|Opisthotonus in a patient suffering from tetanus. Painting by Sir Charles Bell, 1809.|
Opisthotonus or opisthotonos, from Greek roots, ὄπισθεν, opisthen meaning "behind" and τόνος tonos meaning "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. It has been shown to occur naturally only in birds and placental mammals, among existing animals; it is observed in some articulated dinosaur fossils.
Opisthotonus is a symptom of some cases of severe cerebral palsy and traumatic brain injury or as a result of the severe muscular spasms associated with tetanus. It can be a feature of severe acute hydrocephalus, poisoning, and drowning.
Opisthotonus is more pronounced in infants. Opisthotonus in the neonate may be a symptom of meningitis, tetanus, severe kernicterus, or the rare Maple syrup urine disease. This marked extensor tone can cause infants to "rear backwards" and stiffen out as the mother or nurse attempts to hold or feed them. Individuals with opisthotonus are quite challenging to position, especially in wheelchairs and car seats.
Drowning and the "dinosaur death pose"
Opisthotonus is seen with drowning victims – called the "Opisthotonic Death Pose". This pose is also common in complete dinosaur skeletal fossils and it has been suggested that this is due to the animal drowning or being immersed in water soon after death.
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.
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