Tuberculosis

Key Facts

Terminology

Lung infection caused by inhalation of Mycobacterium tuberculosis

Clinical Issues

Primary infection usually asymptomatic and leads to formation of the Ghon complex (apical nodule with calcification) and hilar lymphadenopathy
Progressive primary TB occurs when the primary foci do not involute and progress to disseminated, cavitary lesions
Most common mechanism of secondary TB is reactivation of a latent focus of infection
Often associated with chronic, debilitating diseases or immunosuppressive states (AIDS, transplant, cancer, diabetes, alcoholism)

Microscopic Pathology

Basic pathologic manifestation of TB is granulomatous inflammation, with necrotizing, palisading epithelioid granulomas
Palisading granulomas can progress to complete fibrosis and calcification
Necrotizing granulomas may enlarge to form cavities with central liquefaction that may range from 3-10 cm in diameter
Mycobacteria are slender rods measuring 4 µm in length; they cannot be visualized on routine histology and require special stains

Ancillary Tests

Best stain for M. tuberculosis is the Ziehl-Neelsen stain, which stains acid-fast bacilli (AFB)

Gross appearance of the lung in active pulmonary tuberculosis shows subpleural areas of consolidation with cavitary lesions containing caseous material in the lumen

Typical histologic appearance of pulmonary tuberculosis characterized by necrotizing caseating granuloma shows peripheral palisading of histiocytes surrounding a necrotic center.

TERMINOLOGY

Abbreviations

Tuberculosis (TB)

Definitions

Lung infection caused by inhalation of Mycobacterium tuberculosis

ETIOLOGY/PATHOGENESIS

Infectious Agents

Tuberculosis transmitted through inhalation of airborne droplets contaminated with the bacillus

CLINICAL ISSUES

Epidemiology

Incidence
- About 1/3 of the world’s population is infected with tuberculosis; more prevalent in underdeveloped countries
- About 10% of HIV-infected patients will develop active disease annually

Presentation

Primary TB is defined as an infection occurring in a previously uninfected host
- Primary infection is usually asymptomatic and leads to formation of the Ghon complex (apical nodule with calcification) and hilar lymphadenopathy
- Progressive primary TB occurs when the primary foci do not involute and progress to disseminated, cavitary lesions
- Fulminant infection with dissemination via the bloodstream can lead to miliary tuberculosis
Secondary or chronic TB is defined as active disease developing in a previously sensitized host
- Most common mechanism of secondary TB is reactivation of a latent focus of infection
- May also be the result of massive reinfection from external source
- Symptoms include cough, weight loss, fatigue, and fever with cavitary lesions in the upper lobes
- Often associated with chronic, debilitating diseases or immunosuppressive states (AIDS, transplant, cancer, diabetes, alcoholism)