Atlas of pathophysiology, 2 Edition
Part II - Disorders
An acute or chronic infection caused by Mycobacterium tuberculosis, tuberculosis (TB) is characterized by pulmonary infiltrates, formation of granulomas with caseation, fibrosis, and cavitation. People who live in crowded, poorly ventilated conditions and those who are immunocompromised are most likely to become infected. In patients with strains that are sensitive to the usual antitubercular agents, the prognosis is excellent with correct treatment. However, in those with strains that are resistant to two or more of the major antitubercular agents, mortality is 50%. The incidence of TB has been increasing in the United States secondary to homelessness, drug abuse, and human immunodeficiency virus (HIV) infection. Globally, TB is the leading infectious cause of morbidity and mortality, generating 8 to 10 million new cases each year.
TB develops after inhaling droplets sprayed into the air from a cough or sneeze by a person infected with M. tuberculosis. Although the primary infection site is the lungs, mycobacteria commonly exist in other parts of the body. A number of factors increase the risk of infection, including:
· uncontrolled diabetes mellitus
· Hodgkin's disease
· HIV infection
· treatment with corticosteroids or immunosuppressants.
After exposure to M. tuberculosis, roughly 5% of infected people develop active tuberculosis within 1 year; in the remainder, microorganisms cause a latent infection. Transmission of active disease is by droplet nuclei produced when infected persons cough or sneeze. The host's immune system usually controls the tubercle bacillus by killing it or walling it up in a tiny nodule (tubercle). However, the bacillus may lie dormant within the tubercle for years and later reactivate and spread. Persons with a cavitary lesion (large, granulomatous lesion) are particularly infectious because their sputum usually contains 1 million to 100 million bacilli per milliliter. If an inhaled tubercle bacillus settles in an alveolus, infection occurs, with alveolocapillary dilation and endothelial cell swelling. Alveolitis results, with replication of tubercle bacilli and influx of polymorphonuclear leukocytes. These organisms spread through the lymph system to the circulatory system and then throughout the body.
Cell-mediated immunity to the mycobacteria, which develops 3 to 6 weeks later, usually contains the infection and arrests the disease. If the infection reactivates, the body's response characteristically leads to caseation—the conversion of necrotic tissue to a cheeselike material. The caseum may localize, undergo fibrosis, or excavate and form cavities, the walls of which are studded with multiplying tubercle bacilli. If this happens, infected caseous debris may spread throughout the lungs by the tracheobronchial tree. Sites of extrapulmonary TB include the pleurae, meninges, joints, lymph nodes, peritoneum, genitourinary tract, and bowel.
Signs and symptoms
After an incubation period of 4 to 8 weeks, TB usually produces no symptoms in primary infection but may produce nonspecific symptoms, such as:
· weight loss
· night sweats
· low-grade fever
Physical examination may reveal crackles, decreased breath sounds, and clubbing of the fingers and toes.
In reactivation, symptoms may include a cough that produces mucopurulent sputum, occasional hemoptysis, and chest pain.
Fever and night sweats, the typical hallmarks of TB, may not be present in elderly patients, who instead may exhibit a change in activity or weight.
Diagnostic test results
· Chest X-ray shows nodular lesions, patchy infiltrates (mainly in upper lobes), cavity formation, scar tissue, and calcium deposits.
· Tuberculin skin test reveals infection at some point but doesn't indicate active disease.
· Stains and cultures of sputum, cerebrospinal fluid (CSF), urine, drainage from abscesses, or pleural fluid show heat-sensitive, nonmotile, aerobic, acid-fast bacilli.
· Computed tomography scan or magnetic resonance imaging allows the evaluation of lung damage and may confirm diagnosis.
· Bronchoscopy shows inflammation and altered lung tissue. It may also be performed to obtain sputum if the patient can't produce an adequate sputum specimen.
Antitubercular therapy is the main treatment. Daily doses of multiple drugs may include combinations of rifampin, isoniazid, pyrazinamide, and ethambutol. After 2 to 3 weeks of continuous medication, the disease generally is no longer infectious, and the patient can resume his normal lifestyle while continuing the medication.
APPEARANCE OF TUBERCULOSIS ON LUNG TISSUE