Opisthotonus refers to 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.



It is an extrapyramidal effect and is caused by spasm of the axial muscles along the spinal column. 



It is a symptom of some cases of severe cerebral palsy and traumatic brain injury or severe muscular spasms associated with tetanus, severe acute hydrocephalus, poisoning, and drowning.



It is more pronounced in infants, and in a neonate may be a symptom of meningitis, tetanus, severe kernicterus, or maple syrup urine disease. 



Opisthotonus can be triggered by any attempt at movement: smiling, feeding, speech, or by involuntary movement, such as seizures. 



Opisthotonus can sometimes be seen in lithium intoxication, and it is a rare extrapyramidal side effect of phenothiazines, haloperidol, and metoclopramide.



Opisthotonus with the presence of the risus sardonicus is also a symptom of strychnine poisoning and hydrogen cyanide poisoning.



It is seen in drowning victims.



It can be reproduced 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 hyperextension occurs due to stimulus by the anterior reticulospinal tract caused by the loss of the balancing inhibitory counter-stimulus of corticoreticular fibers.



Opisthotonus is a potential CNS symptom of heat stroke.




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