Introduction
Almost all people with MS develop some degree of problems with coordinated movement during the course of their disease. While weakness and spasticity directly impact the ability to move a limb, ataxia and tremor impact movement even if the strength is normal.
People with ataxia lose control over their movements, so that the movements become less smooth. It can also become difficult to start and to stop a movement. While ataxia affects all movements of a limb, it is especially obvious in skilled movements, such as writing, typing, using cutlery, working with a computer mouse or reaching for small objects.
Tremor is a rhythmic shaking movement that most often affects the arms and hands in people with MS. Tremor in MS is often associated with ataxia and can affect all tasks that require a steady hand.
How frequent are ataxia and tremor in MS?
Ataxia and tremor are both common in MS. One study found that four out of five people with MS experience ataxia at some point of their disease course, and another study showed that one third of a large group of persons with MS had ataxia and tremor that was severe enough to influence their ability to function.
The most common forms of tremor in MS are tremor that occurs when a limb is held up against gravity (postural tremor) and tremor that occurs when grasping for an object (intention tremor).
Intention tremor of the hands can be especially disabling because the shaking movement becomes worse the nearer the hand gets to the object, for instance when taking off one's glasses or when reaching for a cup. Tremor can be detected in one quarter to one half of all people with MS, but is severe in only a minority (about one in ten).
What causes ataxia and tremor?
MS is a disease that causes damage in many different places in the brain and spinal cord. This makes it difficult to link a symptom to a specific area of damage, and consequently to find the specific cause of any symptom.
However, ataxia, intention tremor and postural tremor are all believed to be due to damage to the cerebellum (see opposite) and its connections, mainly because these symptoms in MS are very similar to those seen in other diseases of the cerebellum. Some evidence for this assumption comes from research studies which showed that the disability from ataxia in MS is related to the amount of damage to the cerebellum.
Normal coordinated movement
To make a coordinated limb movement, such aspicking up a pen or reaching for a cup, the actions of the many muscles in the arm are required to be precisely coordinated. The activation of muscles with opposing actions needs to be precisely timed so they do not interfere with each other. This precise control is achieved by a network of nerve cells in the cerebellum and other regions of the nervous system that uses information on the position of the limbs and activation levels of the muscles to generate a plan for smooth movement.
The cerebellum is central in this network and does much of the actual processing work. We can get an idea about the difficulty and complexity of this task from the fact that the cerebellum contains about half of all nerve cells in the brain.
The cerebellum:
● receives detailed information on the threedimensional position of the limbs and joints and about the activation of the muscles via the spinal cord;
● processes this information;
● develops a plan for the next step in the movement, and;
● communicates this plan to the regions of the brain that control the movement.
A simplified scheme of this network is shown in figure 1 (above). Every small step of an ongoing movement is processed via this feedback loop, which makes a smooth movement possible overall.
 What goes wrong in MS?
The network of the cerebellum and its connections is relatively widespread and encompasses the spinal cord, the cerebellum and several further regions in the brain. Damage to any of the components of this network can occur relatively easily, for example when a demyelinating plaque develops in the cerebellum or in a pathway that connects the cerebellum to the other components of the network. Any such damage can impair the working of the whole network and lead to a lack of coordination: ataxia.
Intention tremor is closely associated with ataxia and is also caused by damage to the same system. The cause of postural tremor in MS is less well understood, but some believe that damage to the cerebellum and its connections plays an important role in the development of this form of tremor as well.
Conclusion
Ataxia and some forms of tremor are very common problems in MS. Ataxia, and probably tremor in MS, are the result of damage to the cerebellum and its connections, which form a highly complex network of nerve cells in the brain and spinal cord. These symptoms can cause much disability and remain a major challenge for MS treatment.
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