What is the human nervous system?

The human nervous system is an intricately organized, interconnected system of nerve cells that relays messages to and from the brain and spinal cord (nervous systems are also found in organisms called vertebrates, or, literally, organisms with backbones, a group that includes humans). The human nervous system receives sensory input, processes the input, and then sends messages to the tissues and organs for the appropriate response. In humans (and other vertebrates), there are two parts to the nervous system. They are the central nervous system, consisting of the brain and spinal cord, and the peripheral system, consisting of peripheral nerves that carry signals to and from the central nervous system.

Do some diseases that affect the nervous system have connections to diabetes?

Although there are several diseases of the nervous system, the more well-known ones are epilepsy, multiple sclerosis (MS), and Parkinson’s disease. The two with the most possible connections to diabetes are epilepsy and Parkinson’s disease (see below), but not multiple sclerosis. (MS is an autoimmune disease. It is a chronic, potentially debilitating disease that affects the sheaths surrounding the nerves [called myelin] in the central nervous system. In MS, the body directs antibodies and white blood cells against proteins in the sheaths, causing injury and inflammation to the sheaths, especially those surrounding nerves in the brain and spinal cord.) Overall, there is little evidence that diabetes-either type 1 or type 2-has any connection to MS, although people with either disease can develop the other one.

How are seizures connected to diabetes?

Seizures, or a condition in which something interrupts the normal connections between the nerve cells and the brain, can under some circumstances be connected to diabetes. The definitions of seizures and shocks often differ depending on the research, but either way, a very low blood glucose reading can lead to a seizure. In particular, a hypoglycemic seizure from low blood glucose levels can cause clumsiness, weakness, trouble talking, confusion, loss of consciousness, seizures, or (rarely) death. Diabetic shock (severe hypoglycemia), also called insulin reaction, is a consequence of too much insulin. It can occur anytime there is an imbalance between the insulin in the person’s system, the amount of food eaten, or the level of a person’s physical activity. Hypoglycemic shock is similar (if not the same) as diabetic shock, caused by extremely low blood glucose levels, most often from an excessive amount of injected insulin, failure to eat after an insulin injection, or rarely by an insulin-secreting tumor of the pancreas. Whatever it is called, an extremely low blood glucose level can cause seizures and is considered to be a medical emergency.

Is epilepsy connected to diabetes?

Epilepsy is a nervous system disorder in which clusters of nerve cells (neurons) in the brain sometimes send signals abnormally. When the normal pattern of nerve activity becomes disturbed, it causes strange sensations, emotions, and behavior or sometimes convulsions, muscle spasms, and loss of consciousness. There are thought to be many causes, including certain illnesses, brain damage, abnormal brain development and wiring, an imbalance of nerve-signaling chemicals (called neurotransmitters), or some combination of these factors. As for epilepsy’s connection to diabetes, it is thought that there may be an increased risk-nearly three times more-of epileptic seizures in young people with type 1 diabetes as compared to others without type 1. One study also suggests that there may be an association between epilepsy and diabetic ketoacidosis in children with type 1 diabetes. (For more about diabetic ketoacidosis, see the chapter “Type 1 Diabetes.”) As most researchers mention, more studies need to be conducted to determine whether there is a definite connection.

Is there a relationship between Parkinson’s disease and diabetes?

Several studies suggest a connection between Parkinson’s disease and diabetes, including one report indicating that people with type 2 diabetes were 80 percent more likely to be diagnosed with Parkinson’s disease. Parkinson’s is a progressive neurological disorder (also called a neurodegenerative disease) that results from degeneration of neurons in a region of the brain that controls movement. This degeneration creates a shortage of the brain-signaling chemical (neurotransmitter) known as dopamine, causing changes in body movement-most often shaking of various parts of the body, including hands and head-that characterize the disease. No one knows whether there truly is a relationship, but one study suggests that many lifestyle factors, such as being overweight or obese, smoking, and being sedentary, seem to be associated with both disorders.

Parkinson’s disease is caused when an area of the brain called the substantia nigra deteriorates, which lowers dopamine production, an essential chemical for muscle control. Patients with diabetes are more likely to get Parkinson’s, but a direct connection has not yet been established.

What recent research addresses why there may be a link between Parkinson’s disease and type 2 diabetes?

In 2015, a study conducted at Ben Gurion University in Israel indicated that there may be a link between type 2 diabetes and Parkinson’s based on certain proteins. The researchers were the first to discover the structure of a brain protein (alpha-synuclein) that can trigger Parkinson’s by clumping and causing the death of nerve cells. There is also a protein (a short chain of amino acids) called amylin found in the pancreas in 95 percent of the people with type 2 diabetes-and it is also found in the brain. (In the pancreas, the amylin can harm insulin-producing beta cells, contributing to type 2 diabetes.) The researchers then studied a snippet of the brain protein alpha-synuclein (the snippet was called a non-amyloid-beta component, or NAC). Because of their research, they believe that the NAC and amylin may act together, causing the clumping in the brain and killing nerve cells-a possible link between both diseases. Researchers believe that if there is such a link, a drug may be developed that would prevent the interactions between the NAC component and the amylin. But as with many new discoveries, more research needs to be conducted on the possible link between the two diseases.


What is neuropathy?

Neuropathy is the general term that means nerve damage caused by various conditions that affect the body. It usually refers to the damage of a person’s peripheral nerves (see below), as opposed to the nerves of the central nervous system (the brain and spinal cord). The nerves affected can be singular (mononeuropathy) or in sets (polyneuropathy). For example, in polyneuropathy, multiple nerves simultaneously malfunction, causing weak hands and feet, as well as the loss of sensation in those areas.

What are the three types of nerves in the body that can be affected by neuropathy?

Overall, neuropathy for various reasons can occur if there is damage to three types of nerves (any number of these three types can affect a person at any one time):

Autonomic-The nerves that control the major systems of the body, such as the bladder; they can cause changes in a person’s heart rate and blood pressure, along with sweating.

Motor-The nerves that allow the body to have power and movement; neuropathy of motor nerves can cause weakness in the hands and feet.

Sensory-These nerves control sensation; neuropathy of sensory nerves can cause pain, tingling, numbness, or weakness in the hands and feet.

What are the common types of nerve damage often found in people with diabetes?

Not every person with diabetes will experience nerve damage, but for those who do, there are three different types of neuropathy:

Peripheral neuropathy-This is the most common type of neuropathy-and one of the major complications-in people with diabetes. It affects the long nerves that run from the spine to the arms, legs, and hands. Most researchers believe it is likely caused by damage to the delicate nerve fibers when the body experiences high blood glucose levels (see below). It is often called “diabetic peripheral neuropathy” (or “peripheral diabetic neuropathy”) if these nerves are damaged from diabetes.

Focal neuropathy-This type of nerve damage affects a specific nerve or set of nerves and usually causes weakness in the face, arms, legs, or eye muscles. It often leads to weakness in the hands, double vision, or difficulty in raising the legs. If treated (most often by controlling the person’s glucose levels), it typically disappears in two to six months.

Autonomic neuropathy-This nerve damage affects the autonomic nervous system, which controls such body functions as heart rate, digestion, blood pressure, sweating, and for men, erections. Autonomic neuropathy can cause a multitude of symptoms, including racing heartbeat, dizziness, or light-headedness. It can affect the digestive tract, causing vomiting, diarrhea, and/or constipation. It can also cause a person not to empty his or her bladder efficiently, which can predispose the individual to bladder infections. And for men, especially those who have had diabetes for many years, it may cause impotence (for more about impotence [also called erectile dysfunction] and diabetes, see the chapter “How Diabetes Affects the Reproductive System”).

Why do people with diabetes often develop certain types of neuropathy?

Most cases of chronic neuropathy are found in people who have diabetes. In fact, roughly 60 to 70 percent of people with diabetes have some type of neuropathy. Neuropathies in people with diabetes are thought to be caused by poorly controlled blood glucose levels. High levels of blood glucose can eventually damage delicate nerve fibers, interfering with nerve-signal transmission and damaging the nerves themselves. Although it is unknown why this happens, some researchers speculate that the damage is caused when glucose attaches to or affects the proteins found in nerve cells. This condition may either cause a chemical imbalance inside the nerves or restrict the blood flow to the nerves. Either way, once the network of nerves is damaged, the messages to and from different parts of the body are also affected. The nerve signals may slow down or send the wrong cues, or eventually, the nerves may stop working.

What are some causes of peripheral neuropathy besides diabetes?

Numerous conditions can cause peripheral neuropathy. The most common cause of chronic peripheral neuropathy is diabetes, also called diabetic peripheral neuropathy. Other reasons for peripheral neuropathy include physical trauma (injuries such as broken bones can put pressure on nerves); infections (shingles, HIV infections, and Lyme disease often contribute to peripheral neuropathy); Guillain-Barré syndrome, a specific type of peripheral neuropathy triggered by infection; high consumption of alcohol; chronic liver and kidney diseases (both can cause peripheral nerve damage because of the imbalance in the body’s overall chemistry); cancers (lymphoma can cause peripheral neuropathy); folate vitamin deficiencies (the lack of B2 in the diet can contribute to nerve damage); repetitive injuries (carpal tunnel syndrome); exposure to toxins (insecticides and certain solvents can cause peripheral neuropathy); and some drugs (some chemotherapy medication used to treat HIV often causes damage to the peripheral nerves). If the reason for the neuropathy is unknown-and it is estimated that the reasons for 30 percent of neuropathies are unknown-it is usually referred to as idiopathic peripheral neuropathy.

Are there any treatments for a person who has diabetes and a certain type of neuropathy?

Yes, there are treatments for people with diabetes and neuropathy, depending on where the nerve damage is located. For example, if a person with diabetes has digestive problems because of nerve damage (a type of autonomic neuropathy), then he or she may need to eat more fiber. If a person with diabetes has bladder problems caused by autonomic neuropathy, then oral drugs, or even surgery, may help improve bladder function or reduce incontinence caused by the neuropathy.

Can inflammation affect a person with peripheral neuropathy?

Yes, it is thought that peripheral neuropathy can be worsened by inflammation. This swelling of tissues-often accompanied by the area becoming hot-is the way the body fights off infection. But if there is too much inflammation along with neuropathy, these conditions can damage tissues and often cause pain.

How can nerve damage affect the extremities of a person with diabetes?

Nerve damage can affect the extremities of a person with diabetes in several ways. These can include musculoskeletal complications in the hands, wrists, and shoulders, along with problems in the feet, toes, ankles, and legs. Most of this damage is caused by uncontrolled or high blood glucose levels that damage the delicate nerve endings in the extremities. (For more information about nerve damage and hands, wrists, and shoulders, see the chapter “How Diabetes Affects Bones, Joints, Muscles, Teeth, and Skin.”)

What are some of the dangers if a person with diabetes develops diabetic peripheral neuropathy?

There are some concerns if a person with diabetes develops diabetic peripheral neuropathy. For example, if a person has less sensitivity to vibrations, touch, and pain, owing to diabetic neuropathy (especially in the feet), then it could put him or her at a greater risk for foot injury. Such nerve damage could mean that conditions such as too tight shoes, high-impact exercise injuries, or even stepping on an object that punctures the bottom of the foot would go unnoticed. In extreme cases, a foot wound that goes overlooked for a long period could become infected, and if it does, the infection could lead to gangrene, eventually requiring amputation. In addition, with neuropathy, there may be more bladder and kidney infections or a decrease in muscle mass (or even muscle damage). If the person’s digestive system is affected by neuropathy, along with frequent bouts of vomiting, it can result in poor blood glucose control.

What are some of the symptoms of diabetic peripheral neuropathy?

Symptoms of peripheral neuropathy in people with diabetes include tingling, numbness, and reduced sensitivity to touch. It may also cause itching, weakness, and loss of balance, especially when it affects the legs and feet. In rare cases, it may also cause a burning or painful sensation in the extremities that continues up the arms or legs. People with diabetes and peripheral neuropathy commonly describe the sensation in various ways. For example, peripheral neuropathy in the feet has been described as “walking barefoot on cut glass.”

Why is it important for people with diabetes to care about their feet?

While everyone should practice good foot care, a person with diabetes should be extra vigilant. One of the more common side effects of diabetes is the loss of sensation because of diabetic peripheral neuropathy. Because of this, minor injuries to the feet could become a major problem. For example, if a person with diabetes gets a blister and has lost some of the sensation in the feet, he or she may not treat it properly or even know it exists. If the wound becomes infected, it may lead to ulcerations and, if severe enough, to amputation. (For more about diabetic foot problems, see the chapter “How Diabetes Affects Bones, Joints, Muscles, Teeth, and Skin.”)

Can diabetes cause a burning sensation on the soles of both feet?

Yes, a burning sensation on the soles of the feet is often reported by people who have diabetes. This can be caused by something as simple as ill-fitting shoes. But for a person with diabetes, it may be a sign of diabetic peripheral neuropathy, or damage to the peripheral nerves in the feet. It can also be caused by alcoholism and less commonly by vitamin deficiencies or lead poisoning. This burning sensation has even been attributed to a rare condition called erythromelalgia, a disorder that causes a burning feeling in the extremities brought about by an increase in blood flow to the hands and feet.

A sore on a foot for a diabetic person is much more prone to infection. If left untreated, the infection can get into the bones, even making amputation of toes, feet, or legs necessary.

Why do some foot injuries go unnoticed by people with diabetes?

In most cases, the main reason that foot injuries go unnoticed is nerve damage from the diabetes and/or peripheral artery disease (PAD; for more about PAD, see the chapter “How Diabetes Affects the Circulatory System”). If a foot injury goes unnoticed, it is mostly because peripheral neuropathy often impairs pain sensations, and there is a decrease in the blood flow or complete blockage in the arteries feeding the foot. Thus, a cut, blister, bleeding wart, or even an ingrown toenail can cause an undetected infection to spread.

If the infection becomes severe, the foot’s poor circulation from PAD or diabetic nerve damage exacerbates the problem by not allowing the body’s natural infection fighters-such as white blood cells and antibodies-to attack the invading infection. If the infection remains uncontrolled, a foot ulcer can develop, which is a condition that needs immediate attention. If not treated, the infection can penetrate into the lower layers of the skin and reach the bone, causing a bone infection. If one or more toes, or another part of the foot, becomes involved, then the toes or even an entire foot often has be amputated. In extreme cases, a leg may have to be amputated. This is why it is often stressed that people with diabetes pay close attention to their extremities, especially the feet. (For more about foot ulcers, see the chapter “How Diabetes Affects Bones, Joints, Muscles, Teeth, and Skin.”)

What is Charcot’s arthropathy (or Charcot’s foot and ankle)?

Charcot’s arthropathy, also called Charcot’s foot and ankle or diabetic foot, is a syndrome that often affects a person with diabetes and who has severe diabetic peripheral neuropathy. (It can also affect other people who have loss of sensation in their feet for other medical reasons.) According to Harvard Medical School, this condition occurs when a person’s foot and ankle joints and/or bones are destroyed, including such problems as bone disintegration, fractures, and dislocation of bones. These changes in the bones and joints can eventually cause deformities that interfere with walking. It is also one of the most serious foot problems that a person with diabetes faces.

Initially, there is minor trauma to a joint or bone in the foot from daily wear and tear. If this goes unnoticed-which often happens with severe diabetic peripheral neuropathy-then the damage often continues until the tissues are eventually destroyed. Again, this is why doctors stress that people with diabetes pay close attention to changes in their feet and ankles. If caught in time, the damage can potentially be minimized by changing the way a person walks (sometimes through physical therapy), modifying their footwear, or starting an exercise program.

The above graphic shows where dangerous ulcers most commonly develop on the feet of diabetics.

How does a physician initially diagnose diabetic peripheral neuropathy?

The initial tests for diabetic peripheral neuropathy can usually be conducted during a physician’s office exam. In fact, it is estimated that 60 to 70 percent of people with diabetes have some signs of neuropathy that can usually be detected with a physical exam or special tests. The best way to determine neuropathy is to test the reflexes and sensory perception of the patient. Other means of diagnosing will also be conducted, such as looking at the person’s history (for example, any history of neuropathy in the family, exposure to toxins, medications taken, and alcohol consumption).

What are some special tests to check a person with diabetes for peripheral neuropathy?

Sometimes a nerve conduction test may be needed to see whether certain nerves are affected, such as those in the arms or legs. This is done by attaching special electrodes to the skin over the nerve being tested. Another test is electromyography, which looks at the electrical activity of muscles. This is done by inserting a very thin needle with an electrode attached into the muscle being tested; the way the muscle responds is recorded on an oscilloscope. Not as common are nerve and skin biopsies, in which a small part of the suspect nerve is removed and examined under a microscope to detect damage.

Is there a connection between food and peripheral neuropathy?

Yes, some studies indicate that there may be connections between food and the occurrence of peripheral neuropathy. The following are two of these possible connections (for more about food, see the chapter “Diabetes and Food”):

Several studies indicate that high glucose levels after a meal are strongly associated with peripheral neuropathy. According to the Foundation for Peripheral Neuropathy, the best way to reduce after-meal glucose levels is to eat more foods that are slowly absorbed into the bloodstream. These include fruits, vegetables, beans, and nuts-and not refined starches and sugars. In addition, exercising after a meal will often cause blood glucose levels to decrease, and using fast-acting insulin can lower sugar. The best way to discover how to lower levels after meals is for a person to work with his or her health care professional to make certain medical and/or eating adjustments.

Another culprit that connects food and peripheral neuropathy is inflammation. In fact, many foods are known to increase the amount of inflammation in the body. The greatest problem in determining how a food affects a person is just that-different foods cause different inflammations in different people. Some of the more well-known foods that can cause inflammation are trans fats, saturated fats, sugar, alcohol, and what are called omega-6 oils: corn, safflower, and soybean oils. (For more about omega-6, see the chapter “How Diabetes Affects the Digestive System.”) Of course, other types of food, such as those that contain dairy, nuts, caffeine, and other foodstuffs, can also cause inflammation. To understand which foods may cause inflammation may take time. Again, the best way to uncover foods that cause inflammation is to work with an individual’s health care professional.

Why does having diabetes often increase the risk for amputations?

For some people with diabetes, there is an increased risk for amputation. This can be because of diabetic peripheral neuropathy but may also be from the complication of peripheral artery disease, or PAD. Both conditions raise the risk for nerve damage. If the damage is severe enough, it can eventually lead to amputation-especially if the tissue dies, and the extremity becomes infected. This often happens with the feet as foot injuries and ulcers may not be readily noticed because of nerve damage.

What are some statistics concerning diabetes and amputations?

According to Harvard Medical School, diabetes is responsible for more than 60 percent of lower limb amputations that have not been caused by accidental injuries. The school also notes that a person with diabetes is ten times more likely to have an amputation than a person who does not have diabetes. But not everyone with diabetes will need amputation of extremities, especially if people keep their blood sugar levels under control and especially check their feet daily for possible cuts and wounds. If there is a wound, tend to it immediately so it does not become infected.


Why do human brain cells need glucose?

While most of the cells in the body can adapt-at least temporarily-to using lipids (fats) and proteins as energy sources, the human brain cells require glucose in order to work well. In fact, the brain cells can get energy only from glucose. This means that when the body’s blood glucose levels fall too low for that person, his or her brain cells shut down. This shutdown often results in fainting, which actually increases the flow of blood and glucose to the brain to compensate.

Can diabetes contribute to short- and long-term memory loss?

Short-term memory loss and reduced brain function can occur during periods of low and high blood glucose (hypoglycemia and hyperglycemia, respectively). Over the long term, high blood glucose (hyperglycemia) can affect memory in people with type 1 and type 2 diabetes. This is because high glucose levels can damage nerves, including those of the brain, thereby increasing the risk of memory loss-and sometimes eventually contribute to dementia.

What are the two forms of dementia most often mentioned in the media?

The term “dementia” describes a group of symptoms caused by changes in a person’s brain function and is usually associated with advancing age. The two most common forms of dementia in older people-and the ones most often mentioned in our daily lives-are multi-infarct dementia (sometimes called vascular dementia) and Alzheimer’s disease. These types of dementia are irreversible, which means they cannot be cured. Research on both types now center on mitigation or slowing down their progression.

In multi-infarct dementia, a series of small strokes or changes in the brain’s blood supply may result in the death of brain tissue. The location in the brain where the small strokes occur determines the seriousness of the problem and the symptoms that arise. Symptoms that begin suddenly may be a sign of this kind of dementia. People with multi-infarct dementia are likely to show signs of improvement or remain stable for long periods of time, then quickly develop new symptoms if more strokes occur. In many people with multi-infarct dementia, high blood pressure is to blame. In Alzheimer’s disease, nerve cell changes in certain parts of the brain result in the death of a large number of cells. The most common symptoms range from mild forgetfulness to serious impairments in thinking, judgment, and the ability to perform daily activities.

What is the possible connection between diabetes, Alzheimer’s disease, and something called “type 3 diabetes”?

In 2005, research suggested that there may be something called “type 3 diabetes,” or a term proposed for Alzheimer’s disease, which is thought to result (in part) from the brain’s resistance to insulin. Other studies followed, including several in 2012, indicating that resistance to insulin-and resulting low levels of insulin in the brain-seemed to play a key role in the progression of Alzheimer’s disease. Another study indicated that people with insulin resistance, especially those with type 2 diabetes, have an increased risk of eventually developing Alzheimer’s disease-between 50 and 60 percent higher than people without type 2 diabetes.

Most researchers emphasize that not everyone with type 2 diabetes develops Alzheimer’s disease, but type 2 could be a co-factor in the disease’s progression. (In other words, having type 2 diabetes does not cause Alzheimer’s disease but may contribute to an increase in impaired brain function and eventually Alzheimer’s.) Other researchers suggest that diabetes and Alzheimer’s disease may have the same source: An overconsumption of foods that cause problems with the many roles insulin plays in the body. And since such consumption often leads to obesity, several studies indicate that as the obesity rates have increased in the past several decades, so has the incidence of Alzheimer’s disease. More studies need to be done. If there is a definite connection, some “cures” for type 2 diabetes and Alzheimer’s may be as “simple” as eating the right foods, exercising, and keeping off excess weight.