What is the test for a tortfeasor who causes a plaintiff to suffer from a fractured skull as a result of their negligence?

The most basic principle of tort law is that the plaintiff must be placed in the position he or she would have been in if not for the defendant's negligence, no better or worse. Tortfeasors must take their victims as they find them, even if the plaintiff's injuries are more severe than they would be for a normal person. This is the “thin skull” rule. However, the defendant need not compensate the plaintiff for any debilitating effects of a pre-existing condition which the plaintiff would have experienced anyway, sometimes called the “crumbling skull” rule, Athey v. Leonati, at paras. 32-35.