According to the US National Institute of Health:
- Decorticate posture: "sign of damage to the nerve pathway between the brain and spinal cord".
- Decerebrate posture: "severe injury to the brain". (And brain stem damage according to the Wikipedia article above, that uses paper sources).
Are such brain damage irreversible or is there a chance, even slight, that the person recovers?
1- As this was asked in comments and chat: this does not mean that all kinds of brain damages lead to these postures. Maybe some don't. The question here is limited to the damages these two posture reveal. The reason for focusing on these two postures is that they have a special meaning in first-aid, when evaluating the conscious state of a victim.