Does the defendant have to compensate me for a pre-existing condition that was made worse?

What the Law Says

Canadian tort law does not require a statute to establish liability for worsening pre-existing conditions—the principle is judge-made and grounded in fairness and causation. While no specific federal or provincial statute defines the 'thin skull rule', courts apply it consistently under the common law of negligence.

The law holds that a defendant must 'take their victim as they find them.' This means if your pre-existing condition (e.g., arthritis, prior back injury, or epilepsy) made you more vulnerable to harm, the defendant is still fully liable for the injury caused—even if a person without that condition would have suffered less.

Liability depends on whether the defendant’s wrongful act was a 'material contribution' to the harm—and whether the harm would not have occurred 'but for' that act. This is known as the 'but for' test of causation.

Importantly, the law distinguishes between the 'thin skull' (a pre-existing vulnerability that doesn’t worsen on its own) and the 'crumbling skull' (a condition that would have deteriorated anyway). Only the former triggers full liability for the *incremental* worsening caused by the defendant.

What Courts Have Said

The Supreme Court of Canada clarified how pre-existing conditions affect compensation in Athey v. Leonati—a landmark ruling still applied across all Canadian provinces.

What to Do

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