What is Spinal Muscular Atrophy?

Spinal Muscular Atrophy (SMA) is an inherited neuromuscular condition that very specifically affects nerve cells in the spinal cord called lower motor neurons. Acting as a message delivery system allowing the conscious contraction of muscles, electrical signals originating in the brain are fired down the spinal cord, along upper motor neurons and on to skeletal muscles via the lower motor neurons.

In SMA, the lower motor neurons degenerate, impairing the link between the brain and muscles. These muscles can no longer be excited, which causes them to atrophy, or wither, due to inactivity. The motor neurons mainly affected in SMA are those which enable walking, crawling, arm and hand movement, head and neck movement, and swallowing. The muscles used in breathing can also be affected leading to breathing complications. The brain and sensory nerves, which allow us to feel sensation such as temperature and touch, remain relatively unaffected in SMA.

SMA is a relatively rare disease that affects approximately 1 in 6,000-10,000

 babies born. SMA is a recessive genetic disorder, meaning that two faulty copies of the disease gene have to come together, one from each parent, for the disease to occur. When both parents carry a faulty copy of the disease gene, there is a 1 in 4 chance in each pregnancy of the baby being affected by SMA. About 1 in 40-60 of us carries one of these faulty gene copies that cause the condition. For more information on the population statistics of SMA, click here.

There are 4 main types of SMA - Types 1, 2 and 3 appear in childhood, while Type 4 affects adults, and is known as Adult Onset SMA. The disease gene behind Types 1-4 is called Survival Motor Neuron 1 (SMN1).

Other rarer forms of SMA include SMA with Respiratory Distress (SMARD), Spinal Bulbar Muscular Atrophy (SBMA), and Distal SMA (DSMA). These diseases are genetically distinct and are caused by faults in different genes.

The inheritance pattern of the adults and rarer forms of SMA can be different from the childhood forms. All types of SMA produce muscle weakness but in varying degrees of severity. Of the conditions caused by faults in the SMN1 gene, Type 1 SMA is the most severe and is usually fatal with two years of birth.


Spinal Muscular Atrophy Type I

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 the diagnosis is made before 3 months of age. Some mothers even note decreased movement in of the final months of their pregnancy.

Usually a child with Type I is never able to lift his/her head or accomplish the normal motor skills expected early on in infancy. They generally have poor head control, and may not kick their legs as vigorously as they should, or bear weight on their legs. They do not achieve the ability to sit up unsupported. Swallowing and feeding may be difficult and are usually affected at some point, and the child may show some difficulties managing their own secretions. The tongue may show atrophy, and rippling movements or fine tremors, also called fasciculations. There is weakness of the intercostal muscles (the muscles between the ribs) that help expand the chest, and the chest is often smaller than usual. The strongest breathing muscle in an SMA patient is the diaphragm. As a result, the patient appears to breath with their stomach muscles. The chest may appear concave (sunken in) due to the diaphragmatic (tummy) breathing. Also due to this type of breathing, the lungs may not fully develop, the cough is very weak, and it may be difficult to take deep enough breaths while sleeping to maintain normal oxygen and carbon dioxide levels.


Spinal Muscular AtrophyType II

The Diagnosis of Type II SMA 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 when placed in a seated position, although they are often unable to come to a sitting position without assistance. At some point they may be able to stand. This is accomplished with the aid of assistance or bracing and/or a parapodium/standing frame. Swallowing problems are not usually characteristic of Type II, but vary from child to child. Some patients may have difficulty eating enough food by mouth to maintain their weight and grow, and a feeding tube may become necessary. Children with Type II SMA frequently have tongue fasciculations and manifest a fine tremor in the outstretched fingers. Children with Type II also have weak intercostals muscles and are diaphragmatic breathers. They have difficulty coughing and may have difficulty taking deep enough breaths while they sleep to maintain normal oxygen levels and carbon dioxide levels. Scoliosis is almost uniformly present as these children grow, resulting in need for spinal surgery or bracing at some point in their clinical course. Decreased bone density can result in an increased susceptibility to fractures.


Spinal  Muscular Atrophy Type III

The diagnosis of Type III, often referred to as Kugelberg-Welander or Juvenile Spinal Muscular Atrophy, is much more variable in age of onset, and children can present from around a year of age or even as late as adolescence, although diagnosis prior to age 3 years is typical. The patient with Type III can stand alone and walk, but may show difficulty with walking at some point in their clinical course. Early motor milestones are often normal. However, once they begin walking, they may fall more frequently, have difficulty in getting up from sitting on the floor or a bent over position, and may be unable to run. With Type III, a fine tremor can be seen in the outstretched fingers but tongue fasciculations are seldom seen. Feeding or swallowing difficulties in childhood are very uncommon. Type III individuals can sometimes lose the ability to walk later in childhood, adolescence, or even adulthood, often in association with growth spurts or illness.


Spinal Muscular Atrophy Type IV (Adult Onset)

In the adult form, symptoms typically begin after age 35. It is rare for Spinal Muscular Atrophy to begin between the ages of 18 and 30. Adult onset SMA is much less common than the other forms. It is defined as onset of weakness after 18 years of age, and most cases reported as type IV have occurred after age 35. It is typically 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.

Patients with SMA typically lose function over time. Loss of function can occur rapidly in the context of a growth spurt or illness, or much more gradually. The explanation for this loss is unclear based on recent research. It has been observed that patients with SMA may often be very stable in terms of their functional abilities for prolonged periods of time, often years, although the almost universal tendency is for continued loss of function as they age.
For information on Kennedy's Disease, please see www.KennedysDisease.org.