How Pre-Existing Conditions Impact Personal Injury Claims in Ontario

How Pre-Existing Conditions Impact Personal Injury Claims in Ontario

In personal injury claims, no two plaintiffs are ever truly the same. Behind every injury is a unique person with their own medical history, vulnerabilities, and circumstances.

This is where two (often confused) legal doctrines take center stage, those legal doctrines being the thin skull rule and the crumbling skull rule.

These two principles shape how damages are assessed when pre-existing conditions come into play. While at first glance these two terms may sound like medical metaphors from a hospital drama series, they are in fact powerful doctrines grounded in both fairness and policy.

The Thin Skull Rule: Taking the Victim as You Find Them

The thin skull rule demands that a defendant must “take their victim as they find them,” meaning that the defendant remains fully liable for the plaintiff's injuries, even if those injuries are unexpectedly severe due to a pre-existing fragility, whether physical or psychological.

It is confirmed in SCC cases like Athey v. Leonati and ONCA cases like Cotic v. Gray.

The law does not allow defendants to escape responsibility because the plaintiff happened to be more vulnerable than most.

In contrast, the crumbling skull rule accounts for plaintiffs whose injuries would have occurred over time anyway. If a condition was bound to worsen, damages may be reduced, but only if clear evidence supports the decline was inevitable.

Ontario Law on Thin Skull: Fairness Even When It's Harsh

In Ontario, the thin skull rule is a fundamental doctrine that embodies fairness, even when fairness feels harsh to the defendant.

Taking the victim as you find them dictates that a defendant cannot escape liability simply because the plaintiff was unusually vulnerable.

Why Fragility Doesn’t Equal Fault in Ontario Personal Injury Law

This means that if a person suffers more severe injuries than one might expect from a minor incident, the defendant is still responsible for the full extent of the harm caused.

For example, imagine a fender-bender that normally results in mild whiplash, but in this case a serious neurological episode is triggered due to a rare pre-existing condition.

Under Ontario law, the defendant is still fully liable. You might ask “why?” Well because fragility doesn’t equal fault. Ontario law refuses to penalize a plaintiff for being more susceptible to injury than the average person.

The Crumbling Skull Rule: When Pre-Existing Conditions Limit Damages

While the thin skull rule demands full accountability from defendants, the crumbling skull rule introduces a necessary boundary.

This doctrine acknowledges that not all harm attributed to an accident is entirely caused by it—some injuries may have unfolded naturally due to pre-existing conditions, regardless of the defendant’s conduct.

How Ontario Courts Apply the Crumbling Skull Rule

In Ontario, courts apply the crumbling skull rule where there is credible evidence that the plaintiff’s condition was already deteriorating and would have continued to do so independently of the incident.

The defendant remains liable for the acceleration or exacerbation of the decline, but not for the decline itself.

For example, if a plaintiff had degenerative disc disease that would have impaired them within five years, but a car accident hastened that process, damages may be adjusted to reflect only the portion of harm legally attributed to the defendant.

The Burden of Proof: Why Defendants Must Prove Natural Decline

This is not an easy escape hatch for defendants. The burden is on them to establish—through medical or factual evidence—that the plaintiff’s condition would have worsened regardless. Speculation is not sufficient.

How Plaintiffs Can Strategically Leverage the Thin Skull Rule

Understanding how these doctrines operate is critical for plaintiffs, especially when pre-existing conditions are involved.

In Ontario, both the thin skull and crumbling skull rules frequently arise in personal injury litigation, and distinguishing between them can heavily influence how damages are argued and assessed.

Building a Strong Case Around the True Cause of Injury

From a strategy perspective, plaintiffs benefit from framing their vulnerability through the lens of the thin skull rule: that they were harmed more severely because of who they are, not because they were already on a path toward injury.

Medical records, expert opinions, and functional baselines before the incident become key in demonstrating that the accident was the true turning point—not an inevitable decline.

Anticipating Defense Tactics and Strengthening Your Position

Plaintiffs should anticipate defense attempts to invoke the crumbling skull rule, particularly when there’s any history of injury or illness.

The goal of defense counsel is to limit liability by attributing as much harm as possible to natural degeneration or unrelated health issues.

A plaintiff’s legal team must therefore proactively establish a strong causal link between the incident and the resulting injuries, while challenging any assertion that the plaintiff was already “crumbling.”

Turning Vulnerability into Strength in Personal Injury Claims

Properly handled, the presence of a pre-existing condition can actually strengthen a plaintiff’s position under the thin skull rule.

It reinforces the idea that the defendant is liable for the victim they harmed—not the one they wish they had.

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