What are the causes of acute meningitis?

Causes of Acute Meningitis

Acute meningitis is caused by bacterial, viral, and non-infectious etiologies, with the specific pathogen varying significantly by age, immune status, and vaccination history.

Bacterial Causes

Most Common Bacterial Pathogens by Age Group

In adults, Streptococcus pneumoniae (pneumococcus) and Neisseria meningitidis (meningococcus) are the most common and most aggressive bacterial pathogens causing acute meningitis. 1

• Young adults (late teens to early 20s): Neisseria meningitidis causes a second peak of meningococcal disease in this age group 2
• Adults over 50 years: Streptococcus pneumoniae becomes more common 2
• Adults over 60 years: Listeria monocytogenes increases in frequency, though remains relatively rare 2
• Neonates: Common pathogens include Group B Streptococcus, Escherichia coli, and Listeria monocytogenes 2

Historical Context Post-Vaccination

• Before conjugate vaccines: Haemophilus influenzae type b was the most common cause in children, followed by S. pneumoniae (20%) and N. meningitidis (16%) 2
• After widespread vaccination: Conjugate vaccines in Europe resulted in virtual disappearance of H. influenzae type b, with substantial reductions in pneumococcal and meningococcal disease 2
• Current epidemiology: Bacterial meningitis now affects more adults than infants due to successful pediatric vaccination programs 2

Risk Factor-Specific Pathogens

Certain clinical scenarios predict specific bacterial causes:

• Skull fracture or CSF leak: Streptococcus pneumoniae is the predominant pathogen and represents a risk factor for recurrent meningitis 2
• Co-existing upper respiratory infection (otitis media, sinusitis): Pneumococcal meningitis is frequently associated with these infections 2
• Rash present: Neisseria meningitidis was the causative organism in 92% of meningitis cases with rash (89% petechial), though 37% of meningococcal meningitis patients had no rash 2
• Asplenia: Increased risk from all encapsulated bacteria including S. pneumoniae, N. meningitidis, and H. influenzae 2
• Complement deficiency: Markedly increased risk of meningococcal disease 2
• Immunocompromised states (alcohol dependency, diabetes, malignancy, immunosuppressive medications): Listeria monocytogenes becomes more likely 2

Viral Causes (Aseptic Meningitis)

Viral meningitis is the most common form of meningitis overall, accounting for at least 50% and possibly up to 80% of all meningitis cases. 3

Most Common Viral Pathogens

• Enteroviruses: The single most common cause of aseptic meningitis, accounting for more than half of all cases, with over 90 different enterovirus serotypes existing 3
• Herpes simplex virus type 2 (HSV-2): The second most common viral cause, more frequently causing meningitis than HSV-1 3
• Varicella zoster virus (VZV): Another common herpes virus cause 3
• Other herpes viruses: Cytomegalovirus (CMV) and Epstein-Barr virus (EBV) can also cause aseptic meningitis 3

Epidemiology of Viral Meningitis

• Annual incidence: 7.6 per 100,000 adults in the UK 3, 4
• Age distribution: Viral meningitis commonly occurs in adults in their 20s-40s, particularly in women 2
• Prognosis: Generally self-limited with good prognosis, though specific causes like VZV may require antiviral therapy 3

Recurrent Lymphocytic Meningitis

HSV-2 is the most common cause of recurrent lymphocytic meningitis. 2

Non-Infectious Causes

Systemic illnesses including autoimmune disorders and inflammatory conditions can also cause aseptic meningitis. 3

• These non-infectious causes should be considered when bacterial cultures remain sterile and viral testing is negative
• The term "aseptic meningitis" encompasses both viral and non-infectious inflammatory causes 3

Clinical Implications for Diagnosis

Age-Related Considerations

The elderly present differently than younger patients:

• More likely to have altered consciousness 2
• Less likely to have neck stiffness or fever 2
• Higher likelihood of pneumococcal or Listeria infection 2

Classic Triad Limitations

The classic triad of fever, neck stiffness, and altered consciousness was present in less than 50% of bacterial meningitis cases in the largest published study. 2

• Kernig's and Brudzinski's signs have high specificity (up to 95%) but sensitivity as low as 5%, making them unreliable for diagnosis 2
• Clinical features alone cannot distinguish bacterial from viral meningitis 2

Critical Diagnostic Approach

Because clinical features are unreliable, diagnosis relies on cerebrospinal fluid examination obtained from lumbar puncture. 4

• CSF Gram stain and CSF lactic acid levels provide the most rapid, reliable, and cost-effective tests to diagnose bacterial meningitis 5
• CSF testing for enteroviruses, HSV-1, HSV-2, and VZV by PCR should be performed when viral meningitis is suspected 3
• Unelevated CSF lactic acid levels rapidly and reliably rule out bacterial meningitis 5