Tuberculosis is a disease caused byMycobacterium tuberculosis hominis, an organism that is red and rod-shaped on staining, does not decolorize with acid on Ziehl-Neilson or Kinyoun stains and is therefore acid fast - hence the name acid-fast bacillus (AFB).
What is tuberculosis: Pathogenesis
Tuberculosis is most frequently transmitted by tubercle bacilli-containing sputum droplet nuclei (from sputum droplets that have dried in the air after a patient coughs, sneezes, speaks, or sings). These droplet nuclei float around in the air. If they are inhaled by a susceptible host, evade the ciliary mucus escalator of the bronchial tree, settle in the alveolar area, and are ingested by the alveolar macrophages, a TB infection can occur.
With infection, these AFB-laden macrophages develop a primary TB infection whose host focus is called a Ghon complex , a parenchymal subpleural lesion occurring in the lower part of the upper lobe and the upper part of the lower lobe (above and below the interlobar fissure) and enlarged lymph nodes draining the parenchymal focus.
The macrophages are organized into groups of cells called giant cells (Langhan’s giant cells – large cells with nuclei lined up along the edges of the cell) with lymphocytes and plasma cells surrounding them. The giant cells coalesce to form a granuloma, which has amorphous pink caseous (cheesy or necrotic) material containing TB organisms within it and epitheioid cells, lymphocytes, and fibroblasts around it. The result is a caseating (epithelioid) granuloma. This lesion takes about 2 to 12 weeks to develop and is associated with a conversion of the tuberculin (PPD) skin test from negative to positive.
These lesions can remain localized and heal (fibrosis and/or calcification) which is the usual situation with primary TB. The patients are often asymptomatic or have a minimum of non-specific signs and symptoms. A chest x-ray at first shows some infiltrates in the mid-lung area, which calcify.
Some bacilli disseminate to the apical area of the lung with its high oxygen content which is favorable to the growth of aerobic tubercle bacilli. These areas tend to heal by fibrosis and calcification. However small numbers of organisms can lay dormant in the lung apices for years. A small number of TB bacilli can spread by contiguous or lymphohematogenous dissemination to distant portions of the body and heal by fibrosis or calcification.
What is tuberculosis: Miliary TB
Uncommonly, this lymphohematogenous spread of TB bacilli may not heal and result in miliary TB, which can involve any organ of the body (bone marrow, liver, spleen, etc). Miliary TB is characterized by a chest x-ray picture that looks as if there are tiny millet seeds all over the lung as well as multiple other TB foci throughout the body.
What is tuberculosis: TB Bronchopneumonia
Also very uncommon is the development of TB bronchopneumonia (galloping consumption) which is due to the rapid spread of tubercle bacilli throughout the lung. In this situation, caseating granuloma may not form and the diagnosis must be made by the presence of numerous tubercle bacilli in these areas.
What is tuberculosis: TB in the United States
In the United States the most common presentation of TB is that of reactivation of an old, healed TB focus, usually in the upper lobes or apices of the lung where the ratio of ventilation to perfusion is 3 to 1 and favors oxygenation, a boon to aerobic (oxygen-loving) tubercle bacilli. This form of TB is reactivation (secondary) (post-primary) TB which represents reactivation of an old, often dormant TB focus.
In addition, this may less commonly occur in an old TB focus in other parts of the body (extrapulmonary TB). Reactivation may occur in 5 to 10 % of patients who have had a previously healed lesion, which now increases in size. The result is often an apical infiltrate or apical cavity on chest x-ray. Symptoms in this situation include cough, purulent sputum, weight loss, fatigue, night sweats, and a low-grade fever. Occasionally there is hemoptysis and pleuritic pain.