The crumbling skull rule is a common law concept that defendants in tort cases invoke to limit their liability.
The rule reflects the notion that defendants being sued by a plaintiff who was in a deteriorating state of health at the time of injury should not be held responsible for paying damages to return the plaintiff to a state of perfect health.
Tort law is premised on the idea that a person who is injured by the wrongdoing or negligence of another should be made whole or, in other words, paid by the person responsible for the injury.
There are times, however, when the injured person was already in deteriorating health, or was not whole, at the time of injury. So, the crumbling skull rule holds the defendant responsible for only returning the injured to the state in which the injured existed before the injury, and not to complete health.