What can cause the sudden loss of motor function in your right arm?

There are many possible causes for the arm paralysis (complete loss of motor function) and/or paresis (partial loss of motor function). Other signs and symptoms can help determine the precise cause; e.g. pain, presence or absence of sensory function (feeling in the arm), paresthesias (tingling), symptoms in other parts of the body (headaches, neck pain, etc.). History of illness is important; e.g. sudden or slow onset, presence or absence of trauma, etc. At the lowest level, the motor function of the arm is controlled by the brachial plexus (the "node" or network of nerve fibers near the axilla - armpit). Therefore, any trauma, impingement or severe disease of that plexus can be the cause. Once the disease of the plexus is ruled out, you move "upward" in the nervous system to determine the cause, e.g. trauma or disease of the cervical nerves (also include the T1 nerve), trauma or disease of the spinal cord on the cervical (neck) level). More rarely, such an isolated loss of function can be caused by a disease (e.g. tumor), trauma or stroke (bleeding or vessel blockage) in the brain, albeit it would have to be a very "pinpointed" disease or stroke. However, since the symptom is so isolated (only one arm) and in the absence of information about any other symptoms, it is statistically more likely to have been caused by the damage (due to trauma or disease) to the brachial plexus and/or C5 through T1 spinal nerves.