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Spinal muscular atrophy (SMA)

January 9, 2024 — published by Pyramidale Communication

Key numbers on spinal muscular atrophy

Infantile SMA has an incidence rate of 1 in 6,000, with approximately 120 new cases diagnosed each year.^1,2
It is the 2^nd most common autosomal recessive disorder in children, after cystic fibrosis.¹

What is spinal muscular atrophy?

Spinal muscular atrophy (proximal or infantile), or SMA, is a genetic neuromuscular disorder characterized by the degeneration of alpha motor neurons in the anterior horn of the spinal cord.² Clinically, SMA manifests as progressive paralysis of the proximal limbs and muscle atrophy, leading to hypotonia and generalized muscle weakness. Deep tendon reflexes (DTRs) are typically reduced or absent.^2,3 SMA is caused by mutations in the SMN1 survival of motor neuron 1 gene, located on chromosome 5q11-q13.²

Diagnostic criteria and management

Spinal muscular atrophy is classified into four main types with varying severity and clinical symptoms: infantile SMA types I and II, juvenile SMA type III, and adult-onset SMA type IV.¹ SMA type I is the most severe form, while types II, III, and IV are relatively less severe.³

Type I SMA accounts for 50 to 60% of SMA cases. It is defined by the inability to sit independently, hypotonia, feeding difficulties during the first 6 months of life, and usually leads to death before the age of 2.^2,3
Type II SMA accounts for 30% of SMA cases and manifests between 6 and 18 months of age. Affected children are able to sit independently but never walk. Most patients survive into adulthood.³
Type III SMA accounts for 10% of SMA cases and occurs between the ages of 18 months and 18 years. Affected children learn to walk independently but may lose this ability over time. In most cases with disease onset before the age of three, children lose the ability to walk during childhood or early adulthood.⁴
Type IV SMA is a rare form (<1% of SMA cases) that manifests in adulthood (after age 30)² and whose clinical characteristics are still poorly defined.⁴
Type 0 SMA is an extremely rare variant that appears in utero, with low survival rates (< 6 months).^3,4

The diagnosis of spinal muscular atrophy is based on genetic and molecular analyses confirming the presence of SMN1 gene mutations. Additional investigations may be required, such as electromyography (EMG) or muscle biopsy.

The management of SMA is mainly symptomatic, with a multidisciplinary approach coordinated by a reference or competence center. Key areas of care include: respiratory support, rehabilitation, neuro-orthopedic management, digestive and nutritional support, pain management, etc. Family support is also planned. ²

Would you like to learn more about spinal muscular atrophy?

Click here to access the season dedicated to autoimmune myasthenia gravis on our RARE medical Podcast – RARE à l’écoute.
Listen to interviews with experts Prof. J. Melki, Dr. C. Cances, Dr. F. Audic, and Prof. I. Desguerre, as well as a patient testimonial from Mr. Cottet, director general of AFM-Téléthon.

Find the various webinars and live session series dedicated to the SMA Tour de France by clicking here.

You can also read our article on SMA by clicking here.

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References:

Orphanet. Amyotrophie spinale proximale. Disponible sur le lien : https://www.orpha.net/consor/cgi-bin/OC_Exp.php?Lng=FR&Expert=70
PNDS amyotrophie spinale infantile. Décembre 2020.
Mercuri E, et al. Spinal muscular atrophy – insights and challenges in the treatment era. Nat Rev Neurol. 2020 Dec;16(12):706-715. doi: 10.1038/s41582-020-00413-4.
Nicolau S, et al. Spinal Muscular Atrophy. Seminars in Pediatric Neurology. 2021. 37:100878. https://doi.org/10.1016/j.spen.2021.100878

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