Atypical pneumonia

Atypical pneumonia, also known as
walking pneumonia, is the type of pneumonia not caused by one of the more
traditional pathogens. Its clinical presentation contrasts to that of
“typical” pneumonia. A variety of microorganisms can cause it. When it
develops independently from another disease it is called primary atypical
pneumonia. The term was introduced in the 1930s and
was contrasted with the bacterial pneumonia caused by Streptococcus
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 and lobar pneumonia from those more likely to present with “atypical”
generalized symptoms and bronchopneumonia.
Distinction between atypical and typical pneumonia, however, is medically
insufficient. For the treatment of pneumonia it is important to know the
exact causal organism. Terminology
“Primary atypical pneumonia” is called primary because it develops
independently of other diseases. “Atypical pneumonia” is atypical in that
it is caused by atypical organisms. 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. At the time that atypical pneumonia was first described, organisms
like Mycoplasma, Chlamydophila, and Legionella still were not recognized as
bacteria and instead considered viruses. Hence “atypical pneumonia” was also
called “non-bacterial”. In literature the term atypical pneumonia is still in
use, though incorrect. Meanwhile, many such organisms are identified as
bacteria, albeit unusual types. As the conditions caused by these agents have
different courses and respond to different treatments, the identification
of the specific causative pathogen is important.
Signs and symptoms Usually the atypical causes also involve
atypical symptoms: No response to common antibiotics such
as sulfonamide and beta-lactams like penicillin.
No signs and symptoms of lobar consolidation, 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.
Absence of leukocytosis. Extrapulmonary symptoms, related to the
causing organism. Moderate amount of sputum, or no sputum
at all. Lack of alveolar exudate.
Despite general symptoms and problems with the upper respiratory tract, there
are in general few physical signs. The patient looks better than the symptoms
suggest. Cause
The most common causative organisms are bacteria:
Chlamydophila 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
Usually occurs in younger age groups and may be associated with neurological and
systemic symptoms. Atypical pneumonia can also have a
fungal, protozoan or viral cause. 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=
When comparing the bacterial-caused atypical pneumonias with these caused by
real viruses, the term “atypical pneumonia” almost always implies a
bacterial etiology and is contrasted with viral pneumonia.
Known viral causes of atypical pneumonia include respiratory syncytial virus,
influenza A and B, parainfluenza, adenovirus, severe acute respiratory
syndrome and measles. Diagnosis
Chest radiographs often show a pulmonary infection before physical signs of
atypical pneumonia are observable at all. This is called 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.
Infiltration commonly begins in the perihilar region 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. Epidemiology
Mycoplasma is found more often in younger than in older people. Older
people are more often infected by Legionella.

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