The secondary pulmonary tuberculous lesion is located in the apex of one or both lungs. It begins as a small focus of consolidation, usually less than 3 cm in diameter. Less commonly, initial lesions may be located in other regions of the lung, particularly about the hilus.
In almost every case of reinfection, the regional nodes develop foci of similar tuberculous activity. In the favorable case, the initial parenchymal focus develops a small area of caseation necrosis that does not cavitate because it fails to communicate with a bronchus or bronchiole. The usual course is one of progressive fibrous encapsulation, leaving only fibrocalcific scars that depress and pucker the pleural surface and cause focal pleural adhesions. Sometimes, these fibrocalcific scars become secondarily blackened by anthracotic pigment.
Histologically, coalescent granulomas are present, composed of epithelioid cells surrounded by a zone of fibroblasts and lymphocytes that usually contains Langhan’s giant cells. Some necrosis (caseation) is usually present in the centers of these tubercles, the amount being entirely dependent on the sensitization of the patient and the virulence of the organisms.
As the lesions progress, more tubercles coalesce to create a confluent area of consolidation. In the favorable case, either the entire area is eventually converted to a fibrocalcific scar, or the residual caseous debris becomes totally and heavily walled off by hyaline collagenous connective tissue. In these late lesions, the multinucleate giant cells tend to disappear.
Tuberculosis Disease: Tuberculous Tissue Changes
In cases of suspected tuberculous tissue changes, the diagnosis is confirmed by histologic staining, smears, and cultures of acid-fast organisms. Tubercle bacilli can be demonstrated in the early exudative and caseous phases, but it is usually impossible to find them in the late fibro-calcific stages.
Lesions with sparse organisms can be highly infective: one can estimate that finding a single acid-fast bacillus in a routine histologic sample from a one cubic centimeter granuloma indicates that a total of at least 2,000 organisms are present within the granuloma. Hence, it cannot be assumed that their absence in histologic sections is tantamount to their total destruction because in many of these instances culture of the lesions or inoculation of this material into guinea pigs yields the organisms.