I. INFECTIOUS DISORDERS
A. GENERAL PRINCIPLES
1. Inflammation vs Infection:
- Inflammation is not synonymous with infection. The body responds to
an insult with inflammation; if an infectious agent is present, inflammation
is accompanied by infection. The subject of this unit is the response of
the nervous system to infectious agents.
2. Routes of Infection:
Although the brain is well protected by coverings (meninges) and the blood
brain barrier, these are not absolute barriers.
- Routes of infection include:
- a. Hematogenous Spread: blood-borne spread from a primary
source of infection, often the lung
- b. Direct Inoculation: infection as the result of trauma or
surgery
- c. Direct Spread/Extension: infection from nearby site of suppuration
- d. Spread of viruses along nerves: A few viruses can spread
along peripheral nerves to reach the central nervous system
3. Locations and Definitions:
Infections tend to be confined by the meninges. Terminology for infections
in various compartments of the nervous system are listed below:
- a. Epidural Abscess or Empyema: accumulation of pus in the space
between dura and skull (periosteum)
- b. Pachymeningitis: inflammation of the dura (rare)
- c. Subdural Abscess or Empyema: accumulation of pus in the space
between the dura and arachnoid
- d. Meningitis/Leptomeningitis: inflammation of the pia and arachnoid,
in the subarachnoid space (e.g. meningococcal meningitis)
- e. Encephalitis: generalized inflammation of the brain parenchyma
- f. Abscess or Granuloma: localized inflammation in the brain
or spinal cord parenchyma
- g. Myelitis: generalized inflammation of the spinal cord parenchyma
(e.g. polio myelitis)
- h. Radiculitis: inflammation of the nerve roots
- i. Neuritis: inflammation of a peripheral nerve
- j. Encephalomyelitis: generalized inflammation of the brain
and spinal cord
- k. Ependymitis: inflammation of the ventricles and ventricular
lining
- Note: the terms neuritis and myelitis are synonymous
with neuropathy and myelopathy and can indicate a non inflammatory process
4. Cerebrospinal Fluid:
a. Detection of Meningitis: The cerebrospinal fluid (CSF) is a sensitive
indicator of a leptomeningeal infection, as will be noted later.
- b. Detection of Reactive Meningitis: CSF analysis can detect
an inflammatory response in the leptomeninges due to infection in other
locations. For example, an infection may be localized to the epidural
or subdural spaces, or within the brain or spinal cord parenchyma. However,
some inflammatory reaction, termed reactive leptomenintitis will
be present in the leptomeninges and subarachnoid space, and will be detectable
in the CSF. In this condition, organisms are NOT recovered from the CSF
even though the cell count, protein concentration and pressure may be elevated.
5. Cellular Reaction:
-
- The cellular reaction to infection depends on many factors. These include
the kind of infectious agent, the age of the host, the location of the
lesion and the presence of other diseases.
- a. Bacterial agents usually cause purulent reactions
with polymorphonuclear leukocytes and necrosis.
- b. Fungal agents cause granulomatous inflammation.
- c. Viral agents provoke a lymphocytic response and, rarely,
may precede poorly understood allergic phenomena to myelin protein characterized
by demyelination.
- In the fetus, cellular reaction may be minimal although massive destruction
of the central nervous system may occur.
6. Sequelae & Complications:
-
- Sequelae of a CNS infection depend on the pathogenesis and location
of lesions. Examples include:
- a. Hydrocephalus after obstruction of CSF flow in leptomeningitis
b. Permanent damage to cranial nerves
- c. Focal and generalized signs due to neuronal destruction,
e.g. seizures, movement disorders, mental retardation
d. Thrombosis due to vasculitis
e. Consequences of increased intracranial pressure due to edema
or expanding mass lesions