- Lobar pneumonia refers to consolidation of an entire lobe while bronchopneumonia is scattered solid foci in the same or several lobes.
Bacterial pneumonias occur in three settings:
- Community-acquired pneumonia arises outside the hospital.
- Nosocomial pneumonia represents an infection spread by organisms in the hospital environment.
- Opportunistic pneumonia affects persons whose immune status is compromised.
- Most bacteria are normal inhabitants of the oropharynx and nasopharynx and reach alveoli by aspiration of secretions. Q
- Other routes include inhalation from the environment, hematogenous dissemination.
- A number of conditions predispose to infection by depressing the host defenses, including cigarette smoking, chronic bronchitis, alcoholism, severe malnutrition, wasting diseases and poorly controlled diabetes. Q
- Bacterial pneumonias should be classified on the basis of the etiologic agent, because clinical and morphologic features, and thus therapies, often vary with the causative organism. Q
Pneumonia caused by Streptococcus pneumoniae remains a significant problem.
- Pneumococcal pneumonia is mostly a consequence of altered defense barriers in the respiratory tract.
- The aspiration of pneumococci is also promoted by factors that impair the epiglottic reflex, including exposure to cold, anesthesia, and alcohol intoxication.
- Lung injury caused by factors such as congestive heart failure and irritant gases also renders the lung more susceptible to pneumococcal pneumonia. Q
- The capsule of the pneumococcus provides a defense against phagocytosis by the alveolar macrophages. Q
- In the earliest stage of pneumococcal pneumonia, protein-rich edema fluid containing numerous organisms fills the alveoli.
- Marked capillary congestion leads to massive outpouring of polymorphonuclear leukocytes.
- The firm consistency of the affected lung is reminiscent of the liver, this stage has been aptly named red hepatizationQ
- The next phase, involves lysis of polymorphonuclear leukocytes and appearance of macrophages.
- At this stage, the congestion has diminished, but the lung is still firm (grey hepatization)
A number of complications may follow pneumococcal pneumonia:
- Pleural effusion.
- Empyema. Q
- Endocarditis or meningitis.
- Pulmonary fibrosis is a rare complication
- Lung abscess is an unusual complication.
Commonly associated with alcoholism, diabetes and chronic pulmonary disease are also at risk.
K. pneumoniae has a thick, gelatinous capsule, which is responsible for the characteristic mucoid appearance of the cut surface of the lung. Q
- Community-acquired staphylococcal pneumonia is uncommon. Q
- Pulmonary infection with Staphylococcus aureus is a common superinfection after influenza and other viral respiratory tract infections. Q
- Staphylococcal pneumonia is characterized by abscess development. Q
- Multiple small abscesses.
- Cavitation and pleural effusions are common complications.
- Staphylococcal pneumonia requires aggressive therapy. Q
- The onset of insidious, leukocytosis is absent or slight and the course is prolonged.
- The infection characteristically causes a bronchiolitis with a neutrophilic intraluminal exudate and an intense lymphoplasmacytic infiltrate in the bronchiolar wall. Q
- Mycoplasma difficult to isolate by traditional culture methods.
- The diagnosis is often established on detecting M. pneumoniae antibodies or cold agglutinins. Q
- Erythromycin is effective.
The disease is divided into primary and secondary (or reactivation) tuberculosis.
The disease is acquired from the initial exposure to M. tuberculosis, most commonly as a result of inhaling infected aerosols generated when a person with cavitary tuberculosis coughs.
- The Ghon complex is the first lesion of primary tuberculosis and consists of a peripheral parenchymal granuloma, often in the upper lobes.
- When it is associated with an enlarged mediastinal lymph node a Ranke complex is formed.
- Microscopically, a granuloma with central caseous necrosis shows varying degrees of fibrosis.
- Apical scarring with the appearance of a fibronodular patch or ill-defined reticular shadow in the upper lung fields on chest x-ray is known as "Simon's focus".
- PURL'S LESION - Lesion at the apex of lung in chronic cases.
Assmannâ€™s Focus - Typically apical (site of highest oxygen tension)
Point to be remember:
TB SIMON'S FOCUS - Early hematogenous seedling in apex of lungs
GHON'S COMPLEX - parenchymal subpleural lesion + draining lymphatics + enlarged caseous lymph nodes in primary tuberculosis
RANKE COMPLEX - healed lesions in lung parenchyma and hilar lymph nodes undergoing calcification due to TB
RICH FOCUS - tuberculous caseous foci in brain, meninges and spinal cord
This stage represents either reactivation of primary pulmonary tuberculosis or a new infection in a host previously sensitized by primary tuberculosis. Q
- A cellular immune response occurs after a latent interval and leads to formation of many granulomas and extensive tissue necrosis. Q
- The apical and posterior segments of the upper lobes are most commonly involved.
- A diffuse, fibrotic, poorly defined lesion develops. Some erode into a bronchus creates a tuberculous cavity. Q
- Tuberculous cavities range in size from under 1 cm in diameter to large, cystic areas.
- The tuberculous cavity often communicates freely with a bronchus, and spreads the infection within the lung. Q
- Miliary tuberculosis refers to the presence of multiple, small (size of millet seeds), tuberculous granulomas in many organs.
- Bronchopleural fistula occurs when a subpleural cavity ruptures into the pleural space. In turn, tuberculous empyema and pneumothorax result. Q
- Tuberculous laryngitis.
- Intestinal tuberculosis.
- In patients who have AIDS, the ability to mount a granulomatous reaction may be impaired, and MAI pneumonia is characterized by an extensive infiltrate of macrophages and innumerable acid-fast organisms.
- Actinomycosis is caused by infection with actinomycetes, and the usual pulmonary organism is Actinomyces israelii. They are anaerobic filamentous bacteria. Q
- Lung lesions consist of multiple, small lung abscesses.
- The central necrotic area is purulent and contains colonies of organisms, which form sulfur granules. Q
- The colonies consist of thin, branching, filamentous gram-positive bacteria.
- Nocardia is a gram-positive filamentous bacteria that causes an acute progressive or chronic bacterial pneumonia. Q
- It is frequently encountered in immunocompromised persons, particularly patients with lymphomas, neutropenia, chronic granulomatous disease of childhood and pulmonary alveolar proteinosis. Q
- It is caused by inhalation of Histoplasma capsulatum in infected dust, commonly from bird droppings.
- Clinical and pathologic similarities to tuberculosis.
- Most infections are asymptomatic and result in lesions comparable to the Ghon complex. Q
- The organisms are generally not visible on routine stains and are best seen with a silver stain. Q
- Immunocompromised persons are at particular risk for dissemination of Histoplasma within the lungs and spread to other organs.
- Lesions are limited to a peripheral parenchymal granuloma, with or without lymph node granulomas. Q
- Immunocompromised persons may experience rapid progression of the disease. Q
- Results from inhalation of spores of Cryptococcus neoformans. Q
- Lung lesions range from small parenchymal granulomas to several large granulomatous nodules. Q
- Occur in immunocompromised persons. Q
- Infection of the lungs by Aspergillus species, usually Aspergillus niger or Aspergillus fumigatus. Q
- Invasive aspergillosis: occurring almost exclusively as an opportunistic infection in persons.
- The lungs exhibit patchy, multifocal areas of consolidation. Invasive aspergillosis is a fulminant pulmonary infection that is not amenable to therapy. Q
- Aspergilloma (or mycetoma): Aspergillus species may grow in preexisting cavities.
- They proliferate to form a fungus ball within these cavities.
- The most important symptom being hemoptysis, owing either to the underlying condition, or less commonly, to fungal infection of the cavity wall.