Pre-Existing Conditions: The Aggravation Rule Explained

Almost every adult over thirty has something on a spine MRI. Degenerative changes, old sports injuries, arthritis, a bulging disc that never caused a day of pain — these are ordinary findings in ordinary people, and insurers know it. Which is why “the condition was already there” is the most reliably deployed argument in injury claims, and why understanding the legal answer to it matters more than almost anything else you can learn about your own claim.

Quick answer: In most states a defendant who worsens a pre-existing condition is liable for the worsening — the aggravation principle. Separately, the eggshell plaintiff rule means a defendant takes their victim as they find them and is liable for the full extent of the harm even if an unusually vulnerable claimant was hurt worse than an average person would have been. What you cannot recover for is the condition as it stood before the accident. The whole fight is over the delta, and the word that decides it is usually asymptomatic.

Two Legal Principles Doing Different Work

Aggravation. A condition already existed; the accident made it worse. The defendant is responsible for the worsening, not for the underlying condition. A person with an arthritic knee who could walk two miles before the collision and can now walk two blocks has a claim measured by that difference.

The eggshell plaintiff rule. A claimant was unusually susceptible to injury, and the same impact that would have bruised someone else caused serious harm. Here the defendant is liable for the full extent of the injury — susceptibility is not a discount. The classic formulation is that a defendant “takes their plaintiff as they find them.”

These overlap in practice, and a single claim frequently involves both. What matters is that neither one requires you to have been in perfect health before the accident — a standard almost no adult claimant could meet.

The Word That Decides Most Disputes: Asymptomatic

Here is the pivot the entire argument turns on. A degenerative finding that was asymptomatic — present on imaging, but causing no pain, no treatment, no limitation — and became symptomatic after the collision is a compensable aggravation. Not a defense. The insurer’s argument depends on conflating the condition existed with the condition was already hurting you, and those are entirely different facts.

A related but distinct fight shows up in accidental death and dismemberment insurance, where a pre-existing condition is used to argue the death resulted from “sickness” rather than an accident at all — see our guide to accidental death claims and what counts as an “accident” for how that causation dispute is fought differently.

What establishes asymptomatic status:

• No prior treatment history for that body part — no doctor visits, no physical therapy, no imaging, no medication
• Documented pre-accident activity level — the job you performed, the sports you played, the things you lifted and carried
• Employment records showing full duty without restriction
• Witness testimony from family, coworkers or teammates about what you could do before
• Prior imaging for comparison, where it exists — the single most decisive piece of evidence when it shows the finding absent or milder

The Three Causation Attacks, and What Answers Each

1. “The condition predated the accident.” Answered by the aggravation principle plus asymptomatic-status evidence above. This is the most common attack and, correctly handled, the most defensible.

2. “There’s a gap in treatment, so you must have recovered.” A gap between the accident and treatment — or in the middle of a course of care — is read as evidence that the injury resolved or was never serious. Sometimes there’s a real explanation (no insurance, caring for a family member, a provider waitlist, symptoms that genuinely improved and then relapsed). That explanation belongs in the medical record, stated at the time, not reconstructed for a demand letter two years later.

3. “Something else caused this.” A subsequent fall, a later car accident, a sports injury, or ordinary aging. The answer is the same tool as always: contemporaneous documentation tying symptom onset to the collision, and a treating physician willing to say so explicitly.

What a Causation Opinion Actually Needs to Say

A diagnosis is not a causation opinion. Insurers read the chart looking for a specific sentence, and its absence is treated as its own argument. What carries weight:

• It comes from a treating physician, ideally one who saw you early
• It states the mechanism explicitly — the collision caused or aggravated this condition
• It addresses the pre-existing finding directly rather than ignoring it, distinguishing what was there before from what changed
• It is stated within the medical record, not only in a letter written later at an attorney’s request

This applies across injury types. See how the same fight plays out in specific claims: herniated disc claims against degenerative disc disease, shoulder injury claims against age-related rotator cuff findings, and knee injury claims against pre-existing arthritis and meniscal wear.

Never Conceal a Prior Injury

The temptation is understandable and the consequence is severe. Insurers routinely obtain prior medical records, and a claimant who denied a prior back problem that then appears in those records has handed the defense something more valuable than the pre-existing condition itself: a documented reason to disbelieve everything else they said.

Disclose it, and frame it accurately. “I injured my back in 2019, completed six weeks of therapy, and had no symptoms or treatment for four years before this collision” is a stronger position than a denial that later collapses — and it is exactly the asymptomatic-interval framing the aggravation principle rewards.

This is also why a blanket medical authorization is worth refusing: it lets an insurer fish through your entire medical history for anything usable rather than the records actually relevant to the claimed injury. See dealing with insurance adjusters for what you are and are not obliged to provide.

The Independent Medical Examination

An IME is an exam by a physician the insurer selects and pays for. The name oversells the neutrality. In a pre-existing-condition dispute, the IME report is frequently the document that says your symptoms are degenerative rather than traumatic, and it becomes the insurer’s basis for reducing or denying the claim.

Whether you must attend depends on where the demand comes from: your own insurer can generally require one as a condition of first-party benefits (a UM/UIM or PIP claim), and a court can order one once litigation is underway. A third-party liability insurer generally cannot compel you to attend one before a lawsuit is filed. Ask an attorney which situation you’re in before agreeing.

Practical Steps

1. Tell your treating doctor about the prior condition and ask them to document what changed — new symptoms, new limitations, new findings.
2. Ask explicitly for a causation statement in the chart, addressing the pre-existing finding rather than ignoring it. It is rarely volunteered.
3. Locate prior imaging for comparison if any exists — this can resolve the entire dispute.
4. Document your pre-accident baseline while it’s easy: job duties, activities, the absence of treatment.
5. Do not sign a blanket medical authorization; limit disclosure to records relevant to the claimed injury.
6. Never minimize or conceal a prior injury — the credibility cost exceeds anything the disclosure costs you.
7. Explain any treatment gap in the record, contemporaneously, not afterward.

Sources & Further Reading

• Restatement (Second) of Torts § 461 — the eggshell plaintiff principle: a defendant is liable for harm greater than expected because of the victim’s unforeseeable physical condition
• State pattern jury instructions on aggravation of a pre-existing condition, which most states publish and which set out how a jury is actually told to apportion between the prior condition and the accident
• See our guides to how insurance companies calculate settlements for how causation disputes affect valuation, dealing with insurance adjusters for the medical authorization question, a second accident during a pending claim for this same fight when the “prior condition” is itself an open claim, and the eggshell plaintiff rule glossary entry