What is Spinal Muscular Atrophy (SMA)?
Spinal Muscular Atrophy is a group of genetic disorders that affect the motor neurons in the spinal cord, leading to muscle atrophy and progressive weakness of the muscles of the shoulder, hip, and back. It also affects the voluntary muscles hence affecting the patient’s ability to walk, sit, and stand independently. X-linked infantile Spinal Muscular Atrophy is an autosomal recessive disorder, affecting almost 1 in 10,000 births every year. Due to their inheritance patterns, it only affects males and females to become silent carriers.
Causes of X-linked Spinal Muscular Atrophy
Affected males inherit Spinal Muscular Atrophy through their mother. It is caused by a mutation in the SMN1 gene and SMA2 gene on chromosome 5, both of which are responsible for the production of SMN protein. SMN protein is present all throughout the human body, with the highest concentrations in the spinal cord where it helps in the maintenance of motor neurons. Depending on the number of genes deleted, the phenotype may be milder or severe.
Types of X-linked Spinal Muscular Atrophy
X-linked recessive SMA is further divided into Types 0, 1, 2, 3, and 4. This typing is done based on the age of onset and the amount of function remaining/maintained. Each type is explained below:
Type 0 Spinal Muscular Atrophy, with either partial or complete loss of the SMN1 gene, is the most severe type of Spinal Muscular Atrophy. It presents with decreased fetal movement and can cause swallowing and respiratory problems which may prove to be fatal.
Type 1 Spinal Muscular Atrophy or Werdnig-Hoffman Disease is also a severe type of SMA, with the age of onset before 6 months of age. It can also present with swallowing and breathing problems, which can lead to a worse prognosis. Males affected by this can never sit independently or walk.
Type 2 Spinal Muscular Atrophy, has the age of onset after 6 months of age, and before 2 years of age. Due to this relatively late-onset, affected male infants learn to sit independently, and can even walk short distances. However, as the disease progresses, they might not be able to sit independently.
Type 3 Spinal Muscular Atrophy or Kugelberg-Welander syndrome is another subtype of SMA that has the age of onset between 2-3 years of age (juvenile-onset). These children can sit, and walk. However, they may fall frequently while walking. It is a milder type of SMA but can cause complete loss of movement as the disease progresses.
Type 4 Spinal Muscular Atrophy or Adult-onset Spinal Muscular Atrophy is the mildest type of SMA, with the age of onset after 10 years of life. Affected males may not face severe symptoms like loss of movement till 60 years of life.
Clinical Symptoms of Spinal Muscular Atrophy
Each type of SMA is associated with a different age of onset and severity of the disease. The symptoms of each type are as follows:
Type O SMA presents with poor fetal movements, severe muscle weakness, joint issues, and respiratory failure at birth.
Type 1 SMA presents with muscle weakness, decreased muscle tone, and little to no motor development. Affected males may also have difficulty eating, breathing, and coughing. Some patients may also experience tongue twitching. Based on the severity of respiratory symptoms, life expectancy may increase or decrease. Patients presenting with respiratory difficulty usually die by the age of 2.
Type 2 SMA affected patients present with mild symptoms. They may suffer from areflexia and decreased muscle tone. Some may also suffer from finger tremors. Depending on the severity of symptoms, patients may have a normal life span. However, affected males may lose the ability to sit independently and walk by their teen years.
Type 3 SMA presents with mild symptoms as well. It affects the legs more than the arms of the affected males. Muscle weakness and decreased tone usually present after their teen years.
Type 4 SMA, the mildest form of SMA, presents with muscle weakness in the legs and hips that may ascend to arms. The diagnosis is usually made in the 30s-40s of the affected males, and due to its mild phenotype, many do not lose the ability to walk. However, some may need wheelchair assistance.
Diagnosis of X-linked Spinal Muscular Atrophy
The diagnosis of SMA is based on clinical manifestations of SMA and confirmed with molecular genetic testing. The presence or absence of the SMN1 gene and mutations in SMA2 gene are studied to confirm the clinical diagnosis.
Genetic counseling should also be provided.
Treatment of X-linked Spinal Muscular Atrophy
Therapy for X-linked SMA depends initially on timely and accurate diagnosis. Based on different symptoms, the treatment differs:
- Pulmonary Support: Especially for the SMA1 type, tracheostomy or non-invasive respiratory support can help prolong life.
- Nutrition: Since many patients present with eating and swallowing difficulties, it is important to provide nutritional support. Especially for SMA1 affected individuals. Early gastrostomy should be considered for them, and SMA 2 and 3 types too.
- Scoliosis: SMA Type 2 and 3 may present with scoliosis which can be treated surgically with vertical expandable prosthetic titanium rib implant.
- Hip Dislocation: If asymptomatic, no surgical intervention is required. But if symptomatic, it is an orthopedic concern.
- Sleeping problems: Sleep may be interrupted by breathing difficulties, hence, CPAP with a nasal mask should be used.
The use of BiPAP has shown an increase in life expectancy. General care of these patients requires Physical therapy and Occupational therapy. It is also to monitor the progression or development of any respiratory complications.
Prognosis of X-linked Spinal Muscular Atrophy
The life expectancy of each type differs. SMA0 may not make it past 1 year of life, especially if respiratory complications arise. SMA1 affected patients may survive till 2 years of age, or more depending on the severity of symptoms. SMA type 2, 3, and 4 have a normal life span.
Complications of X-linked Spinal Muscular Atrophy
X-linked SMA, depending on its types, may lead to scoliosis, and joint contractures. Respiratory infections like pneumonia may also arise. Also, metabolic abnormalities such as severe metabolic acidosis and dicarboxylic aciduria may occur.