Opisthotonus or opisthotonos (from and ) 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.
Vivisection
Opisthotonus can be produced experimentally in animals by transection of the midbrain (between the superior colliculus and the inferior colliculus), which results in severing all the corticoreticular fibers.
Severe "arching" (hyperextension) occurs due to stimulus by the anterior reticulospinal tract caused by the loss of the balancing inhibitory counter-stimulus of corticoreticular fibers, which normally act upon the pons reticular formation.
Other causes
Opisthotonus is also described as a potential CNS symptom of heat stroke along with bizarre behavior, hallucinations, decerebrate rigidity, oculogyric crisis, and cerebellar dysfunction.
Opisthotonus is a symptom of "lavender foal syndrome", a lethal genetic disorder in horses.
Sir Rudolph Peters, in Oxford, introduced thiamine-deprived pigeons as a model for understanding how thiamine deficiency can lead to the pathological-physiological symptoms of beriberi. Feeding them polished rice caused opisthotonos, and if not treated they died after a few days. Administration of thiamine at the stage of opisthotonos led to a complete cure within 30 minutes. As no morphological modifications were observed in the brain of the pigeons before and after treatment with thiamine, Peters introduced the concept of a biochemical lesion.