4. Imaging of pneumonias.

Pneumonia is an acute inflammation of the lung parenchyme, with involvement of the bronchioli and alveoli, most often caused by microbes. It can have significant mortality. It’s a very common condition. In young people, it usually have a mild natural course, but for elderly, it has a high mortality.

Classification

According to the pathological distribution:

• Bronchopneumonia
  • Exudate in a patchy distribution
• Lobar pneumonia
  • Exudate involving an entire lobe
• Interstitial pneumonia (pneumonitis)
  • Inflammation localized to alveolar septae

Special types:

• Atypical pneumonia
  • Caused by atypical bacteria
• Aspiration pneumonia
  • Caused by aspiration of gastric fluids
• Chemical pneumonitis

Etiology

Microbial agents

• Community-acquired pneumonia
  • Streptococcus pneumoniae (most cases)
  • H. influenzae
  • Staphylococcus (if after influenza)
  • Gram negatives (if old patient or chronic disease)
• Nosocomial pneumonia
  • Gram negative enterobacteria (E. coli, Klebsiella)
  • MSSA and MRSA
  • Pseudomonas aeruginosa
  • ESBL-producing gram negatives
• Atypical pneumonia
  • Chlamydia pneumoniae
  • Mycoplasma pneumoniae
  • Legionella pneumophila
  • Pneumocystis jirovecii (in AIDS patients) (= pneumocystis pneumonia, PCP)
• Aspiration pneumonia
  • Anaerob bacteria

Pathophysiology

Pneumonia causes hypoxaemia (type I respiratory failure) due to V/Q mismatching. When part of the lung becomes consolidated, that part of the lung receives no ventilation, but it still receives perfusion, causing a ventilation/perfusion mismatch. This effectively forms a shunt, where part of the blood is not oxygenated in the lung.

Diagnosis

In CAP, imaging and microbiology are not necessary, and the condition is rather diagnosed based on clinical features. In nosocomial pneumonia, imaging and microbiology is important judge the severity and to target the treatment.

Imaging

The gold standard for diagnosis is the presence of a new opacity on chest x-ray in the setting of typical clinical features. The opacity is caused by the consolidation of the tissue, caused by presence of exudate in the alveoli. Chest x-ray is negative in the first 24 – 48 hours. The sensitivity of chest x-ray for pneumonia is low, and so a negative x-ray does not rule it out, especially in the early phases.The chest x-ray findings usually persist for weeks even after symptoms have cleared, so they’re usually not used for follow-up for pneumonia. CT may also visualise pneumonia but is unnecessary for uncomplicated cases.

The consolidation usually has indistinct margins, except if they are next to the pleura of interlobar fissures, where the margins will be sharp. It appears "fluffy". If the pneumonia is central, one can see hypodense bronchi in the consolidation, which are called air bronchograms.

Lobar pneumonia is the most common type, in which an entire lobe or almost an entire lobe is consolidated on imaging. Bronchopneumonia also exists, where there are multiple diffuse patchy opacities in one or both lungs.

In atypical pneumonia, chest x-ray may show patchy reticular opacities in the perihilar lung. CT can show also show the pathology, but CT is rarely necessary to make the diagnosis.

In elderly typical lung symptoms are not necessary for diagnosis as they may have atypical symptoms.

If a consolidation has been found on chest radiograph and a diagnosis of pneumonia has been made, a repeat radiograph should be made after resolution of the pneumonia to rule out lung cancer.