Myxoma: This is the most common type of primary cardiac tumor, and it often presents as a filamentous or pedunculated mass in the left atrium. Myxomas are usually benign and can cause obstructive symptoms or embolic events.

Thrombus: A blood clot in the left atrium can appear as a filamentous structure, especially if it is attached to the atrial wall or a cardiac device such as a pacemaker lead. Thrombi can form due to atrial fibrillation, cardiac surgery, or other conditions that predispose to clotting.
Vegetation: Infective endocarditis can cause the formation of vegetations on the mitral valve or other cardiac structures, which can appear as filamentous masses in the left atrium. This diagnosis is more likely in patients with a history of valve disease, intravenous drug use, or other risk factors for endocarditis.

Left Atrial Appendage Thrombus: A thrombus in the left atrial appendage can be a source of embolism and may appear as a filamentous structure. This diagnosis is critical to recognize because it can lead to stroke or other embolic events.
Cardiac Sarcoma: Although rare, cardiac sarcomas can present as filamentous masses in the left atrium. This diagnosis is important to consider because sarcomas are malignant and require prompt treatment.

Papillary Fibroelastoma: This is a rare type of cardiac tumor that can appear as a filamentous mass in the left atrium. Papillary fibroelastomas are usually benign but can cause embolic events.
Cardiac Lymphoma: Primary cardiac lymphoma is a rare condition that can cause the formation of filamentous masses in the left atrium. This diagnosis is more likely in patients with a history of immunosuppression or other malignancies.
Parasitic Infections: In rare cases, parasitic infections such as hydatid disease or schistosomiasis can cause the formation of filamentous masses in the left atrium. These diagnoses are more likely in patients who have traveled to or lived in endemic areas.