Spinal Muscular Atrophy
Spinal muscular atrophy is a group of inherited diseases that affect the muscles responsible for voluntary movement in the body. This disease occurs when there is damage to the motor neurons, specialized nerve cells that facilitate communication with the muscles. This damage keeps the muscle from contracting, which leads to muscle weakness and atrophy.
Depending on the age of onset, severity and genetic cause there are several different types of the disease: spinal muscular atrophy Types 1, 2, 3 and 4, X-linked spinal muscular atrophy, and spinal muscular atrophy, lower-extremity dominant (SMA-LED).
Symptoms of spinal muscular atrophy Type 1 are evident at birth or within the first few months of life. These symptoms include:
- Developmental delays
- Inability to support the head
- Inability to sit up without assistance
- Breathing and swallowing difficulties which may lead to choking
Spinal muscular atrophy Type 2 is less severe than type I and begins to develop in a child’s first 6-12 months of life. Symptoms may include:
- Inability to walk or stand without assistance
- Ability to sit without assistance when placed in that position
Symptoms of spinal muscular atrophy Type 3 can begin to appear in early childhood or well into adolescence. Symptoms may include:
- Difficulty rising from a seated position
- Trouble climbing stairs
- Trembling hands
The onset of spinal muscular atrophy Type 4 often does not occur until after the age of 30. Symptoms of this condition may include:
- General muscle weakness
- Minor difficulty breathing
Because X-linked spinal muscular atrophy is carried on the X chromosome, it only affects men. People with this condition may experience the following symptoms:
- Joint deformities that impede mobility
- Infants may be born with broken bones
Kennedy’s disease is one type of X-linked spinal muscular atrophy. Symptoms generally show up in male patients between the ages of 20 and 40 years old. In the early stages, patients may notice tremors in their outstretched hands, brief visible twitches under the skin, and muscle cramps. As the disease progresses patients may notice limb weakness starting in the pelvis or shoulders, or weakness of the facial and tongue muscles.
Symptoms of SMA-LED often develop in infancy or early childhood. People with this condition may experience:
- Leg weakness, especially in the thigh muscles
- An unsteady gait
- Difficulty standing from a seated position
- Trouble climbing stairs
Causes and Risk Factors
Although spinal muscle atrophy has many forms, they are all genetically passed down from parent to child. Depending on the type of condition the person has been diagnosed with, mutations in certain genes have been linked to the disease. Genetic tests can be used to determine if a person has mutations of any of the following genes:
- SMN1, which is associated with spinal muscle atrophy Type 1, Type 2, Type 3 and Type 4
- UBA1, which is associated with X-linked spinal muscle atrophy
- DYNC1H1, which is associated with SMA-LED
- VAPB, which is associated with adult onset spinal muscle atrophy
People with two parents who are carriers of the abnormal SMN1 gene are at an increased risk of developing spinal muscular atrophy I, II, III and IV. This is because it is a recessive trait, which requires two copies of the abnormal gene to be present in order for symptoms to appear.
Other types of spinal muscular atrophy, such as SMA-LED and most of those that do not appear until adulthood are dominant traits and require only one copy of the abnormal gene in order for symptoms to appear.
Diagnosis of spinal muscular atrophy often begins with a thorough physical exam. The physician will review any family history of the disease to determine if the patient is at an increased risk. Other tests that may be performed include:
- Blood tests
- Genetic testing
- Electromyography (EMG)
- A muscle biopsy
Treatments for spinal muscular atrophy will vary depending on the type of diagnosis and the age of onset. Most of the conditions that develop after the first two years of life include physical therapy in the treatment plan in order to improve mobility and joint movement.
Many people with Type 1 do not survive past the first two years of life due to high rates of respiratory failure. Breathing difficulties can sometimes be managed through the use of non-invasive ventilators and bi-level positive airway pressure support. However, a tracheostomy may be needed if the non-invasive methods are unable to address the issue. For patients who survive past 2 years old, a treatment plan that includes regular respiratory therapy is generally needed to reduce the risk of lung complications.
Patients with type I can also experience difficulty acquiring proper nutrition due to a weaker sucking reflex and the tendency to tire more easily. Naso-gastric or gastric tubes may be used to assist in feeding.
Treatment for patients with Type 2, and those with Type 1 who live past their second year, often includes powered wheelchairs. If symptoms are less severe, other assistive devices such as a vertical stander or a standing wheelchair may be used. People diagnosed with spinal muscular type III or IV are often able to walk and remain fully functional for many years before they begin to need assistance.