Proving Causation in Medical Malpractice Cases

by John McKiggan

Legal gavel

In order to succeed in a medical malpractice case, the patient must prove, on the balance of probabilities, the following:

  • That the medical practitioner in question (physician, nurse, midwife etc.) owed the plaintiff a duty of care (usually not an issue),
  • A breach of the standard of care required of any reasonably competent practitioner of similar training,
  • That the breach of the standard of care caused harm — proving “Causation,” and
  • Compensable damage.

This paper will focus on causation.

In medical malpractice litigation, a great deal of time, money and effort is directed toward proving that the defendant medical practitioners breached standards of care in the treatment provided to the injured plaintiff. This, however, is insufficient to establish a right of recovery. The plaintiff must go further and prove that the breach of the standard of care caused the injury.

A link must be established between the breach of the standard of care and the harm complained of and that link is causation. The law demands that there be a substantial connection between the injuries suffered by the patient and the substandard care of the physician, nurse or midwife.

The Onus of Proof

The basic test can be simply stated. To succeed, the plaintiff must satisfy the “but for” test. This means that the plaintiff must show on a balance of probabilities that “but for” the defendant’s negligent act, the injury would not have occurred. Inherent in the phrase “but for” is the requirement that the defendant’s negligence was necessary to bring about the injury ― in other words that the injury would not have occurred without the defendant’s negligence. This is a factual inquiry. If the plaintiff does not establish this on a balance of probabilities, having regard to all the evidence, the claim will be dismissed.

Theoretically, there is, strictly speaking, no absolute requirement that the plaintiff adduce expert opinion evidence from qualified medical practitioners supporting the causation case. The Supreme Court of Canada has held that the “but for” causation test must be applied in a robust common sense fashion.

There is no need for scientific evidence of the precise contribution the defendant’s negligence made to the injury. Any evidence connecting the breach of duty to the injury suffered may permit the judge, depending on the circumstances, to infer that the defendant’s negligence probably caused the loss.

In malpractice cases, however, as a practical matter, a failure to lead expert opinion evidence will usually result in the claim being dismissed. This is because the defendants will often lead expert opinion evidence saying that the injury would likely have occurred in any event. This argument is often open to them because patients seeking medical care usually have a background of medical problems which often can be implicated in the injury suffered. In order to succeed, therefore, it is necessary for the plaintiff to lead expert opinion evidence, from qualified medical experts, establishing that proper care would have avoided the injury.

In an obstetrical setting it is usually necessary to lead expert opinion evidence on the causation issue for the simple reason that it is often very difficult to establish what happened to cause the injury. Questions of timing (i.e., – did the injury occur at or around the time of delivery or much earlier) and cause (was the injury caused by hypoxia, by infection, by a metabolic problems such as diabetes or by a genetic problem) dominate obstetrical malpractice claims. Expert opinion evidence is usually led from both sides, the plaintiff’s attempting to establish a treatable cause that could and should have been addressed while the defense attempts to establish a cause which was beyond the control of the medical practitioner.

The experts engaged in attempting to resolve these disputes are myriad. Typically, a plaintiff will require the assistance of the following experts in order to put forward a reliable theory of causation:

  • A neuroradiologist, a specialist in reading radiological examinations such as CT scans and MRI scans. This expert and give valuable information as to the pattern of the brain injury. Different types of injuries cause different patterns on imaging and those patterns are often related to the timing of the injury.
  • A neonatologist, a specialist pediatrician who takes care of children during the first month of life. The specialist gives the court expert opinion evidence regarding matters such as the significance and implications of the Apgar scores (an assessment of the condition of the child immediately after birth) and an analysis of the child’s blood gases, looking at matters such as the pH the base excess of the blood which give some indication as to the amount of acid in the baby’s blood. This, again, goes to questions of timing and duration of hypoxic insults. The neonatologist can also give evidence regarding infection and other potential causes of injury.
  • A perinatologist, a specialist obstetrician who deals with high risk deliveries. This expert can give expert opinion evidence regarding maternal causes of injuries to the baby including preeclampsia and gestational diabetes.

In obstetrical cases there is often no direct evidence as to what happened to the child. There are limited means that allow assessment of the fetus before the birth. The primary way to assess fetal well-being is by monitoring the fetal heart rate during labour.

Evidence regarding causation must therefore be gathered from external sources such as the fetal heart strip, cord gases taken immediately after birth and scalp lactate samples taken prior to delivery which measure the amount of lactate in the baby’s blood (another indication of potential hypoxia). Where “but for” causation is established by inference only (which is usually the case in obstetrical cases), it is open to the defendant to argue or call expert opinion evidence that the injury to the infant would have happened without the defendant’s negligence, i.e., that the negligence was not a necessary cause of the injury, which was, essentially unavoidable.

Medical defendants spend a great deal of money and time developing this sort of evidence. A properly prepared case on behalf of an injured baby anticipates this sort of evidence and attempts to undermine it with compelling evidence that the injury was caused or contributed to by the negligence of the medical provider.

Multiple Defendants

In some cases, the injury to the baby may flow from a number of different negligent acts committed by different medical providers, each of which is a necessary or “but for” cause of or jury then apportions liability according to the degree of fault of each defendant pursuant to contributory negligence legislation.

In an obstetrical case, a baby who has suffered an injury as a result of the negligence of a number of different medical providers can collect all of his/her damages from any defendant. The defendant’s and then fight among themselves as to who must contribute to each other so that they each wind up only paying that portion of the damages attributable to their own negligence.

Loss of a Chance

Canadian malpractice law provides that the plaintiff must prove, on the balance of probabilities, that the negligence of the defendant medical provider(s) caused his or her injury. The law does not recognize as compensable “injury” a lost chance for a better outcome unless the chance which was lost is a greater than 50 percent chance.

For example, if the judge concludes, on the basis of the medical evidence, that the baby had a 40 percent chance of recovering from an intrauterine insult (such as, for example, infection) but, due to the negligence of the obstetrician, that chance was reduced to 10 percent. In this scenario, while many people may feel that a 30 percent reduction in the possibility of a full recovery is “damage” or “injury,” Canadian law does not agree.

For that matter neither does English or Australian law. The plaintiff must prove, on the balance of probabilities (the 50 percent plus one test) that but for the negligence of the defendant the injury would not have occurred or that the plaintiff would have recovered.

The Role of the Expert

In any obstetrical malpractice action both sides will typically lead expert opinion evidence from multiple experts. The experts for both sides are serving, at least in theory, one purpose – to assist the court in coming to the correct conclusion. In order to do this, each expert must understand the issues that arise in the case, you must understand their role in assisting the court in coming to the proper conclusion and they must understand the limitations of their expertise.

In other words, they must remain within the bounds of their own area of expertise and not delve into areas best reserved for experts in other fields. For example, the obstetrician should not be providing expert opinion evidence in the area of neonatology, and vice versa.

Most importantly, the expert must not be an advocate for either side. The expert must be neutral as between the parties. For this reason it is important to retain experts who are both experts in their respective fields and who can present their evidence in a way which on the one hand is “neutral” enough to assist the court in coming to the correct conclusion while, at the same time, being compelling enough to allow the parties to put forward their best case.

“Hired gun” experts have no place in this kind of litigation. They rarely stand up to careful cross-examination and often wind up hurting the case of the party that called him/her.


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