An Overview of Spinal Muscular Atrophy (SMA)

Spinal muscular atrophy (SMA) is a rare genetic disease affecting the control nerves branching from the spinal cord have over voluntary muscles. SMA mainly afflicts children.

A child with SMA will experience impairment of crucial functions such as breathing, sucking and swallowing. Additional conditions can develop out of such impairment. For example, abnormal spinal curves may develop due to weak back muscles, further complicating the breathing process by compressing the lungs.

Prior to the advent of feeding tubes, interruption in swallowing often caused death in cases of SMA type 1 (the most severe kind). There are now many assistive devices to help keep children with SMA alive (and comfortable, at least relative to years past).

However, risks still exist. One is choking. Choking is possible because a child with SMA has weak swallowing and chewing muscles. Another risk is aspiration or inhaling of food. Aspiration can block the airway as well as be a source of infection.

SMA manifests in many ways, which will vary especially according to type. In all types of SMA, you can expect muscle weakness, wasting, and atrophy, as well as muscle coordination problems. The reason for this lies in the nature of the disease itself—SMA affects nervous control of voluntary muscles.

There is no cure for SMA. The most promising prognosis comes about with early detection. Advances in medicine can help manage the complications associated with SMA.

Genetics

SMA is a genetic disorder in which the gene that codes for a muscle protein called SMN (survival motor neuron) is faulty. The malfunctioning of the SMN protein leads to the problems seen in SMA.

SMA is inherited in a recessive pattern. This means that in order for SMA to occur, a child must inherit the defective gene from both parents, and therefore, both parents must be a carrier of the defective gene. It is estimated that about one in 40 people is a carrier of this gene. If both parents are carriers, there is a one in four chance that a child born to them will have SMA.

In some people with SMA, other genes can partially compensate for the one that is producing faulty SMN proteins. As a result, the severity of SMA is somewhat variable from person to person.

Types of SMA

Spinal muscular atrophy affects 1 in 6,000 newborns. It is the leading genetic cause of death in children under the age of 2. SMA is non-discriminatory as to whom it affects.

There are several types of SMA, depending on the degree of malfunction seen with the SMN protein. There are also some varieties of SMA that are related to other genetic problems.

SMA is categorized by severity and age of onset of symptoms. The degree of severity, the amount of protein deficiency in the motor neurons, and (early) age of onset all tend to show correlation with one another. Development of sensations and the mind are completely normal in SMA.

Type 1

Type 1 SMA is the most severe, affecting children under the age of 2. Diagnosis of type 1 SMA is usually made in the first six months of life.

Babies with type 1 SMA are never able to attain normal motor development accomplishments, such as sucking, swallowing, rolling over, sitting, and crawling. Children with SMA type 1 tend to die before the age of 2, usually due to associated breathing problems.

Babies with SMA type 1 tend to be limp, motionless, and even floppy. Their tongues move in a worm-like fashion and they cannot hold their head up when placed in a sitting position.

They may also have noticeable deformities, such as scoliosis, and will have muscle weakness, particularly in the proximal muscles which are located close to the spine.

Type 2

SMA type 2, also called intermediate SMA, is the most common type of SMA. It allows for a longer lifespan than SMA type 1, but a shorter-than-normal lifespan overall. Lifespan may go until the 20s or 30s. Respiratory infection is the most common cause of death in type 2.

SMA type 2 begins either between 6 and 18 months or after the child has demonstrated she or he can sit without support (after being placed in a sitting position). Symptoms of type 2 include deformity, motor delay, enlarged calf muscles, and tremors in the fingers.

Proximal muscles that lie closest to the spine are affected with weakness first; the legs will become weak before the arms. Children with type 2 SMA will never be able to walk without assistance. The good news is that a child with SMA will most likely be able to perform tasks with their arms and hands, such as keyboarding, feeding, etc.

It has been observed that children with SMA type 2 are very intelligent. Physical therapy, assistive devices, and power wheelchairs can go a long way toward contributing to a meaningful life for them.

Two main problems with SMA Type 2 include:

  • Weak respiratory muscles causing infection
  • Scoliosis and/or kyphosis developing due to weak spinal muscles

Types 3 and 4

SMA Type 3, also known as mild SMA, begins after 18 months or after a child has taken five walking steps by themselves. People with mild SMA live for a long time and can have professional careers decorated with accomplishments.

People with SMA type 3 are usually dependent upon assistive devices, and throughout their lives need to continually monitor where they are at with regards to their respiratory and spinal curvature risks. They tend to stop walking some time in their lives. When they stop walking varies between adolescence and their 40s.

While children with type 3 SMA can move and walk, there is muscle weakness and wasting of proximal muscles, i.e. those closest to the spine.

There is a 4th type of SMA, adult-onset SMA. Type 4 generally shows up when the person is in her or his 30s. As you may have guessed, SMA type 4 is the mildest form on the continuum of severity of this disease. Symptoms of type 4 are very similar to those of type 3.

Diagnosis and Testing

The first step in obtaining a diagnosis is for parents or caretakers to notice SMA symptoms in their child, noted throughout this article. The physician should take a detailed medical history of the child, including a family history and a physical exam.

There are several types of tests used to diagnose SMA:

  • Blood tests
  • Muscle biopsy
  • Genetic tests
  • EMG

A lot of issues are generated regarding testing for SMA in children, as well as testing parents for carrier status. In 1997, a DNA test, called the quantitative PCR test for the SMN1 gene, came out on the market to help parents determine if they carry the mutant gene that causes SMA.

The test is done by taking a blood sample. Testing the general population is too difficult, so it is reserved for those who have had people with SMA in their family. Testing is possible prenatally via amniocenteses or chorionic villus samples.

Treatment

Treatment for SMA is focused on life support, encouraging independence, and/or improving the patient's quality life. Examples of care and treatment modalities include:

  • Physical therapy
  • Use of assistive devices, such as wheelchairs, breathing machines, and feeding tubes. (There are many assistive devices for SMA. It is best to discuss this with your treatment team.)
  • Surgery for spinal deformity

Doctors recommend that families work with a healthcare team in a multidisciplinary approach. The SMA patient should be medically evaluated quite often during her or his lifetime. Genetic counseling for family members is very important.

Activity is not to be avoided, but rather used in such a way as to prevent deformity, contracture, and stiffness and to preserve range of motion and flexibility. Therefore, it should not be done to the point of exhaustion. Good nutrition will enable the patient to utilize their muscles as well.

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Article Sources

  • Childrens Hospital of Philadelphia. Your Child's Health - Spinal Muscular Atrophy.

  • Doctor's Guide Publishing. Carrier Testing Now Available For Spinal Muscular Atrophy.

  • Families of Spinal Muscular Atrophy Medical Issues.

  • Muscular Dystrophy Association. Facts About Spinal Muscular Atrophy.

  • NINDS Spinal Muscular Atrophy Page.

  • Tsao, Brian, M.D., Stojic, Andrey, M.D., PhD. eMedicine.com Spinal Muscular Atrophy