Estonia Study Looks at Prevalence and Characteristics of Spinal Muscular Atrophy

According to a recent article, a study in Estonia looked at the prevalence and characteristics of spinal muscular atrophy (SMA) over the last 24 years to aid in implementing SMA screenings at birth.

Spinal Muscular Atrophy (SMA)

Affecting approximately 1 in 10,000 people, spinal muscular atrophy is a rare genetic disorder that causes mild to severe muscle weakness and degeneration. The types of spinal muscular atrophy vary based on severity and age the condition was developed.


Most cases of spinal muscular atrophy are caused by a mutation in the SMN1 gene.

This prompts a loss of motor neurons, a certain type of nerve cell, in the spinal cord and brainstem. The loss results in weak and atrophied muscles, particularly those used in crawling, walking, sitting up, moving the head and, depending on severity, breathing and swallowing.


Symptoms of SMA vary by type. Types include:

  • Type I (Werdnig-Hoffman disease): A severe type diagnosed at or soon after birth, type I spinal muscular atrophy causes developmental delay and inability to sit or support the head independently. It also causes breathing and swallowing issues.
  • Type II: Children between six and 12 months of age develop type II spinal muscular atrophy and usually cannot stand or walk independently.
  • Type III (Kugelberg-Welander syndrome): Developed between early childhood and adolescent age, type III spinal muscular atrophy can cause progressive difficulty in walking and climbing stairs.
  • Type IV: This type usually occurs after age 30, resulting in mild muscle weakness, breathing issues, tremors, or twitching.

The Study

The retrospective study took place over 24 years. It looked at the data of patients with SMA who went to the hospital between 1996 and 2020. There proved to be 57 spinal muscular atrophy diagnoses within 53 families. The disease prevalence was found to be 1 per 8,286 births in the country.

Researchers discovered the most common disease type patients were diagnosed with was SMA type 1, with 43.9% of patients having this type. The second most common type was type 3, with 29.8% of patients diagnosed with this type. The least common was SMA type 2, with a total of 22.8% of patients having this type.

Depending on what type of SMA patients had, they were diagnosed at different times in their life. For patients with type 1 SMA, they were diagnosed at around 3 months. Those with SMA type 2 were diagnosed at around 18 months. Finally, those with SMA type 3 were diagnosed at around thirteen years old.

Most of the patients (96.5%) had the homozygous deletion of SMN1, with two other patients (3.5%) having the compound heterozygous for a deletion of a whole SMN1 gene. The compound heterozygous for a deletion of a whole SMN1 gene is rare and was not previously in any databases or published in any other research.

Out of the 57 patients in the study, 51 of them took part in a multiplex ligation-dependent probe amplification (MPLA) analysis. Researches did this in order to understand their disease severity. The results of this corresponded to the published data.

The hope of this study is for hospitals in Estonia to start performing SMA screenings at birth.

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