That distinction sits at the heart of many medico-legal disputes.
The law begins from a straightforward position. A defendant takes the claimant as they find them. Under the “eggshell skull” principle, an underlying vulnerability does not relieve a wrongdoer of responsibility merely because the consequences are more severe than they would have been for a healthier person.
In practice, however, the medico-legal analysis rarely ends there.
Medical experts are not asked simply whether an accident occurred or whether symptoms were reported afterwards. They must separate the effects of the accident from the claimant’s pre-existing condition and determine what changed because of the collision. That task becomes harder as the medical history becomes more complex.
Sometimes the analysis is uncomplicated. A claimant may have suffered a whiplash injury several years earlier, made a full recovery and remained symptom-free until the index accident. Where the records and history support that chronology, the previous injury may carry little significance. The expert may conclude that the claimant sustained a fresh injury with its own prognosis and recovery period.
The greater challenge arises when symptoms were already present before the collision.
A claimant who attended their GP repeatedly for neck pain before an accident presents a different evidential picture from somebody who had enjoyed years of complete recovery. The expert must establish a reliable baseline. How severe were the symptoms? How frequently did they occur? Was treatment being sought? Were work, hobbies or daily activities already affected? Was medication being taken?
Only after establishing that baseline can the expert assess what changed.
The central question may be one of aggravation. Did the collision materially worsen the claimant’s pre-existing condition? If so, by how much and for how long? The answer may support a period of accident-related symptoms even though the claimant would have continued to experience some discomfort in any event.
This is where many compensation disputes become focused. A claimant may experience genuine symptoms after the accident, but not every symptom will necessarily be attributable to it. Equally, the existence of a pre-accident condition does not mean the collision played no meaningful role.
Both extremes can produce unreliable conclusions.
Another common scenario involves acceleration rather than temporary aggravation. A claimant may have mild degenerative changes or intermittent symptoms expected to remain manageable for years. Following the collision, those symptoms become persistent, more severe or functionally limiting.
In such a case, the expert may conclude that the accident brought forward symptoms that would have developed later. The question then becomes one of timing. Did the accident accelerate the condition by months, several years or only a short period? Compensation should reflect the additional loss caused by that acceleration, rather than treating the entire future course of the condition as accident related.
This requires careful reasoning. Degenerative changes on imaging do not automatically prove that symptoms would have occurred at a particular time. Many people have age-related cervical changes without significant pain. An expert should therefore avoid if radiological findings alone establish an inevitable symptomatic outcome.
The records must be considered alongside the claimant’s reported history, functional evidence, treatment pattern and clinical examination.
Pre-accident records are often decisive. They may support the claimant by showing that earlier symptoms were minor, infrequent or fully resolved. They may also reveal a more substantial history than was initially disclosed. Neither outcome should be approached defensively. The purpose of reviewing the records is to establish an accurate clinical baseline.
Inconsistency does not always mean dishonesty. People may forget isolated consultations, underestimate the frequency of earlier symptoms or describe recovery in broad terms. However, where there is a significant difference between the history given to the expert and the contemporaneous records, the discrepancy must be addressed.
A credible medico-legal opinion explains whether the inconsistency affects diagnosis, causation or prognosis. It should not simply list the discrepancy and leave the reader to decide what it means.
The expert must also distinguish vulnerability from inevitability. A claimant may have been more susceptible to injury because of a previous condition, but that does not mean the same symptoms would have occurred at the same time without the accident. Conversely, a claimant may already have been on a clear clinical trajectory towards worsening symptoms or treatment.
The correct opinion may therefore involve a combination of conclusions: a fresh soft tissue injury superimposed on pre-existing degeneration, a temporary aggravation of earlier symptoms, or an acceleration of an underlying condition by a defined period.
Precision matters. Phrases such as “the accident exacerbated the condition” are of limited value unless the expert explains the duration and practical effect of that exacerbation. Was there an additional six months of symptoms? A permanent increase in pain? Earlier loss of function? A need for treatment that would otherwise have been delayed.
Without that explanation, the court is left with a label rather than an opinion.
The same applies to prognosis. An expert should not simply adopt the recovery period normally associated with uncomplicated whiplash if the claimant’s background makes that prognosis inappropriate. Equally, a long pre-existing history should not automatically justify an indefinite accident-related prognosis.
The opinion must remain proportionate to the evidence.
Ultimately, previous injury cases are not resolved by choosing between two simplistic positions: either the claimant was already vulnerable and should recover less, or the defendant must compensate every symptom reported after the accident. The proper approach lies between them.
The claimant must be compensated for the injury and loss caused by the defendant, including additional harm resulting from genuine vulnerability. However, the defendant is not responsible for symptoms, disability or deterioration that would have occurred in any event.
That is why the baseline matters. It allows the expert to identify the difference between the claimant’s condition without the accident and their actual condition afterwards.
In medico-legal reporting, that difference is often the case.
