1. Pathogenesis

a. Hematogenous spread of organisms from primary infection in lungs or elsewhere in body; including infection by organisms in birth canal and aspiration of contaminated amniotic fluid.

b. Iatrogenic (e.g. introduction of organism into CSF during lumbar puncture in patient with septicemia).

c. Direct inoculation secondary to trauma.

d. Spread from contiguous infection.

2. Etiologic Agents:

a. Bacteria are the most common. Common organisms, by age group, are:

E. coli

6 months to 5 years:
H. influenzae (30-40%)
N. meningitidis (30%)
S. pneumoniae (10%)

5 to 50 years:
H. influenzae (10%)
N. meningitidis (30%
S. pneumoniae (10%)

Over 50 years:
N. meningitidis (10%)
S. pneumoniae (40-60%)

Notes:-percentages represent numbers within age-groups
-N. meningitidis is sometimes referred to as "meningococcus"
-S. pneumoniae is sometimes referred to as "pneumococcus"

In general, pneumococcal and meningococcal meningitis are the most common types

b. Viruses, spirochetes, fungi, and M. tuberculosis are less common agents.

3. Pathological changes:

a. Acute pyogenic meningitis (usually bacterial):

1) Purulent exudate in subarachnoid space and along vessels, includes polys and bacteria; the severity of the exudate varies with the organism

2) Inflammatory infiltrate in walls of arteries and veins (vasculitis); in severe cases ischemia may result from subsequent thrombosis

3) Cellular infiltration of cranial nerves and spinal roots, causing demyelination, cranial nerve palsies

4) May be infiltration of ependyma and choroid plexus

5) Edema may occur in brain tissue

This dorsal view of the brain in the skull illustrates purulent meningitis with opaque meninges due to the dense infiltrate.

b. Acute lymphocytic meningitis (viral): mononuclear infiltrate; mild

c. Chronic meningitis - caused by fungi (e.g., candidiasis, cryptococcosis), tubercle bacillus (tuberculosis), T. pallidum (syphilis):

1) Exudate, mononuclear infiltrate with lymphocytes, mononuclear cells, small tubercles (caseous necrosis surrounded by epithelioid cells and giant cells, peripheral ring of lymphocytes)

2) Localized over base of brain

3) Vasculitis common

4) Dense fibrous arachnoiditis, in late stages

4. Spinal fluid changes:
Acute pyogenic meningitis Acute lymphocytic meningitis Chronic meningitis Normal values
(mm H2O)
Increased Normal or mildly increased Increased 70-200
Decreased Normal Sometimes decreased 45-80
Increased Mildly increased Increased 15-45
Up to thousands of PMN's Hundreds of mono-
nuclear cells
Hundreds of mono-
nuclear cells
0-5 mononuclear cells

5. Complications:

a. Hydrocephalus caused by obliteration of subarachnoid space, exit foramina or other sites

b. Edema and increased intracranial pressure

c. Cranial nerve palsies

d. Thrombosis from inflammatory infiltrate in the walls of arteries

e. Brain abscess or subdural abscess

f. Seizures
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