Community-acquired pneumonia overview

# Overview
Community-acquired pneumonia (CAP) is an infection of the lungs which occurs outside the hospital settings. It is the most common form of pneumonia and a major cause of morbidity and mortality. It often causes symptoms such as: breathing difficulties, fever, chest pains, and cough. Community-acquired pneumonia occurs when the alveoli becomes filled with fluid and cannot work effectively.Causes of CAP include: bacteria, viruses, fungi, and parasites. CAP can be diagnosed by its symptoms and physical examination alone, although x-rays, examinations of the sputum, and other tests are also often used. CAP is primarily treated with antibiotic medication. Some forms of CAP can be prevented by vaccination.

# Historical Perspective
Sir William Osler, known as "the Father of Modern Medicine," appreciated the morbidity and mortality of pneumonia, describing it as the "Captain of the Men of Death" in 1918. However, several key developments in the 1900s improved the outcome for those with pneumonia. With the arrival of penicillin and other antibiotics, modern surgical techniques, and intensive care in the twentieth century, mortality from pneumonia dropped precipitously in the developed world. Vaccination of infants against Haemophilus influenzae type b began in 1988 and led to a dramatic decline in cases shortly thereafter. Vaccination against Streptococcus pneumoniae in adults began in 1977, and it began in children during the year 2000; this resulted in a similar decline.

# Pathophysiology
The lower respiratory tract is protected by different pulmonary defense mechanisms . Community-acquired pneumonia connotes a breach of host defense mechanisms and/or an overwhelming inoculation of virulent infectious agents. Modes of transmission include: macro- or micro-aspiration, circulation, local spread, traumatic inoculation, and iatrogenic. Impaired immunity and inability to filter out pathogen increase the risk for developing pneumonia. Causative etiologies vary with age, immune status, geographical area, and comorbid conditions.

# Causes
Community-acquired pneumonia can be caused by viral, bacterial, and fungal organisms. Causative etiology varies with age, immune status, epidemiologic background, and comorbidity. The most common cause of CAP in adult outpatients and inpatients is Streptococcus pneumoniae.  Patients admitted to the intensive care unit tend to have more aggressive organisms such as Staphylococcus aureus and Gram-negative bacilli.  Neonates are most susceptible to Group-B-Streptococcus (GBS) which causes approximately 50% of pneumonias in the first week of life.  Children and elderly patients are more susceptible to viral infections as well atypical bacterial pneumonias (Mycoplasma, Chlamydia, Legionella).

# Differentiating Community-acquired pneumonia from other Diseases
Pneumonia should be differentiated from other non-infectious conditions that cause cough, fever, shortness of breath, tachypnea and lung infiltrates; such other conditions include: interstitial lung disease, CHF, cancer, and pulmonary emboli.

# Epidemiology and Demographics
Pneumonia is the leading cause of death in children younger than 5 years of age worldwide. Both children and the elderly are at a higher risk of pneumonia complications. Developing countries have a higher mortality rate among children with pneumonia.

# Risk Factors
The risk factors of pneumonia include: smoking, age, immunosuppression, exposure to chemicals, and underlying lung disease.

# Natural History, Complications and Prognosis
Complications, including sepsis, respiratory failure, pleural effusion, and empyema, may occur despite appropriate antibiotic treatment. Complications are associated with bacterial pneumonia more frequently than they are with viral pneumonia. Most types of bacterial pneumonia can be cured within one to two weeks of using appropriate medication. Viral pneumonia may last longer, and mycoplasmal pneumonia may take four to six weeks to resolve completely. The eventual outcome of an episode of pneumonia depends on how ill the person is when first diagnosed.

# Diagnosis

## CURB-65
CURB-65 is a clinical prediction rule that has been validated for predicting mortality in community-acquired pneumonia and infection of any site. The CURB-65 is based on the earlier CURB score and is recommended by the British Thoracic Society for the assessment of the severity of pneumonia.

## Pneumonia Severity Index
The pneumonia severity index is a clinical prediction rule that medical practitioners can use to calculate the probability of morbidity and mortality among patients with community acquired pneumonia.

## History and Symptoms
Common symptoms of pneumonia include: cough, fever, and difficulty breathing.  Patients with CAP usually have a history of having close contact with similar symptoms.

## Physical Examination
Physical examination by a health care provider may reveal: fever,or sometimes low body temperature; an increased respiratory rate; low blood pressure; a fast heart rate; or a low oxygen saturation, which is the amount of oxygen in the blood as indicated by either pulse oximetry or blood gas analysis. People who are struggling to breathe, confused, or have cyanosis (blue-tinged skin) require immediate attention. A lack of normal breath sounds, the presence of crackling sounds (rales), or increased loudness of whispered speech (whispered pectoriloquy) can identify areas of the lung that are stiff and full of fluid; this is called "consolidation." The examiner may feel the way the chest expands (palpation) and tap the chest wall (percussion) to further localize consolidation. The examiner may also palpate for increased vibration of the chest when speaking (tactile fremitus).

## Laboratory Findings
Laboratory findings, such as leukocytosis, are helpful for the diagnosis of bacterial pneumonia or the assessment of the patient's status.  Sputum samples need to be collected from every patient and gram staining and culture need to be performed in order to determine the exact pathogen causing the pneumonia.  Other tests include: urine antigen test, PCR, C-reactive protein, and procalcitonin.

## Chest X Ray
An important test for making a diagnosis of pneumonia is a chest x-ray. Chest x-rays can reveal areas of opacity (seen as white) which represent consolidation. Pneumonia is not always seen on x-rays, because the disease may either be in its initial stages or it involves a part of the lung not easily seen by an x-ray.

## CT
A chest CT scan is not routinely done in patients with pneumonia, but it is a diagnostic test that may be useful when a chest X-ray is not conclusive. CT findings may include: lobar consolidation, ground-glass oppacities, pleural effusion, lymphadenopathy, and tree-in-bud appearance.

## Ultrasound
In some cases, ultrasound is used to: diagnosis and follow-up a patient with pneumonia; perform a guided thoracocentesis; quantify the amount of pleural effusion.

## Other Diagnostic Studies
Bronchoscopy with BAL (bronchoalveolar lavage) helps to identify certain uncommon pathogens that cause CAP and aid in the differential diagnosis to exclude non-infectious pneumonia.

# Treatment

## Medical Therapy
Community acquired pneumonia treatment includes using the appropriate antibiotics and managing complications.  An empirical therapy may be started while awaiting culture results.  Once culture results are available, specific treatment may be started.  Empiric therapy is classified according to severity, using the Pneumonia severity scale (PSI) and the CURB-65 score.  Empirical therapy usually includes coverage for atypical and typical bacteria.

## Primary Prevention
There are several ways of preventing infectious pneumonia. Appropriately treating underlying illnesses (such as AIDS), smoking cessation, vaccination against pneumococcal and influenza are the commonly used methods.