Spinal muscular atrophy 1 (SMA1), also known as Werdnig Hoffmann disease, is a genetic neuromuscular disorder that affects the nerve cells that control voluntary muscles (motor neurons). Without treatment, symptoms of SMA1 become apparent before 6 months of age and include worsening muscle weakness and poor muscle tone (hypotonia) due to loss of the lower motor neurons in the spinal cord and brain stem. Feeding and breathing problems are also present. SMA1 is caused by changes (pathogenic variants also called mutations) in the SMN1 gene and is typically inherited in an autosomal recessive manner.
Diagnosis of SMA1 is suspected by symptoms and confirmed by genetic testing. SMA has been added to the list of recommended newborn screening tests in the United States, so that it can be detected prior to symptoms developing. This occurred because treatments are being developed that are changing the course of the disease. In December 2016, nusinersen (Spinraza) became the first FDA approved treatment for SMA1. Continued treatment with nusinersen is allowing many babies with SMA1 to reach and maintain age appropriate developmental milestones, including sitting, crawling, and walking. On average, breathing problems, nutrition problems, and hospital admissions have also decreased. However, response to treatment does vary. Some babies with SMA1 may not respond to the nusinersen at all or may have medical complications that prevent use of the treatment. Other treatments remain supportive.
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