Disorders of sensation (abnormal sensitivity, abnormal sensation, numbness, tingling, etc.) can be divided into: 1) those in which sensation is absent (numbness, a feeling like “after the dentist”), and 2) those in which unpleasant sensations, abnormal sensations or so-called neuropathic pain occur (burning, stinging, tingling, etc.). These usually occur together where there is an absence of sensation, but an absence of sensation can occur on its own.
Numbness
This is a disturbance in the sensation and perception of stimuli such as superficial touch, but also pain and temperature. When the sense of touch is impaired, this is subjectively most disturbing. The causes are manifold and may result from injury to nervous structures in the central nervous system (damage to the brain or spinal cord, e.g. in the case of stroke or multiple sclerosis) or in the peripheral nervous system (e.g. nerve damage seen in metabolic diseases, inflammatory diseases, e.g. in the case of polyneuropathies). Only in very severe cases, the lack of feedback from muscles and joints (effects of gravity or acceleration) to the brain, leads to a coordination disorder and gait instability.
Abnormal sensations
These sensory disturbances often involve unpleasant sensations, sometimes described as “electric shocks” or “a crawling sensation”. Here, too, the causes can lie in either the central or peripheral nervous system. Abnormal sensations can also lead to sleep disturbance, for example in the case of restless legs syndrome. They may also occur in attacks or episodes (e.g., in focal epileptic seizures or in severe forms of migraine). In more severe cases, they can also be very painful if there is structural injury to the nerves. The resulting pain is called neuropathic pain because it is triggered by nerve damage itself.
Neuropathic pain
The nature of this pain characteristically comes in sudden attacks and it is often described as burning, stabbing, or sometimes dull. Nerve pain (also called neuralgia) comes from overexcitability of the nerve, which then produces excessive signals independently, due to injury. “Typical” pain medications (aspirin, paracetamol, diclofenac, etc.) are not very effective for the treatment of neuropathic pain. Antiepileptic drugs are generally used here, since they are the only way of reducing the hyperexcitability of the nerve. If the nerve endings are affected by damage, the brain can identify the exact location of the damage at the site of the pain (e.g., in the case of injuries to the fingertips or in the case of polyneuropathy at the tips of the toes). However, if the nerves are damaged at a location along the course of the nerve, the brain has difficulty locating the site of the damage. Therefore, this pain typically radiates both to the area supplied by the nerve (e.g. radiating to the leg in the case of a herniated disc that presses on the nerve root, or to the face in the case of trigeminal neuralgia, to the fingertips in the case of carpal tunnel syndrome from thumb to ring finger, etc.) or it may also be felt along the course of the nerve towards the body (e.g., nocturnal upper arm pain in carpal tunnel syndrome). Neuropathic symptoms may also be associated with arm or leg weakness or clumsiness.
