CNS Infections

Definition

The Central Nervous System (CNS) includes the brain, brainstem, spinal cord, meninges (membranes covering the brain and spinal cord) and the space within the cranium (skull).

CNS infections result in inflammation in and around the brain and the common ones are categorized as follows: meningitis (inflammation of the meninges i.e. membranes surrounding the brain and the cerebrospinal fluid (CSF)), encephalitis (inflammation of the brain itself) or brain abscess (localised collection of pus within the brain tissues).

Aetiology

Causes of meningitis may be viral, bacterial or fungal. In contrast, the majority of cases of encephalitis are viral in aetiology. Abscesses are localized infections within the brain tissue and may be due to bacteria, viruses or fungi. Viral meningitis usually have a benign course and resolve spontaneously. Some CNS infections are related to surgery or placement of devices in the CNS e.g. shunts, valves etc.

Epidemiology

Meningitis and encephalitis may occur in persons without any pre-exisiting illness. Head injury, recent head and neck surgery are predisposing factors for CNS infections. Patients who are immunosuppressed may have CNS infections due to unusual organisms e.g fungi (Cryptococcal meningitis) or parasites (Toxoplasmic encephalitis).

Clinical

Meningitis and encephalitis have similar symptoms and include:

  • Fever
  • Headache
  • Stiff neck
  • Vomiting
  • Sensitivity to light
  • Confusion and poor mentation
  • Skin rash
  • Seizures
  • Neurological deficit such as limb weakness, facial asymmetry
  • Newborns and infants may have constant crying, drowsiness, irritability, stiff neck, high fever and poor feeding
  • In abscesses, there may only be a dull persistent headache and loss of coordination

Common pathogens

CNS infections can be acquired via different routes depending on the type of micro-organism:

  • Bacterial: Most CNS infections due to bacteria usually spread via the blood stream. Other possible transmission routes include direct trauma (in head injuries or skull fractures), spread from nearby infections (such as ear or dental infections) and introduced during surgery. Common bacterial pathogens include Neisseria meningitidis, Listeria monocytogenes, Haemophilus influenzae and Streptococcus pneumoniae. Less common bacteria include Staphylococcus aureus, Escherichia coli etc. Bacterial infections can cause bacterial meningitis or abscesses. Tuberculous meningitis may occur in young children and also in immunocompromised patients.
  • Viral: Enterovirus, Herpes simplex virus, cytomegalovirus (CMV), HIV are important viral causes of CNS infection. In the endemic countries in South East Asia, Japanese encephalitis virus, Nipah virus and rabies viruses are also causes of encephalitis.
  • Fungal: Candida albicans, Cryptococcus neoformans, Aspergillus fumigatus are opportunistic fungi that can cause CNS infections. This often occurs in immunocompromised patients, such as in HIV, renal failure and organ transplants.
  • Parasites: Toxoplasma gondii is the most common parasitic cause of encephalitis and brain abscesses. Cats are often the reservoirs of these parasites. The immunocompromised patients and infants (perinatal, uterine transmission) are at risk.

Diagnosis

  • Neurological examination
  • Lumbar puncture (spinal tap)
  • Blood cultures
  • Magnetic Resonance Imaging (MRI) scan
  • Cranial Computerized Tomography (CT) scan
  • Biopsy of the brain/skin rash may be required if the infection does not improve with treatment

Treatment

  • Meningitis: Bacterial infections are treated with intravenous antibiotics. Infected sinuses may need to be drained if they were the initial source of infection. The majority of viral meningitis such as those due to Enteroviruses are not responsive to antibiotics and most cases resolve in 14 days with bed rest and medications for symptom relief. The only viral infections that may be treated with antiviral agents are Herpes simplex virus ( HSV ), Cytomegalovirus (CMV ) and HIV. Antifungal therapy is given for specific fungi that are identified.
  • Viral Encephalitis: Mild cases may resolve spontaneously. The more serious cases are given corticosteroids to reduce swelling in the brain and anticonvulsants to control seizures. Antiviral medications with acyclovir are usually commenced first and then their use is reviewed when the diagnostic tests become available.
  • Abscess: Brain abscesses are medical emergencies requiring hospitalisation and aggressive antibiotic treatment. Drainage may be required in large abscesses and to obtain specimens for diagnostic management.

Prevention

The best way to prevent CNS infections is to reduce the risk for any harmful micro-organisms to enter our body. Some forms of meningitis can be prevented by vaccinations. Some recommended childhood vaccinations that have reduced CNS infections include:

  • Haemophilus influenzae type B (Hib) (scheduled at 3 months, 4 months and 5 months, with a booster at 18 months)
  • Pneumococcal conjugate vaccine (scheduled at 3 months, 5 months, with a booster at 12 months

Other vaccinations are recommended if indicated but not mandatory in Singapore. These vaccinations include:

  • Meningococcal conjugate vaccine/polysaccharide vaccine to prevent meningococcal infections
  • Pneumococcal conjugate/polysaccharide vaccine (recommended for adults >65 years, younger adults and children with weak immune systems or chronic illnesses such as heart disease or diabetes)
  • Japanese encephalitis vaccine (recommended for travellers spending 1 month or more in rural areas and for short-term travellers who may spend substantial time outdoors in rural areas after dusk)

It is important for pregnant women to abstain from consuming foods such as unpasteurised milk or cheeses (feta, brie and camembert) as these can cause listeriosis. Prevention of mosquito bites is important as mosquitoes are vectors of viral infections such as Japanese encephalitis, dengue, malaria etc.

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