Natural History

SMA is a rare, progressive neuromuscular disease^1,2

Please note, throughout this website, when discussing “SMA”, we are referring to 5q SMA, the most common form of the disease.

SMA (5q SMA) is a rare, progressive neuromuscular disease caused by the degeneration of motor neurons in the spinal cord, which results in skeletal muscle atrophy and weakness.^1,2

Each individual with SMA is unique. SMA can present with a wide spectrum of symptoms and is often classified by clinicians based on the age of onset, severity of symptoms, and disease progression.²

The natural history of SMA in untreated individuals

Infantile-onset (Type I) SMA
Symptoms typically begin within the first 6 months of life²

Impact of the disease^2,3

In some, a normal development for the first few months, followed by progressive weakness and functional decline;
Individuals are unable to sit unsupported, and most never crawl, stand, or walk;
Morbidity and mortality are common and most often caused by pulmonary complications and bulbar dysfunction.

Expected lifespan^2,3

Typical life expectancy is <2 years of age without respiratory support.

Phenotype⁴

Approximately 58% of newly diagnosed individuals with SMA will be considered Type I.

Later-onset (Type II) SMA
Symptoms typically begin between 7 and 18 months of age³

Impact of the disease³

Standing with support is the highest motor milestone achieved before weakness begins;
Individuals cannot stand or walk independently; delays in achieving motor milestones;
The strongest individuals can stand with a standing frame or long leg braces but cannot walk independently;
Individuals eventually develop kyphoscoliosis.

Expected lifespan^3,5

Individuals generally live past 2 years, and approximately 70% are alive at 25 years of age.

Phenotype⁴

Approximately 29% of individuals with SMA will be considered Type II.

Later-onset (Type III) SMA
Symptoms typically begin after 18 months of age³

Impact of the disease³

Usually reach all major motor milestones, including independent walking, before decline begins;
Individuals may lose the ability to walk later in childhood, during adolescence, or in adulthood;
Scoliosis can develop in these patients.

Expected lifespan⁵

Individuals generally have a normal life expectancy.

Phenotype⁴

Approximately 13% of individuals with SMA will be considered Type III.

In individuals with untreated SMA, the prognosis is often related to maximum motor function achieved.⁶

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References

Butchbach ME. Copy number variations in the survival motor neuron genes: implications for spinal muscular atrophy and other neurodegenerative diseases. Front Mol Biosci. 2016;3:7.
Lunn MR, Wang CH. Spinal muscular atrophy. Lancet. 2008;371(9630):2120-2133.
Wang CH, Finkel RS, Bertini ES, et al; and Participants of the International Conference on SMA Standard of Care. Consensus statement for standard of care in spinal muscular atrophy. J Child Neurol. 2007;22(8):1027-1049.
Ogino S, Wilson RB, Gold B. New insights on the evolution of the SMN1 and SMN2 region: simulation and meta-analysis for allele and haplotype frequency calculations. Euro J Hum Gen. 2004;12(12):1015-1023.
Prior TW, Russman BS. Spinal muscular atrophy. NCBI Bookshelf Web site. Updated November 14, 2013.
Zerres K, Rudnik-Schoneborn S, Forrest E, et al. A collaborative history on the natural history of childhood and juvenile onset proximal spinal muscular atrophy (type II and III SMA): 569 patients. J Neurol Sci. 1997;146(1):67-72.

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Biogen-176301-HKG-09/2022

Natural History

SMA is a rare, progressive neuromuscular disease1,2

The natural history of SMA in untreated individuals

SMA is a rare, progressive neuromuscular disease^1,2