Pneumonia is an abnormal inflammatory condition of the lung. It is often characterized as including inflammation of the parenchyma of the lung (that is, the alveoli) and abnormal alveolar filling with fluid (consolidation and exudation).
The alveoli are microscopic air-filled sacs in the lungs responsible for absorbing oxygen. Pneumonia can result from a variety of causes, including infection with bacteria, viruses, fungi, or parasites, and chemical or physical injury to the lungs. Its cause may also be officially described as idiopathic —that is, unknown—when infectious causes have been excluded.
Typical symptoms associated with pneumonia include cough, chest pain, fever, and difficulty in breathing. Diagnostic tools include x-rays and examination of the sputum. Treatment depends on the cause of pneumonia; bacterial pneumonia is treated with antibiotics.
Pneumonia is a common illness which occurs in all age groups, and is a leading cause of death among the elderly and people who are chronically and terminally ill. Additionally, it is the leading cause of death in children under five years old worldwide. Vaccines to prevent certain types of pneumonia are available. The prognosis depends on the type of pneumonia, the appropriate treatment, any complications, and the person's underlying health.
Classification
Pneumonias can be classified in several ways. Pathologists originally classified them according to the anatomic changes that were found in the lungs during autopsies. As more became known about the microorganisms causing pneumonia, a microbiologic classification arose, and with the advent of x-rays, a radiological classification. Another important system of classification is the combined clinical classification, which combines factors such as age, risk factors for certain microorganisms, the presence of underlying lung disease and underlying systemic disease, and whether the person has recently been hospitalized.
Early classification schemes
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.
- A lobar pneumonia is an infection that only involves a single lobe, or section, of a lung. Lobar pneumonia is often due to Streptococcus pneumoniae (though Klebsiella pneumoniae is also possible.)
- Multilobar pneumonia involves more than one lobe, and it often causes a more severe illness.
- Bronchial pneumonia affects the lungs in patches around the tubes (bronchi or bronchioles).
- Interstitial pneumonia involves the areas in between the alveoli, and it may be called "interstitial pneumonitis." It is more likely to be caused by viruses or by atypical bacteria.
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.
Combined clinical classification
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.
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 ).
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 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.
Community-acquired pneumonia
Main article: Community-acquired pneumoniaCommunity-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 of the fact that the sufferer can continue to "walk" rather than require hospitalization). Walking pneumonia is usually caused by the atypical bacterium, Mycoplasma pneumoniae .
Hospital-acquired pneumonia
Main article: Hospital-acquired pneumoniaHospital-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.
Other types of pneumonia
- Severe acute respiratory syndrome (SARS)
- Bronchiolitis obliterans organizing pneumonia (BOOP)
- Eosinophilic pneumonia
- Chemical pneumonia
- Aspiration pneumonia
- Dust pneumonia
- Necrotizing pneumonia , although overlapping with many other classifications, includes pneumonias that cause substantial necrosis of lung cells, and sometimes even lung abscess. Implicated bacteria are extremely commonly anaerobic bacteria, with or without additional facultatively anaerobic ones like Staphylococcus aureus, Klebsiella pneumoniae and Streptococcus pyogenes. Type 3 pneumococcus is uncommonly implicated.
- Opportunistic pneumonia includes those that frequently strike immunocompromised victims. Main pathogens are cytomegalovirus, Pneumocystis jiroveci, Mycobacterium avium-intracellulare, invasive aspergillosis, invasive candidiasis, as well as the "usual bacteria" that strike immunocompetent people as well.
Signs and symptoms
People with infectious pneumonia often have a cough producing greenish or yellow sputum, or phlegm and a high fever that may be accompanied by shaking chills. Shortness of breath is also common, as is pleuritic chest pain, a sharp or stabbing pain, either experienced during deep breaths or coughs or worsened by them. People with pneumonia may cough up blood, experience headaches, or develop sweaty and clammy skin. Other possible symptoms are loss of appetite, fatigue, blueness of the skin, nausea, vomiting, mood swings, and joint pai
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