Spinal Muscular Atrophy Explained
It’s SMA Awareness Month, so I’m going explain everything I can about SMA. And yes, I have a type of SMA. This is quite long and in depth but please take the time to read it and share if you’d like. To clear things up, I do not like the term “disease” when talking about SMA, I prefer condition but for science sake, they use disease in the medical articles.
What is SMA?
SMA is short for #SpinalMuscularAtrophy, it’s an autosomal recessive neurodegenerative disease characterised by degeneration of spinal cord motor neurons, atrophy of skeletal muscles, and generalised weakness.
Big words, I know so let’s break that down so you all can understand before we get to the good stuff.
Autosomal Recessive – it’s one of 7 ways that a trait, condition, or disease can be inherited. Two copies of an abnormal gene must be present in order for the condition or trait to develop. This means one abnormal gene from mom and one from dad. If only one parent passes the abnormal gene, you get a carrier. (See the picture below)
Carrier – they have the abnormal gene but are unaffected or don’t have the trait but, they can pass it onto their children.
Neurodegenerative Disease – is an umbrella term for a range of conditions which primarily affect the neurons and their functions in the human brain. They are incurable and debilitating conditions that result in progressive degeneration and/or death of nerve cells. Some examples are, but not limited to –
• #AlzheimersDisease (AD) and other dementias
• #ParkinsonsDisease (PD) and PD-related disorders
• Prion disease
• Motor neurone diseases (MND)
• #HuntingtonsDisease (HD)
• Spinocerebellar #Ataxia (SCA)
• #SpinalMuscularAtrophy (SMA)
Neurons – Neurons are the building blocks of the nervous system, this includes the brain and spinal cord. Neurons normally don’t reproduce or replace themselves, so when they become damaged or die they cannot be replaced by the body. Basically, they are VERY important little dudes.
Atrophy – the general physiological process of reabsorption and breakdown of tissues, involving apoptosis. In SMA, the muscles atrophy.
SMA is caused by homozygous disruption of the survival motor neuron 1 (SMN1) gene by deletion, conversion, or mutation.
Homozygous Disruption – homozygous literally means you inherited two identical versions of a gene. Something, usually a deletion, happens to the SMN1 gene on chromosome 5q and results in insufficient levels of SMN protein in motor neurons
SMN1 – The SMN1 gene provides instructions for making the survival motor neuron (SMN) protein. The SMN protein is found throughout the body, the highest levels are in the spinal cord. This protein is one of a group of proteins called the SMN complex, which is important for the maintenance of specialized nerve cells called motor neurons. Motor Neurons are located in the spinal cord and the part of the brain that is connected to the spinal cord,the brainstem. Motor neurons send signals from the brain and spinal cord to tell skeletal muscles to tense (contract), which allows the body to move.
Okay, now the good stuff and I hope you’re still with me lol. Now, there are subtypes of SMA, because why not complicate things lol right? So let’s go into those subtypes as easily as possible.
In chromosome 5-related SMA (there are rare cases where SMA occurs differently), the greater the number of SMN2 gene copies (yes! There’s a backup gene, but only produces about 10% of the SMN protein) a person has, the more functional SMN protein is available, and thus, the later the onset of disease symptoms and the milder the disease course.
SMA type 0 – also called prenatal onset SMA, is the most severe form of SMA. It affects a baby that is still in the womb. It can be fatal before birth and is almost always fatal within the first year of life. It’s characterized by a decrease in fetal movement during pregnancy. At birth, patients with SMA type 0 present with severe weakness and #Hypotonia (abnormally low muscle tone). Often, these patients present with lack of reaction to stimuli, facial diplegia (#FacialParalysis), and congenital heart defects.
SMA type 1 – also called infantile onset or Werdnig-Hoffmann disease. When SMA symptoms are present at birth or by the age of 6 months, the child is classified as a type 1. Babies typically have generalized muscle weakness, a weak cry, breathing distress, difficulty swallowing and sucking and don’t reach the developmental milestone of being able to sit up unassisted. These babies have increased risk of aspiration and failure to thrive. Typically, these babies have two or three copies of the SMN2 gene.
SMA type 2 (this is my type) – also known as intermediate SMA or Dubowitz disease. When SMA has its onset between the ages of 3 – 15 months and before the child can stand or walk independently, they are classified as type 2. Children with SMA type 2 generally have three copies of the SMN2 gene. Muscle weakness is predominantly proximal (close to the center of the body) and involves the lower limbs more than the upper limbs. Usually, the face and the eye muscles are unaffected. They may have respiratory difficulties including hypoventilation (breathing at an abnormally low rate) in sleep. Some also need mechanical ventilation. Many experience tremors of the hands and tongue (I have this) and #Scoliosis extremely comm(curvature of the spine, again I have this). The progression of this type is variable without medical intervention. Life expectancy is reduced but most individuals live into adolescence or young adulthood.
Many of us with type 2 have very big differences in our weakness. Some people my age can still be somewhat independent, I am not. Some don’t need a Trach and ventilator and some do, myself included.
SMA type 3 – also known as #KugelbergWelanderDisease. Patients develop symptoms after 18 months of age and can walk independently. They first show difficulty walking and running, climbing steps, or rising from a chair. The proximal leg muscles (closest to the torso) are most often affected first, with a #Tremor seen in the hands. Complications include #Scoliosis and joint contractures—chronic shortening of muscles or tendons around joints–caused by abnormal muscle tone and weakness, which prevents the joints from moving freely. Individuals with SMA type III may be prone to respiratory infections, but with care most have a normal lifespan.
SMA type 4 – symptoms develop after 21 years of age, with mild to moderate proximal (center of the body) muscle weakness and other symptoms. Patients are able to achieve motor milestones and maintain their mobility throughout life. This type accounts for less than 5% of overall SMA cases.
SMA is the most common genetic cause of infant death. In the United States, approximately 1 in every 11,000 babies are born with a type of SMA.
And an estimated 1 in 40 people are carriers of the gene.
In Canada, approximately 1 in 6000 babies born are affected, and about 1 in 40 people are genetic carriers. Sensation and the ability to feel are not affected. Intellectual activity is normal and it is often observed that patients with SMA are unusually bright and sociable.
As I said before, SMA is a genetic disease where both parents are carriers. 2 carrier parents have a 25% chance of having a child with SMA. Then a 50% chance of having a child that is a carrier of SMA. And finally, a 25% chance of having a child that is not a carrier and also does not have the disease.