Pre-Existing Conditions and the Crumbling Skull Rule

The crumbling skull rule decides how much a defendant pays when the person they injured was already unwell. A defendant takes the victim as they find them, but is not required to pay for a decline that was going to happen regardless. Working out which part of the present condition belongs to the accident is one of the most contested questions in an injury trial.

The Thin Skull Rule Comes First

If a plaintiff is unusually vulnerable, and the accident makes that vulnerability produce a worse injury than an ordinary person would suffer, the defendant pays for the whole of it. A person with a fragile spine who is left disabled by a low speed collision recovers for the disability, not for the collision an average person would have walked away from. Vulnerability is not a discount.

The Crumbling Skull Rule Sets the Limit

The crumbling skull rule is the other side of that principle. Where the plaintiff already had a condition that was going to worsen on its own, the defendant is liable for the difference the accident made and not for the state the plaintiff would have reached anyway. The Supreme Court of Canada drew the distinction in Athey v Leonati, which remains the leading authority and is reported on CanLII.

The Difference Is Proved with Old Records

Applying the rule requires a baseline. Pre-accident clinical records, imaging and employment history show what the plaintiff’s condition and function were before the accident, and the defence will obtain them. Where those records show years of the same complaint the general damages award falls. Where they show none it rises. The medical records chapter explains how that disclosure works.

A Measurable Risk, Not a Possibility

A defendant does not earn a reduction by suggesting the plaintiff might have deteriorated at some point. The reduction depends on evidence of a measurable risk that the pre-existing condition would have produced the same loss without the accident. Speculation does not lower the number. Where an accident aggravated a settled old injury rather than accelerated a degenerative process, the aggravation chapter is the closer fit.

Where This Argument Arises Most

Degenerative disc disease, earlier concussions, prior back complaints and previously treated psychological conditions produce most of these arguments. The firm meets them on car accident and motorcycle accident claims, and the wider assessment framework is set out in how courts assess general damages.