Intracranial infection

  Infections involving the central nervous system (CNS) pre­sent in a variety of ways and unless diagnosed and treated promptly can result in serious morbidity and mortality. Intracranial infections encountered in neurosurgical practice include bacterial meningitis, brain abscess and subdural empyema.

Meningitis

 

Bacterial meningitis, although potentially a life-threatening infection, seldom requires neurosurgical intervention. Treat-

ment involves confirming the diagnosis and identifying the organism by obtaining a CSF sample, usually via lumbar puncture (LP). Broad-spectrum intravenous (i.v.) antibiotics with good CSF penetration should then be commenced without delay (ceftriaxone 2 g 12 hourly and metronidazole 400 mg 8 hourly) and adjusted as cultures and sensitivities become available. In situations where lumbar puncture is deemed impossible or unsafe, unless CSF can be obtained by other means, treatment might have to be empirical. Studies have demonstrated that steroids, commenced early and in combination with antibiotics, can improve outcome by reducing oxygen free radical attack on the brain. Complica­tions are more likely to occur if treatment is not commenced early and may require neurosurgical intervention. The major complications are: cerebral oedema, seizures, hydrocephalus (communicating), subdural effusion, subdural empyema, brain 

abscess and ventriculitis.

  Brain abscess

 

The treatment of brain abscess has been revolutionised by the availability of CT scanning, metronidazole and stereotaxis. Prior to these modalities, the mortality for brain abscesses ranged between 15 and 40 per cent. Pyogenic inflammation of the brain leading to cerebral abscess may result from:

haematogenous spread from a known septic site or occult focus (particularly in patients with right to left cardiac shunts) (Fig. 35.24); contiguous spread from an adjacent infected sinus presumably via retrograde thrombophlebitis; and direct inoculation from trauma. In 20 per cent of patients no cause can be identified (Fig 35.25 ,FIG. 35.26Fig 35.27).

The organisms most commonly responsible vary depend­ing on the aetiology. With otitis media anaerobic strepto­cocci, Haemophilus and Gram-negative anaerobes are involved; with sinusitis Streptococcus milleri and Staphylococcus. ; with traumatic inoculation Staphylococcus; and with haematogenous spread various organisms are involved

  Clinical features

   

Patients can present with either features of raised pressure, seizures, meningeal irritation or focal signs, each of which occurs in 50 per cent of patients. Systemic features of infec­tion including pyrexia are frequently absent but the history is usually short with rapid clinical progression. Diagnosis is confirmed by demonstrating a single or multiple low attenuation and smooth, symmetric, ring-enhancing mass lesions with variable oedema on CT. MRI with enhancement will demonstrate similar findings including high signal on 12-weighted images from the surrounding oedema. These lesions arise in an area of cerebritis and can often be multiloculated. Opacification of the sinuses or mastoids can be recognised on both CT and skull X-ray (SxR). Infection markers will demonstrate a leucocytosis and a raised erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP), but may initially be normal.

 

Treatment

  The principles of treatment are to identify the bacterial organism, institute intravenous antibiotic therapy (see above), drain or excise the abscess, and aggressively pursue and treat the cause. During the stage of acute cerebritis, prior to abscess formation, surgical treatment is seldom indicated. Steroids reduce oedema around these lesions and are thus invaluable in reducing intracranial pressure but can inhibit formation of the abscess capsule.

Unless very small, intracranial abscesses require surgical management. This is performed by either aspiration through a burr hole, with or without stereotaxis (repeated as neces­sary) or formal excision. The latter is undertaken if there is persistent reaccumulation of pus despite serial aspirations. Excision may also be considered if the abscess is superficial and easily accessible, and if there is a well-formed capsule which fails to collapse despite repeated aspirations. Patients should have regular follow-up CT scans with contrast until the abscess stops reaccumulating. Anticonvulsant therapy is routinely instituted.

 

Subdural empyemas

Subdural empyemas, although uncommon, may develop from sinusitis or mastoiditis. The infective agent is frequent­ly S. milleri, although other anaerobic or aerobic streptococci, other anaerobes and Staphylococcus aureus may also be responsible. These patients are systemically unwell with rapidly progressive neurological signs, a depressed level of consciousness and hemi paresis. Epilepsy occurs in most patients and can often progress to status epilepticus.

Despite this being a surgical emergency, diagnosis is often delayed as the collection on CT scanning is usually subtle to the untrained eye and frequently missed. Treatment is as for an intracranial abscess but the collection requires a formal craniotomy for drainage with exploration of the interhemispheric fissure. The combination of fever and seizures with a background of sinusitus is usually diagnostic of this lesion. Complications include refractory status epilepticus and cortical vein/venous sinus thrombosis.

  Human immunodeficiency virus (HIV) infection

 

Surgical management

  The neurological complications of HIV infection have impli­cations for both surgeon and patient. Although transmission of HIV from patients to healthcare workers is extremely rare, the surgeon is at risk from needle stick injury and thus extra care should be taken when dealing with a population at risk. In the acquired immunodeficiency syndrome (AIDS), single or multiple lesions within the brain may develop and patients may present with focal neurological signs, symptoms of raised intracranial pressure or seizures. In the early days of AIDS management, stereotactic biopsies of these lesions were routinely carried out. However, in view of the high proportion of infective lesions, particularly toxoplasmosis, it

has now become routine to treat the patient first with a full course of antitoxoplasma treatment. Should this fail or the patient’s clinical condition deteriorate, or both, then a stereotactic biopsy should be considered. The differential diagnosis is then between infection with an organism thus far not covered with the therapy given, or tumour, principally lymphomas, or the condition of progressive multifocal leucoencephalopathy (CML). The current clinical course is one of rapid and relentless deterioration and, indeed, when a stereotactic biopsy is required for diagnostic purposes in AIDS, then the overall prognosis is poor. However, biopsy and histological diagnosis allow patients, their partners and families to be counselled as regards prognosis. Furthermore, a histological diagnosis also allows a prospective study of new therapies for AIDS which, it is hoped, will lead to improved long-term prognosis.