Possible causes of swelling and pain next to the thumb include trauma, onychomycosis, stenosing tenosynovitis, rheumatoid arthritis and so on. 1. Trauma: most of the thumb injury experience, can appear skin, subcutaneous tissue injury or finger fracture, manifested as swelling and pain in the thumb. 2. Nail groove inflammation: because of finger impingement, trauma and other skin breaks, staphylococcus aureus infection occurs, causing finger pain, redness, swelling, fever, pus and other symptoms. 3. Stenosing tenosynovitis: because of overwork, excessive flexion and extension of fingers, local aseptic inflammation occurs, causing pain, swelling and difficulty in moving the thumb. 4. Rheumatoid arthritis: If there are symptoms such as swelling and pain in both fingers, stiffness of fingers in the morning, relief after activities, muscle pain, etc., it may be rheumatoid arthritis. The symptoms of thumb swelling and pain, if continued unrelieved, it is recommended to consult a doctor in a timely manner to clarify the cause of the disease, under the guidance of the doctor’s treatment.