Pneumonia

Chapter 42 Pneumonia

What are the clinical and pathological features of pneumonia?

The term pneumonia is most commonly used to indicate the presence of a lung infection. However, pneumonia can also sometimes be used to refer to other forms of inflammatory lung disease that interfere with lung function but are not necessarily associated with the presence of an infective organism within the lung. Pneumonia is classically associated with bacterial infection, which most commonly takes the form of acute inflammation involving the alveolar spaces. However, some forms of pneumonia predominantly involve the alveolar walls (interstitial pneumonia). Inflammation may spread to involve the pleura (pleurisy) or pericardium (pericarditis) and bacteria may enter the bloodstream (septicaemia) and travel to other organs (e.g. resulting in a cerebral abscess).

Infections within the lung

Lung tissue is exposed to many pathogens within inhaled air and may become infected with a wide range of microorganisms (Table 3.42.1). Bacteria are the organisms most commonly associated with the development of pneumonia, with certain types particularly characteristic (Table 3.42.2; see the clinical box). Tuberculosis is a very important cause of lung infection and is dealt with separately in Ch. 43. The clinical features of pneumonia include fever, general malaise and a productive cough. The inflammatory process not uncommonly spreads to the pleura (pleurisy) and this is associated with chest pain on inspiration.

Table 3.42.1 TYPES OF ORGANISM INFECTING THE LUNG

Organism	Features
Bacteria	Common; wide range of types (Table 3.42.2); usually severe and potentially life threatening
Viruses	Common but usually less severe than bacterial pneumonia
Fungi	Rare, e.g. Pneumocystis carinii pneumonia in HIV infection; Aspergillus sp. in immunocompromised patients

Table 3.42.2 BACTERIA CAUSING PNEUMONIA

Bacterial type	Characteristics
Gram positive
Streptococcus pneumoniae	Common; CA; LP; BP; ECD
Staphylococcus aureus	Less common; HA; abscess-forming
Gram negative
Haemophilus influenzae	Less common; CA; neonates; ECD
Enteric organisms e.g. Escherischia coli	Less common; severe; HA; BP; ECD
Legionella pneumophila	Legionnaire’s disease; organism lives in tepid/stagnant water
Other
Mycobacteria	Tuberculosis (Ch. 43.); atypical mycobacteria, e.g. in HIV infection
Mycoplasma pneumoniae^a	Quite common; CA; less severe; radiograph changes worse than clinical signs
Chlamydia pneumoniae^a	CA; less severe

CA, community acquired; HA, hospital acquired; LP, lobar pneumonia; BP, bronchopneumonia; ECD, exacerbation of chronic obstructive pulmonary disease.

a Mycoplasma and Chlamydia are important causes of ‘atypical’ pneumonias and are associated with an interstitial pneumonia pattern of inflammation.

Lobar pneumonia refers to pneumonia involving one pulmonary lobe, or part of a lobe, with infection spreading through the lobe via tiny holes in the alveolar walls (pores of Kohn; Fig. 3.42.1). This form of pneumonia may affect any age group but most commonly affects young and middle-aged patients. Bronchopneumonia is a diffuse form of pneumonia that commonly involves both lungs (e.g. both lower lobes) and comprises inflammation centred on respiratory bronchioles. Bronchopneumonia is particularly common in elderly patients or those with an underlying serious disease (e.g. widespread cancer); in these patients, bronchopneumonia is a very common eventual cause of death.