Opisthotonus

Opisthotonus or opisthotonos (from Ancient Greek: ὄπισθεν, romanized: opisthen, lit. 'behind' and τόνος, tonos, 'tension') is a state of severe hyperextension and spasticity in which an individual's head, neck and spinal column enter into a complete "bridging" or "arching" position.

This extreme arched pose is an extrapyramidal effect and is caused by spasm of the axial muscles along the spinal column. Among extant animals it naturally occurs in birds, snakes suffering from advanced boid inclusion body disease, and placental mammals; it is also observed in some articulated dinosaur fossils.