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Last reviewed: January 2024

These pages may be useful if your child starts asking questions about their SMA. As a guide, it is written for children aged 7+ years.

To understand why some people have SMA, you have to know more about:

Our Bones & Muscles

Inside us we all have a skeleton. It supports and protects our body. It is made of bones that grow as we grow.

The bones that go from our bottom to our head are small and moveable and are called vertebra. They make up our backbone or spine.

As we grow, some of our bones join up together. Where these bones meet , is called a joint – like at our knees or elbows.

Our skeleton can bend at these joints because the bones have muscles attached to them. When our muscles move, so do our bones so that we can do things like wave a hand or nod our head. Our muscles need to stay strong and be able to move when we want them to.

A baby’s body has about 300 bones. As they grow, some bones join up. By the time they are an adult, they only have 206 bones.

Everyone has more than 640 muscles attached to their bones.

Our nerve cells and brain

Everyone’s body is made up of cells – trillions of them. Most cells are so small you would need a microscope to see them. There are all sorts of cells with different jobs. Some are building blocks that make up our bones, our muscles, our hair and skin.

Nerve cells are one sort of cell. They are long and thin and different shapes so that they can carry messages from one part of our body to another. Some nerve cells stretch from inside our spinal cord right down to our big toe!

The nerve cells you need to know about to understand SMA are our lower motor neurons. They are inside our spinal cord. This is protected by our backbone.

Our brain sends instructions to our muscles so that they can move and move our bones. It does this by sending electrical messages through our nerve cells.

Our lower motor neurons pick up the messages that come from our brain and down our spinal cord. From there, our lower motor neurons take the messages to our muscles and tell them to move.

Our genes

Inside every cell there are thousands of genes – these are very important. They control lots of the instructions about us and how our bodies are put together – a bit like a cake recipe. They control things like the colour of our eyes and our hair.

When one of our genes does not work properly, this is sometimes called an ‘altered’ gene. Having an ‘altered’ gene is a bit like having a cake recipe with part of the instructions missing.

Lots of people have ‘altered’ genes of one sort or another – for example, people who are colour blind.

The important genes you need to know about to understand SMA are:

  • The Survival Motor Neuron 1 or the SMN1 gene

Everyone has two of these SMN1 genes in all the cells in their body; they were passed to them by their parents.

Half our genes come from our mum, half from our dad. The exact set of genes that mums and dads pass on to their children is different each time. This is why brothers and sisters are different from each other.
  • The other gene is called Survival Motor Neuron 2 or the SMN2 gene.

Unlike most genes, the number of copies of SMN2 on each chromosome can vary from one person to the next; there can be between 0 – 8 copies.

Our proteins

As well as controlling lots of instructions about us, genes also carry information that is needed to make proteins.

Proteins are even smaller than cells and have different jobs to do, like fighting off infections and building muscles.

Our SMN1 genes make an important protein called: the Survival Motor Neuron Protein or SMN Protein for short.

We need SMN protein to keep our lower motor neurons healthy and working so that they can tell our muscles to move. Lower motor neurons pick up the messages that have come from our brain, all the way down to our spinal cord.

Our SMN2 genes are like the SMN1 gene but they can only make a small amount of protein that works. They are also a bit different in another way – we can have between 0 and 8 copies of them. The more copies we have, the more SMN protein we are likely to be able to make. These genes are sometimes called ‘back-up’ genes.

Our SMN1 and SMN2 genes work together to make SMN protein.

Your SMN1 and SMN2 genes

If you have SMA, you have two ‘altered’ SMN1 genes in all the cells of your body. This means these genes cannot make enough SMN protein to keep your lower motor neurons healthy.

The more copies of the SMN2 gene you have to help make up for the ‘altered’ SMN1 genes, the more SMN protein you should be able to make.

The different amounts of SMN protein that people with SMA can make is one of the reasons why SMA affects people differently.

The chance of inheriting two altered SMN1 genes

Everyone has two SMN1 genes in all the cells in their body. These are passed to them by their parents – one from their mum, one from their dad.

Dads pass on their genes through their sperm; mums pass on their genes through their eggs. When a sperm and egg join up in the mum’s womb, they form an embryo which grows into a foetus and then a baby.

If the mum and dad are carriers of SMA:

  • A carrier of SMA has one ‘healthy’ SMN1 gene and one ‘altered’ SMN1 gene.
  • Parents cannot control what genes they pass on, it is like rolling a dice.
  • For each pregnancy, for a mum and dad who are both carriers, their SMN1 genes can be passed on in one of these four ways:

Do mums and dads know if they are carries?

Not many people know they are carriers of SMA. It is usually not until they have a child who is diagnosed with SMA that they find out.

Because of the way those dice roll, their child born with SMA may be their first child or their second, third or fourth. They do not find out they are carriers until then.

If the mum or dad has SMA themselves, the dice roll a bit differently.

SMA is a rare condition; this means that not many people have it.

Nobody knows exactly how many people have SMA, but in the whole world, approximately 1 person in every 10,000 are born with SMA each year.

To make this easier to picture, think about the huge crowds of people that go to the Glastonbury music festival or to Wembley Stadium for football matches:

  • 200,000 people went to Glastonbury in 2024, so about 20 of them could have been born with SMA.
  • 90,000 people can fit in Wembley Stadium, so that means about 9 of them could have been born with SMA.

The first clues

Working out that someone has got SMA is a bit like solving a mystery or puzzle – doctors know what clues to look for.

One of the first clues was when we started to notice that your muscles were not as strong as they usually are for someone the same age as you.

We asked the doctors if they knew why. They did some tests to see what was happening.

One of these tests was to look at a very tiny bit of your blood using a microscope. This is what doctors do a lot of times when they want to work out why someone is not feeling or growing well.

What they could see in your blood were the tiniest things called your genes.

They could see your two SMN1 genes were not working properly. This was the big clue that told them you had SMA.

They could also see your SMN2 genes and count how many you had. This gave them another clue about your SMA.

The doctors put these clues together and because of them were able to tell us:

What Type of 5q SMA you have

This was not the end of the puzzle.

The doctors still cannot be sure what will happen with your SMA because everyone with SMA is different and as children grow things may change.

Doctors can give children a drug treatment that helps make up for the SMA protein that their SMN2 genes cannot make. A drug treatment can help to keep lower motor neurons healthy and working so that they can tell muscles to move. A drug treatment cannot do this for all lower motor neurons. It cannot cure or get rid of your SMA completely.

There are different drug treatments and not all are suitable for all children. We talked to your doctors about drug treatment and about what we thought was best for you.

There are also other ways your SMA can be managed. Some children have:

  • physiotherapy to help them with their movement
  • equipment like a wheelchair to help them get around
  • equipment to help them with their breathing, sleeping and eating

All these things put together mean that you can join in at home and school and go on trips and holidays.