The issue is the assessment of the portion of the plaintiff’s future damages where the plaintiff has a relevant pre-existing condition caused by a non-tortious event. What if the evidence is equally balanced pro and con as to what might happen in the future: the future meaning the period after the trial. Since the plaintiff has the onus of proof, does this mean that the plaintiff’s action fails in relation to the claims for which the evidence is equally balanced?
Assuming that the defendant’s negligence has been found to have caused an actionable loss, the assessment of the portion of the damages that is “future” loss – the loss, if any, that will occur after the trial – is not done on the basis of probability but on the basis of possibility. Moore v. Kyba, 2012 BCCA 361 states at para. 43
 … if the plaintiff had a pre-existing condition and there was a measurable risk that that condition would have resulted in a loss anyway, then that pre-existing risk of loss is taken into account in assessing the damages flowing from the defendant’s negligence. This principle is called the “crumbling skull” rule. As explained in Athey (at para. 35): “This is consistent with the general rule that the plaintiff must be returned to the position he would have been in, with all of its attendant risks and shortcomings, and not a better position.”
Now assume the injured person (P) had an asymptomatic condition before the incident. After the incident, the person has symptoms. P alleges that the incident (the defendant’s negligence) was a but-for cause. P’s expert evidence is that
“it is not possible to predict when, if ever, that degenerative condition might have become symptomatic if the accident had not occurred, and … is equally impossible to predict the severity of any symptoms that might have arisen naturally.”
Is this enough for the plaintiff to receive any award for future non-pecuniary general damages?
Is the court stating that it is impossible to say whether there is a measurable risk the accident would or would not make any difference in the future? In theory, so long as the plaintiff has the onus of proof, and the resultant risk of non-persuasion, shouldn’t the plaintiff’s claim fail in relation to those claims to which that finding applies on the basis the plaintiff hasn’t satisfied the onus?
The quotation comes from Stovel v. Paul, 2013 BCSC 30 at para. 67.
 Part of Ms. Stovel’s neck complaints stem from the pre-existing degenerative arthritic changes to her neck, which were not causing her problems before the accident, and now cause her discomfort. As I understand Dr. Carruthers’ evidence, it is not possible to predict when, if ever, that degenerative condition might have become symptomatic if the accident had not occurred, and I infer from that that it is equally impossible to predict the severity of any symptoms that might have arisen naturally.
The plaintiff’s claim didn’t fail. The award for non-pecuniary general damages was $90,000.
The trial judge didn’t consider the issue as I’ve framed it. Instead, he wrote:
 The starting point for the assessment of damages 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”: Blackwater v. Plint, 2005 SCC 58 (CanLII), 2005 SCC 58,  3 S.C.R. 3 at para. 78.
 This case presents a combination of divisible and indivisible injuries. The Court of Appeal discussed the principles to be applied in Bradley v. Groves, 2010 BCCA 361, 8 B.C.L.R. (5th) 247, and recently restated them in Moore v. Kyba, 2012 BCCA 361.
 … I therefore approach Ms. Stovel’s neck as a combination of divisible injuries – the injuries to the left neck and shoulder, as well as those making symptomatic the pre-existing but asymptomatic degenerative condition of her cervical spine – and indivisible injuries – those aggravating the right neck and shoulder problems from 1998 as well as those contributing to any aggravation of her pre-accident anxiety condition. For the divisible injuries, the plaintiff is entitled to full compensation; for the indivisible injuries, her damages assessment must reflect only the harm caused by the defendants’ negligence.
 … Balancing the factors listed in Stapley, recognizing that the list there was decidedly not exhaustive, and taking into account the evidence as a whole, I conclude that an appropriate award of non-pecuniary damages that would be fair to all parties is $90,000.
That has to mean the trial judge thought the plaintiff’s evidence satisfied the plaintiff’s onus.
Did it? Or did the judge fail to consider the implications of his finding in para. 67.
None of this means that the actual decision was necessarily wrong. It does, however, mean that it’s not clearly right; creating a technical basis for appeal that could have been easily avoided.