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Paralysis - the loss of muscle power
A guide for patients and carers
What are the causes of paralysis?
As explained above, paralysis is caused by something being wrong with the nerves that activate muscles. Another word for nerve is neurone. The nerves that send the electrical messages, or impulses, to muscles to make them move or contract are called motor neurones. When we move a part of our body two types of motor neurone are involved in getting the relevant muscles to move: the upper and lower neurones. The upper motor neurone conveys the impulse from the brain to lower down in the nervous system where it connects with the lower motor neurone. The lower motor neurone then provides the final path for the signal to reach the muscle.
Recognising these two different types of motor neurone is important as the nature of the paralysis varies according to which type is mainly affected.
Even while a person is sitting fully relaxed, the motor neurones (both upper and lower) are sending impulses to the muscles to maintain a normal degree of tightness. This is called resting muscle tone. Broadly speaking, upper motor neurone damage causes an increased tightening up of the muscles (i.e. increased tone or ‘hypertonia’). Another term for this increased tone resulting from upper motor neurone damage is spasticity. By contrast, lower motor neurone damage leads to a reduction in, or loss of, the normal tone in the muscles. This paralysis, which is characterised by slack or floppy muscles, is sometimes called flaccidity.
So generally speaking, if the problem lies in upper motor neurones the result is spastic paralysis, while if it is in the lower motor neurone there is a flaccid paralysis. Needless to say things are not always as straightforward as this, but it does help to separate the two main types of paralysis and helps explain why different diseases or injuries cause different symptoms.
The range of causes of upper and lower motor neurone problems is vast and the issue is further complicated by the fact that in some circumstances, both are affected. For that reason no attempt will be made here to list all the possible causes of paralysis.
Generally, conditions that affect the brain and spinal cord result in upper motor neurone paralysis. Some common examples are stroke, tumours, cerebral palsy, multiple sclerosis, spina bifida, and severe trauma or injury to the head, neck or back. Damage to the right side of the brain causes paralysis of the left side of the body and vice versa. When upper motor neurone damage occurs in the spinal cord it tends to involve both sides of the body. Therefore, when an injury to the spinal cord occurs in the lower back or lumbar area it will lead to paralysis of both legs(paraplegia), while damage higher up in the neck region can lead to paralysis of all four limbs (quadriplegia).
Lower motor neurone damage resulting in flaccid paralysis is less common. This can occur when an injury to a limb involves the nerve on its course to the muscles it supplies - for example an injury to the upper arm could lead to a loss of movement in the forearm and hand. More widespread paralysis occurs in a large number of fairly uncommon disorders that are called polyneuropathies.
Polyneuropathies can be inherited; they may be triggered by allergies, poisons or drugs; or they may be a symptom of diseases such as diabetes. One condition which used to be a major cause of paralysis of this type is poliomyelitis, and while new cases of polio are rare in the UK nowadays because of vaccination, there are still many people having to cope with the effects of getting older with paralysis as a result of the disease.
Motor neurone disease is a condition that often involves both types of motor neurone, resulting in a mixture of the two types of paralysis described. This can also occasionally happen when people suffer severe injuries resulting in a combination of upper motor neurone problems from head or spinal injury and lower motor neurone paralysis from injury to the nerves in the limbs.
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