Spinal Muscular Atrophy (SMA) is a rare autosomal recessive disease caused by a genetic defect in a gene – the Survival Motor Neuron 1 gene (SMN1) – which encodes a protein widely expressed in cells but which is of crucial importance for survival of motor neurons. Reduced levels of the protein result in death of neuronal cells in the anterior horn of the spinal column, resulting in systematic muscle wasting (atrophy).
SMA is the most common genetic cause of infant death. SMA, which affects one in every 6,000 to 20,000 births, is a complex disease with no known cure.
In SMA, the nerve cells of the spinal cord do not have the genetic programming that enables them to send messages effectively or efficiently to the muscles surrounding the spinal cord. Because the muscles are not stimulated, they become weak and the muscle cells atrophy. Muscle weakness becomes worse over time until the muscles no longer respond. Muscles closest to the trunk (shoulders, hips and back) are often the most severely affected, with leg weakness usually more pronounced than in the arms.
Key life functions such as swallowing and feeding can be affected. Because of the involvement of respiratory muscles used for breathing and coughing, SMA sufferers are at significant risk of contracting pneumonia or experiencing other lung problems. Neck, head and facial muscles can also be affected. With poor head control, there is a risk that difficulties in lifting the head can result in airway occlusion.
Some children eventually lose the ability to suck, chew, drink, spit or smile.
There are multiple forms of SMA, each child and each set of parents facing their own unique decisions. Four types of SMA are recognised, referred to as Type 1 (Infantile SMA or Werdnig-Hoffman disease), Type 2 (Intermediate SMA or chronic childhood SMA), Type 3 (Juvenile SMA or Kugelberg-Welander disease) and Type 4 (Adult onset SMA) although there are also rare variants.
These types display varying degrees of severity, although all are associated with general muscle wasting and mobility impairment. Other body systems may be affected as well, particularly in early-onset forms.
Children with Infantile SMA are diagnosed usually before 6 months of age, more often before 3 months of age. Symptoms may even start in the womb. Many mothers later recall the baby not moving as much in the last month or so of pregnancy. Once born, children are not able to hold up their heads, roll over, crawl, sit up without support, or walk. All of their muscles are extremely weak, with the weakest muscles being in the legs, upper arms, and neck. The child’s chest may appear concave, or very skinny at the top, with a big belly, almost bell-shaped. SMA affects all muscle systems as well including the processes of sucking, swallowing, digesting food, and excretion.
Other types of SMA only become apparent at 6-18 months (Type 2), after 18 months (Type 3) or in adulthood, and patients commonly survive into adulthood.
Currently, there is no cure for SMA.