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Causes and Mechanics of CMT

CMT is caused by a genetic fault (mutation) that leads to damage of the nerves in your legs or arms.

In order for you to move with speed and precision, messages must be relayed between your brain and the rest of your body within a fraction of a second. These messages are relayed through your nerves. For example, if you want to move your leg, an electrical message is sent from your brain, via the spinal cord, to the muscles in your leg along a motor nerve. If you cut or burn your leg, you feel it because an electrical signal is sent from the affected area, up the sensory nerves, via the spinal cord, to your brain.

The nerves in your arms and legs, called peripheral nerves, can be compared to electrical cables. The central ‘wire’ is known as the axon and the ‘plastic outer’ is called the myelin sheath.

Axons transmit the electrical signals to and from the brain, and the myelin sheath acts as insulation, speeding up the signal and nourishing the central axon.

nerve

Some forms of CMT affect the axon – making the signal to and from the brain weaker and less efficient. Other forms of CMT affect the myelin sheath, slowing down the signal: if the signal is slowed down, the axon is eventually damaged as well.

Damage to the axon (whether the problem started in the axon or initially in the myelin) causes the symptoms of CMT. Without an intact axon and myelin sheath, your nerves are unable to activate target muscles or relay sensory information from your limbs back to the brain.

As of November 2014, 80+ genes have been found to cause different types of CMT. Each one of these genes is responsible for making particular proteins that are essential to the axon or myelin sheaths.

CMT – the axon and myelin sheath

Some more information about how the myelin sheath and the axon work together may be helpful at this point.

To recap, our nerves can be compared to an electrical cable. The wire running down the inside of the cable is called the axon and the insulating plastic is called the myelin sheath.

Simply put, damage to the axon means that the signal becomes weaker, whereas damage to the myelin sheath slows down the signal (doctors call this nerve conduction velocity).

What is not so commonly known is that it is only when the axon itself is damaged that you get the symptoms of CMT. So, why does CMT1, in which the myelin sheath is damaged, lead to the symptoms of CMT?

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As well as insulating the axon, the myelin sheath also nourishes it. Eventually, if damage to the myelin sheath continues, the axon is damaged as the myelin breaks down. This is known as secondary axonal damage. Only when this happens do the symptoms of CMT become apparent.

What this means is that, if you have CMT1, although the speed that your nerves pass on messages may be slow, this in itself will not cause the symptoms of CMT. In fact you can live with slow nerves for decades with no symptoms or signs of CMT. It is only when the damage to the myelin sheath becomes so severe that the axon is also damaged that you will be affected.

The mechanics of CMT

Whatever form of CMT you have, the mechanics are broadly similar.

The damage caused by CMT to your peripheral nerves may lead to two underlying problems, known as primary symptoms. Problems usually start in the feet as the nerves to the feet are the longest in your body. They can then affect the hands.

Muscle wasting (loss of muscle mass) and weakness (loss of muscle power), usually first noticed in your feet and later in your hands. Because the muscles in your legs and arms stop receiving signals from your brain – due to the damage to the peripheral motor nerves – they start to waste away through lack of use, leading to muscle weakness.

Loss of sensation again usually starting in your feet and later in your hands, although this is often not noticed until it is severe or has caused skin problems.

Muscle wasting and weakness
  • As the muscles become wasted and weak two problems arise in addition to the weakness:
    contractures
  • strain on other muscle groups

Contractures
Nearly all muscles have an opposing muscle that balances the body as it moves. These pairs of muscles are called antagonists and allow for precise body control. For example, your biceps are the muscles that bend your arm at the elbow (the classic body-builder’s pose) and your triceps do the exact opposite, straightening your arm. Antagonistic muscles control every single joint – elbow, ankle, hip and those in your toes and fingers.

Muscles have a natural tendency to contract and tighten, only being stretched by their opposing muscle. Problems arise when a muscle weakens: the opposing muscle that continues to function is not stretched by this weak muscle, allowing the stronger muscle to become tighter and shorter. This mismatch can pull the joint out of shape.

One of the most important goals of managing your CMT is to stop this tightening of a muscle before it damages the joint, reduces flexibility and leads to deformity. This can best be achieved by daily stretches and regular exercise. For more on this see Chapter 3, Managing your CMT: Stretching, exercise and physiotherapy.
Problems caused by an imbalance between muscles usually start as flexible deformities and progress to fixed deformities.

  • Flexible deformities – the joint, although damaged, can still be moved manually and it may be possible to prevent further damage, such as a fixed deformity, through managed stretching, physiotherapy and orthotics.
  • Fixed deformities – the joint has ‘welded’ together. Usually the only effective treatment at this point is surgery, although orthotics and physiotherapy can both help prevent further problems.

In CMT the most common changes are found in the foot and ankle, due to wasting and weakness of the shin muscles. The foot is mainly controlled by the shin muscles – at the front of your lower leg – which pull the foot up, and the calf muscles – large muscles at the back of your lower leg – which pull the foot down.

Because of weakness in the shin muscles, people develop foot drop as it becomes harder for the shin muscles to pull up the foot. Often this is accompanied by the heel turning in so that, when viewed from behind, it looks as though the person is walking on the outside edge of the foot, causing instability and balance problems. Medically this is known as heel varus.

At the same time, the Achilles tendon at the back of the foot and the calf muscles meet less and less resistance from the shin muscles and become shorter and stiffer through lack of use. If the calf muscle and the Achilles tendon are left to tighten and contract, they will pull the foot and toes out of shape, leading to very high arches – medically known as pes cavus or cavus foot – and clawed toes.

 

 

 

References:

A Practical Approach to the Genetic Neuropathies; Rossor AM, Evans MRB, Reilly MM. Pract Neurol Published Online First: [ 10 March 2015] doi:10.1136/practneurol-2015-001095

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