Curriculum · Respiratory
Community-acquired pneumonia
What it is
Pneumonia is classified first by where it was acquired: 1. CAP, community acquired pneumonia, 2. HAP, hospital acquired pneumonia, 3. aspiration pneumonia. CAP is then split into typical and atypical, and beside HAP sits VAP, the pneumonia that develops 48-72 hours after endotracheal intubation. With CAP the score comes first and the drug second. The CURB-65 score sorts patients into three bands, 0 or 1, 2, and 3 or more, and each band carries its own most likely microbial etiology and its own preferred and alternate drug regimen, so the route and the drug follow the band rather than the organisms alone. The organisms are set out in three groups: the usual causes, atypical CAP, and fungal pneumonia.
Causes and risk
Streptococcus pneumoniae is the most commonly identified cause of CAP. HIV patients have a 100-fold increased risk, and risk is also increased in the elderly and in alcoholics, asplenic patients, and patients with B-cell/Abs deficiencies. Klebsiella pneumoniae typically presents in an alcoholic or a diabetic. Staphylococcus aureus is a rare cause of CAP and is more common in children; it can cause pneumonia during or after influenza infection, and in some parts of the country methicillin-resistant S. aureus is causing CAP even in patients without prior influenza. Haemophilus influenzae is less common in the era of vaccination against this bacterium. Mycoplasma pneumonia is classically seen in teenagers or young adults living in close quarters, such as a college dormitory, military barracks or a summer camp. Chlamydia psittaci is contracted from birds, often parrots, and the birds may also show signs of illness with ruffled feathers. Pneumocystis is almost always seen in an HIV patient and can also be seen in patients on corticosteroids. A typical CAP table sets three organisms beside the host each one is found in. Streptococcus pneumonia: smoker, immunocompromised (B-cells), post-splenectomy, chronic HF, pre-existing lung disease. Staphylococcus aureus: young or elderly, IV abusers, leukemia or Lymphoma, CF, and CXR can show cavitation. H. influenza: COPD or bronchiectasis. One table sets each pathogen beside the setting that raises it, and that is what a most likely cause stem asks for. Haemophilus influenzae goes with COPD, so the old smoker with COPD who gets pneumonia has H. influenzae. Staphylococcus aureus goes with a recent viral infection, that is influenza. Klebsiella pneumoniae goes with alcoholism and diabetes, and with current jelly sputum. Anaerobes go with poor dentition and aspiration. Mycoplasma pneumoniae goes with young healthy patients. Chlamydophila pneumoniae goes with hoarseness. Legionella goes with contaminated water sources, air conditioning and ventilation systems. Chlamydia psittaci goes with birds, and Coxiella burnetii with animals at the time of giving birth, with veterinarians and farmers.