The following reasons may exist to cause numbness in the left shoulder: 1. The most common reason for this symptom is that the local soft tissues are stimulated by cold or moisture and other external environments, which can trigger inflammation of the local musculofascia of the shoulder joint, or impaired blood circulation, which may cause painful symptoms and numbness. Depending on the location of the lesion, the farther the lesion is from the nerve, the closer it is to the dermatomal nerve, the more likely it is to cause numbness. The deeper the location of the lesion, the more obvious the pain will be; 2. Patients with cervical spondylosis will also have symptoms of numbness in the shoulder, cervical spondylosis is a series of symptoms produced by the stimulation of the cervical nerve, cervical artery, and even the spinal cord of the cervical segment by osteophytes. There are individual differences in the kind of symptoms that occur, with some people experiencing pain and others simply experiencing numbness. The cervical nerve is widely distributed in the shoulder, and once the cervical nerve is compressed, the symptoms of numbness in the shoulder may occur.