Acute lung injury

What is acute lung injury?

The lungs may become injured by infections, trauma, inhalation of toxic substances, autoimmune diseases, medications, and radiation to the chest. While there are many causes of lung injury, the changes seen when the lungs are examined under the microscope are similar regardless of the cause. Acute lung injury is a general term pathologists use to describe these changes.

The lungs

When you breathe, air enters your body through your mouth and nose and travels down airways into your lungs. You have two lungs, one on the right side of your chest and one on the left. Inside the lungs, oxygen enters the blood and carbon dioxide is removed from the body. As the airways enter the lungs they split into smaller and smaller airways, called bronchioles, ultimately ending in air-filled spaces called alveoli.

Normal lung

The alveoli are extremely small cup-shaped air-filled spaces lined by flat cells called pneumocytes. There are two types of pneumocytes. Type 1 pneumocytes are small and flat. Type 2 pneumocytes are larger and shaped more like a cube. The alveoli are surrounded by a fine network of blood vessels called capillaries which carry blood in and out of the lungs.

Patterns of acute lung injury

Acute lung injury can be further divided into two patterns of injury: organizing pneumonia or diffuse alveolar damage (see below for more information). These two patterns differ in the severity of the injury and in the associated symptoms.

Patients with diffuse alveolar damage can be very sick and may be admitted to the intensive care unit. In the intensive care unit, the goal is to support breathing as patients may struggle to get enough oxygen. This sometimes requires intubation, which involves inserting a tube into the trachea to push more air into the lungs. On the other hand, patients with organizing pneumonia typically have more mild symptoms and may be able to recover at home.

Acute lung injury patterns

Organizing pneumonia

Symptoms of organizing pneumonia can include dry cough, difficulty breathing, fever, and weight loss. When a sample of lung tissue is examined under the microscope, a collection of specialized cells called fibroblasts can be seen within the air spaces. Specialized inflammatory cells such as lymphocytes may also be seen in the alveolar walls.

Organizing pneumonia often only affects parts of the lung while other parts remain normal and healthy. Groups of fibroblasts within the affected areas of the lung prevent air from filling the alveoli. As a result, patients with organizing pneumonia can feel short of breath. Radiology reports often describe these areas of the lung as ground-glass opacities.

Organizing pneumonia is a pattern of acute lung injury that has many causes. The most common causes of organizing pneumonia include:

  • Infections with bacteria, viruses, fungi, and parasites
  • Certain drugs
  • Radiation to the chest
  • Connective tissue disorders and inflammatory disorders
  • Lung injury due to repeated aspiration of stomach contents

Your doctor will consider your symptoms, medical history, recent travel, and medications taken to determine the cause of organizing pneumonia. If no cause can be found, a diagnosis of “cryptogenic organizing pneumonia” is made. Cryptogenic organizing pneumonia is generally associated with a good prognosis and responds well to corticosteroids, such as prednisone.

Diffuse alveolar damage

This pattern of acute lung injury can be very severe. Diffuse alveolar damage (DAD) is seen in acute respiratory distress syndrome (ARDS), which is a clinical term used to describe people with severe shortness of breath, low levels of oxygen in the blood, and fluid in the air spaces of the lungs.

When a sample of lung tissue is examined under the microscope the changes associated with diffuse alveolar damage include hyaline membrane formation, replacement of type 1 pneumocytes with type 2 pneumocytes, and thickened alveolar walls.

These changes stop the lungs from functioning normally by preventing the efficient exchange of oxygen and carbon dioxide in the alveoli. This can result in severe shortness of breath and respiratory failure.

Diffuse alveolar damage is a pattern of acute lung injury that has many causes. The most common causes of diffuse alveolar damage include:

  • Pneumonia caused by infectious micro-organisms such as Pneumocystis jirovecii or SARS-CoV-2
  • Severe bloodstream infection (sepsis)
  • Near drowning
  • Inhalation injuries, such as inhaling toxic gas, vaping induced lung injury, or smoke
  • Drug reaction
  • Aspiration of stomach contents or other foreign material
  • Other forms of shock, such as severe burns
  • Trauma to the chest or lungs

Your doctor will consider your symptoms, medical history, recent travel, and medications taken to determine the cause of the diffuse alveolar damage. If no cause can be found, a diagnosis of “acute interstitial pneumonia” is made. The prognosis for patients with acute interstitial pneumonia is poor and there are no specific treatments available currently.

Tests performed to determine the cause of acute lung injury

If your doctor suspects that you have an acute lung injury, they may perform a procedure called a biopsy which removes a small tissue sample for examination by a pathologist. Your pathologist will look at the tissue sample under the microscope and may order additional tests to help determine the cause.

These additional tests may include a special stain called Grocott’s methenamine silver stain (also called GMS) to look for fungal micro-organisms and immunohistochemistry to look for specific types of viruses. If no specific cause can be found, your pathologist will describe the changes seen in your biopsy and will suggest that your doctor consider these changes along with other information about you in order to make a final diagnosis. This combination of information is called clinical correlation.

by Katherina Baranova MD and Matthew J. Cecchini MD PhD FRCPC (December 11, 2020)
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