This is one of the most difficult areas to address in civil claims, both for lawyers and for their medical experts, even more so if there is a pre-existing condition. In doing so, the medical expert must be aware of, understand and apply several legal tests.
Firstly, the ‘Egg-Shell Skull’ principle means that someone who causes injury to another person must take that other person as they are. Therefore, if the injured person has a vulnerability or pre-existing condition with the result that the impact of the accident or injury on them is more severe than would normally be expected, the person who causes the injury is responsible for all of the consequences as they arise in this individual and damages will not be reduced because of this increased vulnerability.
On the other hand, damages will not be awarded for what would have occurred in any event as a result of this pre-existing condition, but only for the consequences of the breach of duty – how the claimant’s life has been affected by the breach of duty. The expert has therefore to help identify the difference between the situation resulting from the breach of duty (and the likely future prognosis) and the situation as it would have been but for the breach of duty.
Approach to the evidence
The expert must therefore first address the issue of what the claimant’s functioning would have been over time had the breach of duty not occurred. Then the expert addresses what the claimant’s situation has in fact been and will be, and finally describes what the significance of that difference is, in terms of symptoms, function and consequences.
If the breach of duty has long-term effects then it is necessary to continue to compare into the future the situation as it would have been in the absence of the accident with what it will now be. For the future proof is no longer necessary on the balance of probabilities but the likelihood of both situations (the ‘but for’ situation and the situation consequent on the accident) must be compared according to the level of risk.
A claimant with a degenerate spine who suffers a disc prolapse in an accident can recover damages for the symptoms and restrictions consequent on the prolapse, but in assessing damages the fact that a prolapse might well have occurred in any event in the absence of an accident will be highly relevant. The expert must seek to set out what would probably have happened in the absence of the accident – the likelihood of a prolapse, the events likely to give rise to it, it’s timing, it’s treatment etc – and compare that and the consequences to what has in fact occurred. In that way the consequences of the accident (the damage caused by the accident) can be identified and damages assessed. Easily stated as a principle, but not necessarily straightforward to address and to justify in a medical report.
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 See Ch 6 (sections 6.1, 6.2 and 6.5) of Writing Medico-Legal Reports in Civil Claims – an essential guide