Neuropathy Causes, Types and Symptoms
What is Neuropathy?
Nearly 20 million Americans suffer from neuropathy (neuro=related to the nerves; pathy=disease), a chronic condition that results from damage to or compression of the nerves outside the spinal cord and brain. Also referred to as peripheral neuropathy, the disorder can manifest in different forms – as mononeuropathy when only one nerve is affected, or as polyneuropathy when many nerves are involved, often symmetrically, on both sides of the body. Symptoms include pain, numbness, tingling and weakness of the affected extremity.
The peripheral nervous system is a network of 43 pairs of motor and sensory nerves that connect the brain and spinal cord (the central nervous system or CNS) to the entire human body. These nerves control the functions of sensation, movement and motor coordination. Neuropathy is not a single disease. Instead, it is a complication found in a number of different underlying medical conditions. More than 100 types of peripheral neuropathy have been identified, each with its own particular set of symptoms, patterns of development and prognoses.
Mononeuropathy may involve any part of the body and is usually the result of injury, although disorders of the whole body may cause isolated, single nerve damage. Long-term pressure on the nerve due to injury or swelling may lead to damage of the nerve’s myelin sheath or the axon, which is part of the nerve cell. This damage slows or prevents signals from traveling through the damaged nerves.
In the most common forms of polyneuropathy, the nerve fibers (individual cells that make up the nerve) that malfunction first are those that are most distant from the brain and the spinal cord. The pain and other symptoms that often appear symmetrically, for example, may occur in both feet followed by a slow progression up both legs. Next, the fingers, hands, and arms may become affected, and those symptoms can progress into the central part of the body. This example is characteristic of diabetic neuropathy, in which people experience this pattern of ascending nerve damage.
Why Does Peripheral Neuropathy Occur?
The peripheral nervous system sends messages from the brain and spinal cord to the rest of the body: the arms and hands, legs and feet, internal organs, joints and even the mouth, eyes, ears, nose, and skin. Peripheral nerves also relay information back to the spinal cord and brain from the skin, joints, and other organs. Peripheral neuropathy occurs when these nerves are damaged or destroyed, resulting in the symptoms outlined further below.
Three main types of nerves can be involved in peripheral neuropathy: autonomic, motor and sensory. Impaired function and other symptoms depend on which nerve is damaged or if it’s a combination of any of those three:
- Autonomic nerves –These are the nerves that aren’t under conscious control. They are “automatic” or “involuntary” nerves and they control functions, such as heart rate, blood pressure, digestion and sweating. When the autonomic nerves are damaged, it can cause alterations in heart rate and sweating. In addition, autonomic nerve damage may result in dizziness, difficulty in swallowing, nausea, vomiting, diarrhea or constipation, problems with urination, abnormal pupil size, and sexual dysfunction.
- Motor nerves – These nerves send signals to all of the body’s muscles from the brain and spinal cord. This allows activities such as walking, exercising, or using the fingers to perform manual tasks. Damage to motor nerves can result in cramps, spasms, general muscle weakness or difficulty with motor movement, such as walking or moving arms.
- Sensory nerves –These nerves send messages in the reverse direction than motor nerves (i.e., from the muscles, joints and skin, back to the spinal cord and the brain). Sensory nerves allow people to judge if an object is sharp, rough, or smooth; if something is hot or cold; or if a body part is still or in motion. Damage to sensory nerves lessens the ability to feel vibrations and touch, resulting in a general sense of numbness, especially in the hands and feet.
- Combination neuropathies – This is the medical term for those neuropathies with involvement of several types of nerves, such as predominant motor neuropathy or sensory-motor neuropathy.
Common Types of Neuropathy
Diabetes is one of the most common causes of peripheral neuropathy in the U.S. Even over time, and despite treatment, neuropathy will occur in 50 percent of diabetics. There is no consensus on why it occurs, nor any medical treatment to prevent it. With time, the nerves become injured by high blood glucose levels. The excess glucose damages the walls of the small blood vessels that nourish nerves and injures the delicate coating that surrounds nerves. Symptoms are most common in the feet and legs, and can occur from either Type 1 or Type 2 diabetes. Risk rises with age and duration of time a person has had diabetes. Most commonly, neuropathy rates are highest for those who have had the disease for 25 years.
Also known as entrapment neuropathy, and often referred to as a pinched nerve, this is the most common cause of mononeuropathy. Carpal tunnel syndrome is the most widely known compression neuropathy followed by ulnar neuropathy at the elbow, and compression of the peroneal nerve at the fibular head, which causes foot drop.
Idiopathic Compression Neuropathies
Approximately 30 to 40 percent of neuropathy cases are idiopathic (no known cause). Just because the neuropathy is of an undetermined cause does not mean it cannot be treated. In fact, our expertise at The Institute for Advanced Reconstruction is in assessing those patients whom we determine are treatable, and relieving the symptoms of neuropathy regardless of the cause.
Medication-induced neuropathies are uncommon (2 to 4 percent of cases in one outpatient neurology setting), but crucial to recognize because intervention can lead to significant improvement or symptom resolution.
Numerous medications have been associated with neuropathy, but many more agents are suspected of causing neurotoxicity, including peripheral neuropathy, than have been confirmed. Also, many cases likely remain undiagnosed.
(Source: Medication Induced Neuropathy)
Neuropathy is a common side effect of certain medications, especially those used to treat HIV/AIDS and those used in chemotherapy to fight cancer. According to the American Cancer Society, chemo drugs spread through the whole body, and certain types of chemo can damage different nerves. Symptoms tend to start farthest away from the head, but move closer over time. In most cases, people will notice chemo-induced peripheral neuropathy (CIPN) symptoms in the feet, then later on in the hands.
In some people, medications may cause nerve damage that results in a loss of sensation or movement in part of the body. Peripheral neuropathy will often go away if these drugs are changed or discontinued, or if the dose is reduced. In extreme cases, however, the nerve damage may be permanent.
Some of the drugs that may cause peripheral neuropathy include:
- Anti-alcohol drugs
- Cancer medications
- Heart or blood pressure medications
- Infection fighting drugs
- INH (Isoniazid)
- Metronidazole (Flagyl®)
- Phenytoin (Dilantin®)
- Skin condition treatment drugs
This type of ongoing neuropathy results not so much from nerve injury caused directly by the trauma or surgery, but rather, by the subsequent swelling and scar tissue that are a result of that surgery or trauma. This causes the nerve to become neuropathic.
A typical example of traumatic neuropathy is as follows: one cause of foot drop is an inversion sprain of the ankle. Even though the ultimately affected peroneal nerve is about 12 inches ABOVE the ankle, the rapid “snap” that gets transmitted up the nerve when the ankle turns inward causes some trauma to the tissues around the peroneal nerve at the top of the leg. Thus, over time, scar tissue can form around the nerve and cause a neuropathy that results in foot drop. Also, after a major trauma, patients tend to be immobilized, and they hold onto water (edema) for months. This edema affects the nerves, and the swollen nerves can get “trapped” inside their sheaths. Neuropathy results from the pressure that this swelling induces within the nerve’s own covering.