Tuberculosis (TB) is a condition arising from Mycobacterium tuberculosis (MTB) infection. MTB is transmitted from a person with pulmonary TB (PTB) infection. Droplet nuclei containing the tubercle bacilli are aerosolized by speaking, sneezing, or coughing. The droplets dry quickly, remain in the air for several hours, and may be inhaled by other individuals. Other MTB transmission routes are insignificant.
The disease most frequently affects the lungs, though up to a third of TB cases involve other organs. The bacterium is an obligate aerobe and thus typically lodges in the highly oxygenated lung regions, the upper lobe and the lower lobe's superior aspect close to the pleura.
The functional unit of the lungs is the alveolus. The thin alveolar walls are comprised of the capillary endothelium, basement membrane, and alveolar epithelium—components of the blood-air barrier that facilitate gas exchange. Alveolar macrophages are cells derived from monocytes, roaming free inside the alveolar lumen. These phagocytic cells are critical to the lung's immunity but also contribute to TB pathophysiology. Pores of Kohn are alveolar wall perforations where microbes and exudates can spread.
Lung vasculature is comprised of the following:
Pulmonary arteries: arise from the pulmonary trunk and branch into lobar and segmental arteries in the lung parenchyma; these arteries carry deoxygenated blood into the lungs
Pulmonary veins: carry oxygenated blood from the lungs and back into the heart and arterial circulation
Bronchial arteries: arise from the thoracic aorta and posterior intercostal arteries; these vessels supply the root of the lungs and visceral pleura
Bronchial veins: follow the bronchial arteries and drain into the azygos and hemiazygos veins
Pulmonary lymphatic plexuses: the superficial subpleural lymphatic plexus drains into the bronchopulmonary (hilar) lymph nodes; the deep bronchopulmonary lymphatic plexus drains into the intrinsic pulmonary lymph nodes before emptying into the bronchopulmonary lymph nodes; the tracheobronchial lymph nodes drain the bronchopulmonary lymph nodes and ultimately empty into the right lymphatic and thoracic ducts
Parietal pleura lymphatics: parietal pleural lymphatic vessels drain into the thoracic wall and axillary lymph nodes
In immunocompromised patients, failure of the immune system to contain the infection enables MTB to spread from the lungs to the other body organs through the vasculature. Disseminated TB is defined as the simultaneous involvement of at least 2 non-contiguous body organs or infection of the blood, bone marrow, or liver. Miliary TB is a potentially fatal, disseminated form of the disease arising from hematogenous tubercle bacilli spread throughout the lungs and other organs.
The condition results in the formation of millet-seed-sized (1 to 2 mm) tubercular foci. The term "miliary tuberculosis" was first coined by John Jacobus Manget in 1700 while describing a pathological specimen having tiny tubercles resembling millet seeds in appearance. The term originated from the Latin word "miliarius," related to the millet seed. Miliary mottling on a chest radiograph is the classical hallmark that supports the diagnosis of miliary TB. Miliary TB is classified as both pulmonary and extrapulmonary TB.
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