Infection of the Central Nervous System – Brain Abscess (Suppurative Encephalitis)

The common Pyogenic lesions affecting the central nervous system are meningitis, brain abscess, septic thrombophlebitis, subdural empyema (subdural abscess), extradural abscess and cranial osteomyelitis. What are their causes, clinical manifestations, treatments and preventions?

Causes

1. Extension of infection from chronic suppuration of the middle ear and mastoid (Otogenic).

2. Extension of infection from paranasal sinuses (rhinogenic).

3. Matastatic abscesses from lung abscess, empyema, and infective endocarditis (hematogenous). And Congenital cyanotic heart disease.

4. Introduction of infection from outside, eg, head injury.

In the tropics, the vast majority of cases developing into these diseases are secondary to ear or sinus diseases.

Pathology

In the early stage, there is hyperemia, and focal and generalized edema of the Brain. In the next stage foci of necrosis and suppuration form, which coalesce to form abscess. The wall is made up of fibroblasts, microglia and astrocytes. Due to extension of infection, multiple abscesses may develop.

The common sites for otogenic abscesses are the ipsilateral temporal lobe or cerebellum, rarely they may be in the opposite occipital lobe. Rhinogenic abscesses are mainly frontal in location. Metastitic abscesses are seen mainly in the frontal lobe and they are multiple. Hematogenous abscess from cardiac lesions are located mainly in the territory of the middle cerebral artery.

In more than 3/4th of the cases, the causative organisms are single, but in a fourth multiple pathogens are seen. In otogenic and rhinogenic abscesses-streptococci, staphylococci, pneumococci, E.Coli, B.Proteus and B pyocyaneus are common. In metastatic abscesses, staphylococci and streptococcus viridans are the commonest organisms, but anaerobes also may be seen. In congenital cyanotic heart disease, the organisms include Gram-negative bacilli, anaerobic streptococci and H.influenzae.

Clinical presentation: The patient presents with generalized signs of infection, features of raised intra-cranial tension and focal neurological signs depending upon the location of the abscess.

Diagnosis: In any patient having one of the underlying causes, features suggestive of meningitis, raised intracranial tension or focal neurological deficit should suggest the possibility or brain abscess. Lumbar puncture should not be routinely done in such cases since the risk or coning is high. Moreover the CSF may not show diagnostic changes in the case of localized abscess.

The diagnosis can be confirmed by CT Scan which should be done of an early stage. X-ray skull may give evidence of sinusitis and rarely gas in the abscess cavity, if the abscess is caused by gas-producing organisms or the abscess is communicating with the paranasal sinuses.

Course and prognosis: If left untreated, brain abscess is fatal. Complications include rupture into cerebral ventricles producing ventriculitis, meningitis, rise in intracranial tension and secondary epilepsy. If the abscess is diagnosed early and treated, full recovery may occur. Mortality is higher in brain abscess complicating congenital heart disease. The overall mortality in large series is 11-15%.

Treatment: Control of infection is achieved by giving the appropriate antibiotic in high dosage, as in the case of purulent meningitis. Metronidazole given intravenously as a continuous drip (0.5g every 8 hours) helps in overcoming the anaerobic sepsis. Antibiotic therapy has to be continued for 3-6 weeks. General measures to reduce intra-cranial tension such as infusion of mannitol 300 ml of 20% solution, and the administration of 15 ml glycerine orally 6 hours are instituted. Convulsions may have to be treated with proper anticonvulsants. Anticonvulsants may have to be continued for long periods.

Surgical therapy: As soon as the abscess is localized, it is aspirated through a burr hole. Antibiotics can be instilled into the abscess cavity. Excision of the abscess may be necessary in some cases.

Preventions: Proper therapy of mastoiditis and sinusitis prevents the development of brain abscess. In subjects with cyanotic congenital heart disease any neurological disorder should raised the suspicion of brain abscess, and early investigation and treatment should be instituted.