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Medical condition From Wikipedia, the free encyclopedia
Pneumonia can be classified in several ways, most commonly by where it was acquired (hospital versus community), but may also by the area of lung affected or by the causative organism.[1] There is also a combined clinical classification, which combines factors such as age, risk factors for certain microorganisms, the presence of underlying lung disease or systemic disease and whether the person has recently been hospitalized.
Pneumonia | |
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A chest X-ray showing a very prominent wedge-shape bacterial pneumonia in the right lung | |
Specialty | Pulmonology |
Community-acquired pneumonia (CAP) is infectious pneumonia in a person who has not recently been hospitalized. CAP is the most common type of pneumonia. The most common causes of CAP vary depending on a person's age, but they include Streptococcus pneumoniae, viruses, the atypical bacteria, and Haemophilus influenzae. Overall, Streptococcus pneumoniae is the most common cause of community-acquired pneumonia worldwide. Gram-negative bacteria cause CAP in certain at-risk populations. CAP is the fourth most common cause of death in the United Kingdom and the sixth in the United States. The term "walking pneumonia" has been used to describe a type of community-acquired pneumonia of less severity (because the sufferer can continue to "walk" rather than requiring hospitalization).[2] Walking pneumonia is usually caused by the atypical bacterium, Mycoplasma pneumoniae.[3]
Hospital-acquired pneumonia, also called nosocomial pneumonia, is pneumonia acquired during or after hospitalization for another illness or procedure with onset at least 72 hrs after admission. The causes, microbiology, treatment and prognosis are different from those of community-acquired pneumonia. Up to 5% of patients admitted to a hospital for other causes subsequently develop pneumonia. Hospitalized patients may have many risk factors for pneumonia, including mechanical ventilation, prolonged malnutrition, underlying heart and lung diseases, decreased amounts of stomach acid, and immune disturbances. Additionally, the microorganisms a person is exposed to in a hospital are often different from those at home. Hospital-acquired microorganisms may include resistant bacteria such as MRSA, Pseudomonas, Enterobacter, and Serratia. Because individuals with hospital-acquired pneumonia usually have underlying illnesses and are exposed to more dangerous bacteria, it tends to be more deadly than community-acquired pneumonia. Ventilator-associated pneumonia (VAP) is a subset of hospital-acquired pneumonia. VAP is pneumonia which occurs after at least 48 hours of intubation and mechanical ventilation.
Pneumonia has historically been characterized as either typical or atypical depending on the presenting symptoms and thus the presumed underlying organism.[4] Attempting to make this distinction based on symptoms, however, has not been found to be accurate, and The American Thoracic Society does not recommend its use.[4]
Initial descriptions of pneumonia focused on the anatomic or pathologic appearance of the lung, either by direct inspection at autopsy or by its appearance under a microscope.
The discovery of x-rays made it possible to determine the anatomic type of pneumonia without direct examination of the lungs at autopsy and led to the development of a radiological classification. Early investigators distinguished between typical lobar pneumonia and atypical (e.g. Chlamydophila) or viral pneumonia using the location, distribution, and appearance of the opacities they saw on chest x-rays. Certain x-ray findings can be used to help predict the course of illness, although it is not possible to clearly determine the microbiologic cause of a pneumonia with x-rays alone.
With the advent of modern microbiology, classification based upon the causative microorganism became possible. Determining which microorganism is causing an individual's pneumonia is an important step in deciding treatment type and length. Sputum cultures, blood cultures, tests on respiratory secretions, and specific blood tests are used to determine the microbiologic classification. Because such laboratory testing typically takes several days, microbiologic classification is usually not possible at the time of initial diagnosis.
Traditionally, clinicians have classified pneumonia by clinical characteristics, dividing them into "acute" (less than three weeks duration) and "chronic" pneumonias. This is useful because chronic pneumonias tend to be either non-infectious, or mycobacterial, fungal, or mixed bacterial infections caused by airway obstruction. Acute pneumonias are further divided into the classic bacterial bronchopneumonias (such as Streptococcus pneumoniae), the atypical pneumonias (such as the interstitial pneumonitis of Mycoplasma pneumoniae or Chlamydia pneumoniae), and the aspiration pneumonia syndromes.[citation needed]
Chronic pneumonias, on the other hand, mainly include those of Nocardia, Actinomyces and Blastomyces dermatitidis, as well as the granulomatous pneumonias (Mycobacterium tuberculosis and atypical mycobacteria, Histoplasma capsulatum and Coccidioides immitis).[17]
The combined clinical classification, now the most commonly used classification scheme, attempts to identify a person's risk factors when he or she first comes to medical attention. The advantage of this classification scheme over previous systems is that it can help guide the selection of appropriate initial treatments even before the microbiologic cause of the pneumonia is known. There are two broad categories of pneumonia in this scheme: community-acquired pneumonia and hospital-acquired pneumonia. A recently[when?] introduced type of healthcare-associated pneumonia (in patients living outside the hospital who have recently been in close contact with the health care system) lies between these two categories.[citation needed]
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