What is the test for determining whether a plaintiff has a thin skull or crumbling skull injury?

In Filsinger v. ICBC, 2009 BCSC 232 at paras. 24-26 [Filsinger], Rice J. neatly articulated the two categories at paras. 24 to 26: The issue is whether this is a “thin skull” or a “crumbling skull” situation. Both address the circumstances of a pre-existing condition, and its effect upon the accident victim. The law is that the defendant need not compensate the plaintiff for any debilitating effects of a pre-existing condition if the plaintiff would have experienced them regardless of the accident: Athey v. Leonati, 1996 CanLII 183 (SCC), [1996] 3 S.C.R. 458 at para. 35, 140 D.L.R. (4th) 235. The court requires “a measurable risk” or “a real or substantial possibility and not speculation” that the pre-existing condition would have manifested itself in the future regardless of the plaintiff’s (sic) negligence. The measurable risk need not be proven on a balance of probabilities, but given weight according to the probability of its occurrence: Athey v. Leonati, at para. 27. The injury is deemed “thin skull” when there is a pre-existing condition that is not active or symptomatic at the time of the accident, and that was unlikely to become active but for the accident. A “crumbling skull” injury is also one where there is a pre-existing condition, but one which is active or likely to be active. If the injury is proven to be of a think skull nature, then the defendant is liable for all of the plaintiff’s injuries resulting from the accident. If it is of a crumbling skull nature, then the plaintiff is liable only to the extent that the accident caused an aggravation to the existing condition.