Difficulty with walking in MS can result from plaques in different places in the brain stem and spinal cord. The location of the plaques determines, in large part, whether that difficulty is due to the particular problems of weakness, loss of sensation, or incoordination of the legs. In certain places in the brain and spinal cord, plaques can produce weakness; those in the back part of the spinal cord cause certain kinds of sensation loss (position sense); others in the cerebellum and its connections lead to incoordination in the legs. Any or all of these disturbances can contribute to gait difficulty.
The most common problem that causes difficulty in walking is "weakness." When a patient complains of "weakness," he or she is often describing one of several different problems. Muscle weakness in nervous system disease is often the result of messages not getting to the muscles from the brain and spinal cord. A signal or message may begin in the brain (the precentral or motor area of the brain), but it has to travel to neurons located in the spinal cord, perhaps up to 3 feet away. The signal travels down a small part of the spinal cord called the pyramidal tract.
One or more MS plaques along the way may prevent part, or all, of that message from getting to the neuron, resulting in "weakness" and difficulty standing and walking. The larger the plaque, the more likely it is to interfere with many messages to many neurons and produce more weakness of the muscle these neurons supply.
Sometimes patients complain of weakness, but the examining neurologist may not detect any weakness. What is going on? If he or she watches the person walk, the patient may be seen dragging one leg; if the patient walks up and down the hallway without stopping, this same patient may not be able to walk 100 yards. Repeated examination may reveal a lot of weakness in certain leg muscles.
If different nerve fibers are affected, such as those sensory fibers that send messages to the brain telling it where the legs are, then the person will not be able to control the legs. Movement might occur, but it may be clumsy and poorly controlled and, therefore, not be useful. This is a major problem! The patient's legs may or may not feel "numb."
Another problem that contributes to walking difficulty, especially early in the illness, is a lack of coordination because of a plaque in the cerebellum (that part of the brain just above the spinal cord inside the skull). (The cerebellum looks like a couple of tennis balls stuck together on top of something that looks like a fatter spinal cord.) A plaque in the spinal cord can also cut off fibers connecting the cerebellum to neurons in the spinal cord, resulting in gait problems. The patient does not have to have a tremor to have difficulty because of a plaque in the cerebellum or its connections.
In summary, MS plaques can affect several different parts of the brain and spinal cord and cause difficulty in walking.
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