Last updated: 2026-05-16 · Reviewed by: HurtCallMax Editorial Team · Reading time: 10 min · General information about Alberta personal-injury damages and causation. Not legal advice for your specific case. Call 780-900-6022 for a free case review.

Bottom line: The two most heavily-litigated principles in Alberta personal-injury law on the question of pre-existing conditions are the thin-skull rule (the defendant takes the claimant as they find them) and the crumbling-skull rule (the defendant is not liable for damage that would have happened anyway). The Supreme Court of Canada settled the modern Canadian framework in Athey v Leonati, [1996] 3 SCR 458. The thin-skull rule produces full recovery for vulnerabilities that the collision activated; the crumbling-skull rule reduces damages for conditions that were already deteriorating independent of the collision. The distinction is fact-driven and routinely worth tens or hundreds of thousands of dollars per case. The single most important piece of evidence is the medical timeline showing whether the pre-existing condition was asymptomatic immediately before the accident.

1. The thin-skull rule (Smith v Leech Brain)

The thin-skull rule originated in Smith v Leech Brain & Co Ltd, [1962] 2 QB 405. A worker received a minor burn injury from molten metal at work. The burn would have been trivial except that the worker had a pre-existing pre-cancerous condition that, triggered by the burn, developed into terminal cancer. The English court held that the employer was liable for the full extent of the cancer, not just the original burn. The rationale: a tortfeasor must take the victim as they find them, including pre-existing vulnerabilities.

In modern Alberta personal-injury practice, the thin-skull rule applies frequently:

• A claimant with a quiescent prior disc bulge that was managed without symptoms for years, then activated by a collision, recovers for the full chronic-pain consequence even though a person with a healthy spine would not have suffered the same outcome.
• A claimant with prior depression that had been in remission for five years, then triggered into a major depressive episode by a collision, recovers for the full mental-health consequence.
• A claimant with a prior mild traumatic brain injury that had fully resolved, then exacerbated by a second TBI from a collision, recovers for the full cognitive deficit.

The defining feature is that the pre-existing vulnerability was not actively producing symptoms or impairment at the date of the accident. The vulnerability was present; the impairment was not. The collision is what converted vulnerability into impairment.

2. The crumbling-skull rule (Athey v Leonati)

The crumbling-skull rule originated in Athey v Leonati, [1996] 3 SCR 458, the leading Canadian case on causation in tort. Mr Athey suffered a back disc herniation that the trial judge attributed in part to two motor-vehicle collisions and in part to a pre-existing degenerative back condition. The Supreme Court held that the defendants are liable for the part of the loss caused by the tort but NOT for the part that would have occurred anyway due to the pre-existing condition.

The crumbling-skull rule applies when:

• The pre-existing condition was already producing symptoms or impairment at the date of the collision; AND
• The condition would have continued to deteriorate even without the collision.

Examples from reported Alberta cases:

• A claimant whose physiotherapy treatment records in the two years before the collision show ongoing chronic back pain — the same body region injured in the collision — has a crumbling-skull problem. The defendant is liable for the worsening attributable to the collision, not the underlying baseline.
• A claimant whose pre-accident medical records show ongoing depression treatment with active prescriptions has a crumbling-skull problem for any post-accident mental-health claim. The defendant is liable for the worsening attributable to the collision.
• A claimant with a documented prior catastrophic injury that has stable but ongoing impairment cannot recover for that baseline impairment again, even if the new collision aggravates it.

3. The “but for” test — how Athey framed causation

Athey v Leonati clarified the Canadian causation test: would the loss have occurred but for the defendant's negligence? If yes, the defendant is fully liable. If the loss would have occurred anyway, the defendant is not liable for that loss.

In personal-injury practice, this typically produces an apportionment exercise:

• The plaintiff establishes that the collision was a material contributing cause of the current condition (not the only cause, just a material one).
• The defendant then tries to prove that some part of the current condition would have arisen anyway from the pre-existing baseline.
• The court or settling parties apportion the loss: 100% for the part caused by the collision, less for the part attributable to the pre-existing condition.

Apportionments in reported Alberta cases vary widely: 0% reduction (clean thin-skull) is common; 25% to 50% reductions appear in crumbling-skull cases with documented prior impairment; reductions above 50% are rare and require strong evidence that the pre-existing condition was actively and progressively deteriorating.

4. Why this is the most heavily-litigated point in Alberta PI files

The thin-skull / crumbling-skull distinction matters because the reduction is multiplicative across every head of damage. A 30% crumbling-skull reduction applies to pain-and-suffering, past wage loss, future wage loss, future care, housekeeping — the whole settlement. A $300,000 file becomes $210,000 with a 30% reduction. The financial stakes for the insurer to push the crumbling-skull argument are obvious.

Equally obviously, the financial stakes for plaintiff counsel to resist the argument are large. This is why the medical-records discovery in Alberta personal-injury practice is so contested. Insurers routinely request 10+ years of pre-accident medical history. Plaintiff counsel routinely refuses, offering 2-3 years of records limited to body regions injured in the collision. The compromise is negotiated on every file.

5. The medical evidence that wins or loses these arguments

Reviewing reported Alberta Court of King's Bench decisions on CanLII, the medical evidence that drives the outcome falls into a consistent pattern:

• The “asymptomatic before the accident” question. The most powerful evidence for a thin-skull finding is medical records showing the claimant was NOT seeing a doctor for the affected body region in the months immediately before the collision. A 12-month treatment gap before the collision is the gold standard.
• The “deteriorating trajectory” question. The most powerful evidence for a crumbling-skull finding is a documented pattern of worsening symptoms in the months immediately before the collision — increasing physiotherapy frequency, escalating pain medication, new diagnostic imaging.
• The independent medical examiner (IME) report. The defendant's IME will typically frame any prior medical reference as evidence of a crumbling skull. Plaintiff counsel responds with an independent medical opinion that frames the same records as evidence of an asymptomatic baseline.
• Treating physician testimony. A treating family physician who has known the claimant for years and can attest to the asymptomatic baseline before the collision is the single most credible witness in a thin-skull case.

6. Practical implications for an Alberta claimant

• Do not lie about pre-existing conditions to the insurer or the IME. Insurers obtain medical records and find the prior treatment. A lie destroys credibility on the whole file. The right strategy is honesty about the existence of prior conditions plus emphasis on the asymptomatic gap immediately before the collision.
• Do not sign a broad medical-records authorization without legal review. Limit the records authorization to relevant body regions and a reasonable lookback period.
• Continue treatment for the new injuries continuously. Gaps in post-accident treatment are read by insurers as evidence of recovery, which undermines causation and quantum.
• Get a treating physician opinion early. Ask your family physician to document, in writing, that the pre-existing condition was asymptomatic immediately before the collision and that the current symptoms are causally related to the accident.
• Talk to a personal-injury lawyer early. The crumbling-skull argument is the single most expensive issue plaintiff counsel handles. Counsel brought in within the first 90 days can build the medical-evidence record correctly from the start, rather than trying to repair damage at month 18.

For a free case review with a vetted Alberta personal-injury lawyer, call 780-900-6022 or submit the free case review form.

Sources: Athey v Leonati, [1996] 3 SCR 458; Smith v Leech Brain & Co Ltd, [1962] 2 QB 405; Contributory Negligence Act, RSA 2000, c C-27; Alberta Court of King's Bench published decisions on CanLII (sampling 2020-2026). The Athey decision remains the leading Canadian authority on causation in tort and is cited in nearly every contested Alberta personal-injury case involving pre-existing conditions.

Related: Edmonton Car Accident Settlement Amounts · Alberta's Two-Year Limitations Clock · Pain and Suffering Damages and the SCC Trilogy Cap · Section B Accident Benefits Explained · How Edmonton Insurance Adjusters Reduce Settlements.