SMA (Spinal Muscular Atrophy) is a disease that robs people of physical strength by affecting the motor nerve cells in the spinal cord, taking away the ability to walk, eat, or breathe. It is the number one genetic cause of death for infants.
SMA is caused by a mutation in the survival motor neuron gene 1 (SMN1). In a healthy person, this gene produces a protein that is critical to the function of the nerves that control our muscles. Without it, those nerve cells cannot properly function and eventually die, leading to debilitating and often fatal muscle weakness.
SMA affects approximately 1 in 10,000 babies, and about 1 in every 50 Americans is a genetic carrier.
Individuals with SMA have difficulty performing the basic functions of life, like breathing and swallowing. However, SMA does not affect a person’s ability to think, learn, and build relationships with others.
One in every 6,000 babies is born with SMA. Of children diagnosed before age two, 50 percent will die before their second birthday. SMA can strike anyone of any race or gender. One in every 40 people carries the gene that causes SMA.
There are four primary types of SMA—I, II, III, and IV—based on age of onset and highest physical milestone achieved.
Type I Acute (Severe)
Type I SMA is also called Werdnig-Hoffmann Disease. The diagnosis of children with this type is usually made before 6 months of age and in the majority of cases before 3 months, there may be lack of fetal movement in the final months of pregnancy.
Usually a child with Type I is never able to lift his/her head or accomplish normal physical milestones. Swallowing and feeding may be difficult and the child may show some difficulties with their own secretions. There is a general weakness in the intercostals and accessory respiratory muscles (the muscles situated between the ribs). The chest may appear concave (sunken in) due to the diaphragmatic (tummy) breathing. Please note: Although diagnosis may be made before 6 months of age it does not necessarily follow the same course of severity for each patient.
Type II (Chronic)
Diagnosis of Type II is almost always made before 2 years of age with the majority of cases diagnosed by 15 months. Children with this type may sit unsupported although they are usually unable to come to a sitting position without assistance. At some point they may be able to stand. This is most often accomplished with the aid of bracing and/or parapodium/standing frame. Feeding and swallowing problems are not usually characteristic of Type II although in some patients this can occur and a feeding tube may become necessary. Tongue fasciculations are less often found in children with Type II but a fine tremor in the outstretched fingers is common. Children with Type II are also diaphragmatic breathers.
Type III (Mild)
Diagnosis of Type III, often referred to as Kugelberg-Welander or Juvenile Spinal Muscular Atrophy, is made sometime after 18 months of age and as late as adolescence. The patient with Type III can stand alone and walk, but may show difficulty with walking and/or getting up from a sitting or bent over position. With Type III, a fine tremor can be seen in the outstretched fingers but tongue fasciculations are seldom seen.
Type IV (Adult Onset)
Typically in the adult form symptoms begin after age 35. It is very rare for Spinal Muscular Atrophy to begin between the ages of 18 and 30. Adult SMA is characterized by insidious onset and very slow progression. The bulbar muscles, those muscles used for swallowing and respiratory function, are rarely affected in Type IV.
This form also known as Kennedy’s Syndrome or Bulbo-Spinal Muscular Atrophy, occurs only in males, although 50% of female offspring are carriers. This form of SMA is associated with a mutation in the gene that codes for part of the androgen receptor and therefore these male patients often have breast enlargement known as gynecomastia. Also noticeably affected are the facial and tongue muscles. Like all forms of SMA the course of the disease is variable, but in general tends to be slowly progressive or nonprogressive.
What Causes Spinal Muscular Atrophy?
Spinal Muscular Atrophy is an autosomal recessive disease, which means that both parents must be carriers. Both parents must have the gene responsible and these genes must be passed onto their child. When a child has received this gene from each of its parents it will than be affected by SMA. Although both parents are carriers the likelihood of passing this gene along to a child and having an affected child is 25%, or 1 in 4.
Familial Forms (affecting other family members) of Spinal Muscular Atrophy in the older age group can occur as autosomal recessive, mutants or autosomal dominant. The genetic defects underlying these diseases make it necessary to be precise regarding the inheritance pattern in a particular family.
Prognosis – What Does it Mean?
Each type of SMA has variability among individual patients. Please keep this in mind when reading this section. In the acute type of this disease the bulbar muscles are often affected, and this may make feeding and swallowing extremely difficult. Breathing is often labored due to reduced strength of the chest muscles, and most breathing can be seen in the abdominal areas, with the chest appearing sunken in. Because of increasing overall weakness or repeated respiratory infections, the prognosis is poor. Death in the majority of children with Type I SMA usually occurs by 2 years of age. AGAIN IT IS IMPORTANT TO NOTE THE OVERLAP OF TYPES I AND II.
Type II Intermediate
Because of the range of progression seen in patients with Type II it is hard to tell how fast, if at all, the weakness will progress. Some children may learn to walk with the aid of bracing and may survive into adulthood. However, others, due to weakened chest and respiratory muscles may become increasingly weak with probable respiratory infections such as pneumonia. There are many cases in which the initial progressive weakness may remain the same, or there may be periods of worsening followed by long periods of stability. With such variables, age of death can vary greatly. It can take place as early as 3 years as in Type I or not until adulthood.
Although not all children diagnosed with Type II develop respiratory weakness, respiratory failure is usually the cause of death following a bout of pneumonia or other respiratory infection.
Type III Mild
Patients with Type III will again vary greatly. However the prognosis is very good. Often walking will be possible, or the patient will be fully functional for years before assistance is necessary. As with Type I and Type II respiratory infections should be presented and necessary precautions taken.
Type IV Adult Onset
There is nothing unusual or distinctive about the current management of the adult forms of Spinal Muscular Atrophy. Proper diagnosis, genetic counseling and appropriate physical therapy remain mainstays.
Though there is currently no approved treatment for SMA, there’s great reason for hope. We know what causes SMA and what we need to do to develop effective therapies, and we’re on the verge of major breakthroughs that will strengthen our children’s bodies, extend life, and eventually lead to a cure.
Source: Families of SMA