- Atypical pneumonia
Atypical pneumonia Classification and external resources ICD-9 486 DiseasesDB 1132 MedlinePlus 000079
Atypical pneumonia aka "walking pneumonia" is a pneumonia not caused by one of the more traditional pathogens, and with a clinical presentation inconsistent with typical pneumonia. It can be caused by a variety of microorganisms. When developed independently from another disease it is called Primary Atypical Pneumonia (PAP).
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 (such as fever, headache and myalgia) 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. Moreover, S. pneumoniae has become a relatively lesser important cause.
"Primary atypical pneumonia" is called primary to indicate that it developed independently, not following another disease.
"Atypical pneumonia" is atypical in that it is caused by atypical organisms (other than Streptococcus pneumoniae, Haemophilus influenzae and Moraxella catarrhalis).  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, nil consolidation, only small increases in white cell counts and no alveolar exudate.   At the time that atypical pneumonia was described first, organisms like Mycoplasma, Chlamydophila and Legionella still were not recognized as bacteria and instead considered as viruses. Hence "atypical pneumonia" was also called "non-bacterial".  In literature the term bacterial pneumonia contrasted with atypical pneumonia is, although actually incorrect, still in use. Meanwhile, many of such organisms are identified as bacteria, albeit unusual types (Mycoplasma is a type of bacteria without a cell wall and Chlamydias are intracellular parasites). 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 responding on common antibiotics 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 (i.e. non-productive).
- Lack of alveolar exudate.
- Despite general symptoms and problems with the upper respiratory tract like high fever, headache, a dry irritating cough, later a productive cough and radiographs, showing consolidation, there are in general few physical signs. The patient looks better than the symptoms suggest.
- Chlamydia pneumoniae
- Mild form of pneumonia with relatively mild symptoms.
- Chlamydia psittaci
- Causes psittacosis.
- 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 (e.g. rashes) 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.
When comparing the bacterial-caused atypical pneumonias with these caused by real viruses (excluding bacteria that were wrongly considered as viruses), the term "atypical pneumonia" almost always implies a bacterial etiology and is contrasted with viral pneumonia.
Chest radiographs (X-ray photographs) 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 (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. 
- ^ a b C. Walter, M.D. McCoy Primary atypical pneumonia*A report of 420 cases with one fatality during twenty-seven month at Station Hospital, Camp Rucker, Alabama; Southern Medical Journal, 1946 39(9): 696
- ^ "Pneumonia, Atypical Bacterial: Overview - eMedicine". http://emedicine.medscape.com/article/363083-overview. Retrieved 2008-12-21.
- ^ eMedicine article 360090: Pneumonia, Typical Bacterial
- ^ Memish ZA, Ahmed QA, Arabi YM, Shibl AM, Niederman MS (October 2007). "Microbiology of community-acquired pneumonia in the Gulf Corporation Council states". Journal of Chemotherapy 19 Suppl 1: 17–23. PMID 18073166. http://www.jchemother.it/cgi-bin/digisuite.exe/searchresult?range=pubmed&volume=19%20Suppl%201&year=2007&firstpage=17.
- ^ a b Diseases Database Causes of atypical pneumonia
- ^ a b Cunha BA (May 2006). "The atypical pneumonias: clinical diagnosis and importance". Clin. Microbiol. Infect. 12 Suppl 3: 12–24. doi:10.1111/j.1469-0691.2006.01393.x. PMID 16669925. http://www3.interscience.wiley.com/resolve/openurl?genre=article&sid=nlm:pubmed&issn=1198-743X&date=2006&volume=12&issue=&spage=12.
- ^ "Primary atypical pneumonia" at Dorland's Medical Dictionary
- ^ a b c d Commission on Acute Respiratory Diseases, Fort Bragg, North Carolina Primary Atypical Pneumonia American J. of Public Health, April, 1944; Vol. 34
- ^ Gouriet F, Drancourt M, Raoult D (October 2006). "Multiplexed serology in atypical bacterial pneumonia". Ann. N. Y. Acad. Sci. 1078: 530–40. doi:10.1196/annals.1374.104. PMID 17114771. http://www3.interscience.wiley.com/resolve/openurl?genre=article&sid=nlm:pubmed&issn=0077-8923&date=2006&volume=1078&spage=530.
- ^ Hindiyeh M, Carroll KC (June 2000). "Laboratory diagnosis of atypical pneumonia". Semin Respir Infect 15 (2): 101–13. doi:10.1053/srin.2000.9592. PMID 10983928. http://linkinghub.elsevier.com/retrieve/pii/S0882054600000372.
- ^ p714, Robbins and Cotran Pathologic Basis of Disease 8th edition, Kumar et al, Philadelphia 2010
- ^ Cunha BA (May 2006). "The atypical pneumonias: clinical diagnosis and importance". Clinical Microbiology and Infection 12 Suppl 3: 12–24. doi:10.1111/j.1469-0691.2006.01393.x. PMID 16669925.
- ^ MeSH MYCOPLASMA+PNEUMONIAE
- ^ Diseases Database
- ^ Tang YW (December 2003). "Molecular diagnostics of atypical pneumonia". Acta Pharmacol. Sin. 24 (12): 1308–13. PMID 14653964. http://www.chinaphar.com/1671-4083/24/1308.pdf.
- ^ "Severe Acute Respiratory Syndrome (SARS) - multi-country outbreak". http://www.who.int/csr/don/2003_03_15/en/. Retrieved 2008-12-21.
- ^ C.G. Murphy et al. Clinical predictors of occult pneumonia in the febrile child. Acad. Emerg. Med. 14(3), 243–249 (2007).
- ^ Rutman MS, Bachur R, Harper MB (January 2009). "Radiographic pneumonia in young, highly febrile children with leukocytosis before and after universal conjugate pneumococcal vaccination". Pediatric Emergency Care 25 (1): 1–7. doi:10.1097/PEC.0b013e318191dab2. PMID 19116501.
- ^ Schneeberger PM, Dorigo-Zetsma JW, van der Zee A, van Bon M, van Opstal JL (2004). "Diagnosis of atypical pathogens in patients hospitalized with community-acquired respiratory infection". Scandinavian Journal of Infectious Diseases 36 (4): 269–73. doi:10.1080/00365540410020127. PMID 15198183.
- ^ a b National Heart, Lung, and Blood Institute, U.S.A. What Causes Pneumonia?
Pathology of respiratory system (J, 460–519), respiratory diseases Upper RT
Common cold)Headvocal folds: Laryngopharyngeal reflux (LPR) · Vocal fold nodule · Vocal cord paresis · Vocal cord dysfunction
Lower RT/lung disease
(including LRTIs)acute: Acute bronchitischronic: COPD (Chronic bronchitis, Acute exacerbations of chronic bronchitis, Acute exacerbation of COPD, Emphysema) · Asthma (Status asthmaticus, Aspirin-induced, Exercise-induced) · BronchiectasisInterstitial/
restrictiveBy pathogenBy vector/routeBy distributionBroncho- · LobarOther
Other/general Pneumonia Infectious pneumonias Pneumonias caused by
infectious or noninfectious agents
Noninfectious pneumoniaChemical pneumonia
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