cerebral abscess

Question 1

define

Answer 1

Brain abscess is a focal collection of pus within the brain parenchyma. Abscesses can develop as a result of:

• direct spread of an infection from an adjacent site. e.g - meningitis, sinusitis, mastoiditis
• hematogenous spread from a distant location

Question 2

aetiology

Answer 2

1. haematogenous spread - usually, from a primary site in the heart, lung or distal bones - for example:

• subacute bacterial endocarditis - usually causing multiple abscesses
• cyanotic congenital heart disease - loss of pulmonary filtration of organisms
• chronic pulmonary sepsis / bronchiectasis

2. direct implantation of organisms - usually, trauma, neurosurgery
3. local extension from adjacent foci - for example:

• suppurative otitis media } mainly to temporal lobe;
• mastoiditis } less to cerebellum
• frontal sinusitis
• orbital cellulitis

4. impaired immunity / diabetes

Question 3

infective organisms

Answer 3

local causes:from chronic otitis media and mastoiditis:

• Streptococcus spp.
• gram negative bacilli
• Bacteroides
• Actinomyces

acute sinusitis:

• Strep. spp
• Staphylococcus spp.
• Bacteroides
• Haemophilus

penetrating head wounds:

• Staph. spp.
• Clostridium

post-operative wound infection:

• Staph. spp.
• Streptococcus
• Pseudomonas

remote causes: { any of the following:

• congenital heart disease { Staph. spp.,
• endocarditis { Strep. spp.,
• bronchiectasis { Coliforms,
• pneumonia { Bacteroides,
• lung / liver abscess { Nocardia,
• diverticulitis { Aspergillus
• dental caries / dental extraction
• infected needle sites
• boils

Question 4

s/s

Answer 4

Usually, the features of a brain abscess develop over 2-3 weeks; occasionally, they occur more slowly; in the immunosuppressed, onset is rapid.

The typical symptoms & signs of an inflammatory reaction (pyrexia, rigors, and dehydration) are uncommon during the time of presentation

Characteristically:

• headache is usually the first symptom - it arises from raised intracranial pressure and may be accompanied by other typical features such as vomiting, drowsiness.
• toxicity effects, e.g. fever - but its absence does not exclude the diagnosis
• characteristics of the infective source - look for tenderness over the mastoid, sinuses; look for a discharging ear; other diagnostic signs - e.g. cardiac murmurs in SBE
• focal neurological signs:
  
  • frontal abscess - impairment of memory and attention; rarely, hemiparesis +/- dysphasia and motor seizures;
  • temporal lobe - nominal dysphasia, more usual when the abscess is on the left; visual field defect - usually, a homonymous upper quadrantanopia resulting from the involvement of lower fibres of optic radiation;
  • cerebellar - headache, usually occipital; ataxia; cerebellar signs; neck stiffness

Question 5

ix

Answer 5

The investigations of choice for a brain abscess include:

1. chest and skull x-ray - to identify pulmonary, sinus and mastoid infection
2. blood culture
3. CT scanning is the investigation of choice:

• it must precede any consideration of lumbar puncture
• it characteristically reveals a central necrotic area of reduced radiodensity with surrounding area of cerebral oedema
• iv contrast can be used to highlight the oedematous region, so demarcating the abscess
• a ring enhancing lesion on CT may be abscess or tumour; classically abscesses have a smoother outline
• ventricular compression and midline shift may be evident due to the mass effect of the abscess
• a subdural empyema forms a thin but extensive area of reduced radiodensity on the surface of the brain or along the falx
• an extradural empyema is more localised and lentiform

4. N.B. lumbar puncture must not be performed until raised intracranial pressure secondary to an abscess has been excluded because of the risk of coning.

• if the diagnosis is uncertain a diagnostic Burr hole & aspiration (under CT guidance) could be done (1)
• a CT or MRI can also be used to detect an infected source responsible for the abscess (e.g. paranasal sinusitis, ear infection)

Question 6

rx

Answer 6

Classically the treatment is by drainage, culture and antibiotic administration.

Antibiotics:

should be used in

• multiple abscesses
• abscesses located deep inside the brain (1)

consult bacteriologist re: appropriate antibiotics. Blind therapy may involve ceftazidime (cover gram +ve’s and gram -ve’s), metronidazole (cover anaerobes) and flucloxacillin (cover staphylococcus aureus)

switch to more specific treatments once organisms identified

Abscess drainage:

• primary excision of whole abscess - standard for cerebellar abscess; risk of damage to surrounding brain tissue
• burr hole aspiration - simple, safe, may need repeating
• open evacuation - involves craniotomy; avoids damage to surrounding brain

Treatment of primary infection site as required:

• to reduce the risk of recurrence
• surgical treatment - sinus or middle ear infection; cardiac surgery for infective valves
• physiotherapy and antibiotics - for pulmonary infection

Steroids may reduce oedema but reduce antibiotic penetration.

Prophylactic anticonvulsants - around 40-50% of patients with an intracranial abscess develops seizures.

Question 7

complications

Answer 7

Local complications:

• focal neurology
• epilepsy

Wider neurological effects:

• meningitis and empyema is the abscess drains outwards
• ventriculitis is the abscess drains centrally
• raised intracranial pressure:
• mass effect
• oedema
• hydrocephalus

Systemic effects:

• septic

In addition post infection complications include:

• fibrotic tissue formation in the abscessed area
• reduction in brain tissue during surgery
• abscess rupture into the ventricular space
• death

Question 8

prognosis

Answer 8

poor prognostic factors include:

low preop LOC

brain herniation

rupture of abscess into ventricles/subarachnoid space

co-morbidities