Lymphedema: This condition is characterized by the abnormal collection of protein-rich fluid in the interstitial tissue, leading to swelling of the arm. It can be primary (inherited) or secondary (acquired) due to surgery, infection, or trauma. Given its relatively common occurrence, especially in patients with a history of breast cancer surgery or radiation, lymphedema stands out as a probable cause for unilateral arm swelling.

Deep Vein Thrombosis (DVT): Although DVT is more commonly associated with the legs, it can occur in the arms, leading to swelling, pain, and discoloration. Risk factors include central venous catheters, cancer, and immobility.
Cellulitis: A bacterial infection of the skin and subcutaneous tissues that can cause swelling, redness, warmth, and pain in the affected limb.
Venous Insufficiency: Similar to lymphedema, venous insufficiency can lead to swelling due to the veins' inability to send blood back to the heart properly, often resulting from valve problems within the veins.

Axillary Vein Thrombosis: A clot in the axillary vein can cause arm swelling and is a medical emergency due to the risk of pulmonary embolism and long-term complications like post-thrombotic syndrome.
Superior Vena Cava Syndrome: Obstruction of the superior vena cava can lead to swelling of the arm, face, and upper body, and is often associated with malignancies or central venous catheters. It's a life-threatening condition requiring prompt intervention.
Compartment Syndrome: Although more common in the legs, compartment syndrome can occur in the arms, leading to increased pressure within a muscle compartment, which can compromise circulation and function of the tissues within that space.

Thoracic Outlet Syndrome: A condition involving compression, injury, or irritation of the nerves and/or blood vessels (arteries and veins) in the thoracic outlet area, which lies between your collarbone (clavicle) and first rib. This can lead to pain in your shoulders and neck and numbness in your fingers.
Madelung's Disease (Multiple Symmetric Lipomatosis): A rare condition characterized by the growth of non-cancerous tumors composed of fat tissue around the neck and shoulders, which can cause swelling and discomfort.
Klippel-Trenaunay Syndrome: A rare congenital malformation that involves blood and lymph vessels, leading to swelling, port-wine stains, and other vascular anomalies in the affected limb.