Numbness on the left arm could be an indication of different diseases and conditions include Lyme disease, HIV/AIDS, alcoholism, multiple sclerosis, syphilis, Vitamin B12 deficiency, vasculitis, spinal cord injury, Sjogren's syndrome, peripheral neuropathy, Raynaud's disease, ganglion cyst, amyloidosis among others. The numbness occurs due to a damage on the nerve or a part of the nerve in the arm.
Diseases that attack the peripheral nerve, such as diabetes, can also cause the numbness on the arm. Numbness on the arm may begin suddenly with no early noticeable symptoms. It may be followed by other symptoms including weakness, dizziness, severe headache, confusion and difficulty in talking, as stated by Mayo Clinic.
It is necessary to seek medical attention if the condition persists or spreads to other parts of the body. The numbness may come and go or may affect one part of the arm, for example, a finger. Numbness caused by carpal tunnel syndrome can get better by taking certain exercises.
Numbness of the arm can only be treated by treating the underlying cause. The doctor will first need to know the main cause of the numbness prior to making any recommendations. Blood test and imaging test may be required during a doctor's visit.