CNS Infections

CNS Infections

Dr. Mehzabin Ahmed

PREREQUISITE KNOWLEDGE:

• There are several layers of tissue that

separate the brain from the outside world.

First, there is the skin (scalp). Beneath the

skin is the bone or the skull. Below the

skull are three special coverings together

called the meninges.

• Dura mater: The outer layer of the

meninges is called the dura. The dura is

tough and thick and it can restrict the

movement of the brain within the skull.

This protects the brain from movements

that may stretch and break brain blood

vessels.

• Arachnoid: The middle layer of the

meninges is called the arachnoid.

• Pia mater: The inner layer, the one

closest to the brain, is called the pia.

Infections of the nervous system are classified according to the tissues

affected, as:

• Meningitis: inflammation of the meninges

– Pachymeningitis: Meningeal infections, which may involve the dura

primarily

– Leptomeningitis: Meningeal infections, which involve the pia and

arcachanoid mater.

• Encephalitis: Infections of the cerebral parenchyma

• Meningoencephalitis: In many cases, both the meninges and brain

parenchyma are affected varying degrees and thus is known as

meningoencephalitis.

Routes of entry of infectious agents into CNS:

• Direct spread: from an adjacent focus of infection such as the

paranasal sinuses or middle ear or from the open skull fractures.

• Hematogenous spread: Blood borne spread of the infectious agents

through the arteries, veins or through the anatomic connections

between the veins of the face and cerebral circulation.

• Through the peripheral nervous system: occurs with certain viral

infections such as rabies and herpes simplex virus- damage to the

nervous system may be the consequence of direct injury to the nervous

system or may occur through the elaboration of microbial toxins.

• Iatrogenic infection: following introduction of organisms into the CSF

during the time of lumbar puncture.

Acute meningitis

– Acute bacterial meningitis

• common in children below 5 years of age.

• Common organisms are

– E.coli, group B streptococcus, and hemophilus influenza.

– Acute viral meningitis (aseptic)

• this is a mild illness;

• clinical coarse is less fulminant than the bacterial meningitis.

• The common agents responsible are

– enterovirus, coxsackie virus, mumps virus and influenza.

Clinical features:

• The patients present with systemic features of inflammation and of meningeal

irritation (headache, vomiting, neck pain and rigidity).

• In general bacterial meningitis is a serious disease with considerable risk of death.

While viral meningitis is usually a mild self limited infection.

Diagnosis is made by examination of the CSF, which is obtained by lumber puncture.

Complications of bacterial meningitis are- *cerebral infarction, * obstructive

hydrocephalus, * cerebral abscess and * epilepsy.

A fever, together with spots or a rash that do not fade under pressure, is a medical emergency – meningococcemia- meningitis + hemorrhagic rashes/ spots

Chronic meningitis:

• It is caused by mycobacterium tuberculosis (produces a space

occupying lesion called tuberculoma, which clinically presents with

raised intracranial pressure & pressure effects on the adjacent

brain),

• Other agents:

– Fungi

– Treponema pallidium (tabes dorsalis- tertiary syphilis).

Bacterial meningitis- Thick purulent exudate Tuberculous meningitis-

Thick grey shaggy exudate encasting the cranial nerves and blood vessels

An autopsy demonstrating signs of streptococcus pneumoniae meningitis. The forceps are retracting the dura mater (white). Underneath the dura mater are the leptomeningitis, which are edematous and have multiple small hemorrhagic foci (red).

Infections of the brain parenchyma

Viral encephalitis: It is a well-recognized complication of many common viral

infection, most cases are mild, self-limiting condition, but infections such as

rabies and herpes simplex result in extensive tissue destruction and are

often fatal.

Bacterial encephalitis: Streptococci and staphylococci are the most common

offending agents. Caused by direct implantation of organisms, local

extension from adjacent foci (mastoiditis and paranasal sinus infections) or

from hematogenous spread.

Clinically patient presents with features of space occupying lesion,

including evidence of raised intracranial pressure and focal neurological

deficits depending on the location of the abscess.

Fungal infections: Cryptococcosis, candidiasis, aspergillosis and mucor

mycosis

Parasitic infections: Toxoplasma gondii, malaria, Entamoeba histolytica.

Examples of viral encephalitis

• Herpes simplex encephalitis:

– It is caused by HSV type 1& 2.

– HSV infection is common in infants and immunocompromised individuals.

• HIV encephalitis:

– HIV is a neurotropic virus that causes subacute encephalitis. \

– Clinically patient develops dementia.

• Poliomyelitis:

– It is caused by poliovirus.

– The virus is transmitted by feco- oral route; it enters the brain via the blood

stream.

– The virus infects the meninges, producing acute lymphocytic meningitis.

– It also infects the lower motor neurons producing acute paralysis of the affected

muscles.

– In acute cases death can occur from paralysis of respiratory muscles.

• Rabies:

– It is transmitted to humans by the bite of a rabid animal usually a dog.

– The virus enters the cutaneous nerve radicles at the site of inoculation and passes proximally to CNS.

– Rabies virus causes severe necrotising encephalitis

• maximally affecting the basal ganglia, midbrain, and fourth ventricles.

– Clinically the patient presents with fever, generalized convulsions precipitated by air, noise and water.

• Slow virus infection (Spongiform encephalopathies):

– Includes Creutzfeldt-Jacob disease (CJD) and Kuru.

– This disease is caused by subviral transmissible agent known as a Prion. Prion consists of abnormal protein that is infectious and transmissible.

– Clinically both the diseases are characterized by dementia, followed by ataxia, which is progressive and fatal

– The changes seen are

• slowly progressing degeneration of the brain with

– progressive neuronal loss,

– demyelination and

– spongiform change in the cerebral white matter.