Numbness of the thumb and index finger is caused by radial nerve injury or cervical spondylosis in many cases, and in rare cases, it may be caused by brain diseases. It is recommended to improve the CT MRI and other related tests to clarify the condition. 1. Radial nerve injury: radial nerve injury often occurs at the same time thumb and index finger numbness, may be accompanied by finger weakness. When the injury is relatively light, the numbness is usually the main symptom, and when the injury is relatively heavy, the weakness is the main symptom. It is mainly seen in trauma or fracture, and some of them may also have long-term muscle strain. 2. Cervical spondylosis: especially nerve root cervical spondylosis, it is easy to appear compression of the nerve, compression of the nerve, resulting in irritation, will cause numbness of the fingers, if serious, it will cause pain and discomfort as well as lifting weakness and so on. 3. Brain tumor: brain tumor may also show numbness of thumb and forefinger in some cases, but it is rare, and many of them will be accompanied by head symptoms, such as headache and dizziness. It is recommended to seek medical treatment and improve CT, MRI and other related examinations in time to clarify the condition.