Imaging description

Left atrial appendage pseudothrombus is not uncommonly encountered in patients with left atrial enlargement and poor atrial function. In this entity, stasis of blood in the left atrium results in incomplete mixing, or underfilling, of nonopacified blood in the left atrial appendage with intravenous contrast. The result is a low-attenuation region in the tip of the appendage that can be mistaken for thrombus (Figure 6.1). At transesophageal echocardiography, the gold standard for evaluation of left atrial appendage thrombus, underfilling of the left atrial appendage on CT, usually corresponds to spontaneous echo contrast, a sign of blood stasis, with or without thrombus.

Importance

Left atrial pseudothrombus is relatively common in patients with atrial fibrillation who undergo cardiac CT for preablation mapping. In a large study of 402 atrial fibrillation patients with 64-slice contrast enhanced CT, underfilling of the left atrial appendage was seen in 40 (10%), with nine of these patients having true thrombus detected by transesophageal echocardiography. The remaining patients had spontaneous echo contrast. Left atrial underfilling is also important to recognize in patients with a history of stroke, who may have emboli originating from the left atrium. If underfilling is recognized at the time of imaging, acquisition of additional delayed images can improve differentiation of thrombus from blood stasis (Figure 6.1). Distinction of true left atrial thrombus from pseudothrombus is critical as ablation procedures should not be performed in the presence of thrombus, due to the risk of clot dislodgement and distal embolization during the procedure. Incorrect diagnosis of thrombus can result in unnecessary delay of ablation therapy and risks of anticoagulation medications.

Typical clinical scenario

Left atrial pseudothrombus can be seen in any patient, but is most commonly encountered in patients with dilated, poorly functioning atria. These patients have poor mixing of blood in the left atrial appendage due to stasis. Unfortunately, these same characteristics also put the patients at high risk for true left atrial thrombus, increasing the challenge of diagnosis.