Acute lung injury

by Katherina Baranova MD and Matthew J. Cecchini MD PhD FRCPC
June 2, 2023

What is acute lung injury?

Acute lung injury is a life-threatening condition associated with inflammation in the lungs. Pathologists divide acute lung injury into two patterns of injury: organizing pneumonia and diffuse alveolar damage (DAD). These two patterns differ in the severity of the injury and in the associated symptoms.

Organizing pneumonia

What are the symptoms of organizing pneumonia?

Symptoms of organizing pneumonia can include dry cough, difficulty breathing, fever, and weight loss.

What causes organizing pneumonia?

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, drugs, radiation to the chest, connective tissue disorders and inflammatory disorders, and aspiration of stomach contents into the lungs.

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.

What does organizing pneumonia look like under the microscope?

When lung tissue with organizing pneumonia 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.

Diffuse alveolar damage

What are the symptoms of diffuse alveolar damage?

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. 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.

What causes diffuse alveolar damage?

DAD is a pattern of acute lung injury that has many causes. The most common causes of DAD include infections that lead to pneumonia; severe bloodstream infection (sepsis); near-drowning; inhalation injuries, such as inhaling toxic gas, vaping-induced lung injury, or smoke; drug reactions; aspiration of stomach contents or other foreign material; other forms of shock, such as severe burns; and 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 DAD. 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.

What does diffuse alveolar damage look like under the microscope?

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.

What tests may be 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.