Introduction

Pneumonia is a common condition with significant morbidity and mortality. It is estimated to be responsible for approximately three million childhood deaths per year, most of which occur in developing countries.¹ Even in developed countries, pneumonia remains a major cause of acute morbidity and one of the most common reasons for paediatric hospital admissions.² The incidence of pneumonia is approximately 40 per 1000 children per year in those under five years of age, and 15 per 1000 children per year in 5–14-year-olds.^3–5

In spite of how common it is, there is no single clinical or radiological definition that is widely accepted for pneumonia. It is investigated with a range of tests, the usefulness of which is known to be incomplete, and it is then often treated without knowledge of the aetiology. Fortunately, most children recover completely with empiric treatment.

Definition

There are various definitions that can be used for pneumonia. From a pathological point of view, it is defined as inflammation or infection of the lung parenchyma. In the clinical setting, the diagnosis is typically made on the basis of a constellation of clinical features, including fever, cough, tachypnoea and auscultatory findings, and confirmed by radiographic changes.

Aetiology

In the majority of cases of childhood pneumonia, the causative pathogen is not identified. Blood cultures are positive in under 5% of cases of pneumonia.^5–7 Transthoracic lung aspiration yields a cause in up to 69% of cases,^8,9 but is invasive. It is difficult to obtain adequate sputum for microscopy and culture in young children. Other indirect methods of identifying a cause, such as serology or immunofluorescence and culture of nasopharyngeal aspirates are neither sensitive nor specific.

Although an alveolar or lobar infiltrate on chest X-ray is considered by some to be suggestive of bacterial infection, chest X-ray changes cannot reliably predict aetiology.^9,10 Nor is any radiological pattern pathognomonic for viral or Mycoplasma pneumoniae infection.

Age is the best predictor of aetiology of pneumonia. In neonates, where bacterial causes predominate, Group B streptococci and Escherichia coli are the most common pathogens. Viruses, particularly respiratory syncytial virus (RSV), parainfluenza, influenza, metapneumovirus and adenovirus, are the most common cause overall, particularly in young children. The occurrence of recent local outbreaks and the clinical pattern may give a clue to the likely causative virus. These viruses appear to be responsible for approximately 40% of cases of community-acquired pneumonia in children who are hospitalised, particularly in those under 2 years of age, whereas Streptococcus pneumoniae is responsible for 27% to 44% of cases of community-acquired pneumonia.¹¹ Up to 40% of infections are mixed.⁵ Infection with Mycoplasma pneumoniae and Chlamydia pneumoniae is usually considered to cause pneumonia in children of school age and in older patients, although more recent studies suggest that preschool-aged children have as many episodes of atypical bacterial pneumonia as older children.¹¹ Staphylococcal and Group A streptococcal pneumonia are uncommon, but should be considered in children who are severely unwell with invasive disease. These infections are more likely to be seen in indigenous and Pacific Islander children. More recently, infections with community-acquired multiresistant Staphylococcus aureus (CAMRSA) have emerged. CAMRSA results in a necrotising fulminant pneumonia with increased morbidity and mortality.

Gram-negative pneumonia is uncommon in children; non-typeable Haemophilus influenzae is mainly seen in children with underlying lung disease, such as cystic fibrosis and bronchiectasis.

The presence of a pleural effusion does not necessarily indicate more severe disease – Strep. pneumoniae remains the most common bacterial cause, with or without effusion.