In a test case on the highly charged issue of whether the right to a medically assisted death should belong only to those who are already dying, a 25-year-old woman with a muscle-wasting disease is claiming the constitutional right to be protected from suffering that could last decades.

Julia Lamb of Chilliwack, B.C., filed a constitutional challenge to the Liberal government's assisted-dying law in B.C. Supreme Court on Monday, just 10 days after Parliament passed the law. She does not wish to die now, but says the law denies her the peace of mind of knowing that if her suffering becomes intolerable, she has a way out.

"My biggest fear is that if my condition suddenly gets much worse, which can happen any day, I will become trapped," she told a news conference in Vancouver. "If my suffering becomes intolerable I would like to be able to make a final choice for how much suffering to endure."

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Ms. Lamb's condition, spinal muscular atrophy, puts her in a different category from end-stage patients with cancer or amyotrophic lateral sclerosis, who are covered by the new law. She could live for years, even decades, she says.

The law says that to qualify, a patient's natural death must be "reasonably foreseeable," and they must be in a state of advanced decline.

"This law deliberately excludes a class of people: those who are suffering with no immediate end in sight," Grace Pastine, a lawyer with the British Columbia Civil Liberties Association, told reporters, calling the law unconstitutional. "How can we now turn away and ignore their pleas?"

In documents filed with the B.C. Supreme Court, her group said Ms. Lamb's case encapsulated the situation of many people – such as those with multiple sclerosis, spinal stenosis, locked-in syndrome, severe conversion disorder (a primarily psychiatric condition formerly known as hysteria), traumatic spinal injury, Parkinson's disease and Huntington's disease.

Justice Minister Jody Wilson-Raybould said her government will respond in the courts. She said she believes the assisted-dying law, known as Bill C-14, complies with the Constitution. The Senate initially rejected the law, saying it was unconstitutional for much the same reasons as those set out by Ms. Lamb.

"The legislation was carefully and deliberately crafted as a cohesive, responsible and balanced regime," Ms. Wilson-Raybould said in a statement. "It represents the right approach for Canada at this important time in our country's history by striking the balance between personal autonomy for those seeking access, protection of the vulnerable, and respect for the conscience rights of health care providers."

Ms. Lamb was diagnosed when she was 16 months old with spinal muscular atrophy, a progressive, incurable disease that affects breathing, swallowing, speaking and walking. She never crawled or walked, and has been using a wheelchair since the age of 6. As a child, she could stand for periods of time in a device known as a standing frame, but severe osteoporosis made that impossible. She said she has lived a full life.

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"I was a happy child. My parents called me a social butterfly … My family went on many adventures including river rafting camping and trips around the country. I was always told growing up to never let any obstacles deter me or hold me back from what I wanted in life. I took that message to heart and lived life."

She works part-time as a marketing assistant for a clothing company, writing newsletters and managing social-media accounts, mostly from home – though she makes weekly visits, in her wheelchair, to the company's outlet and to competitors to do market research, the court documents say. She can use her hands to type.

She receives help bathing, dressing, using the toilet and preparing meals. Since January, she has lived in her own apartment, with the help of three caregivers. At night, she is unable to roll over in bed, and needs an aide to help adjust her position.

"I feel a shadow looming over me. I know I can lose the ability to breathe well enough on my own and require a ventilator which will affect my ability to speak. I can lose the remaining function in my hands and arms which would lead to the inability to use my computer, write and use my phone. If I reach a point where I require constant care I will lose my independence and freedom. I am terrified by the idea I can become trapped in a state of physical and mental suffering that goes on for months, years or even decades. Having to think about the future causes me immense stress."

The BCCLA successfully challenged the old laws prohibiting doctor-assisted dying in Canada, in a case initiated by 89-year-old Kay Carter, who suffered from spinal stenosis. In February, 2015, the Supreme Court said intolerable suffering and a grievous, irremediable condition were required for a mentally competent adult to qualify for a medically assisted death. That case took four years to reach the top court; the BCCLA said it hopes to expedite Ms. Lamb's challenge.

David Baker, a lawyer representing the Council of Canadians with Disabilities and the Canadian Association for Community Living, says he wonders how far the logic of the legal challenge would extend: "Is someone who is blind condemned to interminable suffering? Is someone who has autism condemned to interminable suffering?"

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Michael Cooper, the Conservative party's deputy justice critic, says the Supreme Court was clear in its assisted-death ruling that "a legislative response would be afforded a high degree of deference."