CNS infections: meningitis and encephalitis

Published on 23/06/2015 by admin

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Last modified 23/06/2015

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8.7 CNS infections

meningitis and encephalitis

1 Bacterial meningitis can be rapidly progressive, and result in substantial morbidity and mortality.

2 A high index of suspicion of the possibility of meningitis must be maintained in any sick infant or child.

3 Symptoms and signs are frequently non-specific, particularly with younger age.

4 Antibiotic treatment must not be delayed, although steroids can improve outcome if given before the first dose of antibiotic, and continued 6-hourly for 4 days.

5 Careful management of fluid and electrolyte balance is critical.

6 Careful evaluation for features suggesting raised intracranial pressure should be done at the initial assessment.

7 Conjugate haemophilus, meningococcal and pneumococcal vaccines have reduced the incidence of bacterial meningitis significantly.

8 HSV encephalitis must be considered in any child with features of meningitis with encephalopathy.

9 Empiric aciclovir must be given early when encephalitis is suspected.

Introduction

Meningitis and encephalitis are medical emergencies that require prompt assessment and treatment. Meningitis is inflammation of the meninges that surround the brain and spinal cord. Inflammatory cells invariably spill into the cerebrospinal fluid (CSF) from the meninges, producing an increased cell count. Encephalitis is inflammation involving the brain parenchyma, which may occur in conjunction with inflammation. There is considerable overlap between the features of meningitis and encephalitis, and the two may co-exist. Encephalitis should be considered if a child has encephalopathy, convulsions or neurological deficits.

Meningitis

Classification

Meningitis is usually broadly classified as bacterial or aseptic. Aseptic meningitis may be the result of a localised or systemic insult, but is most commonly caused by viruses. Fungal meningitis usually occurs in immunocompromised children.

Aetiology

Bacteria

The bacterial causes of meningitis vary with the age of the child. In infants less than 2–3 months old, organisms acquired from the maternal genital tract predominate: group B streptococci, Escherichia coli and Listeria monocytogenes. In older children and adults the most common causes are Neisseria meningitidis and Streptococcus pneumoniae. Other causes, including Staphylococcus species and Gram-negative bacilli, are occasionally seen in immunocompromised hosts or following trauma or neurosurgery. Haemophilus influenzae type b (Hib) is rarely seen as a cause now because of widespread immunisation. Mycobacterium tuberculosis is rare, other than in children who have spent prolonged periods in countries of high prevalence.

In Australia, meningococcal disease, particularly that caused by serogroup C, has declined since introduction of the meningococcal C conjugate vaccine (MenCCV) into the National Immunisation Program (NIP) schedule. The most common N. meningitidis serogroup causing invasive disease in those under 19 years is serogroup B (91%), followed by serogroup C (2–3%).¹ Until the introduction of 7-valent conjugate pneumococcal vaccine (7vPCV) into the NIP schedule, the most common pneumococcal serotypes causing invasive disease were those contained in 7vPCV (14, 6B, 18C, 19F, 4, 23F, 9V).² However, since then, it appears that serotype replacement is occurring; nasal carriage of 7PCV serotypes has been replaced by others, namely 19A and 16F,³ and the rate of invasive pneumococcal disease caused by 7vPCV serotypes has decreased significantly.⁴ However, the rate of invasive pneumococcal disease caused by serotype 19A increased in non-Indigenous people and in the population overall.⁴ Antibiotic resistance is almost exclusively restricted to these serotypes (and remaining 7PCV serotypes).

Viruses

Enteroviruses, including coxsackie and echoviruses, cause 85–95% of cases of viral meningitis. Herpes simplex viruses 1 and 2 (HSV-1 and HSV-2) and other herpes viruses (human herpesviruses 6, 7 and 8, varicella-zoster virus, cytomegalovirus and Epstein–Barr virus) tend to cause meningoencephalitis. HSV-1 and HSV-2 are perhaps the most important causes to consider, as meningoencephalitis caused by these viruses is associated with high morbidity and mortality, which may be reduced with early treatment with antiviral medications.

Fungi

Cryptococcus neoformans is the most common fungal cause of meningitis, but occurs almost exclusively in immunocompromised hosts.

Clinical findings

The classical features of meningitis comprise fever, headache, vomiting, neck stiffness, photophobia and altered mentation. However, the clinical manifestations are often non-specific, particularly in infants and young children. They may include fever, irritability, lethargy, poor feeding or vomiting. Up to 58% of children with meningitis have received antibiotics before the emergency department (ED) presentation.⁵ This may modify the clinical presentation of meningitis.⁶ It is therefore important to consider the possibility of central nervous system (CNS) infection in any sick infant or child, particularly if they are already taking antibiotics.

If the fontanelle is still open, it may be bulging when examined with the infant in a sitting position. Photophobia is difficult to ascertain in young children, and other signs of meningeal irritation may be absent or difficult to elicit. Resistance to being picked up or distress on walking may be the only clues. Kernig’s sign (inability to extend the knee when the leg is flexed at the hip), Brudzinski’s sign (bending the head forward produces flexion movements of the legs) and nuchal rigidity may be present in older children, but have even been shown to have low positive and negative predictive value in adults with meningitis.⁷

Rashes may occur with any bacterial meningitis, although are less common with pneumococcal infection. Petechiae or purpura are suggestive of meningococcal sepsis, but may also occur in Hib and viral meningitis. Enteroviral meningitis may even be associated with florid purpura fulminans.

It is impossible to reliably differentiate between bacterial and viral meningitis on clinical grounds. However, children with enteroviral meningitis are more likely to present in summer or autumn with gradual onset of non-specific constitutional symptoms including diarrhoea, cough and myalgia, in addition to the more typical features.

Investigations

Definitive diagnosis of meningitis relies on examination of the CSF with biochemical analysis, microscopy and culture. Children with suspected meningitis should have a lumbar puncture (LP) performed, unless there is a clear contraindication. The only absolute contraindication is raised intracranial pressure (ICP). It may be difficult to determine whether ICP is raised, but the following signs may be indicative:

• coma (absent or non-purposeful response to painful stimulus);

• abnormal pupillary responses;

• abnormal posturing;

• focal neurological signs or seizures;

• recent (within 30 minutes), prolonged (over 30 minutes) or tonic seizures;

• papilloedema – although this is an unreliable and late sign of raised ICP.

A bulging fontanelle, in the absence of other signs of raised ICP, is not a contraindication to LP.

The threshold for performing an LP should be lower in young children with less specific signs or those who have been taking antibiotics prior to presentation.

Cerebral computerised tomography (CT) should not be used to decide whether it is safe to proceed with LP or not. In a prospective study of bacterial meningitis, CT findings obtained during the acute stages failed to reveal any clinically significant abnormalities that were not suspected on neurological examination.⁸ Moreover, cerebral herniation can occur with a normal CT^9,¹⁰ and the true cause of coning and relationship to prior lumbar puncture is not clearly established.

Other relative contraindications to LP include:

• cardiovascular compromise or shock

• respiratory compromise

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