Opisthotonus can be produced experimentally in animals by transection of the midbrain (between superior and inferior colliculus) which results in severing all the corticoreticular fibers. Hyperextension occurs because facilitation of anterior reticulospinal tract due to removal of inhibitory corticoreticular fibers to the pons reticular formation. It has been shown to occur naturally only in dinosaurs and birds, pterosaurs, and placental mammals.
Opisthotonus is more pronounced in infants. Opisthotonus in the neonate may be a symptom of meningitis or tetanus. This marked extensor tone can cause infants to "rear backwards" and stiffen out as the mother or nurse attempts to hold or feed them. Opisthotonus can be induced by any attempt at movement such as smiling, feeding, vocalization, or by seizure activity. A similar tonic posturing may be seen in Sandifer syndrome. Individuals with opisthotonus are quite challenging to position, especially in wheelchairs and car seats.
Opisthotonus can sometimes be seen in lithium intoxication.