Pneumonia is defined as an acute lower respiratory tract infection of the gas exchanging regions of the lung (alveoli), with accompanying new radiographic changes on x-ray. It is commonly classified, based on the setting in which it occurs, into:
Streptococcus pneumoniae is the most common causative agent in all groups.
A diagnosis of pneumonia requires the presence of symptoms and signs consistent with a lower respiratory tract infection and new radiological shadowing on chest x-ray (CXR). Figure 1 summarises the typical clinical and radiological features of pneumonia.
Importantly, elderly patients may display a non-specific presentation of pneumonia with falls and confusion often being the presenting symptom. Elderly patients are more likely to have co-morbid disease and have a higher mortality associated with pneumonia compared to younger patients.
The majority of pneumonia cases are community acquired. The incidence of CAP varies with age, being highest in the very young and elderly. Approximately 5-11/1000 adults are diagnosed with CAP each year. Globally, CAP is responsible for more childhood deaths than any other illness. CAP accounts for <1% of community, 6-12% of hospital and > 30% of ITU deaths. Figure 2 details the most common causative agents of CAP.
CAP is most commonly transmitted by droplet spread, often infecting a previously healthy host. Pneumonia may also be acquired through aspiration of commensal bacteria from the nasopharyngeal flora (e.g. S. pneumoniae). Several factors have been implicated in increasing susceptibility to pneumonia, mainly involving weakening of host airway defences. These are summarised in Figure 3.
Risk Stratification of CAP:
A variety of tools exist to predict mortality in CAP and a recent meta-analysis (Chalmers et al) showed no significant difference between PSI (pneumonia severity index), CRB-65 & CURB-65 in predicting 30 day mortality. The most commonly used tool for assessing the severity of CAP is the CURB-65 score, details of which are displayed in Figure 4:
CURB-65 scoring is an important predictor of mortality, which may be broadly estimated at approximately:
Management of CAP is summarised in Figure 5.
Intravenous antibiotics should be switched to oral as soon as the patient shows good clinical improvement, has been apyrexial for 24 hours and there are no contraindications to the oral route. The antibiotic regime should be continuously revised in the light of clinical progress and microbiological results. For most patients with uncomplicated pneumonia, a 7-10 day course of antibiotics is adequate.
In examinations it is always wise to add in that your choice of antibiotics would be made with reference to local trust prescribing guidelines and the BNF.
You may find it helpful to remember that in moderate risk pneumonias, the aim is to:
"Cover Atypicals and Pneumococcus", hence "Clarithromycin and Amoxicillin are Prescribed".
Chlamydia pneumoniae infection is most commonly seen in the young and middle-aged. Infection is usually mild and self-limiting. Diagnosis is via serology and 50% of youing adults have evidence of previous infection on serology. Treatment is with erythromycin.
Chlamydia psittaci infection causes 'psittacosis'. This is a zoonotic infection acquired from birds (especially exotic ones). This can result in a protracted illness and hepatosplenomegaly may be present. Infection is disgnosed via serology and erythromycin used to treat.
Mycoplasma pneumoniae causes pneumonia in children/young adults, most commonly in the Autumn, and can occur in epidemics. Certain rare complications can ensue, including haemolytic anaemia, Stevens-Johnson Syndrome, erythema nodosum, pericarditis, meningoencephalitis and Gullaine-Barre syndrome.
Legionella pneumophilia is acquired from contaminated water and water systems (old showers, inadequately maintained air-conditioning) and foreign travel is a major risk factor. It may present with headache, confusion, malaise and diarrhoea as well as respiratory symptoms. Hyponatraemia, hypoalbuminaemia and elevated liver enzymes and creatine kinase may also be found on investigation. Treatment is with clarythromycin (rifampicin may also be added).
Staphylococcus aureus pneumonia is often preceded by influenza virus infection. Lung abscesses may be visualised on CXR and haematogenous dissemination can result in septicaemia, osteomyelitis, endocarditis and brain abscesses.
Pneumocystic jirovecii pneumonia (PJP, formerly P. carinii) is a common life-threatening and defining illness in AIDS. Presentation can be with a 2-3 week history of dry cough, fever or exertional dyspnoea disproportionately severe to the milder accompanying CXR changes which may include peri-hilar shadowing (bat's wings) but is often normal. There is no response to standard antibiotics and treatment is with co-trimoxazole.
The pneumococcal polysaccharide vaccine (PPV) is recommended for any one over 2 years who is considered to have increased susceptibility to pneumococcal infection or who is likely to have increased mortality or morbidity associated with infection. At risks groups who receive the vaccine include those with:
If the patient is at increased risk of a fatal pneumococcal infection, the vaccine should be repeated after 6 years.
Hospital acquired pneumonia is considered if the CXR changes and clinical features typical of pneumonia occur at least 48 hours following admission to hospital. It is the most common fatal healthcare acquired infection and occurs in 1% of inpatients (over-represented in the elderly). Mortality is around 30%. See Figure 6.
Patients with compromised immune defences are at increased risk of pneumonia. Immunocompromise can effect different aspects of the immune system, e.g. leukaemia or cytotoxic drugs can cause neutropenia, myeloma can cause antibody deficiency and AIDS impairs T-cell function. See Figure 7.
Davidson’s Principles and Practice of Medicine (20th ed., 2006, Boon NA, Colledge NR, Walker BR, Hunter JAA). p.695-703.
Oxford Handbook of Clinical Medicine (8th ed., 2010, Longmore M, Wilkinson IB, Davidson EH, Foulkes A, Mafi AR). p.154-195
Oxford Handbook of Infectious Diseases and Microbiology (1st ed., 2009, Torok E, Moran E, Cooke F). p.654-661
British Thoracic Society: Community acquired pneumonia guidelines (2009)
Scottish Intercollegiate Guidelines Network: Community acquired pneumonia management guidelines (2011)
Chalmers JD, Singanavagm A, Akram AR, Mandal P, Short PM, Choudhury G, Wood V & Hill AT. Severity assessment tools for predicting mortality in hospitalised patients with community-acquired pneumonia. Systematic review and meta-analysis. Thorax 2010; 65: 878-883.
W S Lim1, M M van der Eerden et al. Defining community acquired pneumonia severity on presentation to hospital: an international derivation and validation study. Thorax 2003;58:377-382 doi:10.1136/thorax.58.5.377
Aujesky D, Auble TE, Yealy DM, Stone RA, Obrosky DS, Meehan TP, Graff LG, Fine JM, Fine MJ. Prospective comparison of three validated prediction rules for prognosis in community-acquired pneumonia. Am J Med. 2005 Apr;118(4):384-92
Patient UK: Pneumonia http://www.patient.co.uk/doctor/Pneumonia.htm. Accessed 13/09/2011.
[Image reference 1: http://en.wikipedia.org/wiki/File:PneumonisWedge09.JPG]
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