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Routes of Infection:

Organisms may gain access to the meninges by several routes

A. Hematogenous spread:

This is the most common routes, where entry into the subarachnoid space is gained through the chotoid plexus or through other blood vessels of the brain.

B. Direct spread from an infected site:

Direct spread from an infected site present close to meninges–Otaitis media, mastoiditis, sinusitis etc.

C. Anatomical defects in Central Nervous system (CNS) :

It may occur as a result of surgery., trauma, congenital defects which can allow organisms for ready and easy access for CNS

D. Direct intraneural spread along the nerve:

This is the least common route, occurs in case of rabies virus or herpex simplex virus infection.

Predisposing factors:

Pathogenesis of meningitis depends upon various host and microbial factors.

A. Age

Neonates have the highest prevalence of meningitis;probably due to,[1] their immature immune system, [2] acquiring the colonised organisms from mother’s birth canal(e.g. Group B streptococcus) [3] permeability of blood brain barrier.

B. Vaccination:

Widespread vaccination is shown to reduce the incidance of meningitis due to the particular agents, e.g. low incidence of H. influenzae meningitis following implementation of Hib vaccination

C. Factors that promote infection at primary site:

Because respiratory tract is the primary portal of entry for many etiological agents of meningitis, the factors that predisposed to respiratory infections can also increase the likelihood of development of meningitis. Such factors including alcoholism, diabetes, immunosuppression, splenectomy, etc.

D. Present of CSF shunts:

Present of CSF shunts can also directly predisposed to the pathogen entry

E. Breach in the blood brain barrier (BBB) :

Organisms can gain access through BBB by:

Loss of capillary integrity by disrupting rhe tight junction of BBBT

ransport within circulatory phagocytes

Crossing the endothelial cells by transport within the endothelial cell vacuoles

F. Microbial virulence factors:

It may also contribute to the pathogenesis such as:

Capsular polysaccharideL

ipoteichoic acid

IgA proteases

Definition:-

Meningitis is an inflammation of the meninges surrounding the brain and spinal cord. In true sense, it implies to infection of subarachnoid space or the leptomeninges(arachnoid and pia mater) but not dura mater.

Some important things about Meningitis:

1. Etiological classification of meningitis

2. Pathogenesis of meningitis

3. Clinical manifestation of Meningitis

4. Laboratory diagnosis

5. Treatment of Meningitis

1. Etiological classification:-

Based on the chance in Leucocyte in cerebrospinal fluid (CSF), meningitis can be grouped into:

A. Pyogenic meningitis:-

It is characterized by elevated polymorphoneuclear cell in CSF

It is exclusively caused by bacterial agent
According to the age,the agent involved may vary
Overall Streptococcus pneumonia is the most common cause of pyogenic meningitis
However for the neonatal meningitis, the common agents are Group Bhai streptococcus, Escherichia coli, and Listeria.

B. Aseptic meningitis:-

It is characterized by elevated lymphocytes in CSF. The etiological agents include viral, tubercular, fungal, and parasitic agents. The term ‘aseptic’ is frequently a misnomer, implying a lack of infection. On the contrary, with the advent of PCR and other newer diagnostics methods, it is now possible to detect most of these agents.