According to WebMD, pain in the left arm or shoulder can indicate an injury, muscle strain, a degenerative disease, an instability in the joint, tendonitis, pinched nerves or a tumor. These causes occur equally on the left and right sides; left side pain does not indicate anything different from right side pain. Intermittent pain in either arm or shoulder may indicate a heart attack, especially if associated with chest pain.
As the USC Center for Spinal Surgery points out, pain that runs from the neck through the shoulder and down the arm with no related weakness or numbness indicates cervical radiculopathy, commonly known as a pinched nerve. This can be caused by a herniated disc, a bone spur, an infection or a tumor.
WebMD explains that neck and shoulder pain is typically treated with anti-inflammatory pain relievers, heat or ice or corticosteroid injections. Exercise or physical therapy is sometimes useful to relieve pain. Surgery is sometimes required if the pain involves the spinal cord or nerve roots.
According to WebMD, diagnosis of the specific causes of left neck and shoulder pain is often done using X-rays, which can reveal the presence of fractures, arthritis, slipped discs, tumors and problems involving narrowing and instability of the spinal column. MRIs are sometimes performed to diagnose issues with the nerves, ligaments and tendons, and electromyography, or nerve conduction velocity tests, also help diagnose pain, numbness and tingling.