What is the difference between a thin skull and a crumbling skull?

Our Court of Appeal noted the distinction between thin and crumbling skull plaintiffs in Dushynski v. Rumsey, 2003 ABCA 164, 327 A.R. 373, at para. 5: ...to determine the loss requiring compensation, it is necessary to determine the plaintiff's position before the tort and after. To achieve fairness in making this determination the courts have developed some guidelines for special situations including the "thin skull" and "crumbling skull" concepts. Unfortunately these labels often cause more confusion than clarity. Where the defendant's action triggers injury in a plaintiff with a latent condition, the defendant will be liable for the full amount of the damages because he has injured a "thin skull" person. Where the plaintiff is asymptomatic but has a degenerative disease or has a symptomatic existing injury, the defendant is liable only for the further injury his actions cause to the "crumbling skull" plaintiff.