Meningitis and Encephalitis: Causative Agents

• Meningitis and encephalitis are caused by microorganisms; recognize the most common causative agents.

Meningitis

• Meningitis is caused by bacterial, viral, or fungal infections.
• Characterized by inflammation of the meninges.

Common causative microorganisms in bacterial meningitis include:

• Streptococcus pneumoniae: Most common in adults.
• Neisseria meningitidis: Common in adolescents and young adults.
• Haemophilus influenzae type b (Hib): Was common in children, has decreased due to vaccination.
• Listeria monocytogenes: Affects neonates, pregnant women, and immunocompromised individuals.

Common causative microorganisms in viral meningitis include:

• Enteroviruses (e.g., Coxsackievirus, Echovirus): Most common cause of viral.
• Herpes simplex virus (HSV-2).
• Varicella-zoster virus.
• Mumps virus: Common in regions where vaccination coverage is low.

Encephalitis

• Most cases are viral.
• Encephalitis is an acute inflammation of the brain.

Common causative microorganisms in encephalitis include:

• Herpes simplex virus type 1 (HSV-1): Most common sporadic cause of infectious encephalitis.
• Arboviruses: Spread through mosquitoes or ticks (e.g., West Nile virus, Japanese encephalitis virus).
• Varicella-zoster virus: Causes encephalitis in immunocompromised patients.
• Rabies virus: Rare but fatal if untreated.
• Cytomegalovirus (CMV): Typically seen in immunocompromised individuals.

Pathophysiology of Meningitis and Encephalitis

• Involves distinct mechanisms driven by inflammation, infection, and immune responses.
• Affects the brain and its protective coverings.

Meningitis Pathophysiology

• Inflammation of the meninges, the protective layers surrounding the brain and spinal cord.

Bacterial Meningitis:

• Bacteria enter the CNS through hematogenous spread, direct extension from adjacent infections (e.g., sinusitis), or traumatic entry.
• Neutrophils are recruited to the subarachnoid space, leading to the release of inflammatory cytokines and bacterial toxins.
• Disruption of the blood-brain barrier allows fluid and cells to enter the CSF, resulting in cerebral edema and increased intracranial pressure (ICP).
• The inflammatory exudate can obstruct CSF flow, causing hydrocephalus.
• Damage to the blood vessels (vasculitis) can lead to thrombosis, ischemia, or infarction, further contributing to neuronal injury.

Viral Meningitis:

• Viruses typically access the CNS via the bloodstream, peripheral nerves, or infected immune cells.
• Inflammation is less severe than in bacterial meningitis, with viral invasion mainly limited to the meninges.
• The immune response, including cytokine release and increased BBB permeability, combats the virus but can also cause neurological symptoms.
• Viral meningitis often resolves spontaneously but can be severe in neonates or immunocompromised individuals.

Fungal Meningitis:

• Develops slowly, often affecting immunocompromised individuals.
• Fungal pathogens enter via inhalation and hematogenous spread, leading to chronic inflammation in the meninges.

Encephalitis Pathophysiology

• Refers to inflammation of the brain parenchyma, most commonly due to viral infections.

Infectious Encephalitis (often viral):

• Common causes include herpes simplex virus (HSV-1), arboviruses (e.g., West Nile virus), and enteroviruses.
• Viruses invade CNS cells, causing direct neuronal injury and inflammation.
• Neuronal damage is mediated by viral replication, immune cell infiltration, and the release of pro-inflammatory cytokines.
• Inflammatory response can lead to increased BBB permeability, cerebral edema, and neuronal death.
• Severe cases can result in widespread neurodegeneration, seizures, and long-term cognitive deficits.

Autoimmune and Postinfectious Encephalitis:

• Autoimmune forms occur when antibodies attack brain antigens, often triggered by an initial viral infection.
• Postinfectious encephalitis is marked by an inflammatory reaction in the brain following systemic infections, such as rubella or measles.

Clinical Manifestations of Meningitis

• Can be caused by bacterial, viral, or fungal infections.
• Symptoms fall into infectious signs, meningeal signs, and neurological signs.

Infectious Signs:

• Fever
• Tachycardia
• Chills

Meningeal Signs:

• Severe throbbing headache
• Severe photophobia (sensitivity to light)
• Nuchal rigidity (neck stiffness)
• Positive Kernig and Brudzinski signs

Neurological Signs (due to increased intracranial pressure):

• Decrease in consciousness
• Cranial nerve palsies
• Focal neurological deficits (e.g., hemiparesis/hemiplegia, ataxia)
• Seizures
• Projectile vomiting
• Papilledema (swelling of the optic disc)
• In severe cases, progression to delirium, unconsciousness, or death

Special Presentation of Meningitis:

• Meningococcal meningitis may present with a petechial or purpuric rash covering the skin and mucous membranes.

Clinical Manifestations of Encephalitis

• Acute inflammation of the brain, usually viral.
• Can result from autoimmune or postinfectious causes.

General Symptoms of Encephalitis:

• Fever
• Headache
• Altered level of consciousness (confusion to coma)
• Seizures
• Focal neurological deficits (weakness or aphasia)

Specific Viral Features:

• Herpes simplex virus type 1 causes sporadic encephalitis and leads to temporal lobe damage, causing disturbances in memory and behavior.
• Other viral causes (e.g., arboviruses, varicella, and rubella) may present with systemic symptoms like a rash or signs of generalized infection.

Progression of Encephalitis:

• Severe cases may result in permanent neurological damage, such as cognitive impairment or motor deficits.

Explore the common microorganisms responsible for meningitis and encephalitis. Meningitis is often caused by bacterial, viral, or fungal infections, with agents like Streptococcus pneumoniae and Neisseria meningitidis being prominent. Encephalitis is usually viral, caused by different viruses.