The Central Nervous System:
Infections
Classified according to the infected tissue
• (1) Meningeal infections (meningitis), which
may involve the dura primarily
(pachymeningitis) or the pia-arachnoid
(leptomeningitis)
• (2) Infections of the cerebral and spinal
parenchyma (encephalitis or myelitis).
• In many cases, both the meninges and the
brain parenchyma are affected to varying
degrees (meningoencephalitis).
Meningeal Infections
Acute Leptomeningitis
• Acute inflammation of the pia mater and
arachnoid.
• Caused by infectious agents; rarely, release of
keratinaceous contents from an intradural
epidermoid cyst or teratoma causes a chemical
meningitis.
• When the term meningitis is used without
qualification, it means leptomeningitis.
Etiologic Agents in Bacterial Meningitis.
Streptococcus Most common agent in patients over age 40
pneumoniae years
30–50% of cases in adults
10–20% of cases in children
5% of cases in infants
Neisseria
Most common agent in patients aged 5–40
meningitidis years
25–49% of cases in children aged 5–15 years
10–35% of cases in adults
Haemophilus Most common agent in patients aged 1–5
influenzae
years
40–60% of cases in children aged 1–5 years
2% of cases in adults
Acute Viral Meningitis
• 10,000 cases per year in the United States
• 90% of these occur in patients under 30
• Mild, benign illness, which rarely causes
death.
• Enteroviruses, mumps virus, and lymphocytic
choriomeningitis (LCM) virus.
• An acute meningitis occurs in 10% of patients
(HIV) infection
Tuberculous Meningitis
• Typically chronic; however, in the early stages
there may be an exudative phase that
resembles acute meningitis
Routes of Infection of the Meninges
• Bloodstream spread accounts for the majority
of cases;
• The primary entry site of the organism may be
the respiratory tract (N meningitidis, H
influenzae, S pneumoniae, C neoformans,
many viruses),
• skin (bacteria causing neonatal meningitis),
• Intestine (enteroviruses).
• From direct spread of organisms from an
infected middle ear or paranasal sinus,
especially in childhood.
• May be associated with skull fractures,
Especially those at the base of the skull
• Lumbar puncture.
• Organisms may also gain entry through the
intact nasal cribriform plate (eg, free-living
soil amebas in stagnant swimming pools).
Pathology
• Grossly, the leptomeninges are congested and
opaque and contain an exudate.
• Microscopically, acute meningitis is characterized
by hyperemia, fibrin formation, and inflammatory
cells.
• In bacterial meningitis, neutrophils dominate
• in acute viral meningitis, neutrophils are rare and
lymphocytes dominate
• In acute tuberculous meningitis, there is an
inflammatory exudate that contains increased
numbers of both neutrophils and lymphocytes.
Pyogenic meningitis, showing obliteration of the
gyri of the brain surface by the purulent exudate.
Clinical Features
• Acute meningitis presents with fever and
symptoms of meningeal irritation,(headache,
neck pain, and vomiting.)
• Physical examination reveals neck stiffness
and a positive Kernig sign (due to reflex spasm
of spinal muscles, a consequence of irritation
of nerves passing across the inflamed
meninges)
• Bacterial meningitis is a serious disease with
considerable risk of death
• Viral meningitis is usually a mild, self-limited
infection.
• Tuberculous meningitis has an insidious onset
and a slow rate of progression but is frequently
a severe illness with a fatal outcome if not
treated
Encephali Bacterial Viral
tis
Tuberculo Brain
us
Abscess
Pressure Raised
Raised
Raised
Raised
Gross
Clear
appearan
ce
Protein Slightly
elevated
Glucose Normal
Chloride Normal
Turbid
Clear
Clear;
may clot
High
May be
very high
Clear
Slightly Very high Elevated
elevated
Very low Normal Low
Normal
Low
Normal Very low Normal
or low
Cells
Lymphoc Neutroph Lymphoc Pleocytos Pleocytos
ytes or ils
ytes
is2
is
normal
Gram
Negative Positive Negative Negative Occasion
stain
in 90%
ally
positive
Acid–fast Negative Negative Negative Rarely
Negative
stain
positive
Bacteria Negativ Positive Negativ Negativ Occasio
l culture e
in 90% e
e
nally
positive
Mycoba Negative Negative Negative Positive Negative
cterial
culture
Viral
culture
Positive Negative Positive Negative Negative
in 30%
in 70%
or less
Chronic Meningitis
• Facultative intracellular organisms such as
mycobacterium tuberculosis, fungi, and
treponema pallidum.
• It is now relatively uncommon in the United
States
• More prevalent in parts of africa, india, south
america, and southeast asia
Pathology & Clinical Features
• caseous granulomatous inflammation with
fibrosis
• Marked fibrous thickening of the meninges
• The entire brain surface is involved, with the
basal meninges more severely affected in cases
of tuberculosis.
• The causative agent may be identified in tissue
sections specially stained for acid-fast bacilli
and fungi
Complications of chronic meningitis include
• (1) Obliterative vasculitis (endarteritis obliterans),
which may produce focal ischemia with microinfarcts
in the brain and brain stem;
• (2) Entrapment of cranial nerves in the fibrosis as
they traverse the meninges, resulting in cranial nerve
palsies; and
• (3) Fibrosis around the fourth ventricular foramina,
causing obstructive hydrocephalus
• Insidious onset with symptoms of diffuse
neurologic involvement,
• Including apathy, somnolence, personality
change, and poor concentration.
• Headache and vomiting are less severe than in
acute meningitis,
• Fever is often low-grade.
• Focal neurologic signs and epileptic seizures
result from ischemia, cranial nerve palsies, or
hydrocephalus
Diagnosis & Treatment
• Lumbar puncture
• Serologic tests for syphilis performed on both serum
and CSF are positive in meningeal syphilis.
• Culture is commonly positive in cases caused by
tuberculosis and fungal infection
• Skin tests for tuberculosis and fungal infection are
positive unless the patient is anergic.