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Yesterday upon the stair
I met the man that wasn't there.
He wasn't there again today.
Oh, I wish he'd go away!
Paresthesias are among the most common problems in MS. A paresthesia is simply an abnormal sensation without an apparent cause. But, understanding what they really are as applied to ourselves are can be a little tricky. To really understand what can be happening, one needs a basic understanding of what a nerve is doing.
A Little Bit About Nerves and How They Work
Nerves can carry a signal in only "one" direction. Ever. All nerves are this way. The main nerves that we deal with in MS are the motor nerves and the sensory nerves. The motor nerves carry the signal "from" the brain to the muscles. We want to move and the brain sends the signals to cause the muscle to contract. The sensory nerves carry signals of sensation or feeling in the opposite direction, from out in our body "to" our brain. This is how we experience the world.
Sensory nerves come in many flavors depending on the type of nerve ending. There are special nerve endings for pain, soft touch, pressure, heat, vibration and many othersincluding the special senses of sight, smell, hearing, and taste. When a sensory nerve ending is stimulated the nerve carries the information to the brain and we percieve or feel it. If a pain nerve is stimulated by injury, whether by a thorn or a broken bone, we feel pain. The difference in how much pain we feel depends on how many nerves are stimulated and how fast they fire. The faster they fire the more painful something is.
Not Enough Signal
When a nerve is damaged it can do several things. The effect can be Negative. It can have no signal arrive at all or the signal may be reduced in strength. If it is totally interrupted on it's way to the brain no signal at all will get through. This causes numbness. Even if the interruption is high up in the spinal cord or the brain, if it doesn't arrive at its destination in the final area of the brain there is no sensation from it. So, if the foot is injured, there may be a normal pain signal in the nerve almost all the way up, but if it is completely interrupted before reaching the final spot in the brain, there is no pain felt. The area is numb. Numbness is the final result when any of the sensory nerves are completely cut off. So when an area is really totally numb we can't feel touch, pain, hot, cold - anything.
Also, a damaged nerve may have a reduced signal arriving at the brain. In this case we will feel pretty much the correct sensation, but not enough of it. This is also a negative (reduced) effect. If the nerves carrying the sensation of heat are affected by MS, then we may think that very hot bath water is just warm and we may burn ourselves. The sensation of warmth is not an abnormal sensation, it is just not enough of the normal sensation. This is not technically a paresthesia.
PARESTHESIAS - Too Much Or the Wrong Kind of Signal
The opposite of a Negative signal is a Positive error signal. The signal is there, but it is too much or it brings wrong information. It may also bring information when there is no reason for it to be carrying a signal, like the tingling hand that isn't being touched or tickled. These positive error signals are called "paresthesias."
When a sensory nerve is damaged, like in MS, if it can send a bad signal to the brain. We feel this as a paresthesia. It sends signals that are inappropriate to what is happening. Our feet or face may tingle, but there is nothing causing the tingling. We may feel a band-like sensation around our chest or around a limb. but there is nothing wrapping around us to cause it. The tongue might feel burned, but there have been no hot foods. When we flex our neck, a lightening sensation may zap in various areas (L'Hermitte's), but no one is behind us with a cattle prod. A common problem is the sensation of a hot or cold patch (which we sometimes interpret as wet). Many people have experienced itching in an area that is numb after surgery. Scratching the itchy, but numb, area is useless and does not relieve the itch.
A paresthesia may be constant or it may be intermittent. It can also be triggered when something does stimulate the area, as when simple soft touch is felt as unbareably painful. This is a condition of paresthesia known as allodynia.
The special senses of sight, smell, hearing and taste may also be affected in MS and cause paresthesias. In Optic Neuritis, a person may see flashing lights, halos, wavy lines, things that appear to scurry round in the peripheal vision. These are paresthesias. If the visual signal is reduced there my be shadows, visual field defects, or diminished color saturation (intensity). The sense of smell may disappear. Or we may have paresthesias of smells that are not there. For some reason these smells are always odd and usually very unpleasant. (why can't we smell cinnamon or lilacs?). Loss of hearing is also not uncommon in MS. Paresthesias of hearing and taste occur, but are less common.
The Special Case of Pain
In truth, pain can be a paresthesia. If there is no injury to the body part, but it hurts anyway, it is a painful paresthesia. However, pain is just usually called "pain." In medicine we tend to call it nerve pain or neuropathic pain. Just like other paresthesias it can be constant or intermittent. It can be stimulated by touching the painful part or it can also occur spontaneously (all by itself). People tend to think all paresthesias have to do with the skin, but pain especially can occur in any area that has sensation, including deep in the muscles or bones in the abdomen, the throat or anywhere else that has sensation.
Other Examples of Paresthesias
Paresthesias are not harmless. Though the sensations themselves do no harm, their very presence can destroy the quality of life. Many just cannot be ignored. They also can cause secondary problems by giving us bad information about our world. We can be burned or injured if we don't feel/perceive the threatening object. Visual paresthesias may obscure something we need to see to get around effectively or to drive. We may drop something because we didn't have good information about whether we have it securely. The hand holding it was tingling, but not sending touch and pressure information correctly to the brain.
On the other hand, we can become so accustomed to having paresthesias that we ignore real sensations. This can be just plain embarassing when we ignore the syrup on our chin or harmful when there really is a rock in the shoe.
Paresthesias that are affecting the quality of life can be treated, much like nerve pain is treated. See the Health Page listed under TREATMENTS --> Pain for a discussion of this.