Pneumonia overview

# Overview
Pneumonia is an illness of the lungs and respiratory system in which the alveoli (microscopic air-filled sacs of the lung responsible for absorbing oxygen from the atmosphere) become inflamed and flooded with fluid. Pneumonia can result from a variety of causes, including infection with bacteria, viruses, fungi, parasites, and chemical or physical injury to the lungs. Typical symptoms associated with pneumonia include cough, chest pain, fever, and difficulty in breathing. Diagnostic tools include x-rays and an examination of the sputum. Treatment depends on the cause of pneumonia; bacterial pneumonia is treated with antibiotics. Pneumonia is a common illness which occurs in all age groups, and is a leading cause of death among the elderly and people who are chronically and terminally ill. Vaccines to prevent certain types of pneumonia are available. The prognosis depends on the type of pneumonia, the appropriate treatment, any complications, and the person's underlying health.

# Historical Perspective
Pneumonia has been recognized since ancient times. It was initally described by Hippocrates who recorded his observations of its symptoms and complications. Edwin Klebs was the first to identify bacteria in the lungs of patients who died from pneumonia in 1875. This discovery was soon-after substantiated by the works of Carl Friedlander and Albert Frankel who were the first to identify Streptococcus pneumoniae as a causative agent. The introduction of the gram stain subsequently led to the discovery of other causative organisms. Despite being an important cause of mortality before the late twentieth century, the advent of antibiotics, modern surgical techniques, and vaccination drastically lowered the morbidity and mortality of pneumonia with the turn of the century.

# Classification
Several pneumonia classification schemes have been described. The earliest classification was based on the anatomical distribution of the infectious process observed on autopsy and eventually on medical imaging. Advances in microbiology led to a classification based on etiologic group (bacterial, viral, fungal) despite difficulties often encountered in identifying the etiologic agent. With the advent of antibiotics and the rise in resistance, a classification scheme taking into account the setting in which the pneumonia was acquired was introduced to guide empiric therapy. Pneumonia was classified into community-acquired pneumonia (CAP), healthcare-associated pneumonia (HCAP), ventilator-associated pneumonia (VAP), and hospital-acquired pneumonia (HAP). Despite significant overlap, this classification is essential in selecting appropriate antimicrobial therapy.

# Pathophysiology
Bacteria and fungi typically enter the lung with inhalation. Once inside the alveoli, these microbes travel into the spaces between the cells and also between adjacent alveoli through connecting pores. This invasion triggers the immune system response by sending white blood cells responsible for attacking microorganisms (neutrophils) to the lungs resulting in manifestations of pneumonia.

# Causes
Pneumonia can result from a variety of causes including infection with bacteria, viruses, fungi, parasites, and chemical or physical injury to the lungs. The etiology will depend upon various factors such as age, immune status, geographical area, and comorbidities.

# Epidemiology and Demographics
Pneumonia is a common illness in all parts of the world. It is a major cause of death among all age groups. Mortality from pneumonia generally decreases with age until late adulthood. Elderly individuals, however, are at particular risk for pneumonia and associated mortality. More cases of pneumonia occur during the winter months than during other times of the year. Pneumonia occurs more commonly in males than females, and more often in African Americans than Caucasians.  People who are hospitalized for any reason are also at high risk for pneumonia. Following urinary tract infections, pneumonia is the second most common cause of nosocomial infections, and its prevalence is 15-20% of the total number.

# Risk Factors
The risk factors for pneumonia include smoking, age, immuno-suppression, exposure to chemicals, underlying lung disease, and exposure to chemicals.

# Diagnosis

## Diagnostic Criteria
Community acquired pneumonia should be distinguished from healthcare-associated pneumonia as these diseases have different causative organism, prognosis, diagnostic, and treatment guidelines.

## History and Symptoms
People with pneumonia often have a productive cough, fever, shaking chills, shortness of breath, pleuritic chest pain,hemoptysis, headaches, diaphoresis, and clammy skin. Other possible symptoms are loss of appetite, fatigue,blueness of the skin, nausea, vomiting, mood swings, andjoint pains or muscle aches. In elderly people manifestations of pneumonia may not be typical. They may develop a new or worsening confusion or may experience unsteadiness, leading to falls. Infants with pneumonia may have many of the symptoms above, but in many cases they are simply sleepy or have a decreased appetite.

## Physical Examination
Physical examination 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. Patients who are struggling to breathe, who are confused, or who have cyanosis (blue-tinged skin) require immediate attention. Auscultation findings include lack of normal breath sounds, the presence of crackling sounds (rales), or increased loudness of whispered speech (whispered pectoriloquy) with areas of the lung that are stiff and full of fluid, called consolidation. Vital signs are useful in determining the severity of illness and have predictive values. However, a high degree of suspicion should be kept in elderly as the presentation could be subtle in them.

## Laboratory Findings
Laboratory findings such as leukocytosis are helpful for the diagnosis of bacterial pneumonia or to assess the status of the patient.  Sputum samples need to be collected from every patient and sent for gram staining and culture that need to be performed 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, either because the disease is only in its initial stages, or because it involves a part of the lung not easily seen by x-ray.

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

## Other Imaging Findings
Bronchoscopy with bronchoalveolar lavage is useful to obtain samples for gram stain and culture in patients with certain conditions, such as immunocompromised patients, ICU admission or antibiotic failure.

# Treatment

## Medical Therapy
The treatment of pneumonia involves three critical decisions: firstly whether the patient truly has pneumonia, secondly what is the severity of the pneumonia, and lastly whether hospitalization is required for adequate management. Most cases of pneumonia can be treated without hospitalization. Typically, oral antibiotics, rest, fluids, and home care are sufficient for complete resolution. However, people with pneumonia who are having trouble breathing, comorbidities, and the elderly may need more advanced treatment. If the symptoms get worse, the pneumonia does not improve with home treatment, or complications occur, the person will often have to be hospitalized.

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