Crumbling skull rule is a legal theory, similar to the thin skull rule, which limits a tort defendant’s exposure to a plaintiff’s injuries to the plaintiff’s condition at the time of the tort.
The distinction between a thin skull case and a crumbling skull case is that in the former, the skull, although thinner than the average skull, is in a stable condition before the accident and, but for the accident, would have remained so. The crumbling skull is where the skull, whether thick or thin, is not in a stable condition before the accident but in a state of continuing deterioration which the accident has merely accelerated.
In the crumbling skull, the defendant is not to be held responsible for the whole of the post-accident condition of the skull. Further, the defendant's actions are not to be treated as having been the sole cause of the entire post-accident condition of the plaintiff. The defendant's actions are viewed as merely an aggravating cause. Depending upon the circumstances, the degree of aggravation may range from very substantial to very slight.