There are many clinical causes for swelling of the back of the left hand, commonly due to excessive localized mosquito bites in the environment. Of course, also consider the existence of local soft tissue contusions due to trauma, which in turn causes local edema. In addition, there are allergies or venous reflux and other causes of obstruction resulting in local swelling of the back of the left hand: 1, mosquito bites: mosquito bites on the back of the left hand, mosquito venom will lead to local metaplasia, which can lead to local red bruises, swelling, and accompanied by intense itching; 2, trauma: trauma leads to soft tissue damage on the back of the left hand, local swelling and pain can occur. If accompanied by infection, it can make the swelling more serious. For hematoma, local massage, physical therapy and hot compress can be used to improve blood circulation and promote absorption of swollen tissues. In case of wound infection, mupirocin ointment can be used for treatment and debridement and drainage if necessary; 3. Allergy: If the back of the left hand is exposed to allergic substances, such as pollen and dust, an allergic reaction may be induced, resulting in local swelling of the left hand with itching; 4. Obstructed venous reflux: The superficial blood vessels of the left upper extremity are tortuous and dilated, resulting in obstructed venous blood reflux and more blood pooling at the back of the hand, which manifests as swelling of the back of the left hand . Patients are suggested to go to the hospital for examination in time to find out whether the obstruction of venous return is caused by chronic disease or by tumor compression, so as to avoid local embolism caused by prolonged untreated and aggravated symptoms.