Doctors looking at a xray of a skull discussing Thin Skull vs. Crumbling Skull Cases

Thin Skull vs. Crumbling Skull Cases

“The tortfeasor must take his or her victim as the tortfeasor finds the victim”: Athey v. Leonati, [1996] 3 SCR 458 para 34. That is one of the fundamental rules of compensation in tort law. The rule also plays when dealing with causation (see “Causation”). It is typically invoked when a victim suffers from a pre-existing condition, of which there is a variety of possible scenarios. The plaintiff’s condition can be active or dormant. In the case of an active condition, the victim’s life is already affected; in the case of a dormant condition, while the victim’s life is not affected at the time of the accident, there is a likelihood that it would eventually become so.

The case law typically refers to a condition that makes a victim more vulnerable to injuries as a ‘thin skull’, and as a ‘crumbling skull’ the one that would eventually affect the victim even in the absence of an accident. Both can be active or dormant at the time of an accident. Schematically, the various scenarios can be represented thusly:

When dealing with causation, a pre-existing condition can interfere in the tort → accident → injury chain reaction. It may be more difficult to establish that a negligent action is the cause of the ailment if the victim was already suffering from a similar active condition. It may also be as challenging if, although nothing was affecting the victim then, it is shown that a dormant condition would eventually have affected the victim in the same way notwithstanding the accident. A court of law might be tempted to attribute the victim’s health issues to the pre-existing condition. That is where the credibility of the plaintiff and strength of the evidentiary record are the most influential: see Hickey v. Savoie, 2012 NBQB 120 paras 48-52.

‘Thin Skull’ or ‘Glass Jaw’

At the stage of compensation, the thin skull and crumbling skull rules play very different roles. As discussed in more details below, while the thin skull will tend to increase the quantum of damages, the crumbling skull will tend to reduce it.

In essence, this is the scenario of the truly vulnerable victim. This is when the effects of an accident become more difficult to predict. A mere fall could render the victim totally disabled, or some unanticipated consequences of that nature. The tortfeasor will be responsible for all the damages caused by the negligent act, even if they would not generally follow in the case of a normal person. In situations of dormant conditions, the thin skull victim is treated like any other except for the quantum of damages that takes into account the magnified effect on his or her injuries. Of course, where the effects on a victim’s life and work are more severe, the award of non-pecuniary and pecuniary compensation will generally be greater.

One of the challenges, when dealing with the thin skull rule, is accounting for active pre-existing conditions that have been caused by a prior tortious act. In other words, a victim is injured and still has sequelae when another accident overlays a new set of symptoms either identical or very similar to the existing one. To what extent, if any, can a second tortfeasor be responsible for the victim’s ailment? In Athey, supra paras 17 & 19, the unanimous bench of the Supreme Court of Canada explained that a tortious act does not need to be the sole cause of the injury in order to justify compensation:

[17]  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. There will frequently be a myriad of other background events which were necessary preconditions to the injury occurring. … As long as a defendant is part of the cause of an injury, the defendant is liable, even though his act alone was not enough to create the injury.


[19]  The law does not excuse a defendant from liability merely because other causal factors for which he is not responsible also helped produce the harm …. It is sufficient if the defendant’s negligence was a cause of the harm ….

[Underlined in the original]

In subsequent cases, some defendants tried to limit the ratio of the court pre-existing conditions related to non-tortious acts, but to no avail. Namely, in E.D.G. v. Hammer, [2003] 2 SCR 459, a case dealing with a girl sexually abused by a school janitor before being also abused by members of her extended family, the defendant argued that liability should not compensate the torts committed subsequently. Chief Justice McLachlin clarified, albeit in obiter, that the “principle is not confined to cases involving non-tortious preconditions. It applies to any case in which the injuries caused by a number of factors are indivisible” (para 31). In Blackwater v. Plint, [2005] 3 SCR 3, a child was abused at home before being also abused in a residential school. Again, the public authorities tried to split liability between the various sets of tortious acts. Regarding the pre-existing condition from the first abuses, the unanimous bench stated (paras 78-79):

[78]  It is important to distinguish between causation as the source of the loss and the rules of damage assessment in tort. The rules of causation consider generally whether “but for” the defendant’s acts, the plaintiff’s damages would have been incurred on a balance of probabilities. Even though there may be several tortious and non-tortious causes of injury, so long as the defendant’s act is a cause of the plaintiff’s damage, the defendant is fully liable for that damage. The rules of damages then consider what the original position of the plaintiff would have been. The governing principle is that the defendant need not put the plaintiff in a better position than his original position and should not compensate the plaintiff for any damages he would have suffered anyway: Athey. …

[79]  At the same time, the defendant takes his victim as he finds him — the thin skull rule. Here the victim suffered trauma before coming to AIRS. The question then becomes: What was the effect of the sexual assault on him, in his already damaged condition? The damages are damages caused by the sexual assaults, not the prior condition. However, it is necessary to consider the prior condition to determine what loss was caused by the assaults. Therefore, to the extent that the evidence shows that the effect of the sexual assaults would have been greater because of his pre-existing injury, that pre-existing condition can be taken into account in assessing damages.

Those principles were followed by the New Brunswick Court of Appeal in McIntyre v. Matthews, 2020 NBCA 52. The plaintiff had sustained a workplace back injury. She had been able to return to work, albeit with the assistance of medication, and although she had to find alternate employments in less physically demanding jobs. At the time of the car collision, she was working full-time in a bakery as a front-end customer service clerk, employment that she was not able to pursue afterward. At trial, and again on appeal, the tortious car driver argued that the trier of facts should distinguish between the two events and limit liability to the sole effects of the second accident. On the basis of the aforementioned quote from Athey, supra para 17, the appellate court stated that in “‘thin skull’ cases, the question of whether the pre-existing condition was dormant or active is of no moment, provided it was not degenerative” (para. 59). The two levels of court looked at the full effects of the car collision without reducing liability to account for the active pre-existing condition as the evidence showed that the workplace injury had rendered the plaintiff vulnerable. The cases of O’Neill-Jones v. Chabot, 2015 NBQB 143, Hickey v. Savoie, supra para 53, and Crawford v. Galbraith (1995), 165 NBR (2d) 339 (QB), are consistent with the approach adopted in Matthews v. McIntyre.

The cases of Chartier v. Laramée, [1969] SCR 771, and Athey, supra, support the view that a tortious act can render vulnerable a normal victim. In Chartier, the plaintiff suffered a broken leg. After being put in a cast and sent home, he returned to the hospital for further treatments. Then, when walking with crutches while his cast was off, he lost his balance, put weight on the healing leg and broke it again. He had to undergo surgery and a bone graft. The unanimous bench attributed the reinjury to the original accident, holding that it would not have happened but for the vulnerability created by the collision (p 777):

Il est évident que si le jeune homme n’avait pas subi une première fracture par la faute de l’intimé, il n’aurait pas subi la seconde. On ne peut pas lui reprocher d’avoir marché avec des béquilles. Son médecin le lui avait prescrit. Évidemment, il lui avait également dit de ne pas faire porter son poids sur la jambe blessée mais de se supporter avec des béquilles. Cependant, sans le faire exprès, il a perdu l’équilibre en voulant passer dans une porte à ressort. Faut-il voir là une faute? Celui qui est obligé de marcher avec des béquilles n’est évidemment pas entraîné à le faire mais on ne peut sûrement pas le lui reprocher. S’il est obligé de s’y aventurer, c’est comme conséquence du premier accident dont la responsabilité est imputable à l’intimé.


Ici la preuve ne démontre pas que la seconde fracture soit le résultat d’une faute de la victime. Comme elle est évidemment par ailleurs la conséquence de la condition dans laquelle cette dernière s’est trouvée par suite du premier accident, il faut l’y rattacher.

[Underlining added]

Similarly, in Athey, the plaintiff injured his back while stretching at a gym. He had been involved in two prior motor vehicle accidents. The unanimous bench held the two tortfeasors liable for the injuries, as having contributed to the victim’s vulnerability. Technically, as far as the wrongdoers were concerned, the cases dealt with the foreseeability of the effects of an accident, not a pre-existing condition, but they still inform the relationship between an active health issue and a subsequent injury.

Thus, for a thin skull victim, the tortfeasor is liable for all the foreseeable effects of the tortious acts, even when they are greater than what a normal person would suffer. The vulnerability created by a prior tortious act is treated similarly if the sequelae from the two events are indivisible. When two accidents generate separate injuries then courts usually consider and compensate them separately.

‘Crumbling Skull’

The crumbling skull, on the other end, is when the victim has a pre-existing condition that would have affected his or her life even in the absence of the tortious act. When putting that injured party in the same position as he or she would have been but for the accident, courts will consider that the tortious act only triggered or precipitated a condition that would have happened in any event. There will still be full compensation, but strictly up to the point where the degenerative condition would have taken over. As stated in Athey, supra para 35: “if there is a measurable risk that the pre-existing condition would have detrimentally affected the plaintiff in the future, regardless of the defendant’s negligence, then this can be taken into account in reducing the overall award”. In Wallace v. Thibodeau, 2008 NBCA 78 para 49, the New Brunswick Court of Appeal warned that “courts are not to dismiss otherwise meritorious claims in ‘crumbling skull’ cases. Rather, they must reduce the award to reflect the risk that the crumbling skull would have actually crumbled and brought about, in any event, the loss experienced by the claimant [italicized in the original]”.

A wrongdoer bears the burden of showing that a condition would eventually affect a plaintiff to the point where degeneration would have brought about the same result as an accident: Wallace, supra paras 41 & 49; O’Neill-Jones v. Chabot, supra paras 133-134. Again, the pre-existing condition can be active or dormant at the time of injury. Many examples can be referred to, from diabetes, multiple sclerosis to Alzheimer, that could be diagnosed before a car collision but with effects not yet being felt. The main issue becomes to determine at what time it would evolve into the equivalent of what was sustained in the accident. Compensation is reduced or ends when that time is set by the trial judge.

Causation may not be an issue at all dealing with the crumbling skull rule, which can partly explain the burden resting on the tortfeasor. It is generally understood that the accident did not cause the ailment, only whether it accelerated its manifestation. The confusion between thin skull and crumbling skull arises at that level: to what extent, if any, a pre-existing condition would manifest itself but for the tortious act? The crumbling skull is the scenario where the condition would have happened in any event, while the thin skull would not in the absence of the trigger. If a medical condition is not degenerative, it will not usually be considered a crumbling skull. However, even when a condition has a risk of degeneration (i.e. diabetes, multiple sclerosis, degenerative disc diseases, etc.), it may be under control or without symptoms. The determination is factually driven and will typically require expert opinion evidence for each specific claimant.

Wallace v. Thibodeau, supra, illustrates the difference between thin skull versus crumbling skull cases. The plaintiff claimed medical expenses to treat a temporomandibular dysfunction that had been diagnosed before the accident, but which was asymptomatic then. The defendant had failed to show that the pre-existing condition would have eventually degenerated notwithstanding the accident. While the trial judge disallowed that part of the claim, the finding was overruled on appeal by a unanimous bench. The New Brunswick Court of Appeal sided with the plaintiff that the evidence showed a stable and non degenerating pre-existing health condition at the time of the accident. Full compensation was thus ordered. The opposite was found in the case of Furlotte v. Elward, 2011 NBCA 95, where the plaintiff’s condition was already degenerating at the time of the accident.

In the case of McIntyre v. Matthews, supra, the negligent car driver tried to assimilate an active pre-existing condition to the crumbling skull scenario to argue that the award of damages should be reduced accordingly. The plaintiff did not dispute that she was suffering from an active health issue before the accident, but argued that her condition was stable and manageable at the time of the accident. The trial judge agreed with the claimant and refused to apportion damages according to the two events, finding upheld by a unanimous bench of the Court of Appeal. The defendant had called no evidence to support the crumbling skull theory of her case.

Finally, regarding the reduction to be applied in crumbling skull cases, it will inevitably depend on the evidence. A health condition that would degenerate to the point of entirely overtaking the symptoms associated with the tortious act would likely limit compensation to the period prior, while degeneration to a level lower than what is experienced following an accident would likely lead to a reduced compensation for the overlapping period between the two sets of symptoms. The evidence will dictate the appropriate award of damages in each case.

This paper is offered for the purpose of discussion only. It does not constitute legal advice and its distribution does not create a solicitor-client relationship. Please consult a lawyer if you require legal advice.