The term peripheral neuropathy, or just neuropathy, is a universal expression to describe myriad syndromes that cause disease or damage to the peripheral nerves. The nerves that could be affected by peripheral neuropathy are the sensory nerves that receive sensations of heat, pain, or touch; the motor nerves that control voluntary motor movements; and the autonomic nerves that control blood pressure, heart rate, digestion, and bladder function. Depending on the severity of the disorder and/or which nerve is damaged, it can affect sensation, movement, or gland/organ function. This damage results in the nerve’s decreased ability to transmit and/or receive its signals to and from the central nervous system. In mild cases of neuropathy, a person may feel a tingling sensation, numbness, or paresthesia when using a certain body part for movement. A person with severe neuropathy, however, may experience muscle wasting (hypotrophy), paralysis, or organ dysfunction, such as having problems digesting food.
Neuropathies are classified as acute, chronic, or idiopathic. Cases of acute peripheral neuropathies have a sudden onset and progress quickly. In these cases, an urgent diagnosis and treatment must be made to ensure that the neuropathy can resolve. In chronic neuropathies, the condition is long term and symptoms progress slowly and may be subtle. In fact, a person may not even realize that he or she is experiencing any signs until it is more severe. Some patients will have bouts of relief, but the symptoms usually return. In other cases, some persons will have a progression of symptoms that eventually plateau. Chronic neuropathies are usually the cause of another disorder such as diabetes. Lastly, idiopathic neuropathies are named as such because the cause is unknown.
Single Nerve Damage
If a single nerve is damaged, it results in a condition called mononeuropathy. This is usually caused by localized nerve compression or trauma to a region supplied by a single nerve. Physical compression of the nerve is the most common cause of mononeuropathy, like carpal tunnel syndrome where the median nerve is compressed as it travels through the wrist; or when a person is sitting with his or her leg crossed for an extended period of time resulting in the foot “falling asleep.” This is a temporary condition and the pins and needles sensation is caused by a compression mononeuropathy. By moving around and adjusting to a better position, a person can be easily relieved from this condition.
An example of damage to a single nerve is seen in Bell’s palsy. This condition is characterized by unilateral temporary weakness or total paralysis of the facial nerve, which is responsible for controlling the muscles of facial expression and for the sense of taste from the anterior two-thirds of the tongue. It also provides innervation to lacrimal glands, salivary glands, and the stapes.
Multiple Nerve Damage
It is most common for more than one nerve to be affected. This condition, called polyneuropathy, generally affects more than one limb or several regions of a body. Polyneuropathy is a serious condition because of the large regions it affects as well as the fact that it usually produces symmetrical symptoms on both sides of the body. These may include muscle weakness; tingling and/or burning sensations covering large areas; the loss of fine touch like the inability to distinguish between textures; problems feeling temperatures; and difficulty in balance when standing or walking. Most of these symptoms tend to appear first in nerves that are long such as those that supply the feet and hands, and over time progress toward the trunk. Common causes of polyneuropathies are diabetes, Lyme disease, blood disorders, and neurotoxins.
Three types of polyneuropathy include distal axonopathy, demyelinating polyneuropathies, and neuronopathy. In distal axonopathy, the axons are damaged while the cell bodies are normal. This is seen most often in patients with diabetes with a very distinct pattern of progression. The farthest part of the axon is damaged first and the cell death slowly progresses toward the trunk as the disease advances. Thus, a person first loses sensation in the toes and the soles of the feet, then the lower leg, and eventually the entire leg. Next, the hands are affected starting with the fingers, the palms, and then the entire arm. Ultimately, the trunk of the body can be affected.
In demyelinating polyneuropathies, the protective covering of the axon is affected. This results in decreased ability to conduct nerve impulses along the length of the axon. Persons with this type of disease often have muscle weakness and decreased sensation because the nerve impulse is weakened in both motor and sensory nerves. The most common cause of this type of polyneuropathy is multiple sclerosis.
Finally, neuronopathy, a rare polyneuropathy, is the opposite of distal axonopathy. Here the cell body of the neuron is damaged and the axon is relatively fine. Usually only one type of neuron is affected, like the motor neurons, which are located in the spinal cord; their axons are the peripheral nerves. This would produce a syndrome called motor neuron disease with most sensory function being normal. If the sensory neurons are damaged, then it can disturb pain, temperature, touch sensation and its ability to be perceived by the brain, but motor function may be unchanged.
Patricia A. Bloomquist
See also: Bell’s Palsy; Nerves; Nociception; Paresthesia
Donofrio, Peter D. (Ed.). (2012). Textbook of peripheral neuropathy. New York, NY: Demos Medical Publishing.
National Institute of Neurological Disorders and Stroke (NINDS). (2012). Peripheral neuropathy fact sheet. Retrieved from http://www.ninds.nih.gov/disorders/peripheralneuropathy/detail_peripheralneuropathy.htm