When the Food and Drug Administration approved Zolgensma, a lifesaving medication to treat spinal muscular atrophy, parents of young children with the rare and fatal disease rejoiced.

But that relief was quickly tempered by the price tag: $2.1 million — the most expensive ever for a single dose of a drug.

“I was pretty shocked,” Sarah Stanger of Monroe, Ohio, said. “You know, as a teacher we definitely don’t have $2.1 million, and I don’t know anybody who does.”

Stanger’s son, Duke, was diagnosed with the condition an as infant. When Zolgensma was approved in May, doctors said that the medicine was the best option for Duke. But the family's insurance company refused to pay for it.

Duke Stanger with his parents, Logan and Sarah. Courtesy Stanger family

Without the drug, Duke’s future is bleak. In babies with SMA, nerve cells in the brain stem and spinal cord that control the muscles needed for speaking, walking, breathing and swallowing are destroyed because a critical protein is missing. As the disease progresses, muscles weaken and atrophy, and patients lose their ability to walk, eat or even breathe, according to the National Institute of Neurological Disorders and Stroke.

“Once those neurons die, there’s no reviving them,” Stanger told NBC News. “With no treatment, most children will pass away by age 2.”

When the drug was approved, Novartis, the maker of Zolgensma, said that it expected insurance companies would cover the cost of the treatment. Novartis also said that the high cost of the drug was justified, and the one-time treatment was half the cost of 10 years of treatment with an existing SMA drug.

But Butler Health Plan, the Stangers’ health insurance provider, said that their reasoning for refusing to pay for Zolgensma is that this type of therapy has historically been excluded from coverage. Zolgensma is a type of gene therapy.

“To date, gene therapy … has been excluded from the benefits provided under our health benefit plans,” Stephanie Hearn, executive director at Butler Health Plan, said in an email. That’s because, despite gene therapy’s potential to treat or cure debilitating diseases, the therapies are costly, and health insurance providers still need to figure out how to balance out the cost of these expensive treatments without jeopardizing coverage for the rest of the people on the plan, Hearn wrote.

But that math is going to become increasingly difficult to resolve as time goes on, according to David Mitchell, founder of the advocacy group Patients for Affordable Drugs.

"The situation we're seeing right now with access to Zolgensma is a problem that will only get worse,” Mitchell said. There are at least 400 other gene therapies in development, and “if they all come to market with prices of $2 million, we won’t be able to afford them as families or as a nation,” he said.

Novartis told NBC News that a “wide range of patients” have had the drug covered by insurance since its approval, but noted that it’s not uncommon for patients to have to go through an appeals process for any new drug.

There may be some hope, however.

Just a few days ago, a major insurance company, UnitedHealthcare, reversed its decision to deny payment for Zolgensma to two children whose cases had received publicity. The company told NBC News that the reversals took place because they had received more information about the cases, not because of media attention.

Ultimately, these cases are yet another example of how the health care system in the United States is failing patients, said Dr. Albert Wu, an internist and professor of health policy and management at the Johns Hopkins Bloomberg School of Public Health.

“It’s an unfortunate reflection of how our health care system is currently working — or not working — that the only way people can get drugs paid for is through a Hail Mary GoFundMe site or by generating enough bad press that the payer feels it isn’t worthwhile to resist,” Wu said.

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