Numbness refers to the partial or complete loss of sensation. People with numbness may be unable to feel light touch, pain, temperature, or vibration or to know where parts of their body are (position sense). When people do not know where parts of their body are, they have problems with balance, coordination, and walking.
Many people mistakenly use the term numbness when they have abnormal sensations such as tingling, prickling, or a pins-and-needles sensation or when a limb feels weak or is paralyzed—perhaps partly because people with numbness often also have such abnormal sensations and symptoms. The presence of other symptoms depends on what is causing numbness.
If numbness has been present a long time, particularly in the feet, it can lead to other problems. People may have difficulty walking and driving and may be more likely to fall. They may not notice infections, foot sores (ulcers), and injuries because they cannot sense pain as well. In such cases, treatment may be delayed.
For a person to feel sensations normally, sensory receptors (specialized ends of sensory nerve fibers in the skin) must detect information in and around the body. These receptors must then send a signal along the following pathway:
- Through sensory nerves (nerves from the skin to the spinal cord)
- Through spinal nerve roots, formed by sensory nerves joined together into thick short branches that pass through the backbones (vertebrae) to connect with the spinal cord
- Up the spinal cord
- Through the brain stem
- To the part of the brain that perceives and interprets these signals (in the cerebrum)
For some parts of the body, the pathway includes a plexus or the cauda equina.
Plexuses are networks of sensory nerve fibers and motor nerve fibers. In plexuses, these nerve fibers are combined and sorted to serve a particular area of the body. The fibers then branch off from the plexus to become peripheral nerves. There are four plexuses in the torso.
The cauda equina is a bundle of spinal nerve root fibers at the bottom of the spinal cord. This structure resembles a horse's tail, which is what its name means in Latin. It supplies sensation to the thighs, buttocks, genitals, and the area between them, which are called the saddle area because they are the area of the body that would touch a saddle.
Numbness results when one part of the pathway for sensation malfunctions, usually because of a disorder or drug. Many conditions can cause numbness in various ways. For example, they may
- Reduce or block the blood supply to nerves in the body, as occurs in vasculitis, or in the brain, as results from stroke
- Damage part of the pathway for sensation, as may result from injuries or from hereditary disorders that affect nerves (neuropathies), such as Friedreich ataxia
- Put pressure on (compress) part of the pathway
- Infect a nerve, as occurs in leprosy, HIV infection, or Lyme disease
- Cause nerves in part of the pathway to become inflamed and lose their outer layer (called demyelination), as occurs in multiple sclerosis or Guillain-Barré syndrome
- Cause metabolic abnormalities, as may occur in diabetes, vitamin B12 deficiency, or arsenic poisoning or with use of chemotherapy drugs
Pressure on different parts of the pathway has various causes, as in the following:
- On nerves: Repeating specific movements over and over, causing swelling, as occurs in carpal tunnel syndrome, or remaining in one position too long, as when people sit with their legs crossed a long time
- On spinal nerve roots: Rupture or herniation of a disk in the spine, osteoarthritis, or narrowing of the passageway for the spinal cord (spinal stenosis)
- On the spinal cord: A tumor, an injury, or a pocket of blood (hematoma) or pus (abscess) near the spinal cord