Causes
It is most often observed as a sign of tetanus. Poisoning with strychnine may result in a risus sardonicus.
In 2009 scientists at the University of Eastern Piedmont in Italy claimed to have identified hemlock water dropwort (Oenanthe crocata) as the plant responsible for producing the sardonic grin. This plant is the most-likely candidate for the "sardonic herb," which was a neurotoxic plant used for the ritual killing of elderly people in pre-Roman Sardinia.
Read more about this topic: Rictus