infections may, therefore, occur in four potential spaces: epidural, subdural, subarachnoid, and intraparenchymal.