Thin and Crumbling Skulls: The Impact of Pre-Existing Conditions on Workers Compensation in Ontario Canada

Introduction

Workers compensation is premised on the concept that workers should be compensated for injuries or disablements sustained in the course of their employment. Causation is the key factor in determining whether a worker’s injuries are compensable. In determining questions of causation, issues often arise as to whether the injury was truly sustained in the course of employment or was the result of some pre-existing condition suffered by the worker. In such situations, a worker’s entitlement to compensation can be complex.

This article considers the role of pre-existing conditions on the issue of entitlement in workers compensation law in Ontario, Canada. It focuses on the use of the “thin skull” rule by Ontario’s Workplace Safety and Insurance Appeals Tribunal (“Tribunal”) and Workplace Safety and Insurance Board (“Board”). Finally, this article identifies relief available for employers who face situations where the severity of their worker’s injury is enhanced by the existence of a pre-existing injury.

Causation and the Thin Skull Rule

In Ontario, a “significant contributing factor” analysis is used to determine whether a worker’s injury or condition is compensable. This test focuses on whether the work itself made a significant contribution to the worker’s injury. The work need not be the sole contributing factor to the injury. As long as the work makes more than a de minimus contribution to the injury, the worker will be entitled to workers compensation.

Where a worker’s pre-existing condition contributes to an injury sustained in the course of employment, workers compensation adjudicators in Ontario apply a doctrine known as the “thin skull” principle. This doctrine originates from common law principles of causation. It provides that individuals are to be taken as they are found, frailties and all. In the context of workers compensation law, this rule means that where a non-compensable pre-existing factor may have rendered a worker more susceptible to injury, such increased susceptibility will not preclude entitlement to benefits. Providing the work related accident/disablement was still a significant contributing factor to the injury or subsequent disability, the worker will be entitled to benefits.

The rationale behind applying the thin skull rule to workers compensation cases was explained in the Tribunal’s Decision No. 915 at p. 136:

Thus, workers will be entitled to compensation for injuries sustained in the course of employment, even if such injuries resulted from a pre-existing condition, provided that the work itself was a significant factor in the injury.

The Crumbling Skull Exception to the Thin Skull Rule

Canadian common law has recognized a limit to the thin skull principle in situations where a pre-existing condition is so large a causal factor in an injury that it overwhelms the significance of the conduct of the tortfeasor. This concept is known as the “crumbling skull” rule.

The crumbling skull rule has been applied by the Tribunal to limit entitlement for workers with pre-existing symptomatic conditions. Its application was expressed in Decision No. 1870/07:

If a pre-existing condition is symptomatic and is so large of a causal factor in the subsequent disability that it overwhelms the significance of the accident, then triers of fact might conclude that the accident, compensable or not, was not a significant contributing factor to the subsequent disability. In such a case, the disability or condition would not be compensable.

In applying the crumbling skull principle, the Tribunal often focuses on whether the pre-existing condition was symptomatic or asymptomatic. In cases of symptomatic pre-existing conditions, the Tribunal will, on appropriate facts, find that the crumbling skull exception applies.

Relief for Employers with Thin Skulled Employees

The Board operates a Second Injury and Enhancement Fund (SIEF) to provide employers with financial relief where a pre-existing condition enhances or prolongs a work-related disability. By providing employers with such financial relief, the SIEF encourages employers to hire workers with disabilities.

The SIEF is only available to Schedule I employers (those employers, largely in the private sector, who are covered by the Board’s insurance fund). Schedule II employers (those employers, largely in the public sector, who are self-insured) must pay for all Board costs, whether or not they can be attributed to a pre-existing condition.

The Board’s policy regarding the SIEF defines a pre-existing condition as an underlying or asymptomatic condition which only becomes manifest post-accident. This definition implicitly recognizes the distinction between compensable asymptomatic pre-existing conditions, and non-compensable symptomatic pre-existing conditions embodied in the crumbling skull exception to the thin skull rule.

If it is likely that a pre-existing condition either contributed to the work-related accident or prolonged or enhanced the work-related disability, employers may be entitled to relief under the SIEF. The percentage of employer costs transferred to the SIEF depends on a consideration of the medical significance of the pre-existing condition in conjunction with the severity of the accident. While, in most cases, 50% of employer costs are transferred to the SIEF, employer relief under the SIEF can reach 75% and above in appropriate circumstances.

Contributing Author: Joseph Cohen-Lyons is an associate lawyer at the Hicks Morley Toronto, Canada office and currently practices in all areas of labour and employment law providing advice and representation to employers and management. Contact him at: joseph-cohen-lyons@hicksmorley.com or 416-864-7213.

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