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Pneumonia is defined as an infection involving the alveoli of the lungs. It occurs in patients of all age groups, but young children and the elderly, as well as immunocompromised and immune deficient patients, are especially at risk. Causal therapy is with antibiotics.

Signs and symptoms

Symptoms may include:

• Common
  • Cough with greenish or yellow mucus
  • Fever with shaking chills (rigors)
  • Sharp or stabbing chest pain, worsened by deep breaths or coughs
  • Rapid, shallow breathing
  • Shortness of breath
• Rarer
  • Bloody mucus
  • Headache
  • Excessive sweating and clammy skin
  • Loss of appetite
  • Excessive fatigue

Pneumonia can progress to sepsis ("blood poisoning") and acute respiratory distress syndrome if untreated. These are the main causes of death in patients with untreated pneumonia.

Diagnosis

For the diagnosis of pneumonia, an infiltrate on an X-ray of the chest is the gold standard. Supportive diagnostic tests are microbiological culture of sputum and/or blood. Blood tests are generally performed when a pneumonia is suspected: a full blood count often showns neutrophilia (except in some immunocompromised and all neutropenic patients). Renal function may have deteriorated if there is sepsis. Electrolytes can show hyponatremia (low sodium levels); this is often due to secretion of antidiuretic hormone by pulmonary tissue.

In nosocomial (hospital-acquired) pneumonia and the pneumonias of the immunocompromised, diagnosis can be difficult, and CT scanning of the lungs can be required to differentiate possible causes (e.g. pulmonary embolism). CT scanning is also used when the symptoms and physical examination point at possible different causes for the complaints (e.g. vasculitis, sarcoidosis, lung cancer).

Classification

There are several different classification schemes: microbiological, radiological, age-related, anatomical, point of acquiring infection. Generally, the following types are used:

• lobar - pneumonia that results in the consolidation of a pulmonary lobe (generally due to Streptococcus pneumoniae)
• multilobar - pneumonia that results in the consolidation of more than one lobe
• community-acquired - pneumonia in a patient who is not or has not recently been in the hospital
• hospital-acquired or nosocomial - pneumonia in a patient in a hospital (or recently discharged)
• "walking" - outdated term, pneumonia in a patient who is still able to walk, a mild pneumonia, usually due to mycoplasma
• pneumococcal - pneumonia due to S. pneumoniae.
• atypical - pneumonia due to either Mycoplasma, Chlamydia or Legionella.

The main classification used in medical journals is that between the point of infection: community-acquired and hospital-acquired.

Types of pneumonia

Community-acquired pneumonia

• Epidemiology - Community-acquired pneumonia (CAP) is a serious illness. It is the fourth most common cause of death in the UK, and sixth in the USA. 85% of cases of CAP are caused by the typical bacterial pathogens, namely, Streptococcus pneumoniae, Haemophilus influenzae, and Moraxella catarrhalis. The remaining 15% are caused by atypical pathogens, namely Mycoplasma pneumoniae, Chlamydia pneumoniae, and Legionella species. Unusual aerobic gram-negative bacilli (for example, Pseudomonas aeruginosa, Acinetobacter, Enterobacter) rarely cause CAP.
• Clinical features - typical symptoms include cough, purulent sputum production, shortness of breath, pleuritic chest pain, fevers and chills. On examination, one notes rapid respiratory rate and heart rate and signs of pulmonary consolidation. In the elderly, symptoms and signs are vague and non-specific. They may consist of headache, malaise, diarrhea, confusion, falling, and decreased appetite. Diagnosis is confirmed by chest x-ray. In general, patients who present with what appears to be CAP, with findings confined to the lungs and no laboratory evidence of extrapulmonary involvement, have CAP caused by a typical pathogen. Patients who have pneumonia plus extrapulmonary physical findings or laboratory features (such as elevations in liver function test results) have an atypical pneumonia.

Hospital-acquired pneumonia

Hospital-acquired pneumonia, also called nosocomial pneumonia, is a lung infection acquired after hospitalization for another illness or procedure. It is considered a separate clinical entity from CAP because the causes, microbiology, treatment and prognosis are different. Hospitalized patients have a variety of risk factors for pneumonia, including mechanical ventilation, prolonged malnutrition, underlying cardiac and pulmonary diseases, achlorhydria and immune disorders. Additionally, pathogens thrive in hospitals that could not survive in other environments. These pathogens include resistent aerobic gram-negative rods, such as Pseudomonas, Enterobacter and Serratia, resistent gram positive cocci, such as ORSA. Because of risk factors, underlying morbidity and resistent bacteria, hospital-acquired pneumonia tends to be more deadly than its community counterpart. Antibiotics used for hospital-acquired pneumonia include aminoglycosides, flouroquinolones, carbapenems, and vancomycin. Multiple antibiotics are administered in combination in order to cover all the possible organisms effectively and rapidly, before the infectious agent can be known. Antibiotic choice varies from hospital to hospital as the likely pathogens and resistence patterns vary similarly.

Other pneumonias

• Severe acute respiratory syndrome (SARS)
• Pneumocystis carinii pneumonia
• Bronchiolitis obliterans organizing pneumonia
• Eosinophilic pneumonia
• Usual interstitial pneumonia

Pathophysiology

Pneumonia is an infectious disease by definition, and whether a patient is prone to develop pneumonia depends on the presence of pathogens but equally on the patient's immune system and other factors. Most pneumonias are not epidemic, although infection with influenza virus can be defined as such.

Breathing problems, as often present in patients after a stroke, in Parkinson's disease, hospitalisation or surgery and mechanical ventilation can all increase the likelihood of pneumonia. Similarly, inability to clear sputum (as in cystic fibrosis) or retention of sputum (as in bronchiectasis) can lead to pneumonia.

After splenectomy (removal of the spleen), a patient is more prone to pneumonia due to the spleen's role in developing immunity against the polysaccharides on pneumococcus bacteria.

Therapy

Antibiotics are the only causal therapy for pneumonia. The exact type of antibiotics that are used depend on the nature of the pneumonia and the immune status of the patient. Amoxicillin is used as first-line therapy in the vast majority of community patients, sometimes with added clarithromycin. In hospitalised patients and immune deficient patients, local guidelines generally determine which combination of (generally intravenous) antibiotics is used.

Prognosis

The clinical state of the patient at time of presentation is a strong predictor of the clinical course. Many clinicians use the Pneumonia Severity Score to calculate whether a patient requires admission to hospital, based on the severity of symptoms, underlying disease and age (Halm et al).

History of pneumonia

Before the advent of antibiotics, pneumonia was often fatal. When penicillin was discovered in the 20th century, it was the first causal therapy. Most community-acquired strains of S. pneumoniae are still penicillin-sensitive.

References

• Halm EA, Teirstein AS. Management of community-acquired pneumonia. N Engl J Med 2002;347:2039-45.

• Pulmonology