Atypical pneumonia, also known aswalking pneumonia,^[1] is any type ofpneumonia not caused by one of the pathogens most commonly associated with the disease. Its clinical presentation contrasts to that of "typical" pneumonia. A variety ofmicroorganisms can cause it. When it develops independently from another disease, it is called primary atypical pneumonia (PAP).

The term was introduced in the 1930s^[2][3] and was contrasted with thebacterial pneumonia caused byStreptococcus pneumoniae, at that time the best known and most commonly occurring form of pneumonia. The distinction was historically considered important, as it differentiated those more likely to present with "typical" respiratory symptoms andlobar pneumonia from those more likely to present with "atypical" generalized symptoms (such asfever,headache,sweating andmyalgia) andbronchopneumonia.^[4]

Usually, the atypical causes also involve atypical symptoms:

• No response to common antibiotics such assulfonamide^[5] andbeta-lactams likepenicillin.
• No signs and symptoms of lobar consolidation,^[6][7] meaning that the infection is restricted to small areas, rather than involving a wholelobe. As the disease progresses, however, the look can tend tolobar pneumonia.
• Absence ofleukocytosis.^{[citation needed]}
• Extrapulmonary symptoms, related to the causing organism.^[8]
• Moderate amount ofsputum, or no sputum at all (i.e. non-productive).
• Lack of alveolar exudate.^[9]
• Despite general symptoms and problems with theupper respiratory tract (such as high fever, headache, a dry irritating cough followed later by a productive cough with radiographs showingconsolidation), there are in general few physical signs. The patient looks better than the symptoms suggest.^[2][5]
• Silent hypoxia may occur and not be noticed, but will be picked up bypulse oximetry.

The most common causative organisms are (oftenintracellular living) bacteria:^[8]

Chlamydia pneumoniae

Mild form of pneumonia with relatively mild symptoms.

Chlamydia psittaci

Causespsittacosis.

Coxiella burnetii

CausesQ fever.

Francisella tularensis

Causestularemia.

Legionella pneumophila

Causes a severe form of pneumonia with a relatively high mortality rate, known aslegionellosis or Legionnaires' disease.

Mycoplasma pneumoniae

Usually occurs in younger age groups and may be associated with neurological and systemic (e.g. rashes) symptoms. SeeMycoplasma pneumonia.

Atypical pneumonia can also have a fungal, protozoan, or viral cause.^[10][11]
In the past, most organisms were difficult to culture. However, newer techniques aid in the definitive identification of the pathogen, which may lead to more individualized treatment plans.^{[citation needed]}

Known viral causes of atypical pneumonia includerespiratory syncytial virus (RSV),influenza A and B,parainfluenza,adenovirus,severe acute respiratory syndrome (SARS),^[12]Middle East respiratory syndrome (MERS),andmeasles.^[13]

Chest radiographs (X-ray photographs) often show a pulmonary infection before physical signs of atypical pneumonia are observable at all.^[5]This isoccult pneumonia. In general, occult pneumonia is rather often present in patients with pneumonia and can also be caused byStreptococcus pneumoniae, as the decrease of occult pneumonia after vaccination of children with a pneumococcal vaccine suggests.^[14][15]

Infiltration commonly begins in the perihilar region (where thebronchus begins) and spreads in a wedge- or fan-shaped fashion toward the periphery of the lung field. The process most often involves the lower lobe but may affect any lobe or combination of lobes.^[5]

Mycoplasma is found more often in younger than in older people.^[16][17]Older people are more often infected by Legionella.^[17]

"Primary atypical pneumonia" is calledprimary because it develops independently of other diseases.^[18]

It is commonly known as "walking pneumonia" because its symptoms are often mild enough that one can still be up and about.^[19][20]

"Atypical pneumonia" isatypical in that it is caused by atypical organisms (other thanStreptococcus pneumoniae,Haemophilus influenzae, andMoraxella catarrhalis).^[21]These atypical organisms include specialbacteria,viruses,fungi, andprotozoa. In addition, this form of pneumonia is atypical in presentation with only moderate amounts of sputum, no consolidation, only small increases in white cell counts, and no alveolarexudate.^[13][8]

At the time that atypical pneumonia was first described, organisms likeMycoplasma,Chlamydophila, andLegionella were not yet recognized as bacteria and instead considered viruses. Hence "atypical pneumonia" was also called "non-bacterial".^[22]

In literature the termatypical pneumonia is current, sometimes contrasted withviral pneumonia (seeabove) and sometimes, though incorrectly, withbacterial pneumonia. Many of the organisms causative of atypical pneumonia are unusual types of bacteria (Mycoplasma is a type of bacteria without acell wall andChlamydias are intracellular bacteria). As the conditions caused by the various agents have different courses and respond to different treatments, the identification of the specific causative pathogen is important.^{[citation needed]}

• Pneumonia
• Atypical pneumonias

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