Pulmonary Infections and Imaging Flashcards
Basic Facts - Pneumonia
- Pulmonary infections occur more frequently than infections of any other organ.
- Patients are most susceptible during infancy and in old age.
- Pneumonia is frequently the immediate cause of death secondary to other primary diseases (e.g., cancer, Alzheimer’s disease, heart disease).
- Providing effective treatment relies on determining the etiology (organism) and extent of disease involvement.
Types of Pneumonia
- Community Acquired Pneumonia
- Nosocomial Pneumonia
- Aspiration Pneumonia
- Chronic Pneumonias
- Pneumonia in Immunocompromised Hosts
Community Acquired Pneumonia
• 95% are due to viral, Mycoplasma, pneumococcal, or Legionella sp. Infections.
Nosocomial (hospital acquired) Pneumonias
- Occur in 1% of all hospitalized patients.
- ICU patients are at highest risk of infection and mortality.
- Most (60%) nosocomial infections are due to gram-negative bacilli; Staphylococcus aureus infections are also common.
Aspiration Pneumonia
- Caused by aspiration of infective material (oropharyngeal organisms) and/or gastric contents.
- Frequently caused by anaerobic bacteria, admixed with aerobic bacteria.
- Often results in chemical pneumonitis, necrotizing pneumonia, lung abscess, or empyema.
Chronic Pneumonias
•usually granulomatous
Pneumonia in Immunocmpromised Hosts
- Common in patients whose immune system is suppressed by disease (HIV, leukemia, lymphoma), chemotherapy, or iatrogenic immunosuppression (e.g. patients with a bone marrow transplant or solid-organ transplant).
- Often caused by “opportunistic” organisms not normally associated with disease in healthy individuals.
- Also may be caused by the more common organisms listed above for other types of pneumonia.
Pathogenesis of Pneumonia
- Loss of Defense Mechanisms
- Other Factors
Pathogenesis of Pneumonia - Loss of Defense Mechanisms
- Inhibition of the normal cough reflex from neuromuscular disease, drug overdose, intubation, coma, etc. allows aspiration of oropharyngeal flora and/or gastric contents into lungs.
- Injury or impairment of mucociliary apparatus prevents clearance of small inhaled particles/microorganisms
a. Viral destruction of ciliated epithelium.
b. Smoking.
c. Genetic disease (immotile cilia syndrome). - Interference of phagocytic or bactericidal action of alveolar macrophages by alcohol, tobacco smoke, or anoxia impairs clearance of particles/organisms deposited in alveoli.
- Bronchial obstruction due to neoplasm, mucus plugging, etc. creates physical barrier preventing clearance of microorganisms.
- Decreased immunity (primary immunodeficiency syndromes, viral infections, leukemia, lymphoma, immunosuppressive therapy, chemotherapy).
Pathogenesis of Pneumonia - Other Factors and Mechanisms
- Organisms may be directly introduced into normally sterile lung by intubation or by contaminated respiratory care equipment.
- Infections from other sites may spread hematogenously and secondarily infect lungs.
- Bacteria common to hospital environments are often drug resistant.
Classification of Bacterial Pneumonia
Classification of bacterial pneumonia is according to etiologic agent (e.g. staphylococcal, streptococcal, etc.) as well as the anatomic distribution pattern (bronchopneumonia versus lobar pneumonia)
Bacterial Pneumonia - Clinical Presentation
- Classic symptoms/signs are malaise, fever, chills, pleuritic pain, and productive cough (often blood-tinged).
- Auscultation may reveal decreased breath sounds in affected lobes and expiratory rales.
- Laboratory findings may show leukocytosis with a left shift (more immature forms).
- Chest X-rays imaging shows focal opacities and occasionally pleural effusions.
Etiology and Diagnosis of Bacterial Pneumonia
- Sputum gram stains and microbiologic cultures are keys to diagnosis:
a. Streptococcus pneumoniae (“pneumococcus”) is the most common organism causing pneumonia in ambulatory patients.
b. In hospitalized patients (nosocomial infections), gram-negative bacilli are most common (Pseudomonas, Klebsiella, Proteus, E. coli). Organisms may reach lungs via upper airway or through the blood (often bacteremia following urinary tract infection).
c. Staphylococcal and Haemophilus sp. infections often follow upper respiratory viral infections.
d. Legionella pneumophila is associated with aerosols from cooling systems (resistant to chlorine). Multiple small abscesses are frequent. Organism only grows on special media and may be missed on routine culture.
Morphology/Patterns of Bacterial Pneumonia
- Bronchopneumonia (lobular)
- Lobar Pneumonia
Bronchopneumonia
- Gross appearance: Patchy consolidation. Infiltrates are associated with airways and represent extension of a preexisting bronchitis or bronchiolitis. On cut lung sections, lesions are elevated, dry, granular, and firm. Multiple patchy areas may become confluent.
- Microscopic appearance: Alveolar spaces are filled by a suppurative exudate composed of neutrophils, fibrin, edema fluid, red blood cells, and macrophages. Alveolar septa are typically hyperemic and congested but not inflamed.
Lobar Pneumonia
• Gross / Microscopic appearance: Consolidation by fibrinopurulent material is widespread and typically involves an entire lobe or lobes. This type of pneumonia pattern is rarely seen today due to effective antibiotic treatment. Classic stages of progression in lobar pneumonia are now rare.
Complications of Bacterial Pneumonia
- Abscess
- Empyema
- Organization
- Bacteremic Dissemination
Abscess
- This is a local suppurative process characterized by destruction of lung tissue and accumulation of neutrophils.
- Abscesses are commonly associated with aspiration, septic emboli, and bronchial obstruction (“post-obstructive pneumonia”).
- The most common organisms are Streptococcus pneumoniae, Pseudomonas aeruginosa, Staphylococcus aureus, and anaerobes. These organisms contain enzymes which will liquefy lung tissue.
Empyema
•This is purulent inflammation of the pleural space caused by spread of infection into the pleural cavity
Atypical Pneumonia Microscopic Appearance
- Characterized by a mononuclear interstitial inflammatory infiltrate within the walls of the alveoli.
- Alveolar septa are widened and edematous, and alveolar spaces may contain protein-rich fluid.
- Type II pneumocytes are often hyperplastic.
- Alveolar walls may be lined by hyaline membranes.
- Herpes, varicella, and adenovirus sometimes cause necrosis of bronchial and alveolar epithelium.
- Cytomegalovirus, herpes, and measles often show characteristic viral inclusions within infected cells
Coccidioidomycosis
- Fungal infection caused by Coccidioides sp. T
- This infection is endemic to the southwestern U.S., especially Arizona and California, and most commonly involves the lungs through inhalation of organisms from the environment.
- Most primary infections are asymptomatic, while 10% of patients have lung lesions, pleuritic pain, and cough. Most (>80%) of persons living in endemic areas have a positive skin test.
- Organisms are seen in tissue as large double-walled spherules that are 20-60 microns in diameter. Spherules are filled endospores which are liberated when capsule is disrupted.
- Host response is primarily granulomatous inflammation with giant cells and macrophages, although a purulent reaction may also be seen.
Histoplasmosis
- Fungal infection caused by Histoplasma capsulatum.
- This infection is endemic to large portions of the central U.S., especially the Mississippi and Ohio River valleys, and most commonly involves the lungs through inhalation of organisms from the environment (soil contaminated with bird or bat droppings, caves).
- Organisms in tissue appear as small (2-5 microns) oval- shaped yeast forms.
- As with Coccidioides, primary infection is usually innocuous unless the host is immunocompromised. The host response is similar to coccidioidomycosis, with granulomatous inflammation.
Blastomycosis
- Fungal infection caused by Blastomyces.
- This infection is endemic to many parts of the eastern U.S. Infection and the host response are similar in many ways to coccidioidomycosis and histoplasmosis.
- In tissue, organisms appear as medium-sized yeast with broad-based budding.
- Sometimes skin involvement
TB - Pathology
- Caseating granulomas are the hallmark.
- These consist of a collection of modified macrophages (epithelioid histiocytes) surrounded by a rim of lymphocytes, fibroblasts, giant cells and occasionally a fibrous wall.
- Central caseous necrosis is seen.
- Organisms appear as small red rods on acid-fast stains
. Pneumocystis jiroveci (previously carinii)
- This fungus produces an alveolar infiltrate of foamy material and mononuclear cells.
- Diagnosis based on identifying the grouped, 4-5 micron, cup shaped organisms.
- Seen in HIV patients when CD4+ T-cells are <200/mm3 .
- Clinicians say “PCP” (= PneumoCystis Pneumonia).
Aspergillus
- Ubiquitous fungal organism found in soil and commonly inhaled into lungs.
- May colonize old, fibrous-walled cavities from a previous disease and grow as a fungus ball. In immunocompromised hosts, may invade parenchyma producing a necrotizing pneumonia.
- Organism frequently invades veins and arteries producing hemorrhagic infarcts. In tissue, organisms have septate hyphae with branching at a 45-degree angle.
Zygomycetes
- (Mucor, Rhizopus sp., etc.)
- Like Aspergillus, they frequently invade arteries and veins. Hyphae are pauciseptate and branch at a 90-degree angle.
Cryptococcus
•Inhaled encapsulated yeast which causes mild pulmonary symptoms and often spreads to the CNS in immunocompromised patients.
The organism is variable in size (4-10 microns), and has a thick gelatinous capsule which appears as a “halo” after tissue fixation.
Candida and Torulopsis
•These yeasts are part of the normal oropharyngeal flora which can produce bronchitis, bronchopneumonia, hemorrhagic pneumonia, and acute abcesses in immunocompromised patients.
Cytomegalovirus and herpes simplex virus
•These viruses can produce hemorrhagic interstitial pneumonias, particularly in immunocompromised patients.
Actinomyces and Nocardia
•These are filamentous, branching bacteria which can produce acute pneumonias with rapid progression to abscesses.
