Spinal Muscular Atrophy
Spinal muscular atrophy (SMA) is a group of genetic neuromuscular disorders that affect the nerve cells that control voluntary muscles (motor neurons). The loss of motor neurons causes progressive muscle weakness and loss of movement due to muscle wasting (atrophy). The severity of the symptoms, the age at which symptoms, begin, and genetic cause varies by type. Many types of SMA mainly affect the muscles involved in walking, sitting, arm movement, and head control. Breathing and swallowing may also become difficult as the disease progresses in many types of SMA. In some types of SMA, the loss of motor neurons makes it hard to control movement of the hands and feet.
SMA type 1, 2, 3, and 4 are caused by changes (pathogenic variants, also know as mutations) in the SMN1 gene and are inherited in an autosomal recessive manner. Extra copies of the nearby related gene, SMN2, modify the severity of SMA. There are other rarer types of SMA caused by changes in different genes. Other autosomal recessive forms include SMA with progressive myoclonic epilepsy (SMA-PME) caused by changes in the ASAH1 gene and SMA with respiratory distress 1 (SMARD1) caused by changes in the IGHMBP2 gene. Autosomal dominant forms include distal MSA type V (DSMA-V) caused by changes in BSCL2 and GARS, SMA with lower extremity predominance (SMA-LED) caused by changes in DYNC1H1 or BICD2, and adult-onset form of SMA caused changes by VAPB. X-linked forms include X-linked infantile SMA caused by changes in UBA1.
Diagnosis of SMA is suspected by symptoms and confirmed by genetic testing. Treatments are in general supportive aiming to increase quality of life and avoid complications. Treatments may include physical therapy, nutrition support, chest physiotherapy, and, in severe cases, breathing machines (ventilators). In December 2016, nusinersen (Spinraza) became the first FDA approved treatment for SMA types 1, 2, 3, and 4. Continued treatment with nusinersen has been shown to slow the progression of the disease and even improve muscle function, but individual response to the treatment does vary. Due to the success of nusinersen as well as other promising treatments presently in clinical trials, SMA caused by changes in the SMN1 gene has been added to the list of recommended newborn screening tests in the United States, so that treatment may begin before symptoms develop. However, as of July 2018, not all States have added the test to their newborn screening panel.
SMA type 1, 2, 3, and 4 are caused by changes (pathogenic variants, also know as mutations) in the SMN1 gene and are inherited in an autosomal recessive manner. Extra copies of the nearby related gene, SMN2, modify the severity of SMA. There are other rarer types of SMA caused by changes in different genes. Other autosomal recessive forms include SMA with progressive myoclonic epilepsy (SMA-PME) caused by changes in the ASAH1 gene and SMA with respiratory distress 1 (SMARD1) caused by changes in the IGHMBP2 gene. Autosomal dominant forms include distal MSA type V (DSMA-V) caused by changes in BSCL2 and GARS, SMA with lower extremity predominance (SMA-LED) caused by changes in DYNC1H1 or BICD2, and adult-onset form of SMA caused changes by VAPB. X-linked forms include X-linked infantile SMA caused by changes in UBA1.
Diagnosis of SMA is suspected by symptoms and confirmed by genetic testing. Treatments are in general supportive aiming to increase quality of life and avoid complications. Treatments may include physical therapy, nutrition support, chest physiotherapy, and, in severe cases, breathing machines (ventilators). In December 2016, nusinersen (Spinraza) became the first FDA approved treatment for SMA types 1, 2, 3, and 4. Continued treatment with nusinersen has been shown to slow the progression of the disease and even improve muscle function, but individual response to the treatment does vary. Due to the success of nusinersen as well as other promising treatments presently in clinical trials, SMA caused by changes in the SMN1 gene has been added to the list of recommended newborn screening tests in the United States, so that treatment may begin before symptoms develop. However, as of July 2018, not all States have added the test to their newborn screening panel.