Pneumonia is a condition caused by microbial infection within the lung parenchyma. This infection, together with the associated host inflammatory response, impairs normal alveolar function (i.e. gasexchange), which, together with the systemic effects of the infection, causes the clinical features of pneumonia. The gold standard for recognition of pneumonia is the presence of new lung shadowing on the chest radiograph in the setting of a compatible clinical illness.

Community-acquired pneumonia (CAP) is that which occurs in the absence of immune compromise or prior hospital admission within the previous 30 days.

Nosocomial pneumonia can occur in anyone resident in hospital for >48 h. It is especially common in the intensive care unit >48 h after endotracheal intubation (ventilator-associated pneumonia (VAP) with risk being proportional to the duration of intubation. The Clinical Pulmonary Infection Score (CPIS) may be useful in nosocomial pneumonia.

Aspiration pneumonia occurs especially in those with swallowing impairment and neurological impairment. Anaerobic bacteria may be important in aspiration pneumonia.

Two types of immune dysfunction predispose to pneumonia: humoral immune dysfunction, such as immunoglobulin deficiencies; and cell-mediated immune function in, for example, cancer chemotherapy, solid organ transplantation and bone marrow transplantation.

Humoral immune deficiency is associated with bacterial infection and cell-mediated immune defects with viral and fungal infections such as Pneumocystis jirovecii.

Investigations are unnecessary outside hospital but, in those admitted, are performed to aid precise diagnosis, assess illness severity and identify the microbial cause. A chest radiograph is essential to confirm new lung shadowing in those admitted. Classically, such shadowing conforms to a lobar pattern and is associated with air bronchograms. More commonly, shadowing may occupy less than a whole lobe and may also be patchy, multilobar and bilateral. Additional features may include pleural effusion and, less commonly, cavitation and pneumothorax. The lower lobes are most commonly affected.

In routine blood tests, peripheral blood white cell count may be raised, especially in bacterial infection, but C-reactive protein and procalcitonin are probably more specific. Blood urea and creatinine are helpful in severity assessment and the assessment of renal impairment, and liver function tests may be abnormal. Measures of gas exchange, such as oxygen saturation and/or arterial blood gases, also aid assessment of illness severity and guide management.

In routine practice, tests to identify a microbial cause are positive in only about 15% of cases of CAP and hence seldom influence management. They are probably not indicated unless the patient is severely ill. In such cases blood culture, sputum Gram stain and culture, and urine tests for pneumococcal and Legionella antigens are indicated. Blood antibody levels or nose/throat secretion PCR-based tests for microbe-specific nucleic acids can be used for the detection of viruses and less common bacteria such as Legionella, Mycoplasma and Coxiella.In nosocomial pneumonia, and especially in VAP, lower respiratory secretions should be sampled either by tracheal aspirate or from bronchoscopic specimens. The latter may also be of value in the immunocompromised.

Differential diagnosisThe differential diagnosis includes acute bronchitis, COPD exacerbation, left ventricular failure, pulmonary embolism, TB, exacerbation of pulmonary fibrosis and rare lung disorders (e.g. pulmonary eosinophilia).

Microbial aetiology and resistanceThe same 10 pathogens commonly cause CAP worldwide, with Streptococcus pneumoniae being the most common overall and the most important cause of severe illness and death. Mycoplasma pneumoniae is also a common cause of mild illness, especially in young adults. Severe illness is most likely to be associated with S. pneumoniae, Legionella, staphylococcal or Gram-negative bacterial infection. Legionella infection may occur in outbreaks associated with a water aerosol source, such as showers or decorative fountains.Staphylococcal infection is especially common following influenza virus infection and in intravenous drug abusers. Influenza occurs in seasonal outbreaks during the winter months and occasional pandemics. It is the most common viral cause of CAP.Bacterial antibiotic resistance varies in frequency between countries. Clinically significant resistance to penicillins in S. pneumoniae is rare but clinically significant macrolide resistance is more common,especially in Southern Europe. Nosocomial pneumonia is most commonly caused by Gram-negative enterobacteria or Staphylococcus aureus. Pseudomonas aeruginosa and multiresistant bacteria (e.g. methicillin-resistant S. aureus (MRSA) are important causes of VAP.

Severity assessmentSeverity assessment is the key to deciding the place of care and should also guide diagnostic tests and antimicrobial therapy. This should be done through clinical judgement guided by objective severity scores. There are many of these, but the best validated for CAP are CURB65 (and its derivative CRB65) and the pneumonia severity index (PSI). The latter is based on a score from 20 variables and is often not practical in routine practice. The former is simpler and based on the number of severity variables present.

ManagementCorrection of gas exchange and fluid balance abnormalities, and the provision of appropriate antimicrobial therapy are the cornerstones of management. Outside hospital, rest, oral fluids and an oral antibiotic may be all that is required. In hospital, oxygen at a concentration to maintain SaO2 (9295%) should be delivered. If this cannot be achieved, CPAP may be helpful. If there is an unacceptable rise in PaCO2, then assisted ventilation should be considered. A place for NIV in pneumonia management has yet to be proven.Initial antibiotic therapy must be empirical and directed by illness severity according to national or international guidelines. Empirical antibiotics for CAP should always include pneumococcal coverage. Treatment for nosocomial pneumonia should be guided by knowledge of local microbial causes and that for pneumonia in the immunocompromised by the type of immune suppression and likely pathogens. Duration of therapy is usually 7 days in uncomplicated cases but may need to be prolonged in severe illness. Failure to respond should prompt a re-evaluation of the correct diagnosis and a more detailed search for microbial cause, for example by bronchoscopy, as long as gas exchange function will allow.

From: ERS Handbook of Respiratory Medicine, Second Edition- Paolo Palange

Below, from: Richard G. Wunderink, M.D., and Grant W. Waterer, M.B., B.S., Ph.D. Community-Acquired Pneumonia. N Engl J Med 2014;370:543-51.

Richard G. Wunderink, M.D., and Grant W. Waterer, M.B., B.S., Ph.D. Community-Acquired Pneumonia. N Engl J Med 2014;370:543-51.

Richard G. Wunderink, M.D., and Grant W. Waterer, M.B., B.S., Ph.D. Community-Acquired Pneumonia. N Engl J Med 2014;370:543-51.

Richard G. Wunderink, M.D., and Grant W. Waterer, M.B., B.S., Ph.D. Community-Acquired Pneumonia. N Engl J Med 2014;370:543-51.

From: J. Gonzlez-Castillo, et al.Guidelines for the management of community-acquired pneumonia in the elderly patient. Rev Esp Quimioter 2014;27(1): 69-86.

From: J. Gonzlez-Castillo, et al.Guidelines for the management of community-acquired pneumonia in the elderly patient. Rev Esp Quimioter 2014;27(1): 69-86.