Pneumonia is the inflammation of the parenchyma of the lung.
The most common cause of pneumonia is infection and the majority of these are bacterial in nature. Viruses, fungi and parasites may also cause pneumonia.
Bacteria may reach the lungs via one of three routes:
These infections may result in both pulmonary and extrapulmonary symptoms. Diagnosis often involves demonstrating acute consolidation on chest radiograph but may require testing for a specific organism.
Pneumonia may be divided into groups based on the location it was contracted, comorbidities and the immune status of the patient. This helps to identify the likely causative organism and guide management.
CAP is a pneumonia that is contracted in the community. It has as traditionally been divided into typical and atypical pneumonia.
These tend to present with features 'typical' of pneumonia; a productive cough, fever and pleuritic chest pain. Likely infecting organisms include:
These tend to have a more insidious, subacute onset. They often present with a combination of pulmonary and extrapulmonary symptoms. Likely infecting organisms may be divided into nonzoonotic and zoonotic causes:
HAP is defined by NICE as a pneumonia contracted > 48 hrs after hospital admission that was not incubating at the time of admission.
Patients in hospital adopt the local bacterial environment, one that differs from what is encountered in the community. This is known to persist for a number of weeks following discharge. Likely organisms include:
Aspiration pneumonias are caused by the inhalation of oropharyngeal or gastric contents. This brings bacteria found in these environments into the lungs.
Aspiration pneumonias have typically been associated with patients who are unable to adequately protect their airway, it may be seen in patients with:
It is increasingly recognised that many CAPs and HAPs may result from an otherwise inconsequential aspiration event. Studies have shown hospitalised patients on proton-pump inhibitors are more at risk of developing HAP than those who are not, implicating aspiration in the aetiology.
Infective organisms include Streptococcus pneumoniae, Staphylococcus aureus, Haemophilus influenzae, and Enterobacteriaceae, in the hospital Gram-negative bacilli (e.g. Pseudomonas aeruginosa) may be implicated.
Mendelson’s syndrome is a related condition. It is defined as a chemical pneumonitis caused by aspiration of acidic gastric contents classically seen by obstetric anaesthetists.
Patients with pathological and iatrogenic immunosuppression are at risk of infective organisms not usually seen in other patient groups.
We consider a wider range of likely causative organisms in immunocompromised patients:
Numerous organisms may be responsible for a pneumonia, below a few common organisms are discussed.
Streptococcus pneumoniae is a gram-positive alpha-haemolytic streptococci. It is the most common cause of CAP (classically lobar), presenting with a cough, pleuritic pain and pyrexia. It frequently causes a significant leucocytosis and a raised CRP.
It classically gives rust coloured sputum and may be accompanied by the reactivation of cold sores. Urinary antigen tests may be used to diagnose the infection and is unaffected by antibiotics.
Mycoplasma pneumonia is a CAP caused by a rod-shaped bacterium that lacks a cell wall. It tends to affect a younger demographic and occurs in cyclical epidemics.
It causes a pneumonia with a prolonged, insidious onset that may exhibit extrapulmonary features. Extrapulmonary features include:
Diagnosis can be made with PCR of respiratory samples.
L. pneumophila is a gram-negative coccobacillus that may cause the atypical CAP (typically lobar) Legionnaire’s disease.
It is encountered in those exposed to contaminated cooling systems, humidifiers and showers. Chest symptoms may be preceded by several days of myalgia, headache and fever.
Hyponatraemia, secondary to SIADH, is a classical finding in Legionnaire’s disease but is not always present.
Other biochemical abnormalities include hypophosphataemia and raised serum ferritin.
Diagnosis may be made with urinary antigen testing (for serogroup 1). Cultures of respiratory secretions and PCR may also be used.
P. aeruginosa is a gram-negative bacillus that causes HAP. In patients with bronchiectasis (e.g. cystic fibrosis) is may cause CAP. It is often described as an opportunistic pathogen, rarely causing disease in healthy individuals.
Typically it causes pneumonia in immunosuppressed patients and those with chronic lung disease. It should be remembered it may affect many of the bodies systems independent of the lungs and can lead to a bacteraemia.
It tends to cause the symptoms classically associated with pneumonia. Sputum is characteristically green. Delineating between active infection and incidental airway colonisation can be difficult. Bronchoalveolar lavage may be used to obtain samples.
Treatment often involves a cephalosporin and aminoglycoside. Aerosolised antibiotics may be used in patients with cystic fibrosis.
K. pneumophila is a gram-negative bacillus that causes CAP classically seen in alcoholics. It causes a fast moving lobar pneumonia. Symptoms include a cough, fever and flu-like features. Sputum may have the characteristic ‘red-currant jelly’ appearance.
K. pneumophila shows resistance to beta-lactams. Beta-lactamase stable beta-lactams, cephalosporins and aminoglycosides may be used to treat the infection.
Pneumocystis jirovecii (formerly PCP/ pneumocystis carinii), a fungi, is an AIDS-defining illness that may cause a life-threatening pneumonia. It causes fever, cough (frequently non-productive) and exertional dyspnoea. Hypoxia and a raised LDH are also common findings.
It may be diagnosed with sputum stains though bronchoalveolar lavage may be required to obtain samples.
It is now classified as a fungi though was previously described as a protozoa. It does not respond to antifungals and is instead treated with co-trimoxazole (trimethoprim-sulfamethoxazole).
Pneumonia is often characterised by cough, SOB, and the signs of consolidation. In atypical pneumonias extrapulmonary features may predominate.
The course of a pneumonia may be interrupted by any of a number of complications.
Investigations should not delay antibiotic treatment which should be administered within 4 hours of admission or diagnosis.
CRB-65 and CURB-65 are scoring systems that help stratify patients risk of mortality and determine the best place of care.
Each category is worth one point when present. Confusion may be defined as an AMT < 8 (abbreviated mental test) or new-onset disorientation in time, place or person.
The CRB-65 is an aid to clinical judgement that helps to assess the need for hospital admission. It may be calculated in primary care to stratify the severity of the infection.
0 - Low risk (mortality <1%), 1-2 - intermediate risk (mortality 1-10%), 3-4 - high risk (mortality >10%).
CURB-65 is calculated when a patient presents to hospital, it includes a urea measurement and helps establish the best place of care.
0-1 - Low risk (mortality <3%), 2 - intermediate risk (mortality 3-15%), 3-5 - high risk (mortality >15%).
Management is dependent on severity, type of infecting organism, & local hospital guidelines. Below we discuss options for empirical antibiotic treatment, this may vary depending on local guidelines.
Patients may present with pneumonia of varying degrees of severity. In those who are acutely unwell physicians should adopt an ABC approach. A number of general measures should be considered in the treatment of a patient hospitalised with pneumonia.
Treatment should follow local antibiotic guidelines. The advice below is based upon BTS Guideline for the management of CAP in adults (2009) and NICE CG 191: Pneumonia in adults: diagnosis and management (2014).
Research indicates that 11% of smokers over the age of 50 who have pneumonia have lung cancer. This may be incidental or causative, tumours may block parts of the bronchial tree increasing the risk of infective events.
A CXR at 6-8 weeks post-event should be used to screen for underlying lung cancers in this age group.
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