The sudden swelling of the index finger may be due to the following reasons: 1. It may be gout. Gouty arthritis is the deposition of gout crystals around the joints, causing redness, swelling and heat in the joints, so it can cause swelling and appear suddenly.2. Pay attention to whether there is rheumatism, rheumatoid, or ankylosing spondylitis. These rheumatic diseases can also cause sudden swelling of the index finger, often accompanied by pain, high skin temperature, and elevated corresponding examination indicators.3. It may be a local trauma, such as fracture or rupture of ligaments. These can cause bleeding and swelling. 4. It may be tendonitis. Inflammation and exudation of the tendon sheath can cause localized swelling. In addition, it is important to know if there are pathological fractures, bone tumors, tuberculosis of bones and other inflammatory conditions that can cause sudden swelling of the index finger.