What is the natural history of spinal muscular atrophy without treatment?

[Anonymous]

What is the natural history of spinal muscular atrophy without treatment?

[Genetic Counselor]

The untreated natural history of spinal muscular atrophy varies substantially by type, reflecting the underlying difference in SMN2 copy number and residual protein production.

*Type 0 and Type 1: Historically, the natural history of untreated Type 1 disease showed a rapid and relentless decline in motor function after birth. Without ventilator or nutritional support, most infants with Type 1 did not survive beyond two years of age, with respiratory failure being the leading cause of death. Type 0 followed an even more severe course, with limited survival often measured in weeks to months.
*Type 2: Children typically achieve independent sitting but do not walk unaided. Motor function may appear to plateau for a period during early-to-mid childhood, but progressive weakness, scoliosis, and reduced pulmonary function often develop over years, and respiratory complications remain a significant concern into adulthood.
*Type 3: Individuals achieve independent walking, though the ability to keep walking varies. Some children lose ambulation during the school-age years or adolescence, particularly those with onset before age three, while others with later onset retain the ability to walk well into adulthood with gradually increasing fatigue and difficulty with stairs or running.
*Type 4: The disease course is slowly progressive over decades, with proximal weakness developing in adulthood. Respiratory and bulbar involvement are uncommon, and life expectancy is generally unaffected.

Across all types, the natural history is shaped by the balance between ongoing motor neuron loss and the residual survival motor neuron protein produced by the SMN2 gene copies present in that individual. This is also why early genetic testing and SMN2 copy number analysis have become central to prognosis and to decisions around early therapeutic intervention, since starting treatment before significant, irreversible motor neuron loss has occurred is associated with meaningfully better outcomes than starting after symptoms are already established.

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