Section EMP.04.20 — Pre-Existing Medical Conditions

(a) If the worker has a pre-existing medical condition when an injury arises and that condition delays or prevents complete recovery, it shall be ascertained, as nearly as possible:

(1) the period over which the injury would have caused disability, were it not for the pre-existing condition, and

(2) the extent of the impairment, which the injury would have caused, were it not for the pre-existing condition.

(b) Compensation shall only be awarded for the period and extent of the injury not attributable to the pre-existing medical condition.