Medicine – Causes of Meningitis

Meningitis is inflammation of the meninges, the membranes surrounding the brain and spinal cord. The most important causes are infectious, particularly bacterial, viral, fungal, and tuberculous infections.

The likely organism depends strongly on age, immune status, exposure, clinical setting, and CSF pattern.

1. Acute Bacterial Meningitis

Acute bacterial meningitis is a medical emergency because infection can progress rapidly and cause:

Cerebral oedema.

Raised intracranial pressure.

Sepsis.

Neurological damage.

Death.

The most likely organisms vary with age.

2. Acute Bacterial Meningitis in Adults

The two classic major causes in adults are:

Neisseria meningitidis – meningococcus.

and

Streptococcus pneumoniae – pneumococcus.

These remain among the most important organisms to recognise.

3. Meningococcal Meningitis

Neisseria meningitidis can cause meningitis and meningococcal septicaemia.

Typical features may include:

Fever.

Headache.

Neck stiffness.

Photophobia.

Altered consciousness.

A non-blanching petechial or purpuric rash suggests meningococcal infection, particularly when associated with sepsis.

4. Pneumococcal Meningitis

Streptococcus pneumoniae is an important cause of bacterial meningitis, especially in adults.

Risk may be increased by:

Older age.

Asplenia.

Immunocompromise.

CSF leak.

Cochlear implants.

Recent otitis media or sinus disease.

Pneumococcal meningitis can be severe and is associated with substantial neurological morbidity.

5. Acute Bacterial Meningitis in Neonates

The causative organisms in neonates differ from those in adults.

Important organisms include:

Group B Streptococcus.

Escherichia coli.

Listeria monocytogenes.

The original note lists group B streptococci and E. coli, but Listeria is also a classic neonatal pathogen.

6. Group B Streptococcus

Streptococcus agalactiae, or group B Streptococcus, is an important cause of neonatal sepsis and meningitis.

Transmission may occur around the time of birth from maternal genital tract colonisation.

7. Escherichia coli

E. coli is another major neonatal cause.

Certain strains, particularly those with the K1 capsule, have a strong association with neonatal meningitis.

8. Listeria monocytogenes

Listeria monocytogenes is an important cause of meningitis in:

Neonates.

Older adults.

Pregnant patients.

Immunocompromised patients.

It is therefore not simply a rare organism.

This is an important update to the older classification in your note.

9. CSF Pattern in Typical Acute Bacterial Meningitis

The classic CSF findings are:

Neutrophilic or polymorphonuclear pleocytosis.

High protein.

Low glucose.

Raised opening pressure.

Therefore:

Bacterial meningitis → neutrophils ↑↑ + protein ↑↑ + glucose ↓.

10. Bacterial Meningitis with Lymphocytic CSF

Although bacterial meningitis usually produces neutrophils, some bacterial infections may produce a lymphocytic or mixed CSF pattern, especially in subacute or partially treated disease.

Important examples include:

Listeria monocytogenes.

Leptospira species.

Treponema pallidum causing neurosyphilis.

Borrelia burgdorferi causing Lyme neuroborreliosis.

11. Listeria and CSF

Listeria meningitis may show:

Neutrophils.

Lymphocytes.

or a

Mixed cellular response.

Therefore, CSF cell type alone cannot reliably exclude Listeria.

12. Leptospirosis

Leptospira infection can cause an aseptic meningitis picture.

The CSF often shows:

Lymphocytic pleocytosis.

This may occur along with systemic features such as:

Fever.

Myalgia.

Conjunctival suffusion.

Renal dysfunction.

Hepatic dysfunction.

depending on disease severity.

13. Syphilis

Treponema pallidum can involve the CNS at different stages of infection.

Neurosyphilis may produce:

Lymphocytic pleocytosis.

Raised protein.

Usually normal or mildly reduced glucose.

The presentation may be meningitic, vascular, cognitive, sensory, or mixed.

14. Lyme Disease

Borrelia burgdorferi can cause neuroborreliosis.

Neurological manifestations may include:

Lymphocytic meningitis.

Facial nerve palsy.

Radiculopathy.

Therefore, lymphocytic CSF does not always indicate a viral infection.

15. Causes with Predominantly Polymorphs in CSF

A neutrophilic or polymorphonuclear CSF response is classically associated with acute bacterial meningitis.

Important organisms in this category include:

Staphylococcus aureus.

Pseudomonas species.

and many other pyogenic bacteria.

However, some infections traditionally regarded as lymphocytic can be neutrophilic early.

16. Tuberculous Meningitis and Polymorphs

Mycobacterium tuberculosis classically causes a:

Lymphocytic CSF picture.

However, early TB meningitis can initially show neutrophils before evolving toward lymphocytic predominance.

Therefore, TB should not be excluded solely because an early CSF sample is neutrophilic.

17. Staphylococcus aureus

Staphylococcus aureus meningitis is relatively uncommon compared with pneumococcal or meningococcal disease.

It is more likely in situations such as:

Neurosurgery.

Head trauma.

Bacteraemia.

Endocarditis.

Spinal procedures.

Indwelling CNS devices.

The CSF is typically neutrophilic.

18. Pseudomonas

Pseudomonas aeruginosa is an uncommon community cause but is important in:

Hospital-acquired meningitis.

Neurosurgical patients.

Patients with ventricular drains or shunts.

Severe immunocompromise.

It generally produces a neutrophilic CSF response.

19. Chronic Bacterial Meningitis

The classic chronic bacterial cause in your notes is:

Mycobacterium tuberculosis.

Tuberculous meningitis usually has a more gradual onset than acute pyogenic bacterial meningitis.

20. Tuberculous Meningitis

TB meningitis commonly presents over days to weeks with:

Headache.

Fever.

Lethargy.

Confusion.

Cranial nerve palsies.

Altered consciousness.

Complications include:

Hydrocephalus.

Basal meningeal inflammation.

Cerebral infarction due to vasculitis.

21. CSF in Tuberculous Meningitis

The classic CSF findings are:

Lymphocytic pleocytosis.

Markedly raised protein.

Low glucose.

Raised opening pressure.

Early disease may occasionally show more neutrophils.

Therefore:

TB meningitis → lymphocytes ↑ + protein ↑↑ + glucose ↓.

22. Chronic Fungal Meningitis

The classic chronic fungal cause is:

Cryptococcus.

Cryptococcal meningitis is particularly associated with:

Advanced HIV infection.

Other forms of significant immunosuppression.

but it can also occur in apparently immunocompetent patients.

23. Cryptococcal Meningitis

Cryptococcal meningitis often develops subacutely.

Typical features include:

Headache.

Fever.

Altered mental status.

Raised intracranial pressure.

Meningism may sometimes be relatively mild.

24. CSF in Cryptococcal Meningitis

Typical findings may include:

Lymphocytic or mononuclear pleocytosis.

Raised protein.

Low glucose.

Markedly raised opening pressure.

In profoundly immunocompromised patients, the CSF white cell count may be surprisingly low despite severe infection.

25. Acute Viral Meningitis

Viral meningitis is usually less severe than acute bacterial meningitis.

The CSF classically shows:

Lymphocytic pleocytosis.

Mildly raised protein.

Normal glucose.

Therefore:

Viral meningitis → lymphocytes ↑ + protein mildly ↑ + glucose normal.

26. Enteroviruses

Enteroviruses are among the most common causes of viral meningitis.

Important examples include:

Coxsackieviruses.

Echoviruses.

Other non-polio enteroviruses.

The older note states “especially polio,” but modern practice more often implicates non-polio enteroviruses.

27. Poliovirus

Poliovirus is an enterovirus and can cause meningitis.

However, widespread vaccination has made poliomyelitis far less common in many regions.

Therefore, for examination purposes:

Enteroviruses are important viral causes, but non-polio enteroviruses are usually more common than poliovirus.

28. Mumps

Mumps virus can cause aseptic meningitis and encephalitis.

It was historically a more important cause before widespread vaccination.

Possible associated features include:

Parotitis.

Orchitis.

Pancreatitis.

However, neurological disease can occasionally occur without obvious parotid swelling.

29. Herpes Simplex Virus

HSV can cause viral meningitis.

HSV-2 is particularly associated with:

Aseptic meningitis.

In contrast:

HSV-1 is classically associated with encephalitis, especially temporal-lobe encephalitis.

This distinction is useful for exams.

30. Varicella-Zoster Virus

An additional important viral cause is:

Varicella-zoster virus.

VZV may cause:

Meningitis.

Encephalitis.

Myelitis.

Cerebral vasculopathy.

Neurological infection may sometimes occur without a typical shingles rash.

31. HIV and Viral Meningitis

Acute HIV infection can occasionally present with an aseptic meningitis syndrome.

In advanced HIV infection, however, the differential diagnosis expands considerably because opportunistic infections such as:

Cryptococcus.

Tuberculosis.

and other CNS infections become important.

32. Other Non-Infectious Causes

Not all meningitis is infectious.

Aseptic meningitis can occasionally occur with:

Drugs.

Malignancy.

Autoimmune or inflammatory disease.

Examples of drug-associated aseptic meningitis include some:

NSAIDs.

Antibiotics.

IV immunoglobulin.

These are less common but useful to remember when microbiological tests are negative.

33. Adult Causes – Note Form

Common acute bacterial:

Neisseria meningitidis.

Streptococcus pneumoniae.

Important additional organism:

Listeria monocytogenes, especially older or immunocompromised patients.

34. Neonatal Causes – Note Form

Group B Streptococcus.

Escherichia coli.

Listeria monocytogenes.

These are the classic organisms to remember.

35. Lymphocytic CSF Causes – Note Form

Lymphocytic CSF is not synonymous with viral infection.

Important causes include:

Viral meningitis.

Tuberculous meningitis.

Cryptococcal meningitis.

Listeria – sometimes mixed/lymphocytic.

Lyme disease.

Syphilis.

Leptospirosis.

36. Neutrophilic CSF Causes – Note Form

Important causes include:

Acute pyogenic bacterial meningitis.

Meningococcus.

Pneumococcus.

Staphylococcus aureus.

Pseudomonas.

Listeria – sometimes.

Early TB – occasionally.

Therefore, the differential cannot be based on cell type alone.

37. Chronic Meningitis – Note Form

Think particularly of:

Tuberculosis.

Cryptococcus and other fungal infections.

Syphilis.

Lyme disease.

Malignancy.

Inflammatory disorders.

38. CSF Pattern Summary

Acute bacterial meningitis:

Neutrophils ↑↑.

Protein ↑↑.

Glucose ↓.

Opening pressure often ↑.

Viral meningitis:

Lymphocytes ↑.

Protein mildly ↑.

Glucose usually normal.

Opening pressure normal or mildly ↑.

Tuberculous meningitis:

Lymphocytes ↑.

Protein markedly ↑.

Glucose ↓↓.

Opening pressure often ↑.

Cryptococcal meningitis:

Lymphocytes/mononuclear cells ↑.

Protein ↑.

Glucose ↓.

Opening pressure often markedly ↑.

Key Clinical Pattern

For rapid recall:

ADULT ACUTE BACTERIAL → MENINGOCOCCUS + PNEUMOCOCCUS.

NEONATE → GROUP B STREP + E. COLI + LISTERIA.

CHRONIC BACTERIAL → TB.

CHRONIC FUNGAL → CRYPTOCOCCUS.

ACUTE VIRAL → ENTEROVIRUSES ± MUMPS/HSV/VZV.

And remember the CSF pattern:

Bacterial = neutrophils + high protein + low glucose.

Viral = lymphocytes + mild protein rise + normal glucose.

TB/fungal = lymphocytes + high protein + low glucose.