Tuberculosis



Tuberculosis





Anatomic Alterations of the Lungs


Tuberculosis (TB) is a contagious chronic bacterial infection that primarily affects the lungs, although it may involve almost any part of the body. Clinically, TB is classified as either primary tuberculosis, postprimary tuberculosis, or disseminated tuberculosis.



Primary Tuberculosis


Primary TB (also called the primary infection stage) follows the patient’s first exposure to the TB pathogen, Mycobacterium tuberculosis. Primary TB begins when the inhaled bacilli implant in the alveoli. As the bacilli multiply over a 3- to 4-week period, the initial response of the lungs is an inflammatory reaction that is similar to any acute pneumonia (see Figure 15-1). In other words, a large influx of polymorphonuclear leukocytes and macrophages moves into the infected area to engulf—but not fully kill—the bacilli. This action also causes the pulmonary capillaries to dilate, the interstitium to fill with fluid, and the alveolar epithelium to swell from the edema fluid. Eventually the alveoli become consolidated (i.e., filled with fluid, polymorphonuclear leukocytes, and macrophages). Clinically, this phase of TB coincides with a positive tuberculin reaction—a positive purified protein derivative (PPD) skin test result (see discussion of diagnosis later in this chapter).


Unlike pneumonia, however, the lung tissue that surrounds the infected area slowly produces a protective cell wall called a tubercle, or granuloma. In essence, the tubercles work to encapsulate—that is, put in a nutshell-like structure—the TB bacilli (see Figure 17-1, A). Although the initial lung lesions may be difficult to identify on a chest radiograph, the lesions may be seen as small, sharply defined opacities. When detected on a chest radiograph, these initial lung lesions are called Ghon nodules. As the disease progresses, the combination of tubercles and the involvement of the lymph nodes in the hilar region is known as the Ghon complex.



Structurally, a tubercle consists of a central core containing TB bacilli. The central core also has enlarged macrophages with an outer wall composed of fibroblasts, lymphocytes, and neutrophils. A tubercle takes about 2 to 10 weeks to form. The function of the tubercle is to contain the TB bacilli, thus preventing the further spread of infectious TB organisms. Unfortunately, the central core of the tubercle has the potential to break down from time to time, especially in a patient with a depressed immune system. When this happens, the center of the tubercle fills with necrotic tissue that resembles dry cottage cheese. During this stage the tubercle is called a caseous lesion or caseous granuloma (see Figure 17-1, B). The patient is potentially contagious at this stage. In most cases, however, the TB bacilli are effectively contained within the tubercles.


Once the bacilli are controlled—either by the patient’s immunologic defense system or by antituberculous drugs—the healing process begins. Tissue fibrosis and calcification of the lung parenchyma slowly replace the tubercle. This tissue fibrosis and calcification cause lung tissue retraction and scarring. In some cases the calcification and fibrosis cause the bronchi to distort and dilate—that is, to develop bronchiectasis.


Finally, when the bacilli are isolated within tubercles and immunity develops, the TB bacilli may remain dormant for months, years, or life. Individuals with dormant TB (also called latent TB) do not feel sick or have any TB-related symptoms. They are still infected with TB but do not have clinically active TB. The only indication of a TB infection is a positive reaction to the tuberculin skin test, or TB blood test, and the finding of possible residual scarring on the chest radiograph. Individuals with dormant (latent) TB are not infectious and cannot spread the TB bacilli to others.



Postprimary Tuberculosis


Postprimary TB (also called reactivation TB, reinfection TB, or secondary TB) is a term used to describe the reactivation of TB months or even years after the initial infection has been controlled. Even though most patients with primary TB recover completely from a clinical standpoint, it is important to note that live tubercle bacilli can remain dormant for decades. A positive tuberculin reaction generally persists even after the primary infection stage has been controlled. At any time, TB may become reactivated, especially in patients with depressed immune systems. Most new TB cases are associated with the following risk factors:



If the TB infection is uncontrolled, cavitation of the caseous granuloma tubercle develops. The patient progressively experiences more severe symptoms, including violent cough episodes, greenish or bloody sputum, low-grade fever, anorexia, weight loss, extreme fatigue, night sweats, and chest pain. It is this gradual wasting of the body that provided the basis for the earlier name for TB—consumption. The patient is highly contagious at this stage. In severe cases a deep tubercle cavity may rupture and allow air and infected material to flow into the pleural space or the tracheobronchial tree. Pleural complications are common in TB (see Figure 17-1, C).



Disseminated Tuberculosis


Disseminated TB (also called extrapulmonary TB, miliary TB, and tuberculosis—disseminated) refers to infection from TB bacilli that escape from a tubercle and travel to sites other throughout the body by means of the bloodstream or lymphatic system. In general, the TB bacilli that gain entrance to the bloodstream usually gather and multiply in portions of the body that have a high tissue oxygen tension. The most common location is the apex of the lungs. Other oxygen-rich areas in the body include the regional lymph nodes, kidneys, long bones, genital tract, brain, and meninges.


Genital TB in males damages the prostate gland, epididymis, seminal vesicle, and testes; and in females, the fallopian tubes, ovaries, and uterus. The spine is a frequent site of TB infection, although the hip, knee, wrist, and elbow can also be involved. Tubercular meningitis is caused by an active brain lesion seeding TB bacilli into the meninges. Over time, the infection may cause mental deterioration, permanent retardation, blindness, and deafness. When a large number of bacilli are freed into the bloodstream, the result can be the presence of numerous small tubercles—about the size of a pinhead—scattered throughout the body. This condition is commonly called miliary TB.


TB is primarily a chronic restrictive pulmonary disorder. The major pathologic or structural changes of the lungs associated with TB (mainly postprimary TB) are as follows:




Etiology and Epidemiology


TB is one of the oldest diseases known to man and remains one of the most widespread diseases in the world. Unmistakable evidence has been provided from mummies from the Stone Age, ancient Egypt, and Peru that TB is an ancient human disease. In early writings, the disease was called “consumption,” “Captain of the Men of Death,” and “white plague.” In the nineteenth century, the disease was named tuberculosis, a term that derives mainly from the tubercle formations found during postmortem examinations of victims of the disease.


According to the Centers for Disease Control and Prevention (CDC), there were 13,299 new cases of TB reported in the United States in 2007—the lowest since the reporting of TB began in 1953. The number of TB cases reported annually in the United States dropped 74% between 1953 and 1985 (84,304 to 22,201). Starting in 1986, however, the incidence of TB trended upward each year in the United States, with a peak of 26,673 reported cases in 1992. The resurgence of TB during this period is well correlated with (1) increased immigration from endemic areas, (2) the sudden rise of the HIV infection epidemic, and (3) the increased use of immunosuppressive drugs. From 1994 to 2007 the yearly incidence of TB again trended downward to its lowest level of 13,299. The decline of TB in the United States is believed to be the result of a number of factors, including new TB medications, a better understanding of the disease, and better public health education. The mortality rate from TB in the United States is currently 0.6 deaths per 100,000, which represents approximately 1700 deaths per year. In 1953 the mortality rate was 12.5 deaths per 100,000 per year.


Globally, TB is still very prevalent. According the World Health Organization (WHO) 2008 report, a third of the world’s population is infected by TB. In 2006, WHO estimated that 9.2 million new cases of TB occurred, and about 709,000 (7.7%) were among HIV-positive individuals. WHO reported that the following counties account for nearly 75% of all TB cases: India, China, Pakistan, the Philippines, Thailand, Indonesia, Bangladesh, and the Democratic Republic of Congo. In the European region, it is estimated that 49 new TB cases and seven TB-related deaths occur every hour. According to WHO, the global incidence of TB per capita peaked around 2003 and appears to have stabilized or begun to decline. However, the fall in the TB incidence per capita will likely be more than offset by the expected global population growth. In other words, the overall absolute number of new TB cases each year can be expected to increase (e.g., new TB cases increased from 9.1 to 9.2 million from 2005 to 2006.


In humans, TB is primarily caused by M. tuberculosis. The mycobacteria are long, slender, straight or curved rods. The M. tuberculosis organism enters humans via the following three routes: the respiratory tract, the gastrointestinal tract, and an open wound in the skin. Most TB infections are contracted via the airborne route (e.g., inhalation of aerosol droplets containing organisms of the tubercle bacillus from an infected individual).


The TB bacilli are highly aerobic organisms and thrive best in areas of the body with high oxygen tension—especially in the apex of the lung. When stained, the hard outer layer of the tubercle bacilli resists decolorization by acid or alcohol; therefore the bacilli are called acid-fast bacilli. In addition, the hard outer coat of the tubercle bacillus also protects the organism against killing and digestion by phagocytes and renders the bacilli more resistant to antituberculous drugs.


The TB bacilli are almost exclusively transmitted within aerosol droplets produced by the coughing, sneezing, or laughing of an individual with active TB. This accounts for the use of strict isolation procedures

Stay updated, free articles. Join our Telegram channel

Jun 11, 2016 | Posted by in RESPIRATORY | Comments Off on Tuberculosis

Full access? Get Clinical Tree

Get Clinical Tree app for offline access