Overview • Def: pneumonia is a cellular, exudative reactive condition within the lung Natural history ends in resolution (return to normal) or organization (fibrosis) Classification • Distribution: lobar pneumonia, bronchopneumonia, or interstitial • Etiology: infectious , aspiration, obstructive
Bacterial pneumonia • Bronchopneumonia • Patchy distribution, ie multiple foci of inflammation at airspace-airway interfaces Caused by streptococci and staphylococci, pseudomonas, coliform bacteria Usually does not consolidate into lobar pneumonia
Histology: peribronchial inflammation, infiltrate in bronchial lumen, Lobar pneumonia Entire lobe is uniformly affected Mostly caused by S. pneumoniae, as well as by Klebsiella • Histology: uniform intra-alveolar infiltrate
Gram negative organisms: inhibits PMN influx causing congestion in blood vessels on histology • Legionella: Mf rich infiltrate, gram positive coccobacilli detectable by silver stain • Viral: eg CMV, adenovirus, herpes; hyaline membranes, inclusion bodies, lymphocytic infiltrate Pneumocystis carinii: affects immunocomp. esp. HIV, frothy/bubbly acellular exudates in alveoli
Aspiration pneumonia, due to aspiration of: • Foreign bodies: can cause full or partial obstruction • Gastric contents: acid or food (granulomatous response) • Hydrocarbons: diffuse alveolar damage, bronchiolitis • Lipid materials • Exogenous: eg mineral oil aspiration, granulomatous response • Endogenous: obstructive pneumonia Mf exit route blocked congestion of Mf which degenerate and become fat-filled endogenous lipid pneumonia
Airway infections • Bronchiolitis: an inflammation of the small airways but airspaces mostly spared; viruses and bacteria are most common cause, but can be due to noninfectious causes • Bronchiectasis: multiple inflammatory foci near large airways organization/scarring traction on bronchi permanent dilation of bronchi
Tuberculosis: typically upper lobe lesion, granulomatous inflammation w/ caseous necrosis, organisms are found in center of debris (not on edge!), acid fast stain can demonstrate organism • Histoplasmosis: granulomatous w/ concentric rings, small budding yeast forms
Cryptococcus: mucoid capsule • Blastomyces: big organism, double wall • Coccidioides: small organisms found in big cysts (aka spherules
Aspergillus: found in immunocompromised, Y-shaped septated hyphal forms, invasion of blood vessels causing occlusion and pulmonary infarct • Lung abscess • Defined as focal infection with necrosis and inflammation leading to cavitation and walling-off Causes “cross country” destruction, ie communication with multiple airways
Pulmonary Infections – Pneumonia • Classification of pneumonia by distribution and outcome: • Bronchopneumonia (intra-alveolar, with patchy distribution through multiple lobes) • --Lobar pneumonia (intra-alveolar) • --Resolution • --Organizing (aberrant healing with fibrosis)
Classification of pneumonia by etiology: • Infectious • Bacterial pneumonia is most often caused by Strep pneumo (Gram+ encapsulated diplococci, positive Quelung reaction). Bacterial pneumonia exudate has tons of PMNs. • Gram- rods (Pseudomonas, Klebsiella) may cause a necrotizing pneumonia with infarction.
Viral and Mycoplasma pneumonia exudates have tons of lymphocytes, but few PMNs. • There may be hyaline membranes and intra-alveolar fibrin. • PCP causes an airway-filling pneumonia with a “frothy” acellular pink exudate. • Histoplasmosis creates a white granulomatous nodule that looks like a tree-trunk cross-section.
• Aspiration • Usually from inhalation of vomit. In older patients, aspiration of mineral oil is very common, as it does not elicit a gag response when swallowed. • Obstructive • Usually from obstruction by a tumor or inhaled object. Macrophages accumulate in alveoli, and become fat-filled, giving obstructive pneumonia the name “golden pneumonia.”
Other pathological features of pneumonia --Granulomas are part of a delayed hypersensitivity reaction with the accumulation of epithelioid macrophages and giant cells. There may be central necrosis. If the granuloma is white and looks like a tree-trunk cross-section, think Histoplasmosis. • Infarcts may be caused by vaso-invasive fungi or Pseudomonas that initiate pulmonary thrombosis.
Lung abscess often leads to cavitation that communicates with multiple airways (“cross-country” cavitation). Lung abscesses are usually caused by aspiration of food or foreign bodies, along with poor dental hygiene (source of anaerobic bacteria). • Bronchiectasis is a condition characterized by permanent dilation of bronchi with scarring and retraction of lung parenchyma. The bronchi are permanently pulled open.