Sardonicus

Risus sardonicus or rictus grin is a highly characteristic, abnormal, sustained spasm of the facial muscles that appears to produce grinning. It may be caused by tetanus, strychnine poisoning, or Wilson's disease, and has been reported after execution by hanging. The condition's name derives from the appearance of raised eyebrows and an open "grin", which can appear sardonic or malevolent to the lay observer, displayed by those experiencing these muscle spasms.