Meningitis

By acute meningitis is meant acute leptomeningitis; that is, an infection of thepia mater and arachnoid, which inevitably involves the subarachnoid space and the cerebrospinal fluid. There are three principal ways in which the meninges may become infected:
    1. After fracture of the skull. In the case of a penetrating injury of the cranial vault organisms may be carried in from the scalp. When the base of the skull is fractured, they may spread to the meninges from the nasopharynx, middle ear, or mastoid.

    2. Extension to the meninges of a pre-existing pyogenic infection of one of the nasal sinuses, the middle ear, or mastoid.

    3. Infection through the blood stream. Sometimes meningitis is the only or the principal manifestation of a bacteraemia, as in meningococcal meningitis and some cases of pneumococcal meningitis, or the infection of the meninges may be secondary to focal infection elsewhere in the body, for example pneumonia, empyema, osteomyelitis, enteric fever, etc. Tuberculous meningitis may be part of a general miliary dissemination of the infection. Sometimes meningitis is secondary to a blood-borne infection which first settles in the brain, as in the case of metastatic cerebral abscess or tuberculoma. It is probable that the infection is blood-borne in most cases of meningitis caused by a virus.
 

Sometimes meningeal inflammation plays a subordinate part in the picture of encephalitis or myelitis, for example poliomyelitis. Such a condition may be described as a meningo-encephalomyelitis. Less frequently encephalitis is secondary to meningitis, as in some cases of meningococcal and tuberculous meningitis.
  Spinal meningitis, that is meningitis arising in, and at first limited to, the spinal canal, is rare and is usually secondary to osteitis of the vertebral column. Rarely also meningitis may be caused by an organism introduced into the subarachnoid space at lumbar puncture.
  The organisms commonly responsible for meningitis are Neisseria meningitidis, Diplococcus pneumoniae, Haemophilus influenzae, Streptococcus, Staphylococcus, Escherichia coli, all of which cause pyogenic meningitis; Mycobacterium tuberculosis; and various viruses which cause so-called 'acute aseptic' or 'lymphocytic' meningitis. Other organisms less frequently the cause of meningitis are Salmonella typhosa, Bacillus anthracis, Brucella abortus, Pseudomonas aeruginosa, Leptosplra, and yeasts such as Cryptococcus neoformans (Torula histolytica).
It is convenient to consider the commoner forms of acute meningitis under three headings:

    1.Acute pyogenic meningitis (Figure 1)   

    2.Tuberculous meningitis (Figure 2)  

    3.Acute aseptic meningitis

Figure 1

Figure 2