Where a plaintiff claims damages for aggravation of a pre-existing condition, the trial judge must consider the probable course of the pre-existing condition and limit damages to the loss occasioned by the defendant's negligence, rather than attributing the plaintiff's total loss of earning capacity to the aggravation. A discount for 'vicissitudes' does not cure the failure to account for the pre-existing condition's probable trajectory. The definition of 'serious injury' in s.135A(19)(a) focuses on impairment of body function, not the injury itself, so an aggravation may qualify as 'serious' even where the pre-existing condition would eventually have caused the same impairment.
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