Montana’s high court ruled that a Liberty Mutual Insurance Co. unit that covered an injured employee’s carpal tunnel syndrome when it was first diagnosed is also liable for coverage when the injury later worsened, even though it was no longer the company’s insurer.

The ruling by the Montana Supreme Court, which was released Tuesday, overturns an earlier ruling in favor of the insurer by a workers compensation court.

Kim Wiard began working at Saint Regis, Montana-based Tricon Timber L.L.C. in 2002 and was diagnosed with bilateral carpal tunnel syndrome in 2010 or 2011, according to the ruling in Montana State Fund v. Liberty Northwest Ins. Corp.

At that time, Liberty Northwest was Tricon’s workers comp insurer and accepted liability for treatment of the occupational disease in August 2011. Ms. Wiard transferred to a different job position at Tricon and her symptoms largely dissipated. She used over-the-counter pain medication, ceased wearing wrist braces and did not seek additional treatment for her carpal tunnel syndrome, court papers say.

In 2012 or early 2013, Ms. Wiard, switched to a grader position at Tricon, which she found more mentally and physically demanding than her previous positions, the ruling states.

On Nov. 1, 2013, the Montana State Fund replaced Liberty Northwest as Tricon’s workers comp insurer, according to the ruling.

After working in her new position for some time, Ms. Wiard sought medical treatment for pain in both wrists on Feb. 17, 2014. She was taken off work and scheduled for follow-up care. Her condition worsened, however, and later that night she went to an emergency room complaining of severe right wrist pain radiating into her shoulder. She was diagnosed with an acute exacerbation of carpal tunnel syndrome, given medications, and referred to an orthopedic surgeon. Two days later, after another emergency room visit, she underwent an emergency carpal tunnel release on her left wrist and in March 2014 the same surgeon performed a carpal tunnel release on her right wrist.

Ms. Wiard filed an occupational disease claim for the emergency surgery with the Montana State Fund, which denied the claim, asserting that her occupational disease diagnosis preceded the fund's insurance coverage. Ms. Wiard then submitted her claim to Liberty Northwest, who denied liability on the basis that fund was liable.

The Montana State Fund paid Ms. Wiard's benefits and the insurers then filed cross motions for summary judgment on the issue of liability in the Workers Compensation Court, which granted summary judgment in favor of Liberty Northwest. The court concluded Ms. Wiard had reached maximum medical improvement from her earlier carpal tunnel syndrome diagnosis and that her later job duties materially aggravated her occupational disease.

On appeal, the Montana Supreme Court found that whether Ms. Wiard had reached maximum medical improvement and whether she sustained a material aggravation to her underlying occupational disease, while relevant in determining liability in a multiple-employer scenario, were irrelevant when determining liability in a single-employer, multiple-insurer scenario.

A worsening or material aggravation of the underlying disease is insufficient to shift liability, meaning the only way an originally liable insurer can avoid liability is if it demonstrates that the injured worker is suffering from a new and different occupational disease, the court ruled.

“There is no dispute here that Wiard is not suffering from a new and different OD,” the ruling stated. “She is suffering from a recurrence of the same OD, while working for the same employer. The WCC’s determination that Wiard’s later work materially aggravated her CTS is insufficient to shift liability from Liberty to State Fund.”

Attorneys for the insurers could not be immediately reached for comment.