The symptoms, limitations and overall impact of an injury will vary significantly from one person to the next. This is particularly apparent when a person has pre-existing injuries which can exacerbate the accident injury. Unfortunately, it is not uncommon for a plaintiff seeking compensation to have been injured in a previous motor vehicle accident or slip and fall. Having a pre-existing injury can also make a person more susceptible to injury in the future. While a pre-existing injury may impact a current claim, it is important to understand how these factors interact so that compensation is not unknowingly forfeited. 

How might a pre-existing injury impact a current claim?

There are various factors which can impact how a plaintiff’s pre-existing injury will affect a subsequent personal injury claim, such as:

• When the pre-existing injury first occurred;
• The stability and prognosis of the pre-existing injury;
• The impact of the current injury;
• The connection between the pre-existing injury and current injury; and 
• The plaintiff’s overall medical history.  

The laws regarding pre-existing injuries in Alberta are complex, particularly when it must be determined whether a pre-existing condition has interfered with the chain of caution. A compensation claim may be limited due to a pre-existing condition, depending on if it falls under the crumbling skull rule. Alternatively, an argument may be made that the thin skull rule should apply, and compensation may not be affected. 

The “thin skull rule”

If a plaintiff has suffered an injury due to someone else’s negligence, and the plaintiff’s pre-existing injury worsened the consequences of the recent injury, is the negligent person responsible for the entirety of the consequences? Sometimes, the answer is yes based on the “thin skull rule.” 

In Athey v. Leonati, the Supreme Court of Canada identified a thin-skull plaintiff as an individual who has a pre-existing condition or vulnerability to injury which is affected by a negligent act of someone else and would otherwise not have occurred otherwise. Therefore, the negligent individual is “liable for the plaintiff’s injuries even if the injuries are unexpectedly severe owing to a pre-existing condition.” 

It is important to note that compensation for the plaintiff cannot be restricted simply because they had a pre-existing injury or condition which made them more susceptible to the accident injury. 

Plaintiff must establish that the defendant’s negligent action was part of the cause of an injury

Application of the thin skull rule can become more complex when attempting to account for active pre-existing injuries caused by a prior tortious (negligent) act. This issue may arise when a plaintiff who was injured is still experiencing symptoms of the first injury when a second accident overlays symptoms similar or identical to the first injury. 

The Supreme Court of Canada provided guidance on this matter in Athey v. Leonati, stating that “it is not now necessary, nor has it ever been, for the plaintiff to establish that the defendant’s negligence was the sole cause of the injury.” The Court also noted that other background events could often contribute to the injury occurring. Therefore, “as long as a defendant is part of the cause of an injury, the defendant is liable.”   

However, when two different accidents create separate injuries and symptoms, the courts will normally consider the resulting injuries separately and appropriate compensation for each injury separately. 

Crumbling skull rule: a caveat

The crumbling skull rule contrasts the thin skull rule by limiting the plaintiff’s compensation where they were injured due to the defendant’s negligent act, but they had a pre-existing condition which would have impacted them eventually, regardless of whether the defendant’s act had occurred or not.

In these cases, courts must consider whether the negligent act triggered or precipitated a pre-existing condition or injury which would have happened anyway. While a plaintiff can still be entitled to compensation, it will typically only be calculated for up to the point where the pre-existing injury or condition would have impacted the plaintiff on its own. 

In Athey v. Leonati, the Court held that a reduction in compensation will be considered if there was a “measurable risk that the pre-existing condition would have detrimentally affected the plaintiff in the future, regardless of the defendant’s negligence.” In other words, an injury caused by the defendant may have only accelerated the degenerative process of the plaintiff’s pre-existing injury.

Which rule applies to which circumstances?

As every personal injury case is different and turns on its own facts, determining which rule applies will be based mainly on medical evidence. A general rule of thumb is that if the evidence supports the fact that a pre-existing condition made the plaintiff more vulnerable to tortious injury, the thin skull rule may apply. However, if, on a balance of probabilities, it can be found that the plaintiff’s pre-existing condition would have occurred “but for” the defendant’s negligent act, and the accident merely caused this to happen sooner, then the crumbling skull rule will likely apply.

Due to the intricate nature of these rules, it is important to speak with an experienced personal injury lawyer who can evaluate the likelihood of either of these rules applying to your unique circumstances. 

Cuming & Gillespie LLP in Calgary Helps Albertans Understand Their Rights After an Accident 

At Cuming & Gillespie LLP, our personal injury lawyers know that injury law can be complicated, and the consideration of pre-existing injuries can add to the intricacy of a claim. Therefore, it is important to obtain the advice of a knowledgeable personal injury lawyer and tell them about any relevant pre-existing conditions at the outset of the claim to ensure that you are not wrongly limiting your compensation. Our personal injury team appreciates that each case, and each client, is unique, so we utilize our experience and resources to help you recover the compensation you deserve. If you have been involved in an accident and are concerned about the impact of your pre-existing injuries on your claim, contact us online or call us at 403-571-0555 to speak with a member of our team.