There are several possible causes of pain in the lateral side of the left arm: first, shoulder disease, when the shoulder joint has frozen shoulder or rotator cuff tears, there can be pain in the lateral side of the arm, often in the distal side of the arm, at this time, the relevant X-ray and shoulder MRI should be performed for diagnosis and treatment; second, cervical spondylosis, when cervical spondylosis compresses the nerve root, there can be pain in the lateral side of the arm. Third, myofasciitis, patients with myofasciitis can experience pain on the medial side of the scapula, which can be radiated to the ipsilateral shoulder and upper limb, with symptoms of pain, soreness and weakness, especially at night when the pressure is applied and when the pain is more pronounced when the patient is cooled or tired. If the pain is more pronounced at night when the pressure is applied, or when the pain is cold or tired, it is necessary to conduct relevant investigations and tests to exclude other diseases for diagnosis.