Where a plaintiff has a pre-existing degenerative condition and the accident materially contributes to the deterioration of that condition, the consequences of the accident are not 'subsumed' merely because the pre-existing condition would independently have caused deterioration over time. The defendant bears the evidential burden of disentangling the consequences of the accident from those of the pre-existing condition, and where the rate of independent deterioration is uncertain, the appropriate approach is to increase the vicissitudes discount rather than to find that the consequences of the accident have been entirely spent. A 40% vicissitudes discount may be appropriate where there is considerable uncertainty about the rate of independent deterioration. A plaintiff's refusal to undergo major surgery carrying significant risks and potential complications is not necessarily unreasonable and does not break the chain of causation.
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