Calcified pulmonary thromboembolism in a child with sickle cell disease: value of multidetector CT in patients with acute chest syndrome
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- Staser, J.A., Alam, T. & Applegate, K. Pediatr Radiol (2006) 36: 561. doi:10.1007/s00247-006-0161-y
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The incidence of pulmonary embolism in children is not clearly known, but is believed to be low. Risk factors for pulmonary thromboembolism include central venous catheter, malignancy, surgery, infection, trauma, and congenital hypercoagulable disorders. Children with sickle cell disease are prothrombotic and are at an increased risk of thromboembolism. The incidence of this event is unknown because these children are often not thoroughly imaged. We report here a case of a calcified pulmonary thromboembolism in a child with sickle cell disease and emphasize the use of multidetector CT in detection of pulmonary thromboembolism in children with sickle cell disease.
KeywordsSickle cell diseaseChildrenPulmonary embolismCT angiography
Children with sickle cell disease (SCD) frequently present with acute chest syndrome (ACS), and it is often impossible to distinguish among the many causes, which include pneumonia, atelectasis and pulmonary embolism. ACS is the second most common cause for hospital admission and the leading cause of death in this patient population . Multiple etiologies of ACS have been proposed, including infection, infarction and thromboembolism, particularly fat embolism . The incidence of pulmonary thromboembolism in these children is unknown but might be higher than that of the general pediatric population .
The incidence of pulmonary embolism in children is not clearly known but is believed to be low. The literature usually draws on adult studies and scant case reports of thromboembolism in children , but it might not be possible to accurately extrapolate such studies to the pediatric population . The Canadian registry reported in 1994 that the incidence of pediatric deep venous thrombosis and pulmonary embolism at tertiary medical centers was 5.3 per 10,000 hospital admissions .
The imaging workup is usually limited to chest radiography. Although findings are often abnormal, they are nonspecific. Because children with ACS are unlikely to undergo chest CT angiography, the incidence of pulmonary thromboembolism is unknown and likely underdiagnosed. With the increasing availability of multidetector CT (MDCT), it is now possible to easily image children with ACS who have not responded to initial treatment and provide a more accurate diagnosis, including the presence of pulmonary thromboembolism. Calcified pulmonary thromboembolism in a child with SCD is an extremely rare phenomenon. We present a child with SCD and clinically suspected pneumonia who had a calcified thromboembolism at MDCT.
The most common risk factor for pediatric pulmonary thromboembolism is central venous catheter, but other factors include malignancy, surgery, infection, trauma, and congenital hypercoagulable disorders such as SCD . SCD is a prothrombotic disorder, and patients are at increased risk for thromboembolism, particularly fat embolism . ACS is diagnosed when a patient has respiratory distress, fever, chest pain and an infiltrate on imaging studies [1, 2]. The causes of ACS are multifactorial, and include infection, infarction, and pulmonary embolism secondary to occlusion of pulmonary vessels by damaged red blood cells and fat emboli from infarcted bone marrow [1, 2].
The incidence of pulmonary thromboembolism in children with SCD is not known. Most reports are single case reports . Fat emboli in the pulmonary vessels have been identified in post-mortem studies and in evaluation of fat-laden pulmonary macrophages from bronchoalveolar lavages [2, 4]. The fat embolizes secondary to bone marrow infarction . Another source of pulmonary artery occlusion in SCD patients is in situ thrombus formation, in which there is occlusion of the vessels by sickled red blood cells . Again, the incidence of in situ thromboembolism is unknown.
Chest radiograph findings in patients with ACS are usually abnormal but nonspecific. It is difficult to attribute the lung infiltrate to atelectasis, infection or infarction. Radiographs are of little value in identifying those with pulmonary thromboembolism.
V/Q lung scans are very sensitive but not specific for the detection of pulmonary thromboembolism, with a reported sensitivity of 98% and specificity of 5% . Normal scans essentially rule out a pulmonary thromboembolic event. A major disadvantage of V/Q lung scans is the inability to characterize the nature of the thromboembolism. An acute thromboembolism cannot be differentiated from chronic pulmonary artery occlusion. In our case, A V/Q scan likely would have indicated a high probability for thromboembolism to the right lower lobe. This finding would certainly prompt the referring physicians to treat the patient with anticoagulants yet this would likely have had little therapeutic benefit in the case of a calcified thromboembolism.
Studies have evaluated single-detector helical CT and showed a sensitivity of 53% to 100% and a specificity of 78% to 100% . In one meta-analysis, the pooled sensitivity for helical CT was 86% and the specificity was 94% . The sensitivity and specificity of the newer MDCT equipment for the detection of pulmonary thromboemboli is unknown. This is being evaluated in the PIOPED II study . Given the improved visualization of the pulmonary arteries with MDCT, the sensitivity and specificity are likely higher.
Further potential advantages of MDCT include the availability of CT 24/7 and the ability to make alternative diagnoses. Infectious and inflammatory conditions can be diagnosed while the arteries are examined for thromboemboli. Pitfalls of CT angiography for diagnosing pulmonary embolism include misidentification of artery versus vein and pulmonary artery versus hilar lymph nodes. Multiplanar reconstruction and thin collimation in our case minimized the chance of error.
In summary, children with SCD and clinical presentation of ACS are likely underdiagnosed with pulmonary thromboembolism and under-imaged with MDCT. MDCT provides a more comprehensive evaluation of the lungs and pulmonary arteries and might be useful to further assess SCD patients presenting with ACS who do not respond to initial therapy. In our patient, it is unclear how long the calcified thromboembolism was present prior to his first hospital presentation. It might have embolized from a leg vein thrombus and produced the initial chest symptoms that were initially treated as right lower lobe pneumonia.