How does the infection occur?
The main infection pathway starts at the nasal cavity where various organisms can live on the mucous surface. The mucous surface normally acts as a barrier against the external environment. However, this barrier can be broken down, usually from a viral infection. Thus the bacteria gains access to the bloodstream.[16]
Once the bacteria is in the bloodstream, the bacteria can start to multiply and eventually find itself in the subarachnoid space through an entrance where the blood brain barrier is vulnerable, such as the choroid plexus. The bacteria can also gain access to the subarachnoid space from head injury or neurosurgery through direct access from the environment. Since, the subarachnoid space contains CSF, the bacteria therefore has access throughout the whole CSF circulation network including the brain and spinal cord.[16]
Video 1: How bacterial meningitis affects the body [10]
How does the infection affect the body?
The infection itself is not actually directly responsible for the symptoms that occur. Inflammation that occurs in the subarachnoid space is mostly due to the response of the immune system when bacteria enters the central nervous system (CNS). The CNS responds by releasing cytokines when atrocyes and microglia detect the components of a bacterial cell. Cytokines are hormone-like mediators that recruit other immune cells and stimulate other tissues to participate in an immune response. The blood–brain barrier becomes more permeable, leading to "vasogenic" cerebral edema (swelling of the brain due to fluid leakage from blood vessels). Large numbers of white blood cells enter the CSF, causing inflammation of the meninges and leading to "interstitial" edema (swelling due to fluid between the cells). In addition, the walls of the blood vessels themselves become inflamed (cerebral vasculitis), which leads to decreased blood flow and a third type of edema, "cytotoxic" edema. [16]
The three forms of cerebral edema all lead to increased intracranial pressure and together with the lowered blood pressure often encountered in acute infection, this means that it is harder for blood to enter the brain, thus brain cells are deprived of oxygen and undergo apoptosis (automated cell death). [18]
The severity of bacterial meningitis can range. Sometimes the inflammatory process is confined to just the subarachnoid space. The pial barrier is not penetrated and the underlying tissue remains untouched. However, if the pial barrier is broken, the inflammatory process accesses the underlying tissue and can lead to cortical destruction. [18]