Classification of pneumonia

Pneumonia
A chest X-ray showing a very prominent wedge-shape bacterial 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.

By location acquired

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).^[2] Walking pneumonia is usually caused by the atypical bacterium, Mycoplasma pneumoniae.^[3]

Hospital-acquired

Hospital-acquired pneumonia, also called

Chemical pneumonia (usually called chemical pneumonitis) is caused by chemical toxicants such as pesticides, which may enter the body by inhalation

or by skin contact. When the toxic substance is an oil, the pneumonia may be called lipoid pneumonia.

Aspiration pneumonia (or aspiration pneumonitis) is caused by aspirating foreign objects which are usually oral or gastric contents, either while eating, or after reflux or vomiting which results in bronchopneumonia. The resulting lung inflammation is not an infection but can contribute to one, since the material aspirated may contain anaerobic bacteria or other unusual causes of pneumonia. Aspiration is a leading cause of death among hospital and nursing home patients, since they often cannot adequately protect their airways and may have otherwise impaired defenses.

Dust pneumonia

dust storms, particularly during the Dust 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 as cavitary pneumonia or cavitatory necrosis, is a rare but severe complication of

Streptococcus pneumonia, Staphylococcus aureus, Klebsiella pneumoniae.^[11]

People with

Mycobacterium avium-intracellulare complex, Streptococcus pneumoniae, Haemophilus species. Less frequent pathogens are Cryptococcus neoformans, Histoplasma capsulatum, Coccidioides immitis, cytomegalovirus (CMV), and Toxoplasma gondii.^[13]

Chemotherapy-induced immunodeficiency may lead to severe lung infections.

influenza B, and cytomegalovirus), and fungi (eg, Aspergillus, Fusarium, and Mucorales species, and Pneumocystis jirovecii).^[14]

This is a historical term for

acute lung injury (ALI) or acute respiratory distress syndrome (ARDS).^[15]

However, the term was and, especially by lay people, 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.

SARS coronavirus, a previously unknown pathogen

.

• 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.)^[16]
• 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.

• 
  Normal AP CXR
• 
  Normal lateral CXR
• 
  AP CXR showing left lower lobe pneumonia associated with a small left sided pleural effusion
• 
  AP CXR showing right lower lobe pneumonia
• 
  AP CXR showing pneumonia of the lingula of the left lung
• 
  Right upper lobe pneumonia as marked by the circle.
• 
  Left upper lobe pneumonia with a small pleural effusion.
• 
  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.^{[citation needed]}

Chronic pneumonias, on the other hand, mainly include those of