Pneumonia

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Pneumonia

Definition

Pneumonia is an acute or subacute infection of the lungs characterized by inflammation of the alveolar spaces and/or interstitial tissue, leading to impaired gas exchange. It remains a leading cause of infectious mortality in industrialized nations.

Key point: Diagnosis requires new pulmonary infiltrates on imaging plus clinical signs of infection (fever, cough, dyspnea).

Etiology and Pathogens

1. Based on Acquisition

TypeCommon Pathogens
Community-acquired pneumonia (CAP)Typical: Streptococcus pneumoniae (most common, esp. nursing home residents and IV drug users), Haemophilus influenzae, Moraxella catarrhalis, Klebsiella pneumoniae, Staphylococcus aureus
Atypical: Mycoplasma pneumoniae, Chlamydia pneumoniae, Chlamydia psittaci, Legionella pneumophila, Coxiella burnetii, viruses: RSV, influenza, adenovirus, CMV, SARS-CoV-2
Hospital-acquired pneumonia (HAP)Gram-negative bacilli: Pseudomonas aeruginosa, Enterobacteriaceae, Acinetobacter spp.; Staphylococci: S. aureus; S. pneumoniae
Ventilator-associated pneumonia (VAP)Similar to HAP; often multidrug-resistant organisms
Aspiration pneumoniaAerobic Gram-positive/negative (community), Gram-negative bacilli (hospital), anaerobes (Fusobacterium, Peptostreptococcus, Bacteroides)

Mnemonic for atypical bacteria: Atypically, Legions of Clams Mind their P’s and Q’s!

  • L: Legionella pneumophila

  • C: Chlamydia pneumoniae

  • M: Mycoplasma pneumoniae

  • Ps: Psittacosis (Chlamydia psittaci)

  • Q: Q fever (Coxiella burnetii)

2. Based on Lung Involvement

PatternDescriptionCommon Pathogens
Lobar pneumonia

Consolidation of a single lobe, typical presentationS. pneumoniae
Bronchopneumonia

Patchy inflammation around bronchi and bronchioles; lower lobes often affectedS. pneumoniae, S. aureus, H. influenzae, Klebsiella
Interstitial pneumoniaDiffuse interstitial inflammation; often atypicalMycoplasma, Chlamydia, Legionella, viral pathogens, Coxiella
Miliary pneumoniaHematogenous spread; diffuse micronodulesTB, fungal infections
Cryptogenic organizing pneumoniaNoninfectious, inflammatory; bronchioles/alveoli involvedAutoimmune or idiopathic

3. Special Populations

  • Neonates: Group B Streptococcus, E. coli, S. pneumoniae, H. influenzae

  • Children (4w–18y): C. trachomatis, C. pneumoniae, RSV, Mycoplasma

  • Young adults (18–40y): Mycoplasma, C. pneumoniae, influenza, S. pneumoniae

  • Adults (40–65y): S. pneumoniae, H. influenzae, anaerobes, viruses

  • Elderly: S. pneumoniae, H. influenzae, Gram-negatives, influenza virus

  • Immunocompromised: Encapsulated bacteria, Pneumocystis jirovecii, Aspergillus fumigatus, CMV, Candida, Histoplasma, Coccidioides

Risk Factors

  • Host factors: Age >65, immobility, chronic cardiopulmonary disease (COPD, asthma, HF), cystic fibrosis, immunosuppression (HIV, chemotherapy), diabetes, alcoholism, malnutrition

  • Aspiration risk: Altered consciousness, neurodegenerative disorders, GERD, dysphagia, NG tube

  • Environmental: Crowded conditions, exposure to toxins

Pathophysiology

  1. Entry of pathogens:

    • Microaspiration of oropharyngeal secretions (most common)

    • Inhalation of droplets (viral/bacterial)

    • Aspiration of gastric contents (chemical pneumonitis → secondary bacterial infection)

    • Hematogenous spread (rare)

  2. Host defense failure:

    • Impaired mucociliary clearance, alveolar macrophage dysfunction

  3. Pulmonary effects:

    • Alveolar/interstitial inflammation → exudate → consolidation

    • Ventilation-perfusion mismatch → hypoxemia

    • Dependent lung positioning worsens hypoxia

  4. Lobar pneumonia stages:

    • Congestion (1–2 days): Red-purple parenchyma, serous exudate

    • Red hepatization (3–4 days): Firm, red-brown, fibrin-rich exudate

    • Gray hepatization (5–7 days): Gray, neutrophilic/macrocytic exudate

    • Resolution (8–28 days): Macrophage-mediated clearance

Clinical Features

TypeSymptomsSigns
TypicalSudden onset, fever, chills, malaise, productive cough (yellow-green), pleuritic chest painCrackles, bronchial breath sounds, dull percussion, tactile fremitus, egophony
AtypicalGradual onset, dry cough, dyspnea, fatigue, myalgia, sore throat, headacheOften unremarkable auscultation

Aspiration pneumonia: Often silent initially; later foul-smelling sputum, fever, dyspnea

Diagnostics

  1. Laboratory

    • CBC: Leukocytosis

    • CRP, ESR ↑

    • Procalcitonin: >0.25 mcg/L → bacterial etiology

    • ABG: ↓PaO2

  2. Microbiology

    • Blood cultures (2 sets)

    • Sputum Gram stain/culture

    • Urinary antigens: pneumococcal, Legionella

    • Viral PCR: influenza, SARS-CoV-2

  3. Imaging

    • CXR: Lobar consolidation (typical), diffuse interstitial infiltrates (atypical)

    • CT scan: For inconclusive cases, recurrent pneumonia, poor response

    • Bronchoscopy/thoracentesis: For mass, biopsy, or empyema evaluation

Key point: Radiographic pattern alone cannot reliably identify the pathogen.

Management

Supportive Care

  • Oxygen therapy, hydration

  • Antipyretics and analgesics

  • Airway management if aspiration or respiratory failure

Empiric Antibiotic Therapy

Community-acquired pneumonia (CAP)

SettingRegimen
Outpatient, healthyAmoxicillin OR Doxycycline OR Macrolide (areas with low resistance)
Outpatient, comorbiditiesβ-lactam (amox/clav, cefuroxime, cefpodoxime) + macrolide OR monotherapy with respiratory fluoroquinolone (moxifloxacin, levofloxacin)
Inpatient, non-ICUβ-lactam + macrolide OR respiratory fluoroquinolone monotherapy
ICU / Severe CAPβ-lactam + macrolide OR β-lactam + fluoroquinolone; add MRSA/Pseudomonas coverage if risk factors

Hospital-acquired pneumonia (HAP) / Ventilator-associated pneumonia (VAP)

  • Broad-spectrum β-lactams with MRSA coverage; double antipseudomonal coverage if high-risk

Aspiration pneumonia

  • Antibiotics covering anaerobes; aspiration pneumonitis usually supportive only

Duration: Typically 5–7 days for CAP; tailored for severity and pathogen

Complications

  • Parapneumonic effusion, empyema

  • Lung abscess

  • ARDS, respiratory failure

  • Sepsis, multiorgan failure

Prognosis

  • Mortality increases with age, comorbidities, severity

  • CURB-65 scoring predicts risk:

    • 0: ~1%

    • 1–2: ~10%

    • 3: ~14%

    • 4: ~40%

  • HAP: >20% mortality

Prevention

  • Vaccinations: Pneumococcal, influenza, COVID-19

  • Smoking cessation

  • Aspiration precautions: head-of-bed elevation, oral hygiene, dysphagia diet

  • Infection control: hand hygiene, prevent VAP in ventilated patients

High-yield points for exams / clinical practice:

  • S. pneumoniae = most common CAP pathogen in all adults

  • Mycoplasma = most common atypical CAP in young adults

  • Aspiration pneumonia favors right lower lobe in supine patients

  • Lobar consolidation + acute high fever → classic bacterial pneumonia

  • CURB-65 ≥ 3 → consider ICU; score 0–1 → outpatient therapy

  • Procalcitonin helps guide antibiotic duration

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