Causation, apportionment, and aggravation of pre-existing conditions
What this domain tests
This domain tests whether you can distinguish work-related injury from underlying disease, natural progression, and unrelated causes. It also tests whether you can explain apportionment without speculation and use the correct terms for aggravation, exacerbation, and acceleration.
Core rules and concepts
- A causation opinion should address diagnosis, mechanism, temporal relationship, biologic plausibility, dose or exposure, consistency of findings, and alternative causes. The conclusion is usually stated to a reasonable degree of medical probability, with the reasoning shown.
- Aggravation is a permanent worsening of a pre-existing condition caused by the work event. Exacerbation is a temporary flare that returns to baseline. Acceleration is earlier onset of a condition that would have occurred anyway. Recurrence is return of a prior condition.
- Apportionment allocates impairment or damages among causes. It requires a credible baseline and an understanding of natural history. It is not the same as causation: a work event may be a substantial cause even when pre-existing disease contributes.
- Legal rules vary. Some systems allow apportionment of permanent impairment; others require the employer to take the worker as found. In Colorado workers compensation, confirm the current statute and Division rule before apportioning; do not assume tort apportionment rules apply.
- The eggshell plaintiff principle may make a defendant liable for unforeseeable consequences of aggravation, but the medical expert still must separate work effects from baseline disease when the jurisdiction permits.
Worked example
A 60-year-old with known knee osteoarthritis falls at work and develops a meniscal tear and effusion. The examiner should not assign all knee impairment to the fall. The acute tear and swelling may be work-related, while cartilage loss, osteophytes, and chronic pain predate the event. The report should cite prior imaging and examinations, describe the acute findings, state which impairment is attributable to the tear, and explain any apportionment basis. If the jurisdiction prohibits apportionment of permanent impairment, the report should still describe the baseline for causation and treatment decisions.
Traps that cost marks
Equating apportionment with causation; assigning percentages without a baseline or method; calling a temporary flare a permanent aggravation; ignoring natural progression; using the eggshell rule to avoid medical analysis; and applying tort rules to an administrative compensation system.