Classification of pneumonia

Community-acquired 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). Walking pneumonia is usually caused by the atypical bacterium, Mycoplasma pneumoniae. Hospital-acquired 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. ==By cause==

Pneumonia has historically been characterized as either typical or atypical depending on the presenting symptoms and thus the presumed underlying organism. 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. In necrotizing pneumonia, there is a substantial liquefaction following death of the lung tissue, which may lead to gangrene formation in the lung. In most cases patients with NP have fever, cough and bad breath, and those with more indolent infections have weight loss. Often patients clinically present with acute respiratory failure. Opportunistic pneumonia People with weakened immune defense, such as HIV/AIDS patients, are highly susceptible to opportunistic infections affecting the lungs. Most common pathogens are Pneumocystis jiroveci, Mycobacterium avium-intracellulare complex, Streptococcus pneumoniae, Haemophilus species. Less frequent pathogens are Cryptococcus neoformans, Histoplasma capsulatum, Coccidioides immitis, cytomegalovirus (CMV), and Toxoplasma gondii. Chemotherapy-induced immunodeficiency may lead to severe lung infections. Double pneumonia (bilateral pneumonia) Double pneumonia is a historical term for acute lung injury (ALI) or acute respiratory distress syndrome (ARDS). However, the term was, and still is used to denote pneumonia affecting both lungs. Accordingly, the term double pneumonia is more likely to be used to describe bilateral pneumonia than it is ALI or ARDS. Severe acute respiratory syndrome 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 by SARS-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 a pandemic by the WHO on 11 March 2020. SARS is caused by the SARS coronavirus, a previously unknown pathogen. ==By area of lung affected==

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. File:Normal AP.JPG|Normal AP CXR File:Normal lateral chest x-ray.jpg|Normal lateral CXR File:LLL_pneumonia_with_effusionM.jpg|AP CXR showing left lower lobe pneumonia associated with a small left sided pleural effusion File:RLL_pneumoniaM.jpg|AP CXR showing right lower lobe pneumonia File:Pneumnia1.jpg|AP CXR showing pneumonia of the lingula of the left lung File:PneumoniaRUL.jpg|Right upper lobe pneumonia as marked by the circle. File:PneumoLLL.PNG|Left upper lobe pneumonia with a small pleural effusion. File:RLLpneumonia.PNG|Right lower lobe pneumonia as seen on a lateral CXR ==Clinical==

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. ==References==