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  Disease summary:

  
  
  
  
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    Bronchitis is the inflammation of the bronchial airway. Bronchitis can be divided into two categories, acute and chronic, each of which has different etiologies, pathologies, and therapies.

    
    
    
    
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      Acute bronchitis is often caused by infection. Viruses cause about 90% of cases of acute bronchitis, whereas bacteria account for fewer than 10%.

      
      
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      Chronic bronchitis is characterized by the presence of a productive cough that lasts for 3 months or more per year for at least 2 years. Chronic bronchitis most often develops due to recurrent injury to the airways caused by inhaled irritants. Cigarette smoking is the most common cause, followed by air pollution and occupational exposure to irritants. Acute exacerbations of chronic bronchitis are frequently caused by an infection.

    
    
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    Pneumonia is an inflammatory condition of the lung associated with fever, chest symptoms, and a lack of air space (consolidation) on a chest x-ray. Pneumonia is typically caused by bacteria but there are a number of other causes. Infectious agents include bacteria, viruses, fungi, and parasites.

    
    
    
    
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      Clinical presentations of bronchitis and pneumonia overlap; the two may have fever, cough, dyspnea, or extrapulmonary symptoms.

      
      
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      A diagnosis of pneumonia requires a chest radiograph showing consolidation in the lung parenchyma.

      
      
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      Anatomic involvement: bronchitis involves bronchial airway, between trachea and lung parenchyma. Pneumonia involves lung parenchyma, especially alveoli, distal airway, and interstitium.

      
      
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      This chapter is focused on acute respiratory infections, including acute bronchitis and pneumonia, and excludes acute exacerbations of chronic bronchitis which is recognized as a different condition by itself.

  
  
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  Hereditary basis:

  
  
  
  
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    Susceptibility to pulmonary infectious diseases and its adverse outcomes has a strong genetic influence.

    
    
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    Although there are numerous reports of positive associations between gene polymorphisms and clinical outcomes, most of these are probably spurious or at best in linkage disequilibrium with the key genetic markers rather than being the “true” sites of polymorphic interest.

    
    
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    Genetic variation in mannose-binding lectin (MBL2), the IgG2A receptor (FCGR2A), and CD14 are the best-validated markers of susceptibility to adverse outcomes from acute respiratory infections.

    
    
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    Rather than a few key polymorphisms determining risk, it appears that dozens, if not hundreds, of polymorphisms interact and contribute to total risk.

  
  
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  Differential diagnosis:

  
  
  
  
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    Chronic obstructive pulmonary disease

    
    
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    Congestive heart failure

    
    
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    Lung cancer

    
    
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    Diffuse interstitial lung diseases

    
    
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    Pulmonary embolus

    
    
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    Pulmonary vasculitis

Diagnostic Criteria

Acute respiratory infection is a clinical diagnosis defined by the presence of at least two of the following clinical signs:

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  Cough

  
  
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  Fever

  
  
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  Shortness of breath

  
  
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  Tachypnea

  
  
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  Phlegm production of yellow, green, or bloodstained color

A chest radiograph is necessary to differentiate between bronchitis and pneumonia:

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  The absence of a consolidation supports the diagnosis of bronchitis.

  
  
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  The presence of a consolidation in lung parenchyma suggests the diagnosis of pneumonia.

Clinical Characteristics

The most common symptoms of acute respiratory infections are fever, nonproductive or productive of purulent cough, dyspnea, pleuritic chest pain, chills, and mental status changes.

Patients present less commonly with diarrhea, new-onset or worsening confusion in elderly patients, and headache.

Clinical signs include fever or hypothermia, dullness to percussion, increased tactile and vocal fremitus, egophony, crackles, whispered pectoriloquy, and pleural friction rub.

Screening

No clear evidence for screening of family members exists. Genetic testing for disease-associated single-nucleotide polymorphisms (SNPs) is not clinically validated. Apart from polymorphisms in MBL2, FCGR2A, and CD14 there is no consensus on which polymorphisms are truly important.

Counseling