Diffuse Alveolar Hemorrhage (DAH) is a medical emergency involving widespread bleeding into the lungs’ smallest air sacs, the alveoli. This condition arises from damage to the tiny blood vessels surrounding these sacs, causing blood to fill spaces normally reserved for air. The result is a sudden onset of severe difficulty breathing, coughing up blood, and a drop in red blood cell count, known as anemia.
Because the lungs’ ability to transfer oxygen is compromised, DAH requires immediate medical attention to control the bleeding and support breathing. While symptoms and laboratory tests provide initial clues, medical imaging is used to confirm the presence of DAH and understand its extent within the lungs.
The Role of CT Scans in Diagnosis
A standard chest X-ray may show hazy areas in suspected DAH, but its two-dimensional nature limits its ability to fully characterize the problem. A Computed Tomography (CT) scan is more effective, taking a series of X-ray images from different angles to create detailed, cross-sectional views of the lungs.
This method is more sensitive for detecting subtle signs of bleeding within the alveoli. It allows physicians to see the precise location and distribution of the hemorrhage, which an X-ray cannot provide. The patterns on a CT scan can suggest DAH over other conditions and guide patient care.
The detailed images help clinicians determine if an invasive procedure like a bronchoscopy is needed for confirmation. During this procedure, where a thin tube with a camera is inserted into the airways, a bronchoalveolar lavage (BAL) can be performed. This involves washing the air sacs with sterile fluid and collecting the sample to confirm active bleeding.
Key CT Findings in Acute Diffuse Alveolar Hemorrhage
During an active DAH episode, radiologists look for specific patterns on a CT scan. The most common finding is ground-glass opacities (GGOs), a hazy increase in lung tissue density that does not obscure underlying blood vessels. This haziness, which looks like frosted glass, represents partial filling of the alveoli with blood.
Where bleeding is more extensive, the CT scan shows regions of consolidation. These appear as dense, opaque white patches, indicating the air sacs are completely filled with blood. The combination of GGOs and consolidation is a characteristic feature of acute DAH, affecting large portions of both lungs in a patchy distribution.
The distribution of these opacities is also a characteristic. The bleeding follows a lobular or sublobar, non-segmental pattern, meaning it does not confine itself to the lungs’ anatomical segments. The outer edges of the lungs are frequently spared, a feature known as “subpleural sparing.”
Radiologists may also identify centrilobular nodules, which are small, poorly defined nodules scattered throughout the lung tissue. They represent the accumulation of blood in and around the smallest airways at the center of the lung lobules. The presence of these nodules, combined with diffuse GGOs and consolidation, is highly suggestive of DAH.
Evolution of CT Findings Over Time
The appearance of DAH on a CT scan is not static; it changes as the body responds to the initial bleed. In the first 24 to 48 hours, the scan is dominated by the ground-glass opacities and consolidation that signify active bleeding into the alveolar spaces.
Between two and ten days after the event, as the body clears the blood, the CT images evolve. During this subacute phase, fine lines called interlobular septal thickening become superimposed on the existing GGOs. This combination creates a distinctive look that radiologists call the “crazy-paving” pattern.
The crazy-paving pattern is caused by fluid and blood breakdown products collecting within the lung’s supporting framework (the interstitium). While not exclusive to DAH, its appearance with the right clinical symptoms indicates a resolving hemorrhage and marks a transition to the repair stage.
If the underlying cause is treated and bleeding stops, the CT findings continue to resolve. The lungs can return to normal within 7 to 14 days as immune cells clear the remaining blood. However, severe or recurrent bleeding can lead to fibrosis, which is permanent lung scarring.
Differentiating DAH from Other Conditions on CT
Several other lung conditions can mimic DAH on a CT scan. One common mimic is cardiogenic pulmonary edema, a fluid buildup in the lungs from heart failure. While both cause widespread GGOs, edema is also accompanied by smooth septal line thickening, pleural effusions (fluid around the lungs), and an enlarged heart, which are not characteristic of DAH.
Acute Respiratory Distress Syndrome (ARDS) also causes diffuse lung damage and can appear similar to DAH, with extensive GGOs and consolidation. However, in ARDS, these changes are more pronounced in the dependent parts of the lungs due to gravity. The prominent centrilobular nodules seen in DAH are not a feature of ARDS.
Widespread pneumonia can also cause consolidation and GGOs. A distinguishing feature is the distribution, as pneumonia follows a more classic lobar or segmental pattern. In contrast, DAH has a more random, non-segmental distribution. Distinguishing these conditions requires integrating CT findings with the patient’s complete clinical picture, including symptoms and lab results.