Intracranial Abscesses

Intracranial abscesses are uncommon, serious, life-threatening infections. They include brain abscess and subdural or extradural empyema. A high number of brain abscesses are polymicrobial.¹

• Brain abscesses are rare in developed countries, but are a significant problem in the developing world, they are twice as common in males and have a peak incidence at around 40 years old.²
• A decrease in meningitis due to the Haemophilus influenzae vaccine has reduced the prevalence in young children.
• The prevalence of brain abscess is higher in patients with HIV infection. They are usually caused by opportunistic fungal or protozoal infection.

Causative organisms include:

• Bacteria: common bacterial causes include Staphylococcus aureus, Streptococci, Bacteroides species and Listeria. Approximately 40% abscesses originate from infection of adjacent structures, e.g. otitis media, dental infection, mastoiditis, sinusitis.³
• Fungi: Aspergillus, Candida, Cryptococcus, Coccidioides, Histoplasma, Blastomyces. The frequency of fungal brain abscess has increased because of the frequent administration of broad-spectrum antimicrobials, immunosuppressive agents or illness (HIV or TB⁴), and corticosteroids.
• Protozoa: e.g. Toxoplasma gondii, Entamoeba histolytica, Trypanosoma cruzi, Schistosoma.
• Helminths, e.g. Taenia solium.

Abscess formation can also develop following blood-bourne spread from a remote site, e.g. in patients with cyanotic congenital heart disease, endocarditis, dental caries.⁵ In at least 20% of cases, no source can be identified.

Symptoms

Onset may be sudden, or subacute over several weeks.

• Common presenting symptoms include fever, headache, changes in mental state (drowsiness, confusion), focal neurological deficits, grand-mal seizures, nausea and vomiting, neck stiffness.
• A suddenly worsening headache, followed by emerging signs of meningism, is often associated with rupture of the abscess.

Signs

• Fever
• Focal motor or sensory deficits
• Raised blood pressure and bradycardia associated with raised intracranial pressure
• Papilloedema
• Ataxia
• Confusion, drowsiness
• Bulging fontanelle in infants

• Meningitis
• Encephalitis
• Brain tumour or other intracranial space occupying lesion

• Full blood count: marked leucocytosis
• Raised ESR and CRP
• Renal function and electrolytes: serum sodium levels may be lowered as a result of inappropriate antidiuretic hormone production.
• Blood cultures: at least 2, and preferably before antibiotics are started.
• Serological tests: are available for some pathogens.
• Cerebrospinal fluid: lumbar puncture (LP) is rarely helpful (unless required to rule out meningitis) and is contraindicated if increased intracranial pressure is present. LP in the presence of raised ICP can precipitate a tentorial or tonsillar herniation.
• CT scanning is the investigation of choice.⁶Cerebral abscesses appear as a radiolucent space occupying lesion:
  • As the disease progresses, a distinctive "ring enhancement" appears on contrast-enhanced CT, as the abscess wall thickens.⁷
  • They are often surrounded by oedema.
  • The position, size and number of abscesses may suggest underlying pathology.
  • MRI scans provide greater contrast between cerebral oedema and the brain and early detection of satellite lesions.
• Aspiration of abscess for culture
• Biopsy of cerebral lesion

The principles of treatment are:

• Drain intracranial collection; supratentorial abscesses can be drained via a burr hole. Pus should be sent for culture.
• Administer effective antibiotic therapy; early treatment is essential.
• Eliminate primary source of infection.

Initial antimicrobial therapy should be started immediately and then modified according to the results of cultures.

• Initial therapy should be guided by local guidelines and the advice of the microbiologist. Initial therapy choices include high doses of penicillins, metronidazole, either gentamicin or chloramphenicol, vancomycin, meropenem and cephalosporins (e.g. cefotaxime). If fungal cause is suspected then amphotericin, flucytosine, fluconazole or voriconazole are indicated. The treatment of choice for toxoplasmosis is a combination of pyrimethamine and sulfadiazine.
• Therapy should be given intravenously for at least the first week.
• Patients presenting with seizures require intubation and hyperventilation. Seizures should be treated aggressively to decrease the risk of increases in intracranial pressure.
• Corticosteroids: intravenous dexamethasone is used if massive cerebral oedema is seen on the CT scan.

Surgical

• Once an abscess has formed, surgical excision or drainage through a burr hole, combined with prolonged antibiotics (usually 4-8 weeks), remains the treatment of choice.
• Aspiration is the most common procedure and is often performed using a stereotactic procedure with the guidance of CT scanning or MRI. Craniotomy is generally performed in patients with larger, multiloculated abscesses and for those whose conditions failed to resolve.
• Management of subdural or epidural empyema requires prompt surgical evacuation of the infected site and antimicrobial therapy.

• Intracerebral abscesses may rupture into the ventricular system and produce ventriculitis.
• Epilepsy occurs in around 30%, particularly with temporal lobe abscess and subdural empyema. Anticonvulsants may be required.
• Mainly depending on the speed of diagnosis and treatment, 20-80% of survivors have neurological sequelae, e.g. hemiparesis, visual field loss.

• Prompt treatment results in mortality less than 10%, but a delay in diagnosis increases mortality to above 50%.
• Rupture of a brain abscess is associated with mortality up to 80%.
• 50% of survivors have neurological sequelae which may include hemiparesis, visual field losses and epilepsy.⁶