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Spinal Muscular Atrophy | Causes | Diagnosis | Treatment

Updated: Jan 7, 2022


Spinal Muscular Atrophy (SMA) is a genetic disorder that occurs because of the mutation of the SMN 1 gene on chromosome 5q. It is a progressive neuromuscular disease, which worsens with time.

Motor neurons are nerve cells in the spinal cord that transmit the impulses from the brain to muscles. In SMA, there is a gradual loss of motor neurons, resulting in muscle weakness and muscle wasting (atrophy) of the shoulders, hip, and back. This impacts feeding, swallowing, and breathing.

Role of motor neurons and causes of spinal muscular atrophy


Mutation in the SMN1 gene results in SMA. SMN1 gene and SMN2 gene codes for the survival motor neuron (SMN) protein. The majority of the SMN protein is produced from the SMN1 gene. SMN protein is vital for the maintenance of motor neurons. Patients with SMA lack the SMN1 gene. Deletion of the gene results in a lack of SMN protein, thereby causing the death of motor neurons. Muscles do not receive the signal, thereby causing the symptoms of SMA.


The common symptoms in patients with SMA are

  • muscle weakness

  • worsening physical disability

  • mobility loss

  • respiratory dysfunction

Spinal Muscular Atrophy is of five main types.

  • Type 0 - Rare form of the disease that impacts babies before birth

  • Type 1 - Patients get impacted with severe muscle weakness, patients cannot sit or rollback

  • Type 2 - It gets diagnosed between 7 months and 18 months. Patients can sit at diagnosis. However, there will be progressive loss of motor function. The life expectancy is less compared to the general population.

  • Type 3 - There is a varying degree of muscle weakness

  • Type 4 - It impacts adults; there will be a mild impact on the motor functions


The diagnosis of SMA is by genetic testing. The main objective of genetic testing is to check for a mutation in the SMN1 gene. SMA types 0, 1, 2, 3, and 4 are caused because of the SMN1 gene loss, and 95% of patients will show deletion of exon seven or exon eight.


Treatment of SMA includes physical therapy, nutrition support, and breathing machines.

Spinraza (nusinersen)

In 2016, FDA approved Spinraza (nusinersen) for treating SMA 1, 2, 3, and 4. Treatment with Spinraza slowed the progression of the disease and improved muscle function.

Zolgensma (onasemnogene abeparvovec)

In 2019, FDA approved Zolgensma, one-time intravenous gene therapy for treating patients with SMA. It is an adeno-associated virus vector-based gene therapy. Vector delivers the functional copy of the SMN gene.


In 2020, the FDA approved Evrysdi (risdiplam) for two months and older patients. It can be administered orally for patients with SMA.



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