CNS Infections

A CNS infection is one that involves the central nervous system in some way. This includes the meninges, cerebellum, ventricular system and spinal cord, among others. Meningitis is the most common form of CNS infection. You should have a low threshold for starting empirical treatment in suspected cases of meningitis or encephalitis, particularly in patients who are immunocompromised.

CNS infections can be classified in many different ways: organism (aetiology), location and route of transmission. Here, we will classify them by location.


Meningitis is the most common CNS infection. It is an infection of the meninges: the membrane which lines the brain and spinal cord. It can be caused by bacteria, viruses or fungi. Strep pneumoniae accounts for 70% of acute bacterial meningitis. The causative organisms are listed below:

Neisseria meningitidis
Strep pneumoniae
Staph aureus
Group B strep
Listeria monocytogenes
Gram –ve bacilli
Mycobacterium tuberculosis
Enteroviruses (e.g. Coxsackie)
Herpes simplex
Candida albicans
Cryptococcus neoformans

Microbes reach the meninges either by direct extension from the ears/nasopharynx, traumatic cranial injury (patients should be given prophylaxis with the pneumococcal vaccine) or from haematogenous spread (sepsis).

There is a higher prevalence in developing countries due to a lack of consistent vaccination programmes. Other risk factors include:

  • <5 years (particularly the neonatal period) or >65 years (fatalities are higher at the extremes of age)
  • Low socioeconomic background
  • Crowding
  • Immunosuppression
  • Alcoholism
  • IVDU
Clinical Features

Meningism triad: headache, neck stiffness, and fever

Photophobia and vomiting are often present. In acute bacterial meningitis, fever, rigors and intense malaise are accompanied by the above and develop within hours, or even minutes. Babies with meningitis are irritable and may lie still. Often doctors will look for Kernig’s & Brudzinski’s signs but whilst they are specific, they have low sensitivity. Do not be falsely reassured by the lack of either sign!

Kernigs and brudzinski sign
(A) Kernig’s sign: Hip is flexed to 90 degrees. Positive when pain is elicited on straightening the knee.
(B) Brudzinski’s sign: Forced neck flexion elicits pain and reflex flexion of the hips

Other neurological symptoms include decreased GCS (can lead to coma), seizures and focal neurology e.g. cranial nerve palsies. Focal neurology occurs due to the build-up of pus between the pia and arachnoid from polymorphs. The pus can organise and form adhesions.

Features of sepsis such as shock, increased cap refill time and fever are often present.

A key sign of meningococcal meningitis is a petechial, non-blanching rash but this is a late sign.

Viral meningitis is usually less severe and self-limiting with symptoms lasting for around 4-10 days. Usually, there are no serious complications unless encephalitis develops.


If a patient comes in with suspected meningitis, the first thing which needs to be done is bloods (FBC, U&Es, coagulation screen, glucose, blood cultures and potentially an ABG, depending on how sick they are).

Next, a lumbar puncture needs to be done with CSF sent for MC&S, glucose, virology/PCR and lactate. An LP should only be performed if there is no suspicion of a mass lesion or increased ICP. Do not delay antibiotics for an LP.

CSF results:

AppearanceClear/turbidTurbid/purulentViscous, fibrin web
CellsMild increase mononuclear cellsPolymorphsMod-high increase mononuclear cells
ProteinMild increaseMod increaseHigh increase
Glucose>1/2 blood glucose<1/2 blood glucose<1/2 blood glucose

Meningococcal meningitis (petechial rash and/or meningism and clinical probability) is a medical emergency and requires immediate treatment, DO NOT WAIT for results of investigations.

Immediate treatment at the first point of contact, prior to transfer to hospital or investigation with usually a penicillin or cephalosporin depending on local guidelines (for example benzylpenicillin or cefotaxime). Dexamethasone is often given alongside this for cases of suspected bacterial meningitis. ICU should be involved early as the disease can progress quickly in all patients.


Once immediate treatment (above) has been given, cefotaxime 2g or benzylpenicillin should be continued. IVFs & analgesia/anti-pyrexials (paracetamol) are also given alongside. Antibiotics should be continued for a minimum of 7 days, usually 14, and tailor to any cultures results with involvement of the microbiologist. Immunocompromised or elderly patients are often covered additionally for listeria with amoxicillin/ampicillin +/- gentamicin.

Remember to look at local guidelines as these are key for how to investigate in your hospital and which antibiotics to use.

Meningococcal infection is a notifiable disease therefore public health should be contacted. Household contacts need to be given prophylaxis with rifampicin.


This is an acute infection of the brain parenchyma. It usually has a viral cause:

  • Herpes simplex 1 and 2
  • Varicella zoster
  • CMV
  • EBV
  • Rabies

There can be haematogenous spread (secondary encephalitis).

Fever and meningism are usually present, but unlike in meningitis, clinical features are dominated by inflammation of the brain parenchyma. Other versions of encephalitis include autoimmune and post-infectious.

Clinical Features

Personality and behavioural changes, often bizarre, are a common early presentation, alongside developing confusion which progresses to decreased consciousness and coma. Seizures are common, as well as focal neurological deficits (more common than meningitis) e.g. speech disturbance, tremors.

  • Bloods: CRP, FBC, U&Es, blood cultures, viral PCR
  • LP: send for the same as meningitis, results show elevated lymphocytes +/- causative organism on PCR
  • MRI is a useful tool and shows areas of inflammation and oedema, signs of increased ICP and midline shift. HSV & VZV can cause temporal lobe changes.
  • EEG can be a useful adjunct
  • Treat immediately with acyclovir for 10-14 days in suspected cases prior to result availability to reduce mortality and permanent neurological deficits.
  • The mainstay is supportive therapy with IVF, paracetamol etc.
  • Keep HDU/ICU teams involved.
  • Long term complications include memory impairment, epilepsy and personality change.

Brain Abscess

This is a focal bacterial infection that behaves the same as any space-occupying lesion. A brain abscess is 10 times rarer than a brain tumour in the UK.

Risk factors
  • Spread from local infection – ear, sinus, dental
  • Skull fracture
  • Congenital heart disease
  • Endocarditis
  • Bronchiectasis – haematogenous spread
  • Immunosuppression, especially HIV

Common organisms include strep anginosus, bacteroides (sinuses and dental infections), staph (trauma) and fungi with mixed infections being common.

Clinical Features

Headache, fever (swinging), seizures, focal signs such as hemiparesis, aphasia, visual disturbance (e.g. hemianopia) and signs of raised ICP (headache worse in morning, vomiting etc).

  • Bloods: as above, plus an ESR which may be raised
  • CT/MRI: ring-enhancing lesion with surrounding oedema
ring enhancing lesion ct brain
Ring-enhancing lesions on a contrast CT brain
  • Antibiotics! Ceftriaxone is common, but tailor to local guidelines or with microbiology guidance.
  • Requires referral to neurosurgery as the abscess may need surgical resection or decompression. HDU/ICU team involvement if necessary.
  • Any increase in ICP requires treatment with mannitol.

Useful Resources & References

Written by Amy Meadows (CDF in Paediatric Surgery)
Edits by Niraj Doshi (Y3 Medical Student)

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