The Gatekeeper Never Leaves: Expert Evidence Revisited Mid-Trial
In the recent decision in Wallis v. Wawanesa Mutual Insurance Company, 2026 ONSC 2636, the Ontario Superior Court of Justice revisited its earlier decision to qualify a treating psychologist as a litigation expert mid-trial.
The decision is an important reminder that the trial judge’s gatekeeping role does not end once an expert is qualified. Where concerns arise during testimony about an expert’s opinion, the court may revisit admissibility and limit the evidence that can be considered by the jury.
Background
The action arose from a motor vehicle accident that occurred on December 22, 2016. The plaintiff alleged that she sustained serious injuries after the vehicle was struck from behind by an unidentified driver.
The trial commenced on March 30, 2026, and was approaching its sixth week when the court addressed issues arising from the testimony of the plaintiff’s treating psychologist, Dr. McDermid.
In an earlier mid-trial ruling, the court permitted Dr. McDermid to testify as both a participant expert and a litigation expert. Dr. McDermid had treated the plaintiff for approximately nine years and had prepared three reports. Although the reports did not strictly comply with Rule 53.03 of the Rules of Civil Procedure, the court initially held that, taken together, they effectively provided the defendants with sufficient notice of her opinion evidence.
However, during cross-examination, it became apparent that the factual foundation for Dr. McDermid’s litigation opinions was more limited than initially understood. The court had believed that Dr. McDermid had reviewed a fulsome medical brief before preparing her main opinion report, when in fact, she had only received two plaintiff medical reports and a medical brief index, but not the underlying documents listed in that index.
Shortly before trial, Dr. McDermid obtained additional records, however, she could not clearly identify which documents she had reviewed, when she had reviewed them, or how they had informed her opinions.
The difficulty was that, although Dr. McDermid maintained that she was providing only a psychological opinion, her reports went further. They addressed the plaintiff’s physical, cognitive, psychological, occupational, and functional impairments, as well as causation and the plaintiff’s pre- and post-accident functioning.
The court held that any meaningful comparison of the plaintiff’s functioning before and after the accident required an adequate understanding of both periods. Dr. McDermid had not reviewed the pre-accident records before preparing her reports and instead relied heavily on the plaintiff’s self-reports, along with summaries contained in reports prepared by other experts retained by the plaintiff.
The Court’s Analysis
Justice Nicholson emphasized that a litigation expert’s duty to provide fair, objective, and non-partisan evidence continues from the time of retainer until the completion of the expert’s evidence. The court found that Dr. McDermid was unable to provide objective litigation opinion evidence because she had only reviewed a fraction of the available records and could not identify with certainty what she had reviewed.
The court also noted that difficulty that can arise when a treating practitioner is asked to step into the role of a litigation expert. While this is not prohibited, the treating relationship can make it difficult to separate concern for the patient from the independent duty owed to the court.
Relying on Buff-Murphy v. Gunawerdena[1], the court confirmed that a trial judge’s gatekeeping role does not end once expert evidence is admitted. The gatekeeping role continues throughout the trial. If concerns arise during testimony that were not apparent at the qualification stage, the court may revisit the issue.
Ultimately, the court concluded that Dr. McDermid could not continue as a Rule 53.03 litigation expert. The prejudicial effect of her litigation opinion evidence outweighed its probative value. The court gave the jury a mid-trial instruction advising on what parts of Dr. McDermid’s evidence were still admissible, and what parts they were to disregard.
Conclusion and Key Takeaways
This decision offers several important reminders for counsel dealing with treating practitioners and litigation experts:
- Experts must know what they reviewed. An expert who cannot identify what documents were reviewed, and when, risks having their opinion evidence limited or excluded.
- The gatekeeping role continues throughout trial. Even after an expert is qualified, the court may revisit admissibility if cross-examination reveals concerns that were not previously apparent.
- Counsel should ensure experts have the necessary materials before reports are finalized. Providing records shortly before trial may be too late, especially if the expert cannot clearly explain how those records affected their opinions.
[1] 2017 ONCA 502.