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Pneumonia in the normal host 

Pneumonia in the normal host

Chapter:
Pneumonia in the normal host
Author(s):

John G. Bartlett

DOI:
10.1093/med/9780199204854.003.180402
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date: 24 April 2017

Pneumonia is an acute or chronic infection involving the pulmonary parenchyma and is the most important infectious disease in terms of morbidity and mortality, which is 14% for patients who are hospitalized with community-acquired pneumonia.

Aetiology—most cases are caused by microbial pathogens, the commonest being Streptococcus pneumoniae, Haemophilus influenzae, Mycoplasma pneumoniae, Chlamydia pneumoniae, legionella, anaerobic bacteria, and viruses (influenza, parainfluenza, and respiratory syncytial virus). Staphylococcus aureus is an important superinfecting pathogen in influenza, and is the most common form of embolic pulmonary infection with injection-drug use and tricuspid valve endocarditis.

Prevention—the main preventive measures are influenza and S. pneumoniae vaccination, the best data for the latter being in favour of giving the protein conjugated vaccine to children under 2 years of age to protect both themselves and adults.

Clinical features—the classic presentation of pneumonia is with cough and fever, with variable sputum production, dyspnoea and pleurisy. Most patients have constitutional symptoms such as malaise, fatigue and asthenia, and many also have gastrointestinal symptoms. Clinical examination may reveal features indicative of the severity of respiratory compromise—appearance of exhaustion, use of accessory muscles, inability to talk in sentences, tachypnoea (or even more worryingly when associated with exhaustion, a low respiratory rate), cyanosis—and (in some cases) of consolidation, in particular localized dullness to percussion and bronchial breathing. The ‘CURB-65’ score—based on compromised Consciousness, elevated blood Urea nitrogen, increased Respiratory rate, reduced Blood pressure and age over 65 years—is a useful predictor of severity and need for hospitalization.

Diagnosis—the key test to confirm the diagnosis of pneumonia is the chest radiograph, which will virtually always show an infiltrate. Most patients with symptoms of pneumonia and a negative chest radiograph have acute bronchitis. The use of laboratory studies for identifying pulmonary pathogens in pneumonia is controversial: even in studies with extensive use of diagnostic resources a likely aetiological agent is only detected in 40 to 60% of cases. Empirical therapy is generally advocated for outpatients; blood cultures (taken before the initiation of antibiotic treatment) and Gram stain and culture of expectorated sputum (if any) are recommended for inpatients. Rapid urinary antigen tests for legionella (which detect the subgroup responsible for 80% of cases) and S. pneumoniae are available. Pleural effusions should be sampled to exclude empyema.

Management—supportive treatment includes (as appropriate) intravenous fluids, supplementary oxygenation and ventilatory support. Antibiotics are the mainstay of therapy, with recommendations for empirical treatment of community-acquired pneumonia typically as follows (but local hospital protocols and policies may vary): (1) outpatients—doxycycline, or macrolide (erythromycin, clarithromycin, azithromycin), or fluoroquinolone (levofloxacin, moxifloxacin or other fluoroquinolone with enhanced activity against S. pneumoniae); (2) general hospital inpatients—β‎-lactam (cefotaxime, ceftriaxone) plus macrolide, or fluoroquinolone alone; (3) intensive care unit—β‎-lactam plus macrolide, or β‎-lactam plus fluoroquinolone; (4) special circumstances: aspiration pneumonia—clindamycin, or β‎-lactam-β‎-lactamase inhibitor; structural lung disease—include agent with activity against Pseudomonas aeruginosa.

A relevant case history from Oxford Case Histories in Respiratory Medicine has been added to this chapter.

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