Atypical pneumonia
Background
- Pneumonia caused by organisms not covered by standard beta-lactam antibiotics
- Classic atypical organisms: Mycoplasma pneumoniae, Chlamydophila pneumoniae, Legionella pneumophila
- Frequently presents with more gradual onset and extrapulmonary symptoms compared to typical bacterial pneumonia
- Can be difficult to distinguish clinically from typical community acquired pneumonia; many guidelines recommend empiric coverage for both
Clinical Features
- Gradual onset (days) vs abrupt in typical pneumonia
- Dry, nonproductive cough (early — may become productive later)
- Prominent extrapulmonary symptoms: headache, myalgias, arthralgias, malaise, pharyngitis
- Low-grade fever
- Exam may be less impressive than CXR findings (walking pneumonia)
- Organism-specific clues:
- Mycoplasma: Young adults, bullous myringitis, erythema multiforme, cold agglutinins
- Legionella: Older adults, smokers; diarrhea, hyponatremia, relative bradycardia, elevated LFTs
- Chlamydophila: Hoarseness, biphasic illness (pharyngitis → pneumonia)
Differential Diagnosis
Causes of Pneumonia
Bacteria
Viral
- Common
- Influenza
- Respiratory syncytial virus
- Parainfluenza
- Rarer
- Adenovirus
- Metapneumovirus
- Severe acute respiratory syndrome (SARS)
- Middle east respiratory syndrome coronavirus (MERS)
- 2019-nCoV (COVID-19)
- Cause other diseases, but sometimes cause pneumonia
Fungal
- Histoplasmosis
- Coccidioidomycosis
- Blastomycosis
- Pneumocystis jirovecii pneumonia (PCP)
- Sporotrichosis
- Cryptococcosis
- Aspergillosis
- Candidiasis
Parasitic
Evaluation
- CXR: patchy infiltrates, often bilateral; may show diffuse interstitial pattern
- Labs: CBC, BMP, procalcitonin
- Legionella urinary antigen (only detects serogroup 1 — covers ~70% of cases)
- Consider Mycoplasma IgM if diagnosis unclear
- Severity scoring: CURB-65 or PSI to guide disposition
Management
- Empiric coverage for atypicals included in standard CAP regimens per guidelines:
- Outpatient (healthy, no comorbidities): Azithromycin or doxycycline monotherapy
- Outpatient (comorbidities): Respiratory fluoroquinolone (levofloxacin or moxifloxacin) OR beta-lactam + macrolide
- Inpatient: Beta-lactam + macrolide OR respiratory fluoroquinolone
- See community acquired pneumonia for detailed antibiotic dosing via templates
Disposition
- Per CURB-65 or clinical judgment
- Most atypical pneumonias are mild and managed outpatient