Hamman-Rich syndrome overview

# Overview
Acute interstitial pneumonitis  is a rare, and  fulminant disease  leading to acute respiratory failure and, or death.  Acute interstitial pneumonitis is an entity of a group of Idiopathic interstitial lung diseases first described by two pathologists Hamman and Rich. The etiology is unknown (idiopathic). Acute interstitial pneumonitis occurs typically previously healthy individuals in the age group of 50 to 55years with out pre eexisting lung disease. It affects men and women equally.  Acute interstitial pneumonitis shows the histopathologic appearance of diffuse alveolar damage. On gross examination, lungs appear firm, heavy and have a dark red or beefy appearance and show irregular areas of consolidation and fibrosis. On microscopic examination, acute interstitial pneumonitis shows bilateral, temporal uniformity of the diffuse alveolar damage, hyaline membrane deposition and extensive fibroblastic and myofibroblastic proliferation. Acute interstitial pneumonitis may be considered as an idiopathic cause of ARDS. Acute interstitial pneumonitis must be differentiated from other diseases that present with respiratory failure and show diffuse alveolar damage on histopathological examination. These include  ARDS, acute eosinophilic pneumonitis, Infections, hypersensitivity pneumonitis, connective tissue diseases, and drug-induced lung toxicity. Patients with acute interstitial pneumonitis usually present with flu-like viral illness or upper respiratory tract infection, which progresses very rapidly to acute respiratory failure. Common symptoms include fatigue, headache, myalgia, cough, fever, and dyspnea. The acute onset of symptoms is characteristic of acute interstitial pneumonitis. Physical examination shows tachypnea, tachycardia, crackles, wheezing and signs of hypoxemia. Chest radiograph of patients with Acute interstitial pneumonitis shows bilateral airspace opacifications. Most of the patients with acute interstitial pneumonitis on HRCT will show bilateral ground-glass attenuation, traction bronchiectasis, airspace consolidation, architectural distortion. Bronchioalveolar lavage and surgical lung biopsy can be helpful in diagnosing other diseases that causing diffuse alveolar damage that present same as acute interstitial pneumonitis There is no effective treatment for acute interstitial pneumonitis, Management in general includes supportive therapy and administration of glucocorticosteroids and Immunosuppressive agents. Lung transplantation may be considered as an alternative treatment for patients with acute interstitial pneumonitis if the conventional therapy fails. Prognosis is very poor.

# Historical Perspective
In 1935, Hamman and rich first described cases with rapidly progressing pulmonary fibrosis of unknown etiology. After that, the eponym, Hamman-Rich syndrome have been used to describe idiopathic pulmonary fibrosis. In 1975, Liebow came up with classification to distinguish between pulmonary fibrosis and idiopathic interstitial lung diseases. In 1986, Katzenstein coined the term acute interstitial pneumonitis. Further studies helped to differentiate acute interstitial pneumonitis from pulmonary fibrosis.

# Classification
According to American Thoracic Society/European Respiratory Society (ATS/ERS) 2002 consensus, Acute interstitial pneumonitis is an entity of a group of Idiopathic interstitial lung diseases. The classification is based on clinical, radiological and histopathologic findings. The classification has been updated by ATS/ERS International multidisciplinary panel recently based on the literature review on idiopathic interstitial lung diseases published between 2000-2011.

# Pathophysiology
Acute interstitial pneumonitis shows the histopathologic appearance of diffuse alveolar damage. On gross examination, lungs appear firm, heavy and have a dark red or beefy appearance and show irregular areas of consolidation and fibrosis. On microscopic examination, acute interstitial pneumonitis shows bilateral, temporal uniformity of the diffuse alveolar damage, hyaline membrane deposition and extensive fibroblastic and myofibroblastic proliferation.

# Causes
There is no specific etiology (idiopathic), that is responsible for developing acute interstitial pneumonitis.

# Differentiating  from other Diseases
Acute interstitial pneumonitis must be differentiated from other diseases that present with respiratory failure and diffuse infiltrates on chest radiographs. Some of the differentials include ARDS, acute eosinophilic pneumonitis, Infections, hypersensitivity pneumonitis, connective tissue diseases, and drug-induced lung toxicity.

# Epidemiology and Demographics
- The world wide incidence of acute interstitial pneumonitis is approximately 97 cases per 100,000 individuals. The prevalence of acute interstitial pneumonitis is not known.

## Age
- Acute interstitial pneumonitis occurs typically previously healthy individuals in the age group of 50 to 55years with out pre-existing lung disease.

## Gender
- Acute interstitial pneumonitis affects men and women equally.

## Race
- In general there is no racial predilection to acute interstitial pneumonitis.

# Risk Factors
- There are no established risk factors associated with acute interstitial pneumonitis.

# Natural History, Complications and Prognosis
If left untreated, patients with acute interstitial pneumonitis have high fatality rate and die because of severe respiratory failure. Most of the survivors after initial hospitalization may develop recurrent disease or chronic lung fibrosis. Acute interstitial pneumonitis usually has a very poor prognosis.

# Diagnosis

## Diagnostic Criteria
- Abrupt onset of respiratory symptoms resulting in acute respiratory failure
- Chest radiographs show bilateral lung infiltrates
- The absence of an identifiable etiology
- Absence of predisposing condition
- Organising diffuse alveolar damage seen on histopathological examination

## Symptoms
- Patients with acute interstitial pneumonitis usually present with flu-like viral illness or upper respiratory tract infection, which progresses very rapidly to acute respiratory failure. Common symptoms include fatigue, headache, myalgia, cough, fever, and dyspnea. The acute onset of symptoms is characteristic of acute interstitial pneumonitis.

## Physical Examination
- Patients with acute interstitial pneumonitis usually appear ill. Physical examination shows tachypnea, tachycardia, crackles, wheezing and signs of hypoxemia.

## Laboratory Findings
- There are no diagnostic laboratory findings associated with acute interstitial pneumonitis. However, routine laboratory tests may help in identifying alternative diagnoses rather than making a diagnosis of acute interstitial pneumonitis, include abnormal arterial blood gases, physiologic lung testing, complete blood count, and sputum examination, and microbiologic tests.

## Imaging Findings
- Chest radiograph of patients with Acute interstitial pneumonitis shows bilateral airspace opacifications.
- Most of the patients with acute interstitial pneumonitis on HRCT will show bilateral ground-glass attenuation, traction bronchiectasis, airspace consolidation, architectural distortion. This pattern of abnormality is typically found in acute interstitial pneumonitis but it is not diagnostic.

## Other Diagnostic Studies
- Bronchioalveolar lavage and surgical lung biopsy can be helpful in diagnosing other diseases that causing diffuse alveolar damage that present same as acute interstitial pneumonitis.

# Treatment

## Medical Therapy
- There is no effective treatment for acute interstitial pneumonitis, Management in general includes supportive therapy and administration of glucocorticosteroids and Immunosuppressive agents

## Surgery
- Lung transplantation may be considered as an alternative treatment for patients with acute interstitial pneumonitis if the conventional therapy fails.

## Prevention
- There are no sufficient guidelines for the primary prevention of acute interstitial pneumonitis. However, preventing general triggering agents that leads to fibrotic changes in lungs including smoking cessation and vaccination against influenza may be helpful in preventing pulmonary fibrosis and other idiopathic fibrotic lung conditions