Thin Skull and Crumbling SkullAthey v. Leonati,  3 SCR 458
Thin skull and crumbling skull cases deal with plaintiffs that have pre-existing medical conditions.
The thin skull rule makes the defendant liable for the plaintiff's injuries even if the injuries are unexpectedly severe owing to a pre-exisiting yet stable condition. The defendant must take the victim as they find them with whatever peculiar weaknesses and predispositions they might have, and is liable even though the plaintiff's losses are more dramatic than they would be for the average person.
The crumbling skull rule deals with a plaintiff that has an unstable pre-existing condition. The defendant need not compensate the plaintiff for the effects of their condition, which they would have experienced anyway. The defendant is liable for additional damage, but not the pre-existing damage.
Athey v. Leonati,  3 S.C.R. 458, 140 D.L.R. (4th) 235,  1 W.W.R. 97
Dulieu v. White & Sons,  2 K.B. 669
Owens v. Liverpool Corp.,  1 K..B. 394 (available in Vancouver and regional courthouse libraries)
Canadian Tort Law, 8th edtion (available in most BC Courthouse Libraries)
"Causation in Tort in Law: Back to Basics at the Supreme Court of Canada" Alberta Law Review (1997) 35: 1013-34 (available in HeinOnline and the Reading Room and available in print in the Vancouver and Kamloops courthouse libraries)
"The Test for Causation for Statutory Accident Benefits: Monk v. ING" The Advocates Quarterly, (2008) 34: 360-375 (available in the Vancouver Courthouse Library and in HeinOnline and the Reading Room)