What is the test for determining whether a plaintiff is a "crumbling skull" rather than a thin skull?

On the issue of causation, defendants’ counsel agrees that Athey v. Leonati, 1996 CanLII 183 (SCC), [1996] 3 S.C.R. 458, 140 D.L.R. (4th) 235, sets out the general test but it is only a general and not the conclusive test for causation. Quoting from paragraph 35 of Athey, counsel pointed out that the defendant need not put the plaintiff in a position better than his or her original position and although a defendant is liable for the injuries caused, the defendant need not compensate the plaintiff for any debilitating effects of a pre-existing condition which the plaintiff would have experienced anyway. The defendants submit that the plaintiff was a “crumbling skull” rather than a “thin skull” and any injuries that may have been suffered as a result of the accident were additional to existing damage. In these circumstances, the defendants are only liable for any additional damage.