| Pneumonia | |
|---|---|
 | |
| A chest X-ray showing a very prominent wedge-shapebacterial pneumonia in the right lung | |
| Specialty | Pulmonology  |
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.
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 includeStreptococcus pneumoniae, viruses, the atypical bacteria, andHaemophilus 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 callednosocomial 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, includingmechanical ventilation, prolongedmalnutrition, underlyingheart andlung 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 asMRSA,Pseudomonas,Enterobacter, andSerratia. 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 ofintubation andmechanical ventilation.
Pneumonia has historically been characterized as either typical oratypical 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, andThe American Thoracic Society does not recommend its use.[4]
Bronchiolitis obliterans organizing pneumonia (BOOP) is caused by inflammation of the small airways of the lungs. It is also known as cryptogenic organizing pneumonitis (COP).
Eosinophilic pneumonia is invasion of the lung byeosinophils, a particular kind ofwhite blood cell. Eosinophilic pneumonia often occurs in response to infection with aparasite or after exposure to certain types of environmental factors.
Chemical pneumonia (usually calledchemical pneumonitis) is caused by chemicaltoxicants such aspesticides, which may enter the body byinhalation or by skin contact. When the toxic substance is an oil, the pneumonia may be calledlipoid pneumonia.
Aspiration pneumonia (or aspiration pneumonitis) is caused byaspirating foreign objects which are usually oral or gastric contents, either while eating, or after reflux or vomiting which results inbronchopneumonia. The resulting lung inflammation is not an infection but can contribute to one, since the material aspirated may containanaerobic bacteria or other unusual causes of pneumonia. Aspiration is a leading cause of death among hospital andnursing home patients, since they often cannot adequately protect their airways and may have otherwise impaired defenses.
Dust pneumonia describes disorders caused by excessive exposure todust storms, particularly during theDust Bowl in the United States. With dust pneumonia, dust settles all the way into the alveoli of the lungs, stopping the cilia from moving and preventing the lungs from ever clearing themselves.
Necrotizing pneumonia (NP), also known ascavitary pneumonia orcavitatory necrosis, is a rare but severe complication oflungparenchymalinfection.[5][6][7] In necrotizing pneumonia, there is a substantialliquefaction following death of the lung tissue, which may lead togangrene formation in the lung.[8][9] In most cases patients with NP havefever,cough andbad breath, and those with more indolent infections have weight loss.[10] Often patients clinically present with acute respiratory failure.[10] The most common pathogens responsible for NP areStreptococcus pneumoniae,Staphylococcus aureus,Klebsiella pneumoniae.[11]
People withweakened immune defense, such asHIV/AIDS patients, are highly susceptible toopportunistic infections affecting the lungs.[12] Most common pathogens arePneumocystis jiroveci,Mycobacterium avium-intracellulare complex,Streptococcus pneumoniae,Haemophilus species. Less frequent pathogens areCryptococcus neoformans,Histoplasma capsulatum,Coccidioides immitis,cytomegalovirus (CMV), andToxoplasma gondii.[13]
Chemotherapy-induced immunodeficiency may lead to severe lung infections.[14] Pathogens commonly associated with lung infectioins are bacteria (likePseudomonas aeruginosa,Stenotrophomonas maltophilia, andNocardia species), viruses (e.g.,respiratory syncytial virus,parainfluenza virus,influenza virus A andinfluenza B, and cytomegalovirus), and fungi (e.g.Aspergillus,Fusarium, andMucorales species, andPneumocystis jirovecii).[14]
Double pneumonia is a historical term foracute lung injury (ALI) oracute respiratory distress syndrome (ARDS).[15] However, the term was, and still is used to denote pneumonia affecting both lungs. Accordingly, the termdouble pneumonia is more likely to be used to describe bilateral pneumonia than it is ALI or ARDS.
Severe acute respiratory syndrome (SARS) is a highly contagious and deadly type of pneumonia which first occurred in November 2002 after initial outbreaks in China caused bySARS-CoV, which had almost disappeared by the month of May 2004 (it was later called SARS-CoV-1 to distinguish it from other similar viruses). The second outbreak,SARS-CoV-2, started in December 2019 from Wuhan, China and was declared pandemic by the WHO on 11 March 2020. SARS is caused by theSARS coronavirus, a previously unknownpathogen.
Initial descriptions of pneumonia focused on theanatomic orpathologic appearance of the lung, either by direct inspection atautopsy or by its appearance under amicroscope.
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 aradiological 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 asStreptococcus pneumoniae), the atypical pneumonias (such as the interstitial pneumonitis ofMycoplasma pneumoniae orChlamydia pneumoniae), and the aspiration pneumonia syndromes.[citation needed]
Chronic pneumonias, on the other hand, mainly include those ofNocardia,Actinomyces andBlastomyces dermatitidis, as well as the granulomatous pneumonias (Mycobacterium tuberculosis andatypical mycobacteria,Histoplasma capsulatum andCoccidioides 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 ofhealthcare-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]