Atypical pneumonia

Atypical pneumonia
Other names: Walking pneumonia;[1] atypical pneumonia syndrome[2]
CXR of atypical pneumonia showing right central peribronchial infiltrate
Specialty	Infectious disease, pulmonology
Symptoms	Fever, cough, tiredness, headache[3]
Duration	Few weeks[2]
Causes	Bacteria, viruses, fungi[1][2]
Risk factors	Smoking, crowded places, inhaled steroids[1]
Diagnostic method	Based on symptoms and CXR[1]
Treatment	Antibiotics (azithromycin or doxycycline)[1]
Prognosis	Generally good[2]
Frequency	Common[1]

Atypical pneumonia, also known as walking pneumonia, is a type of community acquired pneumonia caused by pathogens that are less usual and difficult to isolate.^[3][2] In may start as an upper respiratory infection which progresses to a lower respiratory infection.^[2] Symptoms generally include sore throat, fever, cough, tiredness, and headache.^[3][1] Often it is less severe than typical pneumonia.^[1]

It occurs due to the bacteria Mycoplasma pneumoniae, Chlamydophila pneumoniae, or Legionella pneumophila.^[3] Some also include viruses, such as influenza or RSV or fungi as causes.^[1][2][4] The distinction was considered important as symptoms, X-rays appearance, and antibiotic sensitivity may differ.^[5] Making the separation based on symptoms alone; however, is inexact.^[2]

Treatment is generally with antibiotics, such as azithromycin or doxycycline.^[1] NSAIDs may be taken to help with fever.^[1] Hospitalization is not generally needed.^[1] Outcomes are generally good, with improvement over 1 to 2 weeks, with complete resolution taking up to 6 weeks.^[2][1]

Atypical pneumonia is common.^[1] Children and young adults are more commonly affected.^[3][2] It most commonly occurs in the fall and often occur as part of an outbreak.^[2] While the term was had been used since at least the early 1900s, it only came into common usage in the 1930s following a paper by Hobart Reimann.^[4] Originally it referred to pneumonia that was not the typical lobar pneumonia seen with pneumococcus.^[4] Some feel the term should be abandoned due to its non-specific nature.^[4]

Signs and symptoms

It may present with atypical symptoms:

• No lobar consolidation,^[6][7] meaning that the infection is restricted to small areas, rather than involving a whole lobe. As the disease progresses, however, the look can tend to lobar pneumonia.
• Extrapulmonary symptoms, related to the causing organism.^[8]
• Moderate amount of sputum, or no sputum at all (i.e. non-productive). Lack of alveolar exudate^[9]
• Despite general symptoms and problems with the upper respiratory tract (such as fever, headache, a dry irritating cough followed later by a productive cough with radiographs showing consolidation), there are in general few physical signs. People may look better than the symptoms suggest.^[10][11]

Cause

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

Chlamydia pneumoniae

Mild form of pneumonia with relatively mild symptoms.

Chlamydia psittaci

Causes psittacosis.

Coxiella burnetii

Causes Q fever.

Francisella tularensis

Causes tularemia.

Legionella pneumophila

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

Mycoplasma pneumoniae

^[12] Usually occurs in younger age groups and may be associated with neurological and systemic (e.g. rashes) symptoms.

Atypical pneumonia can also have a fungal, protozoan or viral cause.^[13][14]
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.

Viral

Viral causes of atypical pneumonia include respiratory syncytial virus (RSV), influenza A and B, parainfluenza, adenovirus, severe acute respiratory syndrome (SARS),^[15] Middle East respiratory syndrome (MERS), COVID-19^[16] and measles.^[17]

Diagnosis

Chest X-rays often show a lung infection before physical signs are observable.^[11] This is occult pneumonia. In general, occult pneumonia is rather often present in patients with pneumonia and can also be caused by Streptococcus pneumoniae, as the decrease of occult pneumonia after vaccination of children with a pneumococcal vaccine suggests.^[18][19]

Infiltration commonly begins in the perihilar region (where the bronchus 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.^[11]

Treatment

Treatment is usually with NSAIDs, antibiotics, and rest. There is typically no response to the antibiotics sulfonamide or penicillin.^[4]

Epidemiology

Mycoplasma is found more often in younger than in older.^[20][21] Older people are more often infected by Legionella.^[21]

Terminology

"Primary atypical pneumonia" is called primary because it develops independently of other diseases.

It is commonly known as "walking pneumonia" because its symptoms are often mild enough that one can still be up and about.^[22][23]

"Atypical pneumonia" is atypical in that it is caused by atypical organisms (other than Streptococcus pneumoniae, Haemophilus influenzae, and Moraxella catarrhalis).^[24] These atypical organisms include special bacteria, viruses, fungi, and protozoa. 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 alveolar exudate.^[17][8]

At the time that atypical pneumonia was first described, organisms like Mycoplasma, Chlamydophila, and Legionella were not yet recognized as bacteria and instead considered viruses. Hence "atypical pneumonia" was also called "non-bacterial".^[25]

In literature the term atypical pneumonia is current, sometimes contrasted with viral pneumonia (see below) and sometimes, though incorrectly, with bacterial pneumonia. Many of the organisms causative of atypical pneumonia are unusual types of bacteria (Mycoplasma is a type of bacteria without a cell wall and Chlamydias 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.

References

External links