creation date: 2025-07-03 15:17 tags: Pathologies
Community-Acquired Pneumonia
Background
Definitions
Community-acquired pneumonia, hereafter referred to as pneumonia, is an acute infection of the pulmonary parenchyma acquired from outside the hospital.
This is in contrast to nosocomial pneumonia which refers to hospital-acquired (acquired ≥48 hours post admission) and ventilator-associated pneumonia (≥48 hours post endotracheal intubation).
Note that pneumonia acquired in nursing homes, hemodialysis centres, and other health care facilities are considered community-acquired as their prior classification as “HCAP” was too sensitive and promoted unnecessary antibiotics use.
Etiology
Both bacteria and viruses can cause pneumonia. The most common causes can be grouped as follows.
Typical bacteria
- Streptococcus pneumoniae (most common bacterial cause)
- Haemophilus influenzae
- Moraxella catarrhalis
- Staphylococcus aureus
- Group A streptococci
- Aerobic gram-negative bacteria (eg. Klebsiella spp or E. coli)
- Microaerophilic bacteria and anaerobes Atypical bacteria
- Legionella spp
- Mycoplasma pneumoniae
- Chlamydia pneumoniae
- Chlamydia psittaci
- Coxiella burnetii Respiratory viruses
- Influenza A and B virus
- Coronavirus
- Rhinoviruses
- Parainfluenza viruses
- Adenoviruses
- Respiratory syncytial virus
- Human metapneumovirus
- Human bocaviruses
The relative prevalence of each pathogen varies by geography and other factors such as vaccination rates and seasons.
Of note:
- Decrease in S. pneumoniae incidence due to vaccination rates
- Pathogen detection is overall low with no pathogen identified in up to half of cases
Pathogenesis
Historically, with a sterile perspective of the lungs, infection was thought to have been due to transmission of respiratory pathogen followed by colonization from the nasopharynx.
With current models of the lung microbiome, pathogenesis is thought to be a balance between pathogenic microbes and resident microbiome. Following the introduction of respiratory pathogens, factors such as host immune response, virulence factors, and damage of the parenchyma predisposes the development of pneumonia.
In other cases, pneumonia may arise from uncontrolled replication of a resident microbe, possibly due to exogenous insults (eg. viral infection or smoke exposure) that disrupts the normal microbiome.
Clinical Presentation
Signs & Symptoms
The presentation of pneumonia can range significantly based on severity and the intensity of local and systemic immune response.
Pulmonary features Due to the accumulation of WBCs, fluids, and proteins in the alveolar space:
- Cough with or without sputum production
- Dyspnea
- Pleuritic chest pain
- Tachypnea
- Increased work of breathing
- Abnormal breath sounds including crackles and rhonchi
- Tactile fremitus, egophony, and dullness to percussion
Systemic features
- Fever
- Chills
- Fatigue
- Malaise
- Chest pain
- Anorexia
- Tachycardia
- Leukocytosis with leftward shift or leukopenia
- Elevated inflammatory markers (ESR, CRP, procalcitonin)
- Sepsis (and associated symptoms)
History & Physical Exam
Exam may point towards pneumonia but findings listed above are generally nonspecific and shared with many respiratory disorders.
It should be noted that signs and symptoms can be subtle with age.
Severity of symptoms can be determined by the Pneumonia Severity Index or the CURB-65.
Risk factors
A number of risk factors increases the risk of pneumonia:
- Older age
- Chronic comorbidities, especially COPD, chronic lung disease, chronic heart disease, stroke, diabetes, malnutrition, and immunocompromising conditions
- Viral respiratory tract infection
- Impaired airway protection
- Smoking and alcohol overuse
- Crowded living conditions and environmental exposures
Diagnosis
Criteria
Diagnosis is made on chest imaging with a clinically compatible syndrome.
In most patients, posteroanterior (PA) and lateral chest radiograph is sufficient:
- Lobar consolidations
- Interstitial infiltrates
- Cavitations (gas filled space within lung tissue; abscess/pneumatocele sequelae)
It should be noted that while certain radiographic features suggest certain etiologies, it is not reliable enough to differentiate.
In cases of negative chest x-ray but pneumonia is still suspected (eg. immunocompromised patients where response mounted is not great enough to show up on chest x-ray), a CT chest can be obtained.
Work-up
In the outpatient setting, workup is generally complete following diagnosis with chest imaging.
In moderate pneumonia that require admission the following is typically obtained:
- Blood cultures
- Sputum gram stain and culture
- Urinary antigen testing for S. pneumoniae
- Testing for Legionella spp
- SARS-CoV-2 testing
Vigilance should remain for alternative diagnoses especially if pulmonary infiltrates resolve rapidly (pneumonia infiltrates can take weeks to resolve) or if there are suspicion of sepsis.
Follow-up imaging is not routine.
Differential
Noninfectious illnesses that appear like pneumonia include:
- Congestive heart failure with pulmonary edema
- Pulmonary embolism
- Pulmonary hemorrhage
- Atelectasis
- Aspiration or chemical pneumonitis
- Drug reactions
- Lung cancer
- Collagen vascular diseases
- Vasculitis
- Acute exacerbation of bronchietasis
- Interstitial lung diseases (eg. sarcoidosis, asbestosis)
Rapidly resolving infiltrates could be:
- AECOPD
- Influenza or other respiratory viral infections
- Acute bronchitis
- Asthma exacerbation
Red Flags / Complications
Pneumonia is a common cause of hospitalization and in severe cases, can lead to tissue injury, sepsis, acute respiratory distress syndrome, and/or multiorgan dysfunction.
Nonresolving pneumonia even with empiric antibiotic use can be due to:
- Delayed response to antibiotics due to cormorbidities
- Loculated infection (eg. abscess)
- Bronchial obstruction (eg. by tumour) can slow response
- Certain pathogens that may not response to empiric treatment
- Alternative diagnosis (eg. malignancy)
Management
Lifestyle / Social
Lifestyle management consist of supportive therapy:
- Fluids
- Avoid smoking
Management also includes prevention of repeat episodes:
- Smoking cessation
- Influenza vaccination
- Pneumococcal vaccination for at-risk patients
- SARS-CoV-2 vaccination
Pharmacological / Interventional
The primary treatment of pneumonia is antibiotics. They are discussed in more detail separately.
Outpatient Patient is otherwise healthy with normal vitals (except fever) and no concerns for complications (PSI class I-II (≤70), CURB-65 0 or 1 if ≥65).
In most cases monotherapy is adequate with:
- Amoxicillin
- Macrolide (erythromycin, azithromycin, or clarithromycin)
- Doxycycline
In cases of comobid illnesses (eg. COPD, heart failure, smoking):
- Combination of an amoxi-clavulanate plus either a macrolide or doxycycline
Note, in cases amoxicillin cannot be used consider:
- 3rd generation cephalosporins (eg. cefpodoxime) instead
- Respiratory fluoroquinolone (eg. levofloxacin) or lefamulin
Most treatments are for 5 days with little benefit shown for treatment extending over 7 days.
Hospital inpatient Patient with peripheral O2 sats <92 on room air or those with PSI ≥III (>70) and CURB-65 ≥1 (≥2 for age >65).
Following admission, empiric antibiotic treatments are promptly to avoid increased mortality. Treatment choice depends on suspicion for MRSA or Pseudomonas.
For patients without suspicion for MRSA or Pseudomonas:
- Beta-lactam plus macrolide
- Respiratory fluoroquinolone
- Beta-lactam plus doxycycline (last-line option)
For patient with known colonization or prior infection of Pseudomonas, hospitalization with IV antibiotics use, or strong suspicion for current pseudomonal infection:
- Anti-pseudomonal beta-lactam (eg. piperacillin-tazobactam, cefepime, ceftazidime, meropenem, or imipenem) plus an antipseudomonal fluoroquinolone (eg. ciprofloxacin or levofloxacin)
For patients with known colonization or prior infection with MRSA, or strong suspicion for MRSA infection:
- Add anti-MRSA antibiotic to one of the above regimes
- Eg. Vancomycin or linezolid
Hospital ICU Patients with respiratory failure requiring mechanical ventilation, sepsis requiring vasopressor support and/or other criteria of severe illness.
In more severe cases, antibiotic selection is similar to normal inpatient care except antibiotics are started much quicker and monotherapy is never used.