Overview

Acute interstitial pneumonia (also referred to as acute interstitial pneumonitis or Hamman-Rich syndrome) is a rare, severe form of idiopathic interstitial pneumonia. The term “idiopathic” means the underlying cause of the disease is unknown. This disease is characterised by an acute onset of dyspnoea (shortness of breath), which then rapidly progresses to respiratory failure.¹ 

Currently, acute interstitial pneumonia (AIP) has neither a known definitive cause nor an effective cure available. Because of this, the prognosis of this disease is often poor with high fatality rates of over 50%. In this article, we will explain what AIP is and the various treatment options that are offered.²

Understanding acute interstitial pneumonia (AIP)

What is AIP?

AIP is a rare form of idiopathic interstitial pneumonia, belonging to a larger group of interstitial lung diseases (ILDs). It is classified as “idiopathic” because this condition has an unknown cause. ILDs are said to be “known” if the predisposition to the disease is clear, for instance, if you have a pre-existing condition or have high exposure to certain medications, radiation or harmful substances. AIP is thereby distinct from other ILDs because of its rapid onset of respiratory failure without patients having preexisting lung disease or any risk factors.¹

AIP causes scarring and inflammation of the lung interstitium. The lung interstitium is a network of connective tissue between the air sacs (alveoli) and the blood vessels surrounding those alveoli. The build-up of scar tissue in the lungs is referred to as pulmonary fibrosis. As the scarring and damage increases, your lungs lose their elasticity as they become thicker and stiffer. This makes it more difficult for your lungs to expand sufficiently to allow normal oxygen intake.

AIP manifests with similar symptoms to acute respiratory distress syndrome (ARDS) and is found to affect females and males assigned at birth equally. Most patients tend to be otherwise healthy individuals and are usually over 40 years old. The mean age affected by AIP is reported to be around 55.^1,2

This disease is characterised by organising diffuse alveolar damage (DAD), which is indistinguishable from the DAD found in ARDS. DAD has two phases:³

An acute phase

• Oedema (fluid build-up) in the lung interstitium 
• Formation of hyaline membranes
• Inflammation

An organising phase

• Alveolar septal fibrosis
• Type II pneumocyte hyperplasia 

What are the symptoms of AIP?

Just like other respiratory diseases, the initial symptoms of AIP are often non-specific; this makes it difficult to prevent and predict AIP’s onset. The first symptoms are usually viral-like and develop within 1 to 2 weeks. They may include the abrupt onset of:

• High temperature (fever)
• Dry/non-productive cough (i.e., no mucus/phlegm is produced)
• Shortness of breath (dyspnoea)

In most patients, the severity of these symptoms will increase over 1 to 2 weeks and rapidly progress to respiratory failure.^1,2

Diagnosis of AIP

Physical examination of those with AIP is nonspecific. Patients may have:¹

• Hypoxia (insufficient oxygen supply)
• Tachypnoea (rapid, shallow breathing)
• Bilateral diffuse/bibasilar crackles (crackling sounds in the lungs) 

Please note, that a majority of patients with AIP have severe hypoxia and require mechanical ventilation. 

It is vital for healthcare providers to conduct a thorough history and medical examination, to confirm AIP and exclude other possible causes of DAD (listed below). Following a physical examination, your healthcare provider will create a differential diagnosis to help determine what is causing your symptoms. A differential diagnosis comprises a list of possible conditions that could be causing your condition instead. Differential diagnosis of AIP includes:¹

• Acute exacerbation (sudden aggression of symptoms) of underlying interstitial lung disease 
• Acute heart failure
• Diffuse alveolar haemorrhage (bleeding in the lungs)
• Drug-induced lung injury
• Infections
• Pulmonary exacerbation of connective tissue
• Radiation-induced lung injury
• Other causes of interstitial pneumonia (e.g., cryptogenic organising pneumonia (COP), acute eosinophilic pneumonia, or hypersensitivity pneumonitis) 

No specific test can confirm AIP. In the case that no alternative diagnosis can be made, a lung biopsy is usually conducted to determine AIP. A lung biopsy refers to a procedure where samples of your lung tissue are removed (this can be surgically or via a biopsy needle) to confirm AIP. Other diagnostic tests you may undergo include chest x-ray and high-resolution computed tomography (CT) scans.^1,2

Treatment and management 

Currently, there is no effective therapy for AIP. The mainstay of treatment primarily focuses on supportive care. Management of AIP in a hospital intensive care unit is required following the worsening of the initial viral-like symptoms. Mechanical ventilation is used to help provide adequate oxygenation for patients.^1,2

Although no significant benefit has been established, steroid therapy (anti-inflammatory medications) may be offered, such as corticosteroids or glucocorticoids. Multiple research studies have also indicated lung transplants as another potential treatment modality for ILDs.⁴ In general, lung transplants are reported to have good outcomes and can improve quality of life and extend survival. However, this treatment is not without its limitations. Lung transplants are an expensive procedure with significant risks and long waiting lists. Moreover, post-transplant survival varies greatly depending on factors like age and comorbidities (having more than one medical condition at a time), meaning this treatment is viable for only a small subset of patients. Another drawback involves the fact that AIP is an acute condition with no clear diagnostic tests, meaning early referral and recommendation for a transplant is both difficult and unlikely.^5,6

FAQs

How serious is interstitial lung disease?

ILDs can range from mild to life-threatening conditions. The damage caused is often irreversible; in severe cases, the condition worsens progressively. Symptoms include:

• Shortness of breath (dyspnoea)
• Extreme fatigue
• Dry cough
• Chest pain

Overall, the prognosis and life expectancy of someone with ILD are highly dependent on the cause and severity of their condition. If you have severe forms of ILD, then life expectancy is around 3 to 5 years following diagnosis. 

Most causes of ILD aren’t preventable, especially with AIP where the cause is unknown. Nevertheless, your outcome can be improved if you are treated as early as possible. You can reduce your risk by consulting your healthcare provider on how to manage any underlying conditions. Avoiding harmful substances (e.g., asbestos, dust, and mould) and cessation of smoking can help you prevent further damage. 

What is the prognosis for acute interstitial pneumonia?

In general, AIP has a poor prognosis. That said, patients who do survive AIP may have complete recovery of lung function, and recurrence of AIP is said to be rare. However, you may experience chronic, progressive interstitial lung disease instead.⁷  

Is interstitial pneumonia the same as pneumonia?

Interstitial pneumonia is another term used to describe interstitial lung disease, which is a condition that causes inflammation and scarring of the structural spaces of your lungs, such as your interstitium.  

Interstitial pneumonia is not to be confused with pneumonia, which refers to inflammation of the lung tissue following an infection. Pneumonia is a more common condition associated with viral or bacterial infections.  

How can you tell the difference between pneumonia and pneumonitis? 

Pneumonitis refers to inflammation of the lungs that isn’t caused by infection. Causes of pneumonitis include certain medications (e.g., aspirin), radiation therapy, and airborne moulds and bacteria. It is characterised by inflammation of the alveoli walls. However, this doesn’t cause fluid or pus to build up. The most common symptoms include breathing difficulties and a dry cough. 

Pneumonia is distinct from pneumonitis because it is caused by viral, bacterial, or fungal infection in your lungs. In this condition, inflammation results in the build-up of fluid or pus within your lung tissues. This leads to a productive or wet/chesty cough (i.e., you cough up yellow, green, or bloody mucus).

Summary

Acute interstitial pneumonia (AIP) is a rare lung condition with an unknown underlying cause. It manifests abruptly and develops rapidly into respiratory failure. This condition occurs as a result of inflammation and scar tissue build-up within the lungs, which results in breathing difficulties due to the thickening of your lung tissue. Symptoms involve acute onset of shortness of breath, a drug cough, and fever. Immediate medical attention is fundamental for increasing your chances of having improved outcomes. Presently, AIP has no effective therapy available. Nevertheless, recent research studies demonstrate lung transplants as a viable treatment option. However, more research is required to make this a feasible option for a larger subset of patients.