Pathophysiology

Pattern of involvement

• Classic (typical) pneumonia of an entire lobe; primarily caused by pneumococci
  • Characterized by inflammatory intra-alveolar exudate, resulting in consolidation
  • Can involve the entire lobe or the whole lung
• Bronchopneumonia: mostly commonly a descending infection that affects the bronchioles and adjacent alveoli
  • Primarily caused by pneumococci and/or other streptococci
  • Characterized by acute inflammatory infiltrates that fill the bronchioles and the adjacent alveoli (patchy distribution)
  • Usually involves the lower lobes or right middle lobe and affects �?1 lobe
  • Manifests as typical pneumonia
  • Necrotizing bronchopneumonia and pneumatocele are caused by Staphylococcus aureus and are often preceded by an influenza infection.
• Interstitial pneumonia: interstitial inflammation, typically caused by Mycoplasma and viral infections
  • Characterized by a diffuse patchy inflammation that mainly involves the alveolar interstitial cells
  • Bilateral multifocal opacities are classically found on chest x-ray.
  • Manifests as atypical pneumonia
  • Often has an indolent course (walking pneumonia)
• Miliary pneumonia: multiple small infiltrations caused by hematogenous dissemination (e.g., of tuberculosis)
• Cryptogenic organizing pneumonia: characterized by inflammation of the bronchioles and surrounding structures

Clinical features

Typical pneumonia

Typical pneumonia is characterized by a sudden onset of symptoms caused by lobar infiltration.

• Severe malaise
• High fever and chills
• Productive cough with purulent sputum (yellow-greenish)
  • Crackles and bronchial breath sounds on auscultation
  • Decreased breath sounds
  • Enhanced bronchophony, egophony, and tactile fremitus
  • Dullness on percussion
• Tachypnea and dyspnea (nasal flaring, thoracic retractions)

Atypical pneumonia

Atypical pneumonia typically has an indolent course (slow onset) and commonly manifests with extrapulmonary symptoms.

• Nonproductive, dry cough
• Dyspnea
• Auscultation often unremarkable
• Common extrapulmonary features include fatigue, headaches, sore throat, myalgias, and malaise.

Pathogen-specific pneumonia

Mycoplasma pneumonia

Microbiology

• Incomplete/absent cell wall (not visible on gram stain)
• Bacterial membrane stabilized by cholesterol (obtained from host cell)
• Grows on Eaton agar �?“fried egg�?appearance

Epidemiology

• One of the most common causes of atypical pneumonia
• More common in school-aged children and adolescents
• Outbreaks most commonly occur in schools, colleges, prisons, and military facilities.

Clinical features

• Generalized papular rash
• Erythema multiforme
• Pneumonia#Atypical pneumonia

Diagnostics

• Subclinical hemolytic anemia: associated with elevated cold agglutinin titers (IgM)
  • Mycoplasma pneumoniae infection is common in adolescents and usually presents with tracheobronchitis or walking pneumonia. The pathogen attaches to the respiratory epithelium using surface antigens (I-antigen) that are also present on the plasma membrane of erythrocytes. Therefore, patients with M pneumoniae typically develop cross-reactive IgM antibodies that can attach to red blood cells, activate the complement system, and cause erythrocyte lysis. These cross-reacting antibodies are called cold agglutinins because they bind to erythrocytes most strongly at temperatures below core body temperature. Testing for cold agglutinins can be done at the bedside by drawing blood into an edetate disodium–containing tube and placing it in a cup of ice. Removing the tube after several seconds will reveal clumping/agglutination that resolves as the tube warms.
• Interstitial pneumonia, often with a reticulonodular pattern on chest x-ray
• Chest x-ray can show extensive pulmonary involvement in patients with mild pneumonia.

Aspiration pneumonia

Definitions

• Aspiration pneumonia: a type of pneumonia that occurs as a result of upper airway or stomach microbes (anaerobes) aspiration
• Aspiration pneumonitis
  • Aspiration of gastric acid that initially causes tracheobronchitis, with rapid progression to chemical pneumonitis
  • May cause ARDS in extreme cases

Etiology

• Gram-positive and gram-negative aerobic bacteria predominate in community-acquired infections (e.g., S. pneumonia, S. aureus, H. influenza, Enterobacteriaceae).
• Gram-negative bacilli predominate in hospital-acquired infections (e.g., P. aeruginosa, Klebsiella spp.).
• Mixed infections with anaerobic organisms may occur (e.g., Fusobacterium, Peptostreptococcus, Bacteroides).

Clinical features

• Aspiration pneumonitis
  • Present hours after aspiration event
  • Range from no symptoms to nonproductive cough, �?O2, respiratory distress
  • CXR infiltrates (one or both lower lobes) resolve without antibiotics
• Aspiration pneumonia
  • Present days after aspiration event
  • Fever, cough, �?sputum
  • CXR infiltrate in dependent lung segment (classically RLL)
  • Can progress to abscess

Diagnostics

Imaging: The lung region in which the infiltrates are seen depends on the patient's position on aspiration.

• Supine position: posterior segments of the upper lobes and the superior segments of the lower lobes (most common site of aspiration)
• Standing/sitting: posterior basal segment of the right lower lobe
• Right lateral decubitus position: posterior segment of the right upper lobe or right middle lobe