Medicolegal issues surrounding proof of causality

The concept of causation is central to every medicolegal case. Even if duty of care and breach can be demonstrated, causality still needs to be proven. A doctor may admit to a breach of care, but if it can be successfully argued that this did not alter the outcome of treatment, the case will fail. Furthermore, establishing the cause of a patient’s condition is crucial as it determines liability and therefore financial responsibility. For medicolegal cases, it is not necessary to prove causality beyond any reasonable doubt, but on the basis of it being more probable than not that a specific exposure, event, action or omission caused the claimant’s injury or illness. However, even this level of proof is not always easy to obtain.

Establishing the legal causation of a medical condition requires testimony from a suitably qualified healthcare professional, who is an expert on the medical and environmental causes of illness and injuries. In complex cases, opinions from more than one expert witness may be required. However, this testimony will only be considered reliable if it helps the judge or jury to understand the evidence or determine a fact, it is based on sufficient facts or data, it is the product of established methods or principles, and these methods have been reliably applied to the facts of the specific case. Particular attention should be paid to whether the medical or scientific theory the expert relies upon has been subject to testing, publication and peer review, and whether it is accepted by the majority of other medical practitioners in the same field of medicine. A doctor may be regarded as an expert in their field due to extensive training and experience, but if the opinion he or she gives is not regarded as mainstream by most of his or her peers, it may not be considered reliable or legally admissible in court.

The standard test for proving causation in most medical negligence lawsuits is known as the ‘but for’ test. According to this, the defendant’s conduct can be considered causal if the claimant’s condition would not have resulted but for the defendant’s actions, or lack thereof. Thus, if the condition being claimed for would have occurred irrespective of the defendant’s actions or omission, the but for test fails and causation cannot be proved.

There also needs to be a natural and continuous sequence of events arising from the defendant’s actions or omission that results in injury, and without which the injury would not have occurred, and this must not be broken by any intervening cause. If another event does occur, particularly if it worsens the outcome for the patient, it may be impossible to assign causation to the negligent act.  

Proving the but for test in patients who are claiming for exacerbation of existing symptoms or acceleration of a pre-existing condition can be particularly problematic. It can be extremely difficult to show that the worsening of the patient’s condition represents a deviation from the natural course of the disease unless the difference is extreme. Careful scrutiny of the patient’s medical records and a thorough understanding of the disease or condition involved are vital if causation is to be proved.

In some cases, there may be multiple possible causes of the injury and in these circumstances, the standard but for test cannot be applied. Instead, the court will consider whether the alleged negligence contributed directly to the injury, rather than just increasing the risk of injury occurring, and whether this contribution was ‘material’, the definition of which is usually left to the court as it may vary according to each specific case. In addition, there must be no other alternative established complete cause of the injury. However, it is not always possible to determine the specific cause that led to the disputed injury, and these types of injuries are described as ‘indivisible’.

In order to assign causality following exposure to a specific agent, a causation assessment may be carried out. This refers to the toxicological, epidemiological or industrial hygiene-related methods used to connect or eliminate a drug, procedure, medical device or other agent or event as a cause of disease. It is vital that proper methodology be applied during the assessment so that the conclusions are reliable. As a minimum, the assessment should consider what condition the patient is actually suffering from, whether the agent or event under investigation is capable of causing this condition, and, finally, whether it did so in this specific patient.

The scientific proof that an agent or event is capable of causing the symptoms or health condition being claimed for is usually provided by the results of epidemiologic studies. If there is no evidence that a certain agent is carcinogenic, it cannot be proved that it caused cancer in any particular patient. It must also be shown that the patient received a sufficient dose of any agent, or the specified event was serious enough, to have caused the claimed condition. Perhaps most importantly, all other potential causes must have been considered and ruled out. This last factor is where many cases fail; the cause of many conditions is multi-factorial and it is often impossible to rule out potential causes in addition to the claimed exposure.