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A Cautionary Tale: The Consequences of Eliciting Previously Undisclosed Opinions from Participant Experts

By Suganiya Sivabalan

In its recent decision in St. Marthe v. O’Connor, 2021 ONCA 790, the Ontario Court of Appeal provided guidance on the boundaries of appropriate expert evidence at trial, particularly evidence from participant experts.


The plaintiff in this action was injured in a motor vehicle accident in November of 2011, in which he sustained soft tissue injuries that rendered him incapable of continuing to work in the construction industry.

In November of 2014, the plaintiff underwent an assessment with Dr. Mussett, an orthopedic surgeon, in the context of his related accident benefits claim. Dr. Mussett opined that the plaintiff’s upper back pain and right arm pain, caused by the subject accident, resulted in the inability to perform expected duties at work without experiencing pain and discomfort.

Trial Level

Four years later, this action proceeded to trial. Liability had been admitted, and the only issues to be determined were whether the action was barred by the expiry of the applicable limitation period, and the quantum of the plaintiff’s damages.

At trial, the plaintiff called Dr. Mussett as a participant expert witness. His orthopedic assessment report was also included in the joint document brief.

On cross-examination, Dr. Mussett was examined with respect to his opinion regarding the plaintiff’s inability to perform his employment duties and inability to participate in his recreational activities. The defendant’s counsel questioned Dr. Mussett on the plaintiff’s return to work and participation in recreational activities such as soccer, following the report he provided in November 2014.

In this context, the defendant’s counsel elicited an opinion from Dr. Mussett that a muscle spasm complained of by the plaintiff was not disabling, and would not have prevented the plaintiff from returning to work. This opinion was not contained in Dr. Mussett’s assessment report.

Following this line of questioning, plaintiff’s counsel re-examined Dr. Mussett, but then brought a motion for an order discharging the jury, on the basis that defendant’s counsel had unfairly elicited opinion evidence from Dr. Mussett on cross-examination that was not previously disclosed in Dr. Mussett’s report.

The trial judge granted the plaintiff’s motion and discharged the jury. The trial judge completed the trial without the jury, and awarded the plaintiff damages of $205,662.23 plus interest, costs and disbursements. The defendant appealed the trial judge’s decision to the Court of Appeal.

Court of Appeal

The focus of the appeal was whether the trial judge had erred in finding that counsel for the defendant had improperly elicited inadmissible opinion evidence from Dr. Mussett, and whether it was appropriate to discharge the jury in response.

The Court of Appeal explained that in civil proceedings, experts retained to testify must prepare and share their expert report with all parties prior to trial. These experts are known as “litigation experts”. Their expert reports include an outline of their expert opinion and their anticipated testimony. This limits the scope of the expert’s testimony, and provides a roadmap for the expert’s opinion evidence.

A participant expert, in contrast, is “a witness with special skill, knowledge, training or experience who has not been engaged by or behalf of a party to the litigation.” Non-party experts are subject to different rules than litigation experts. These experts are able to give opinion evidence for the truth of its contents, only so long as the opinion was formed as part of the exercise of their skill and knowledge while observing or participating in the events at issue. Non-party expert opinions are typically contained in clinical notes and records, or reports prepared for the purpose of consultation and treatment.

The Court of Appeal applied the two-part framework from White Burgess Langille Inman v. Abbot and Haliburton Co., 2015 SCC 23, [2015] 2 S.C.R. 182, in considering whether the impugned opinion evidence from Dr. Mussett was admissible. First, the court must assess whether the opinion evidence meets the threshold requirements of admissibility: relevance, necessity, absence of any exclusionary rule and has been provided by a properly qualified expert. If it does, the court then engages in a discretionary analysis to determine if its probative value outweighs its prejudicial effect.

In this case, the Court of Appeal held that the admissible scope of Dr. Mussett’ opinion evidence was limited to his opinions based on his skill and knowledge that he exercised while observing the respondent during his one and only assessment in 2014. At no time prior to his cross-examination at trial was Dr. Mussett asked to assess the plaintiff’s ability to return to work.

In the four years between his assessment of the plaintiff and trial, Dr. Mussett did not see the plaintiff, nor did he review any further medical documentation. Therefore, his opinions needed to be limited to the information he had at the time of his 2014 assessment.

Dr. Mussett’s testimony regarding the plaintiff’s subsequent ability to return to work was accordingly not probative, as it was not based on a current assessment of the plaintiff’s accident-related impairments. However, Dr. Mussett’s opinion was very prejudicial, as it could have affected the jury’s consideration of the central issue at trial of the plaintiff’s past and future income loss. Therefore, the prejudicial effect of the impugned opinion exceeded its probative value, and did not meet the test for admissibility from White Burgess.  

With respect to the trial judge’s decision to discharge the jury, the Court of Appeal confirmed that the right to a jury trial is a substantive right. Upon determining that particular evidence heard by a jury is prejudicial, the trial judge must then determine whether the prejudice is so severe that specific instructions cannot dispel it. In other words, the jury should only be discharged if the prejudice cannot be cured, as discharging the jury is a “drastic remedy”.

A trial judge’s decision to discharge a jury is a discretionary decision entitled to deference, and will not be interfered with on appeal unless it “was exercised arbitrarily or capriciously or was based upon a wrong or inapplicable principle of law.”

The trial judge concluded that Dr. Mussett’s opinion evidence could have a “serious adverse impact” on the case. He held that, “Dr. Mussett’s evidence was simply too impactful for the jury to disregard it and it would inevitably influence their assessment of the other expert testimony in this case” and that justice to the parties would be best served by discharging the jury and continuing the trial before him alone.

The Court of Appeal concluded that there was no basis to interfere with the trial judge’s discretionary decision to discharge the jury in this case. Accordingly, the appeal was dismissed.


The St. Marthe decision articulates the permissible bounds of expert opinion evidence. A party cannot elicit an opinion from an expert that has not been previously disclosed in a report or record. The Court of Appeal seems to suggest that this may even be so on cross-examination of an opposing party’s witness, though the ambit of this holding may arguably be limited to opinions of participant experts. At the very least, previously undisclosed expert opinion evidence may be considered prejudicial, and therefore subject to scrutiny as to whether it should be admitted.

A participant expert can only provide an opinion at trial based on the exercise of the non-party expert’s skill and knowledge while actually observing or participating in the events at issue. If a party wants to have an expert provide opinions based on subsequent events or information, counsel must ensure that the expert is in compliance with Rule 53.03 of the Rules of Civil Procedure, or risk having all or part of the expert’s evidence excluded at trial.