Atypical Pneumonia

Definition And Etiology

Characterized by pulmonary infection caused by organisms lacking peptidoglycan cell wall.
Unresponsive to beta-lactam antibiotics.
Major causative organisms include Mycoplasma pneumoniae, Chlamydophila pneumoniae, and Legionella pneumophila.
Chlamydia trachomatis causes atypical presentation in infants.

Primary involvement localizes to pulmonary interstitial tissue.
Characterized by round cell infiltration.
Alveolar septa become edematous.
Bronchiolar mucosa undergoes inflammation, necrosis, and potential ulceration.
Terminal bronchiole obstruction directly causes secondary emphysema and atelectasis.
Pleural surfaces may exhibit patchy fibrinous exudates.

Pathogen	Peak Age Group	Transmission Route	Epidemiological Features
Mycoplasma pneumoniae	Children >3 years	Droplet infection via direct contact.	Epidemics occur primarily during winter months in overcrowded environments.
Chlamydophila pneumoniae	Children >3 years	Droplet infection via direct contact.	Similar seasonal pattern to Mycoplasma.
Legionella pneumophila	Very rare in children <19 years	Aerosolization from humidifiers and hot water heaters.	Human-to-human spread uncommon.
Chlamydia trachomatis	Infants (1-6 months)	Vertical transmission during birth.	Presents exclusively in early infancy.

Incubation period spans 12 to 14 days.
Gradual onset features malaise, fever, sore throat, headache, and myalgia.
Dry cough progresses to yield mucoid or blood-streaked expectoration lasting 2-3 weeks.
Dyspnea unusual.
Clinical presentation often mild, characterized as “walking pneumonia” despite diffuse lung involvement.
Physical signs minimal; few mild crepitations or wheezing noted.
Extrapulmonary manifestations frequent: Skin lesions, central nervous system involvement, hemolytic anemia, cardiac issues, gastrointestinal disturbances, and joint inflammation.

Presents acutely with high-grade fever, productive cough, and chest pain.
Rapidly progresses to severe alveolar disease featuring cavitation.
Extensive extrapulmonary organ dysfunction common: Dyselectrolytemia, renal failure, liver failure, and rhabdomyolysis.

Distinctive presentation featuring concurrent conjunctivitis and staccato cough.

Modality	Key Findings And Utility
Chest Radiograph	Bronchopneumonia involving perihilar areas and lower lobes. Patchy diffuse infiltrates. Hilar lymphadenopathy often present. Degree of consolidation often exceeds expectations based on mild clinical severity.
Complete Blood Count	Leukocyte count remains normal.
Serology	Elevated cold agglutinins found in 1/3 to 3/4 of patients. Isotype Enzyme-Linked Immunosorbent Assay (ELISA) superior to cold agglutinins for Mycoplasma. Four-fold rise in Immunoglobulin M (IgM) indicates acute infection, though IgM persists for up to a year.
Molecular Methods	Polymerase Chain Reaction (PCR) from appropriate respiratory specimen remains gold standard for definitive diagnosis.
Urinary Antigen Assay	Most common and effective method for Legionella detection.

Organisms lack peptidoglycan cell wall, requiring protein synthesis inhibitors (macrolides, tetracyclines) or Deoxyribonucleic Acid (DNA) synthesis inhibitors (fluoroquinolones).
Macrolides remain treatment of choice due to low minimum inhibitory concentration and pediatric safety profile.

Pathogen/Age Group	Primary Drug And Dosage	Alternative Agents (For Macrolide Resistance)
Chlamydia trachomatis (1-6 months)	Azithromycin PO, 20 mg/kg once daily for 3 days.	Not applicable.
Mycoplasma or Chlamydophila ( $\geq$ 6 months)	Azithromycin PO, 10 mg/kg on day 1, followed by 5 mg/kg once daily from days 2-5.	Levofloxacin or Doxycycline.

Consider Macrolide-Resistant M. pneumoniae (MRMP) if no clinical response observed after 72 hours of appropriate macrolide therapy.
Levofloxacin: <5 years: PO/IV 8-10 mg/kg twice daily. $\geq$ 5 years: PO/IV 10 mg/kg once daily.
Doxycycline: PO/IV 2.2 mg/kg every 12 hours.
Restrict second-line agents due to adverse effects: Tetracycline-induced permanent teeth discoloration and fluoroquinolone-induced tendinopathy.