The causes of swelling of the left arm include trauma or sprain of the elbow joint, venous thrombosis of the upper limb, and obstruction of the lymphatic vessels of the upper limb, etc., as follows: First, trauma or sprain of the elbow joint: swelling of the elbow joint occurs when the local soft tissue of the left elbow joint is edematous and congested with blood. Second, upper limb venous thrombosis: When thrombosis occurs, the venous return of the upper limb will be blocked and blood will pool in the hand and forearm of the arm, so the swelling of the left arm will occur. Third, upper limb lymphatic vessel obstruction: when the upper limb lymphatic vessels are obstructed, lymphedema of the upper limb will appear, manifesting as swelling of the entire upper limb, and the degree of swelling is often more pronounced, generally more serious than the swelling of the upper limb caused by the above two conditions.