Interstitial (atypical) pneumonia
Summary
Diffuse, patchy inflammation of the alveolar septa/interstitium rather than alveolar consolidation. Caused by 'atypical' organisms (Mycoplasma, Chlamydia, Legionella, viruses); presents with dry cough, low-grade fever, and CXR worse than exam findings.
Detail
Atypical pneumonia features mononuclear interstitial infiltrates, with relatively spared alveolar spaces, producing diffuse reticulonodular or patchy infiltrates on CXR that look more impressive than the patient's clinical exam ('walking pneumonia'). Mycoplasma pneumoniae is the classic agent in young adults/military recruits and is associated with cold agglutinins (IgM against RBC I antigen) and erythema multiforme/Stevens-Johnson. Chlamydia pneumoniae and psittaci, Legionella pneumophila (water-source, hyponatremia, GI symptoms), and viruses (influenza, RSV, adenovirus, SARS-CoV-2) also cause interstitial patterns. Treat with macrolides, doxycycline, or fluoroquinolones; beta-lactams are ineffective against Mycoplasma (no cell wall) and Chlamydia (intracellular).
Sources
- First Aid for USMLE Step 1 2024
- Robbins Basic Pathology 10th ed
- Sketchy Micro
Reviewed by AnkiBoss editorial — medical student review. Information here is for study reference only and is not medical advice. Spotted an error? Let us know.