Pre-existing Conditions


Entitlement for a work-related injury/disease will not be denied due to the existence of a pre-existing condition. Once initial entitlement is established, the decision-maker considers the impact, if any, of pre-existing conditions on the worker’s ongoing impairment.


The Workplace Safety and Insurance Act, 1997 (WSIA) directs that compensation be provided for work-related injuries/diseases. Entitlement is not granted for injuries/diseases resulting from other factors, such as non-work-related pre-existing conditions.

Work-relatedness is established when determining initial entitlement. Decision-makers continue to evaluate the work-relatedness of a worker’s ongoing impairment throughout the life of a claim.

The “thin skull” and “crumbling skull” doctrines are well-established legal principles that are components of decision-making at the WSIB.

The WSIB makes its decisions based on the merits and justice of each case, see 11-01-03, Merits and Justice.

When the evidence for and against an issue related to a worker’s claim are evenly balanced, the worker must be given the benefit of the doubt, see 11-01-13, Benefit of Doubt.


The purpose of this policy is to define a pre-existing condition and provide direction to decision-makers regarding how to assess the impact, if any, of a pre-existing condition after an initial entitlement decision has been made.



Pre-existing condition

A pre-existing condition is any condition that existed prior to a work-related injury/disease, and may include injuries, diseases, degenerative conditions, and psychiatric conditions. The existence of the condition must be confirmed by pre-injury or post-injury clinical evidence and may have been evident prior to the occurrence of the work-related injury/disease or it may become evident afterwards.

Pre-existing conditions include but are not limited to

  • conditions that have produced periods of impairment/disease requiring health care and have caused a disruption in employment prior to the workplace/injury or disease, as defined in 15-02-04, Aggravation Basis
  • underlying or asymptomatic conditions which only become manifest post-accident as defined in 14-05-03, Second Injury and Enhancement Fund (SIEF), and
  • work-related permanent impairments for which the WSIB has granted a permanent disability (PD) or non-economic loss (NEL) benefit.


An impairment is a physical or functional abnormality or loss (including disfigurement) which results from an injury and any psychological damage arising from the abnormality or loss.

Initial entitlement

The decision-maker first determines entitlement in the claim, see 11-01-01, Adjudicative Process.

Consistent with the “thin skull” doctrine, the fact that a worker may have a pre-existing condition that could increase susceptibility to injury/disease is not considered during the initial determination of entitlement in a claim. In such cases, workers are compensated for the work-related injury/disease and the claim is not denied due to the existence of a pre-existing condition.

Continuing entitlement where there is a pre-existing condition

The presence of a pre-existing condition does not necessarily mean it is a cause of the worker’s impairment or that the impairment is no longer work-related. When a pre-existing condition is evident, or becomes evident, the work-relatedness of the ongoing impairment must be monitored.

If the pre-existing condition is not impacting the worker’s impairment, there is no effect on the worker’s benefits.

If the pre-existing condition is impacting the worker’s impairment, benefits will generally continue as long as the work-related injury/disease continues to significantly contribute to the worker’s impairment.

Determining work-relatedness of ongoing impairment

Once the existence of a pre-existing condition has been established, ongoing work-relatedness is determined by considering the relationship, if any, between the pre-existing condition, the work-related injury/disease, and the worker’s impairment, based on the clinical evidence.

The decision-maker determines whether the pre-existing condition is impacting the worker’s ongoing impairment by considering factors such as whether

  • the impairment affects the same body part or system as the pre-existing condition
  • the impairment continues beyond the expected recovery period, given the work-related injury/disease
  • the impairment is unexpectedly severe given the work-related accident
  • there is a change in the worker’s ability to perform the pre-accident work, beyond what was expected given the work-related injury/disease.

When assessing the impact of the pre-existing condition on the worker’s ongoing impairment, the decision-maker determines whether the work-related injury/disease continues to be a significant contributing factor. To make this determination, the decision-maker considers the significance of the work-related injury/disease and the pre-existing condition, relative to each other.

In some cases the clinical evidence may demonstrate that the significance of the pre-existing condition is so great it has overwhelmed the impact of the work-related injury/disease, rendering it insignificant. When this occurs, the work-related injury/disease cannot be considered to be of sufficient significance in comparison to the pre-existing condition, for benefits to continue.

To determine if a pre-existing condition has overwhelmed the work-related injury/disease, decision-makers consider whether the worker’s current level of impairment would continue despite the work-related injury/disease and whether the work-related injury/disease on its own would cause a similar level of impairment. If the work-related injury/disease is causing a similar level of impairment, it is still considered a significant cause of the ongoing impairment.

If the pre-existing condition has overwhelmed the work-related injury/disease, and the pre-existing condition resulted from a prior work-related accident, the decision-maker considers whether there has been a recurrence, see 15-02-05, Recurrences.

In cases where the pre-existing condition is not contributing to the ongoing impairment but is prolonging the recovery from the work-related injury/disease, benefits continue as long as the ongoing impairment is work-related, even if recovery takes longer due to the pre-existing condition.

Where the clinical evidence demonstrates that a pre-existing condition has been aggravated as a result of a work-related injury/disease, benefits continue until the worker recovers from the aggravation of the pre-existing condition.

Clinical opinion

If, in the opinion of the decision-maker, the existing clinical evidence does not clearly demonstrate whether the ongoing impairment is work-related, the decision-maker may seek a clinical opinion to assist in making this determination.

Permanent impairment

In cases where it appears the worker has an impairment that continues to exist after maximum medical recovery (MMR) is reached, caused by both the work-related injury/disease and a pre-existing condition, the work-related impairment is determined, see 11-01-05, Determining Permanent Impairment.

The work-related injury/disease does not need to be the sole cause of the impairment for the degree of the work-related permanent impairment to be assessed.

Application date

This policy applies to all decisions made on or after November 1, 2014, for all accidents.

Policy review schedule

This policy will be reviewed within two years of the application date.

Document History

This is a new policy.


Legislative Authority

Workplace Safety and Insurance Act, 1997, as amended
Sections 2(1), 13(1)

Workers’ Compensation Act, R.S.O. 1990, as amended
Sections 1(1), 4(1)


Board of Directors
#3(f), September 18, 2014, Page 7317