Trauma or Injury: This is often the most common cause of unilateral hand swelling due to accidents, sports injuries, or other forms of physical trauma that can lead to swelling, bruising, and pain in the affected hand.

Infection (Cellulitis or Abscess): Bacterial infections can cause swelling, redness, warmth, and pain in one hand, often related to a break in the skin or after a puncture wound.
Deep Vein Thrombosis (DVT): Although more common in the legs, DVT can occur in the arms, leading to swelling, pain, and discoloration of the affected limb.
Lymphedema: This condition involves the abnormal collection of protein-rich fluid in the interstitial tissue, leading to swelling of one arm, which can be due to surgery, infection, or congenital conditions affecting the lymphatic system.

Compartment Syndrome: A medical emergency characterized by increased pressure within a muscle compartment, leading to pain, swelling, and potentially permanent damage if not promptly treated.
Necrotizing Fasciitis: A severe infection that destroys tissue under the skin, including fat and the tissue covering the muscles (fascia), which requires immediate medical attention.
Venous Obstruction: Blockage of a vein, which can cause swelling, pain, and discoloration, and may lead to more serious conditions like pulmonary embolism if the clot breaks loose.

Klippel-Trenaunay Syndrome: A rare congenital condition characterized by abnormal development of blood vessels, soft tissue, and bones, leading to swelling and other abnormalities in the affected limb.
Sarcoidosis: A disease in which granulomas (abnormal masses of inflamed tissue) can form in various parts of the body, including the skin, which may cause swelling among other symptoms.
Gorham-Stout Disease: A rare condition characterized by the replacement of bone with lymphatic or vascular tissue, leading to swelling and other complications in the affected area.