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- Main article: Somatosensory disorders
Paresthesia or paraesthesia (paraesthesia in British English) is a sensation of tingling, pricking, or numbness of a person's skin with no apparent long-term physical effect, more generally known as the feeling of pins and needles or of a limb being "asleep".
Transient paresthesia is the temporary sensation of tingling, pricking, or numbness of the skin -- "pins and needles." It is usually felt in the extremities (hands, arms, legs, or feet), but can also occur in other parts of the body. This temporary sensation is usually caused by inadvertent pressure placed on a superficial nerve. The sensation gradually goes away once the pressure is relieved.
Chronic paresthesia indicates a problem with the functioning of nerve cells, or neurons. This malfunction, which is especially common in older individuals, is often the result of poor circulation in the limbs (such as in peripheral vascular disease), or may be caused by atherosclerosis—the build up of plaque on artery walls. Without a proper supply of blood and nutrients, nerve cells can no longer adequately send signals to the brain. Because of this, paresthesia is also a symptom of vitamin deficiency and malnutrition, as well as metabolic disorders like diabetes and hypothyroidism.
Another cause of paresthesia, however, may be direct damage to the nerves themselves, or neuropathy, which can stem from injury or infection such as Lyme disease, or which may be indicative of a current neurological disorder. Chronic paresthesia can sometimes be symptomatic of serious conditions, such as a transient ischemic attack, a brain tumor, motor neurone disease, or autoimmune disorders like multiple sclerosis or lupus erythematosus. A diagnostic evaluation by a doctor is necessary to rule these out.
Other known causes of paresthesia (aside from pressure on the affected area):
- Anticonvulsant drugs such as topiramate, sulthiame, and acetazolamide
- Heavy metals
- Lidocaine poisoning
- Multiple sclerosis
- Radiation poisoning
- Withdrawal from certain SSRIs, such as Paroxetine
- Vitamin B12 deficiency
In some cases, rocking the head from side to side will painlessly remove the "pins and needles" sensation in less than a minute. A tingly hand or arm is often the result of compression in the bundle of nerves in the neck. Loosening the neck muscles releases the pressure. Compressed nerves lower in the body govern the feet, and standing up and walking around will typically relieve the sensation. 
An arm that has "fallen asleep" may also be "awoken" more quickly by clenching and unclenching the fist several times; the muscle movement increases blood flow and helps the limb return to normal.
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