Sudden pain on the outer side of the arm may be caused by shoulder diseases, cervical spondylosis, dermal neuritis and other reasons. 1. Shoulder diseases: for example, frozen shoulder, etc., the extensive inflammation of the periapical region will easily affect the lateral cutaneous nerve of the upper arm in front of the shoulder joint, which will stimulate the cutaneous nerve, causing the lateral part of the upper arm to have nerve pulling pain, or numbness, weakness and some other symptoms. 2. Cervical spondylosis: when cervical spondylosis compresses the nerve root, it will also cause the patient to have pain on the lateral side of the arm. 3. Dermal neuritis: when the dermal nerve of the lateral arm has chronic inflammation, there can be pain in the skin of its innervated area, and there can be tingling or numbness. There are other causes of sudden pain on the outer side of the arm, it is recommended to go to the hospital to find out the cause.