28th July 2023

Neurological Infections

Bacterial infection of the neurological system usually manifests as meningitis or encephalitis. A brief overview is given here, with more detailed information in the Neurology topics.

Meningitis

Meningitis is caused by inflammation of the meninges. Infectious causes include bacteria, viruses, and mycobacteria. It can also be caused by malignancy. This section focuses on bacterial causes.

The classical meningitis triad is headache, fever and neck stiffness. It may also cause confusion, drowsiness, and other focal neurological signs and symptoms.

The most common causative organism of acute bacterial meningitis is Neisseria meningitidis (meningococcus), followed by Streptococcus pneumoniae (pneumococcus).

Meningococcal disease is usually rapid in onset, with symptoms progressing over hours, and may present as florid sepsis with the classical non-blanching petechial rash, which is diagnostic of this bacteria. The rash indicates meningococcus rather than being a feature of all meningitis. The pneumococcal disease may be more subacute over a few days, and a purpuric rash is absent.

The investigation of choice is lumbar puncture (LP). This should ideally be done before initiating antibiotic therapy, or the results may not be accurate. Contraindications include evidence of raised intracranial pressure (as LP can cause uncal herniation in this case), coagulopathy, and a rapidly involving rash. CT is generally done prior to LP, providing there is time to rule out any intracranial mass that can cause raised intracranial pressure, and is an absolute necessity if the patient has any risk factors for a mass or any clinical features suggesting one. If arranging these investigations leads to a significant delay in treatment (i.e. more than 6 hours), antibiotics should be first commenced.

The cerebrospinal fluid analysis helps confirm acute bacterial meningitis as it should show a (very) high neutrophil count, with high protein and low glucose compared to venous blood. Microscopy may also identify the organism.

First line treatment is with intravenous ceftriaxone (or chloramphenicol if there is an allergy history to beta-lactams). Steroids are also indicated for pneumococcal disease. In a non-inpatient setting, a single dose of intramuscular benzylpenicillin is recommended as empirical treatment in suspected bacterial meningitis if there is a delay in reaching a hospital.

Tracing for prophylaxis is only required for close household contacts of Neisseria meningitidis, for instance, those who share a room or young children. Acute bacterial meningitis is a notifiable disease independent of microbiological causes.

Meningitis with Listeria monocytogenes and Mycobacterium tuberculosis can cause lymphocytosis rather than neutrophilia on CSF analysis. Listeria causes invasive disease in groups with a risk factor, as outlined in the Gastrointestinal Infections section. Treatment is with high dose amoxicillin (or co-trimoxazole if penicillin allergic).

Encephalitis

Encephalitis refers to the inflammation of the brain (i.e. cerebrum or cortex). It presents with a clinical syndrome of headache, fever, focal neurological deficits, and seizures, with the latter being more common than in meningitis.

It is most commonly viral in the UK, with herpes simplex virus (HSV) type 1 being the most common causative organism. The same pathogens which cause bacterial meningitis may also cause bacterial encephalitis.

Meningitis and encephalitis share symptoms, and there may be an overlap between the two; encephalitis is therefore managed in much the same way as meningitis. The key points are prompt diagnosis with CSF analysis and prompt initiation of treatment. As HSV is the most common cause in the UK, empirical treatment of encephalitis includes the antiviral drug aciclovir until the causative organism is identified.

Cerebral abscesses

Abscesses in the brain generally arise secondary to infections, such as otitis media, mastoiditis or sinus infection. They may also result from facial or head trauma, including surgery and implantation of foreign bodies, or due to trauma. The most common causative organisms are Staphylococcus and Streptococci species.

Cerebral abscesses generally present predominantly with a severe headache and often have a subacute presentation of 1-2 weeks. Fever, confusion, drowsiness, focal neurological deficits and seizures may also be seen. There may be localised pain at the site of the abscess.

Imaging findings vary depending on the stage of the disease. The characteristic finding is a ring-enhancing solitary lesion on CT. MRI is more accurate and can be used to further characterise the lesion following CT.

A brain abscess is a medical emergency. The risk of raised intracranial pressures necessitates urgent neurosurgical referral. Antibiotic treatment is similar to the approach taken in meningitis and should be tailored as per sensitivities whenever possible.