Alisina Shahi, Mary K. Mulcahey, Emmanuel Thienpont.
Response/Recommendation: Pulmonary arterial filling defects seen on cross-sectional imaging studies are not always indicative of pulmonary embolism (PE). There are several other conditions that can lead to arterial filling defects on cross-sectional imaging studies such as primary pulmonary arterial (PA) neoplasm, pulmonary vascular involvement of IgG4-related disease (IgG4-RD), Behcet’s disease, Takayasu’s arteritis (TA), Hughes–Stovin syndrome (HSS), and pulmonary arterial streak artifact.
Strength of Recommendation: Strong.
Rationale: The most common cause of PA filling defects on either computed tomography pulmonary angiography (CTPA) or magnetic resonance imaging (MRI) is pulmonary thromboembolism. However, not infrequently, the presentation of this finding is not associated with a PE. Given high rates of morbidity and mortality, timely diagnosis is essential. Unusual or more rarely encountered etiologies must be considered when clinical manifestations and imaging findings are not consistent.
CTPA and MRI studies have become the primary methods for diagnosing PE, largely replacing the previous method of choice, pulmonary angiography or ventilation–perfusion V/Q scans. They are less invasive, faster, and less expensive1,2. A pulmonary arterial filling defect is the major finding on CTPA that can be indicative of PE, with a reported sensitivity of 83 – 91% and a specificity of 89 – 96%3,4. However, there are other clinical conditions that may present with similar findings on CTPA (intraluminal filling defect), mimicking PE, and can lead to inappropriate diagnosis and possibly inappropriate intervention. Some of the conditions that may cause filling defects include primary pulmonary arterial neoplasm, pulmonary vascular involvement of IgG4-RD, Behcet’s disease, TA, HSS, and pulmonary arterial streak artifact.
PE is the third most common cardiovascular condition with high mortality and morbidity rate after coronary artery disease and stroke4. Fresh thrombus in acute PE is formed by red blood cells (RBC) and platelets binding together in a fibrin mesh, this presents on CTPA as a filling defect leading to complete or partial stenosis of the lumen.
Primary pulmonary arterial neoplasms: This is a very uncommon condition. The pathology in the majority of cases is pulmonary artery sarcoma (PAS), which has poor prognosis. On CTPA, PAS also manifests as a filling defect that resembles an acute PE. There are specific findings on CTPA that can help differentiate PAS from PE, including a filling defect involving the entire main PA or one of its principal branches, the proximal margin of the filling defect with the “lobulated sign” and the grape-like appearance of the distal PA, and a filling defect with heterogeneous enhancement5. A magnetic resonance angiogram can be very helpful when there is suspicion for PAS.
IgG4-RD of PA: This is an autoimmune condition that results from chronic fibrotic inflammation. The main findings of PA IgG4-RD include massive filling defects without enhancement or PA aneurysm on CTPA. Definite diagnosis of PA IgG4-RD is only possible with an intrathoracic surgical biopsy6,7.
Takayasu’s arteritis (TA): TA is an idiopathic inflammatory disease that affects large vessels such as the aorta and PA. Studies reported PA involvement in approximately 63.3% of cases8. When there is isolated PA involvement, vessel stenosis or complete occlusion of PA mimics PE9,10. One way to distinguish TA from PE is that on post-enhanced CTPA images, a “double-ring sign” can be seen in TA patients, which is resulted from arterial mural thickening9. MRI is the alternate imaging modality that can help differentiate TA from PE.
Behcet’s disease: Behcet’s disease is an idiopathic syndrome characterized by vasculitis and recurrent ulcers of the oral and genital mucosa, with relapsing uveitis11. Vascular involvement can be seen in 5 – 30% of cases12. Aneurysms are the most common finding when there is vascular involvement. PA aneurysm is the most common finding (in up to 10% of cases) and tends to be multiple and bilateral13. The pulmonary thrombosis of the aneurysm forms an in situ partial or complete filling defect that resembles PE.
Hughes-Stovin syndrome (HSS): HSS is a rare disorder with an unknown etiology14. It is characterized by multiple PA and/or bronchial artery aneurysms as well as deep venous thrombosis, however, unlike Behcet’s disease, it does not have mucocutaneous involvements15. PA filling defects can be seen in the CTPA of patients with HSS and should be differentiated from PE events.
Pulmonary arterial streak artifact: Chronic lung diseases such as tuberculosis, interstitial lung disease, and bronchiectasis as well as conditions like pulmonary vein stenosis, systemic artery-PA shunt, and pulmonary hypertension can affect the hemodynamics of the PA and cause “streak artifact,” which could mimic a PA filling defect. It is important to take patients’ past medical history into consideration when interpreting CTPA results. Using a dual-phase scan protocol, a filling defect in early phase contrast-enhanced imaging, which resolves in the late phase, strongly suggests a pulmonary arterial streak artifact rather than a PE event.
CTPA and MRI studies are reliable tools for diagnosing PE events with a high accuracy3. However, false-positive and false negatives are not uncommon. The presence of a filling defect is not always indicative of PE and other diagnostic tests, as well as past medical history, are necessary to confirm the diagnosis especially when there is a concern for one of the aforementioned conditions.
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