That distinction sits at the centre of most medico-legal work, yet it remains one of the area’s most frequently handled poorly.
The temptation is understandable. Clinicians are trained to diagnose and treat. Courts, insurers, and solicitors, however, are not primarily interested in diagnosis alone. They need to understand what caused the condition and, just as importantly, what would have happened had the event under consideration not occurred.
That difference may appear subtle, but in practice it can determine the outcome of an entire case.
A diagnosis is not a causation opinion.
One of the most common errors in medico-legal reporting is the assumption that a diagnosis answers the causation question. It does not.
Take chronic low back pain as an example. The fact that a claimant reports symptoms and meets the diagnostic criteria tells us little about why those symptoms developed. The existence of a condition and the cause of that condition are separate questions.
The same applies in psychological injury claims. A diagnosis of post-traumatic stress disorder does not automatically establish that the index event caused the disorder. The expert must consider alternative explanations, pre-existing vulnerabilities, intervening events, and the chronology of symptoms before reaching an opinion.
This may sound obvious yet reports still appear in which the reasoning moves directly from diagnosis to causation without showing the steps in between. Courts are entitled to expect more than that.
The difficulty of pre-existing conditions.
Few claimants arrive with a blank medical history.
Individuals presenting for medico-legal examination have experienced previous injuries, earlier episodes of psychological illness, degenerative change, or longstanding symptoms. The task for the expert is not to ignore those factors, but to assess their significance.
That assessment is rarely straightforward.
One individual with asymptomatic degenerative changes may develop symptoms following an accident. Another may have experienced intermittent symptoms for years before the index event. A third may have been progressing towards difficulty regardless of the incident in question.
The challenge lies in determining where the balance of probability sits.
In my experience, reports devote extensive attention to the claimant’s current presentation while giving only limited consideration to the medical history. That approach carries obvious risk. Causation opinions are only as dependable as the evidence on which they are based. A conclusion reached without proper consideration of the claimant’s previous health is unlikely to withstand detailed scrutiny.
The missing counterfactual.
The strongest causation opinions usually contain an explicit, or at least clearly implied, counterfactual analysis.
What would have happened had the accident, treatment, exposure, or incident not occurred?
It is remarkable how often this question is overlooked.
Without a counterfactual, it becomes difficult to separate the consequences of the event from the natural progression of disease, the effects of ageing, or unrelated life circumstances. This issue arises frequently in clinical negligence work, but it is equally relevant in personal injury and employment-related claims.
A report that merely states that symptoms followed an event may establish chronology. It does not necessarily establish causation.
After does not mean because of.
Chronology can mislead.
Experienced practitioners become cautious about temporal associations.
Claimants genuinely associate the onset of symptoms with a particular event because that is when they first became aware of a problem. Sometimes they are correct. Sometimes they are not.
Human beings are naturally inclined to connect events that occur close together in time. The medico-legal expert must resist that instinct and examine whether the proposed causal link is medically plausible and supported by the wider evidence.
This is particularly important in cases involving chronic pain, psychological injury, neurological symptoms, and conditions with complex or multifactorial origins.
A temporal relationship may support a causation opinion. It rarely settles it.
Showing the reasoning
Perhaps the most overlooked aspect of causation reporting is transparency.
Experts occasionally reach conclusions that may be reasonable but fail to explain how they arrived there. The opinion appears in the report as a destination without a map. Readers are left to infer the reasoning, but that is seldom satisfactory.
A well-constructed causation opinion should identify the evidence relied upon, acknowledge competing explanations, explain why alternative causes are considered less likely, and demonstrate how the conclusion has been reached. The reader should be able to follow the analysis even if they disagree with it.
This is not simply a matter of style. It goes directly to the weight that can be attached to the opinion.
An uncomfortable reality.
Many disputes presented as arguments about diagnosis are, in truth, arguments about causation.
The medical condition may not be particularly controversial. What is controversial is whether the defendant’s conduct, the accident, the treatment, or the workplace exposure materially contributed to it.
That is why causation remains one of the most demanding aspects of medico-legal practice. It requires clinical knowledge, careful analysis of records, an understanding of probability, and a willingness to engage with uncertainty.
Most importantly, it requires the expert to distinguish between what can be demonstrated and what is merely possible.
The reports that stand up best under scrutiny are rarely those that express the strongest opinions. They are usually the ones that show their workings. A court may accept or reject the conclusion, but it should never be left wondering how the expert reached it.
