Myoclonic jerks?
Myoclonus may develop in response to infection, head or spinal cord injury, stroke, brain tumors, kidney or liver failure, lipid storage disease, chemical or drug poisoning, or other disorders.
http://www.ninds.nih.gov/disorders/myoclonus/detail_myoclonus.htm
If anything it would be stimulus-sensitive myoclonus. It is triggered by a variety of external events, including noise, movement, and light. Surprise may increase the sensitivity of the individual.