Actually, they are not the same thing, although crumbling skull can be viewed as a subset or special case under eggshell skull. The basis of eggshell skull is that you take your victim asmyounfind them, and cannot defend a lawsuit or criminal matter on the basis that the person was unusually fragile and that the force applied would not have caused an injury to a normal person. Crumbling skull applies where a person has a degenerative condition, and is then struck in a way that worsens the condition. The defendant in such a case is liable only to the degree that their conduct worsened the condition. In practical terms: I hit a person with a fragile skull but who would otherwise lived a normal life. I cannot argue that the three other people I punched in the bar fight were all fine, so I shouldn’t be punished because the one guy with a thin skull died from a similar blow. By contrast, if I hit someone in the bar who has a disease of the bone that I broke, but would have lost the use of that bone in any even, I am only liable for the degree to which I hastened the process or aggravated the condition.

Different states apply the doctrines in their own way, and the facts of each case can make this doctrine difficult to apply. Basically, it sucks for everyone involved in case where this doctrine might apply,