Inflammatory diseases of central nervous system. Meningitis, encephalitis - types according to etiology and pathomorphology
A word first on what injuries to the CNS look like generally. Within 12 hours of an irreversible hypoxic ischemic insult, acute neuronal injury becomes evident on H&E staining, there is shrinkage of nucleus, loss of Nissl substances and often the nucleus changing shape. Many neurological disorders have specific neuronal inclusions (e.g. Lewy bodies in parkinson's, tangles in Alzheimer's). Astrocytes are the principle cells responsible for repair and scar formation in a process known as gliosis, astrocytes undergo hypertrophy and hyperplasia in order to deal with insult.
Meningitis
Meningitis is an inflammatory process involving the meninges within the subarachnoid space, where this spreads to the brain it is meningoencephalitis. Generally meningitis is caused by bacterial or viral infection, although chemical injury can also cause it in rare cases. Types can be as follows:
- Acute pyogenic (usually bacterial)
- Aseptic (usually viral)
- Chronic (usually TB, spirochetal or cryptococcal)
Bacterial (acute pyogenic)
- E. coli in neonates and N.meningitidis in adolescents and young adults (S. pneumoniae in older people)
- Symptoms are that of infection with neurological impairment (headache, photophobia, irritability, clouding of consciousness and neck stiffness)
- Bacteria on lumbar puncture
- Gross morphology shows enlargement of vessels on brain surface, with pus often visible
- Microscopic examination shows neutrophils in entire subarachnoid space
Acute pyogenic (pneumococcal) meningitis at autopsy: dura lifted with forceps to show yellow-green pus in the subarachnoid space and congested surface vessels.
Image: “Streptococcus pneumoniae meningitis, gross pathology 33 lores” by Photo Credit: Content Providers(s): CDC/ Dr. Edwin P. Ewing, Jr., Public domain, via Wikimedia Commons.
Viral (aseptic)
- Clinical term for meningeal irritation, fever and alterations of consciousness
- Onset generally less severe than bacterial form
- CSF shows lymphocytosis with protein elevation, disease generally self limiting
- Identification of the virus is often difficult
TB (chronic)
- TB is by far the most common, only a moderate increase in CSF cellularity, notably difference is inclusion of intraparenchymal mass (tuberculoma) which may be associated with meningitis
- Subarachnoid space contains gelatinous or fibrinous exudate often at the base of the brain
- White granules may appear over leptomeninges, arteries show obliterative endarteritis
- Microscopic examination shows lymphocytes, plasma cells and macrophages, often with well formed granulomas, caseous necrosis and giant cells.
Remember: the CSF tells the types apart:
- Bacterial: many neutrophils, high protein, low glucose
- Viral: lymphocytes, mildly raised protein, normal glucose
- Tuberculous: lymphocytes (moderate count), markedly raised protein, moderately low glucose