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The meninges are the three membranes that along with the cerebrospinal fluid, enclose and protect the structures of the nervous system like the brain and the spinal cord. These consist of the pia mater (closest to the central nervous system organs), the arachnoid and the dura mater (farthest from the brain and spinal cord).
They also include blood vessels and contain cerebrospinal fluid. These are the structures involved in meningitis, an inflammation of the meninges, which, if severe, may become encephalitis, an inflammation of the brain.
The bacteria or infective organism spreads through the blood. They reach the meninges by one of two main routes: through the bloodstream or through direct contact between the meninges and either the nasal cavity or the skin. The infection begins in one part of the body – e.g. throat or lungs and spreads to the brain.
Normally the brain is protected by the blood brain barrier that is a thick membrane that filters out impurities from blood and does not allow entry into the brain. In some persons with decreased immunity the infection crosses the blood brain barrier.
Once bacteria have entered the bloodstream, they enter the subarachnoid space in places where the blood-brain barrier is vulnerable—such as the choroid plexus. Meningitis occurs in 25% of newborns with bloodstream infections due to group B streptococci; this phenomenon is less common in adults.
This leads to activation of the immune system that leads to swelling of the meninges to stop the spread of the infection. This swelling damages the brain and the nervous system.
The organism may also affect the cerebrospinal fluid (CSF). This adds to the injury and there is increased pressure on the brain and on the skull. This is called raised intracranial pressure. Direct contamination of the cerebrospinal fluid may arise from indwelling devices, skull fractures, or infections of the nasopharynx or the nasal sinuses.
With the inflammation, the immune system identifies the bacteria by its cell wall. The immune cells of the brain (astrocytes and microglia), respond by releasing large amounts of cytokines that are hormone-like mediators that recruit other immune cells. This stimulates 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). The blood vessels are also inflamed leading to cerebral vasculitis which leads to a decreased blood flow another type of edema, "cytotoxic" edema.