There are a number of challenges in proving causation against a doctor. Medicine is often described as an art, not a science, because it is imprecise and unpredictable. Diagnostic procedures and tests can often be imperfect. Patients may present with confusing or inconsistent signs and symptoms. Treatments often carry risk of poor outcomes or side effects. Doctors commonly have differences of opinion in terms of treatment. If there is an unsuccessful outcome for a patient, a claim for medical malpractice will only succeed if causation is proven.

In a medical malpractice claim, the injured claimant must prove not only that the doctor failed to meet the standard of care expected of him/her, but also that this negligence in failing to meet the standard of care caused the injury. This is what is known as causation.

Causation is an essential element in not only medical malpractice claims, but also in all personal injury claims. The standard of proof is the “balance of probabilities” in which the injured claimant must show that “but for” the doctor’s negligence, the injury would have have occurred. Medical malpractice claims are inherently challenging because of the inherent complexity and difficulty in proving causation against a doctor which requires scientific evidence and expert opinions.

The issue of causation was the central issue in Reyes v. Marsden 2015 BCSC 884. In this case, the injured claimant brought a medical malpractice claim against an emergency room doctor for failing to diagnose a brain tumor on a head TC scan which then led him to carry out a lumbar puncture procedure which resulted in a loss of his vision.

On the issue of causation, expert witnesses testified that if the doctor had identified the tumor on the CT scan, he would not have carried out the lumbar puncture which caused the vision loss. The question for the trial judge was whether the injured claimant would nevertheless have lost his vision just as he did; or, in the absence of the lumbar puncture, would his vision have been preserved pending removal of the tumor.  In short, but for the lumbar puncture, would he still have had functional vision?

There were several expert witnesses called by both the injured claimant and the doctor. Their evidence differed between them on the issue of causation. The injured claimant’s experts testified that the lumbar puncture caused the vision loss. On the contrary, the doctor’s experts testified that the tumor itself was developing to the point where it would have resulted in vision loss with or without the lumbar puncture. The trial judge acknowledged that the answer was not one that could be determined with certainty and that he was required to weigh the evidence on the balance of probabilities.

On the evidence, the trial judge concluded that the injured claimant successfully proved causation in that the lumbar puncture procedure did cause the permanent visual impairment.

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