Due to a change in the SMN1 gene, there is a deficiency of the SMN protein. The SMN protein is important for the motor neurons in the spinal cord, which transmit information from the brain to the muscles. Over time, this deficiency leads to destruction of the motor neurons and atrophy of the muscles - resulting in muscle weakness.
In people with SMA, the formation of SMN protein is limited to the activity of the SMN2 gene. However, 90% of the SMN protein produced by the SMN2 gene is unstable and rapidly degrades (see figure 1). The copy number of the SMN2 gene varies from person to person. Therefore, the lower the SMN2 gene copy number, the more severe the disease.2,3
SMA is an autosomal recessive inherited disease. This means that both parents must carry a defective SMN1 gene for their offspring to develop SMA. Thus, the probability of the offspring developing SMA is 25%. Studies have shown that about 1 in 50 people are carriers (see figure 2).3-4
Traditionally, SMA has been classified into three or four types, depending on the age of onset and the most advanced motor stage of development acquired by the individual (see figure 3). This classification does not provide any information about the actual current motor abilities of the affected person, nor about any associated limitations.
In approximately 60% of cases, it is SMA type I. The onset of SMA in adulthood (type IV) is much more rare than the other forms (not described in figure 3).5 In addition, there is another form of SMA (type 0) that begins in utero, is very severe and is usually fatal before the age of six months, sometimes already in utero. 6
Figure 3: Classification of SMA in 3 types (types 0 and IV are not described here) - this table describes the natural evolution of the disease, without a disease-modifying treatment. 7-13
Spinal muscular atrophy is a complex disease and therefore requires integrated treatment by a multidisciplinary care team (see figure 4). Coordination should be provided by the neurologist or neuro pediatrician. The type of management depends on the severity of the disease and individual patient complications. Treatment of SMA takes place in specialized treatment centers.
What does the multidisciplinary management of SMA usually include?
A disease-modifying treatment for SMA
Symptomatic treatment of potential complications
Orthopedic care, support and aids
Verhaart IEC et al. Prevalence, incidence and carrier frequency of 5q-linked spinal muscular atrophy - a literature review. Orphanet J Rare Dis 2017;12:124
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Bowerman M, et al. Therapeutic strategies for spinal muscular atrophy: SMN and beyond. Dis Model Mech 2017; 10:943-954. Pubmed-Link
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Farrar, M., Park, S., Vucic, S., Carey, K., Turner, B., Gillingwater, T., Swoboda, K. and Kiernan, M., 2017. Emerging therapies and challenges in spinal muscular atrophy. Annals of Neurology, 81(3), pp.355-368.
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Bladen CL, et al. Mapping the differences in care for 5,000 spinal muscular atrophy patients, a survey of 24 national registries in North America, Australasia and Europe. J Neurol 2014; 261:152-163. Pubmed-Link
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