It is also known as myoclonus or myoclonic jerk, which is the medical word for that particular involuntary muscle twitch. These jerks are unexpected and involuntary, thus you have no control over them. Some individuals associate them with feeling frightened or as though they're falling. Others may feel pain from the movement of these muscles. Myoclonus can be caused by many different conditions including epilepsy, brain tumors, vitamin B12 deficiency, multiple sclerosis, narcolepsy, and toxic substances such as alcohol or certain medications. The most common cause of myoclonus among adults is epilepsy. It is estimated that about 70 percent of people with myoclonus have some form of neurological disorder such as dementia, Parkinson's disease, or multiple sclerosis. The other 30 percent of patients do not have a clear-cut medical condition that can be diagnosed based on physical examination and tests performed so far. These individuals are called "idiopathic" meaning "no apparent cause". Idiopathic myoclonus usually affects both sides of the body at the same time. It can also appear in groups - bursts of several minutes duration during which time both arms or legs will jerk repeatedly. This is called "biphasic" myoclonus. Or only one side of the body may jerk, such as when reading or writing and then suddenly moving or shaking that arm. This is called "unilateral" myoclonus.