Atelectasis overview

# Overview
Atelectasis is characterized by incomplete lung expansion leading to diminution of lung volume. It affects the whole lung or a part of it. Atelectasis may be classified based on etiology into obstructive and non-obstructive types. Obstructive atelectasis, which is the most common type of atelectasis may develop due to obstruction by foreign bodies, tumors and mucus plugs. Causes of non obstructive atelectasis include lung scarring caused by necrotizing pneumonia or granulomatous diseases, lung infiltration, extrinsic lung compression and diminished levels of surfactant. The most common cause of atelectasis is postoperative atelectasis, due to the effect of surgical manipulation or general anaesthesia on the patient. Chest trauma, general anaesthesia, thoracic surgery, cystic fibrosis, prematurity and respiratory distress syndrome are some of the common risk factors that predispose patients to developing atelectasis. Common symptoms of atelectasis include breathlessness, chest pain, and cyanosis. Symptoms in patients with atelectasis depend upon rapidity of bronchial occlusion, lung area affected and presence of any overlying infection. Physical examination of patients with atelectasis is usually remarkable for decreased chest expansion, mediastinal displacement towards the affected side and elevation of the diaphragm. Patients may develop dullness to percussion over the involved area, wheezing and diminished or absent breath sounds on auscultation. An x-ray may be helpful in the diagnosis of atelectasis. Findings on an x-ray suggestive of atelectasis include displacement of fissures, rib crowding, elevation of ipsilateral diaphragm, volume loss on ipsilateral hemithorax, hilar displacement and compensatory hyperlucency of the remaining lobes. CT scan of the chest without contrast is the gold standard diagnostic modality.The primary treatment for atelectasis is management of the underlying cause. Besides this, supportive therapy for atelectasis includes chest physiotherapy, breathing and coughing exercises, early ambulation, nasotracheal suctioning, humidifiers, nebulized bronchodilators and supplemental oxygen in order to maintain an arterial oxygen saturation of greater than 90 percent. Intubation, mechanical support, positive pressure ventilation and continous positive airway pressure (CPAP) help in the prevention of alveolar collapse, thereby assisting in lung inflation in patients with atelectasis.

# Historical Perspective
The Greek words ateles and ektasis form the term atelectasis which translates into incomplete expansion.

# Classification
Atelectasis may be classified based on etiology into obstructive and non-obstructive types. Obstructive atelectasis, which is the most common type of atelectasis may develop due to obstruction by foreign bodies, tumors and mucus plugs. Causes of non obstructive atelectasis include lung scarring caused by necrotizing pneumonia or granulomatous diseases, lung infiltration, extrinsic lung compression and diminished levels of surfactant. Atelectasis may also be classified based on duration into acute and chronic types. Acute atelectasis is associated with airlessness due to recent lung collapse while chronic atelectasis involves a combination of infection, bronchial destruction, and fibrosis, in adition to airlessness. If left untreated, atelectasis may be fatal in patients and progress to pneumonia, sepsis, and respiratory failure.

# Pathophysiology
The pathophysiology of obstructive and non-obstructive atelectasis is determined by several factors. Obstructive atelectasis, the most common type of atelectasis and occurs due to obstruction from the trachea to the alveoli at any level. Foreign bodies, tumors, and mucus plugs are causes of obstructive atelectasis. Non obstructive atelectasis occurs due to severe lung scarring caused by necrotizing pneumonias or granulomatous diseases leading to cicatrisation atelectasis. Lung infiltration by a tumor (bronchoalveolar carcinoma) may cause replacement atelectasis, thoracic space occupying lesions can cause compression atelectasis, diminished levels of surfactant can lead to adhesive atelectasis presenting as ARDS. Passive atelectasis occurs due to absence of contact between the parietal and visceral pleurae due to fluid (pleural effusion), air (pneumothorax), blood (hemothorax) etc. Patients undergoing upper abdominal and thoracic procedures may develop postoperative atelectasis which may arise as a complication of surgery or anaesthesia leading to decreased surfactant activity and dysfunction of the diaphragm.

# Causes
The most common cause of atelectasis is postoperative atelectasis, due to the effect of surgical manipulation or general anaesthesia on the patient. Obstructive atelectasis mostly develops due to blockage within the bronchiole or bronchus, which may be within the airway (foreign bodies, mucus plugs), arising from the wall (tumors such as squamous cell carcinoma) or space occupying lesions within the thoracic cavity (tumor, lymph node, tubercle). Causes of non-obstructive atelectasis include impaired surfactant formation or activation, leading to alveolar collapse due to increased surface tension.

# Differentiating Hereditary pancreatitis from Other Diseases
Atelectasis must be differentiated from other diseases that cause acute dyspnea, fever, and chest pain, such as asthma, bronchitis, and interstitial lung disease.

# Epidemiology and Demographics
The incidence and prevalence of atelectasis are not known. Patients of all age groups may develop atelectasis. Rounded atelectasis commonly affects individuals at sixty years of age. Atelectasis affects men and women equally.

# Risk Factors
Chest trauma, general anaesthesia, thoracic surgery, cystic fibrosis, prematurity and respiratory distress syndrome are some of the common risk factors that predispose patients to developing atelectasis. Children less than three years of age and adults over sixty years of age are also at increased risk of alveolar collapse and impaired surfactant production in response to pulmonary stressors such as toxins, hyperoxia, hypoxia and ischemia.

# Screening
There is insufficient evidence to recommend routine screening for atelectasis.

# Natural History, Complications, and Prognosis
If left untreated, atelectasis may be fatal in patients and progress to pneumonia, sepsis, and respiratory failure. Common complications of atelectasis include pneumonia, bronchiectasis, hypoxemia, respiratory failure, and sepsis. Depending on the extent of lung involvement at the time of diagnosis, the prognosis may vary. However, the prognosis is generally regarded as good. Involvement of a small portion of the lung is not associated with life threatening complications, as the remaining lung compensates for the hypoxemia. However, atelectasis is associated with poor prognosis if the surface area of lung involvement is very large, in the presence of pre-existing lung disease, in case of failure to remove obstruction and in case of atelectasis due to cancer.

# Diagnosis

## History and Symptoms
The majority of patients with atelectasis are asymptomatic. Common symptoms of atelectasis include breathlessness, chest pain, and cyanosis. Symptoms in patients with atelectasis depend upon rapidity of bronchial occlusion, lung area affected and presence of any overlying infection.

## Physical Examination
Patients with atelectasis usually have non specific signs on physical examination. Physical examination of patients with atelectasis is usually remarkable for decreased chest expansion, mediastinal displacement towards the affected side and elevation of the diaphragm. Patients may develop dullness to percussion over the involved area, wheezing and diminished or absent breath sounds on auscultation.

## Laboratory Findings
Patients with atelectasis have hypoxemia with low levels of PaO2 and normal/low PaCO2 levels on ABG analysis. Raised peak and end-inspiratory plateau pressures due to decreased lung compliance are also present in these patients.

## X-ray
An x-ray may be helpful in the diagnosis of atelectasis. Findings on an x-ray suggestive of atelectasis include displacement of fissures, rib crowding, elevation of ipsilateral diaphragm, volume loss on ipsilateral hemithorax, hilar displacement and compensatory hyperlucency of the remaining lobes. Complete lung atelectasis and atelectasis involving different parts of the lung have their own characteristic appearance. While complete atelectasis of the lung may lead to opacification of the entire hemithorax and ipsilateral shift of the mediastinum, a right middle and lower lobe atelectasis may show subpulmonic effusions along with right hemidiaphragmatic elevation on X-ray.

## Ultrasound
There are no significant ultrasound findings associated with atelectasis.

## CT scan
CT findings suggestive of atelectasis include hilar displacement, elevation of ipsilateral diaphragm, rib crowding, displacement of fissures, and compensatory hyperlucency of the remaining lobes. CT findings associated with complete atelectasis of an entire lung include opacification of the entire hemithorax and ipsilateral shift of the mediastinum. Collapse of different parts of the lung have their own characteristic appearance. For example, a collapsed right middle lobe has a tilted icecream sign which appears as a triangular opacity against the border of the right heart with a laterally pointed apex. On the other hand, RUL collapse appears as a right paratracheal opacity, with a concave lateral appearance of the minor lung fissure.

## MRI
MRI may be helpful in distinguishing obstructive causes of atelectasis from non-obstructive causes. MRI does not play a significant role in the diagnosis of atelectasis.

## Other Imaging Findings
There are no other imaging findings associated with atelectasis.

## Other Diagnostic Studies
Flexible fibreoptic bronchoscopy may be helpful in the diagnosis of atelectasis. This technique assists in determining the cause of obstruction and removal of mucus plugs. Biopsies taken during fiberoptic bronchoscopy helps in the diagnosis of malignancy and allergic bronchopulmonary aspergillosis.

# Treatment

## Medical Therapy
The primary treatment for atelectasis is management of the underlying cause. Besides this, supportive therapy for atelectasis includes chest physiotherapy, breathing and coughing exercises, early ambulation, nasotracheal suctioning, humidifiers, nebulized bronchodilators and supplemental oxygen in order to maintain an arterial oxygen saturation of greater than 90 percent. Intubation, mechanical support, positive pressure ventilation and continous positive airway pressure (CPAP) help in the prevention of alveolar collapse, thereby assisting in lung inflation in patients with atelectasis.

## Surgery
Surgical intervention is not recommended for the management of atelectasis.

## Primary Prevention
Judicious use of anaesthetic agents known to cause narcosis, use of epidural analgesia in patients with underlying pulmonary disease, incentive spirometry, early ambulation, humidifiers, breathing exercises, coughing exercises and supplemental oxygen are effective measures of primary prevention in patients undergoing operative procedures, in order to prevent post operative atelectasis.

## Secondary Prevention
Effective measures for the secondary prevention of atelectasis include incentive spirometry, deep breathing exercises, coughing exercises and early ambulation. These measures help prevent complications in patients such as  post obstructive infection, bronchiectasis and fibrosis.