Post-thrombotic syndrome: This condition often results from deep vein thrombosis (DVT) and can lead to chronic swelling, pain, and skin changes in the affected limb, making it a common cause of unilateral hand or arm edema.

Cellulitis or skin infection: Bacterial infections of the skin can cause localized swelling, redness, and warmth, which might be confined to one hand.
Lymphedema: This condition, characterized by the abnormal collection of protein-rich fluid in the interstitial tissue, can result from surgery, infection, or trauma, and often affects one limb.
Venous insufficiency: Similar to post-thrombotic syndrome, primary or secondary venous insufficiency can lead to edema due to the inefficient return of blood from the limb to the heart.

Superior vena cava syndrome: Although rare, this condition, characterized by obstruction of the superior vena cava, can cause facial, arm, and upper body swelling and is a medical emergency.
Axillary vein thrombosis: Thrombosis in the axillary vein can cause sudden onset of swelling and pain in the arm and hand, necessitating prompt anticoagulation.
Cancer-related lymphedema: Certain cancers, like breast cancer, and their treatments can lead to lymphedema, making this a critical diagnosis not to miss, especially in patients with a history of cancer.

Sarcoidosis: This autoimmune disease can cause swelling in various parts of the body, including the hands, though it is less common.
Complex regional pain syndrome (CRPS): Characterized by intense burning or aching pain in the hands or feet after an injury, CRPS can also cause swelling and changes in skin temperature and color.
Filarial lymphedema (Elephantiasis): Caused by parasitic infections, this condition leads to severe swelling of the affected limb but is rare in areas without endemic filariasis.