Paralysis is a loss of strength in and control over a muscle or group of muscles in a part of the body. Most of the time, this is not due to a problem with the muscles themselves. It is more likely due to a problem somewhere along the chain of nerve cells that runs from the body part to your brain and back again. These nerve cells deliver the signals for your muscles to move.

There are many types and degrees of paralysis. The condition can be:

• Partial, when you still have some control of your muscles (sometimes called paresis).
• Complete, when you can’t move your muscles at all.
• Permanent, when muscle control never comes back.
• Temporary, when some or all muscle control returns.
• Flaccid, when the muscles get flabby and shrink.
• Spastic, when the muscles are tight and hard and jerk around oddly (spasm).

Paralysis can occur in any part of the body and is either localized, when it affects only one part of the body, or generalized, when it affects a wider area of the body.

Generalized paralysis is broken down based on how much of the body is paralyzed:

• Monoplegia affects one limb only, such as one arm or one leg.
• Hemiplegia affects one side of the body, such as the leg and arm of the same side of the body.
• Diplegia affects the same area on both sides of the body, such as both arms or both sides of the face.
• Paraplegia affects both legs and sometimes parts of the trunk.
• Quadriplegia affects both arms and both legs and sometimes the entire area from the neck down. The function of the heart, lungs, and other organs might also be affected.

Causes

Muscle movement is controlled by trigger signals relayed from the brain. When any part of the relay system — such as the brain, spinal cord, nerves, or junction between the nerve and the muscle — is damaged, the signals to move do not make it through to the muscles and paralysis results. There are many ways the relay system can be damaged.

A person can be born with paralysis due to a birth defect such as spina bifida, which occurs when the brain, spinal cord, and/or the covering that protects them do not form the right way. In most cases, people get paralysis as the result of an accident or a medical condition that affects the way muscles and nerves function. The most common causes of paralysis include:

• Stroke
• Spinal cord injury
• Head injury
• Multiple sclerosis