Chapter 2 – NEUROPATHY OF THE DIABETIC FOOT.

Neuropathy is defined as damage to the nervous system, and one of the common causes is diabetes. There are three types of neuropathy associated with diabetes: sensory neuropathy, motor neuropathy, and autonomic neuropathy. Approximately 50% of diabetic patients experience some form of neuropathy, with the feet and legs being most commonly affected.

The underlying cause of diabetic neuropathy is often prolonged exposure to high blood sugar levels, which damages nervous tissue over time. When blood sugar is elevated, excess sugar enters the nerves, where it is converted into substances like sorbitol and fructose. These compounds accumulate within the nerve cells, causing inflammation that eventually leads to irreversible nerve damage.

Sensory neuropathy in the feet initially causes a burning sensation, sharp knife-like pain, a feeling of pressure, and other mild discomforts that can worsen at night. As the condition progresses, it eventually leads to a complete loss of sensation, resulting in numbness. Without the protective sensation of pain, any trauma to the feet may go unnoticed for a long time. You might not be able to distinguish between mild discomfort and sharp pain when they occur in the feet. A simple pinprick test, comparing sensation between your foot and your thigh, can help you detect any differences; if you notice diminished sensation, you should inform your doctor at your next visit. Decreased sensation in the feet significantly affects the prognosis for diabetic foot disease. In people without neuropathy, discomfort in the feet typically prompts changes in gait to avoid pain. However, if you lose the ability to feel pain, stepping on a nail or piece of glass can go unnoticed, potentially leading to serious complications (See Fig. 2-1).

If your shoes cause blisters on your feet, you might not feel the pain due to sensory neuropathy. Sensory neuropathy often occurs alongside motor neuropathy. Motor nerves control the muscles and tendons that allow you to move your toes up and down based on signals from your brain. When these nerves are damaged—whether by traumatic injuries or chronic high blood sugar—you may experience weakness, muscle wasting, and changes in the shape of your feet.

Each foot contains a total of 44 muscles and tendons. When motor neuropathy affects the foot, these structures become weak, leading to an imbalance in the joints. This imbalance can result in deformities such as hammertoes, bunions, calluses, and flat feet. In particular, hammertoes are often accompanied by calluses on the top of the knuckle due to repeated irritation from shoes. (See Fig. 2-4)

Fatty tissue on the bottom of the foot may become thinner over time, which can lead to the formation of calluses. Additionally, bunions may become irritated due to the pressure exerted by shoes. Because motor neuropathy usually occurs alongside sensory neuropathy, calluses may not be as painful as they should be, allowing them to go unnoticed. As these neuropathies progress, the loss of protective sensation can lead to significant foot deformities.

The ongoing irritation from shoes combined with the constant pressure during walking can eventually result in the formation of calluses, which may later develop into ulcers. Diabetic foot ulcers are particularly challenging to treat because the surrounding tissue often becomes necrotic, and the underlying bones may become infected.

Therefore, a daily visual inspection of your feet is essential. Check for blisters, any fluid accumulating under calluses, or discoloration—especially redness around callused areas. It's also important to inspect your shoes for any foreign objects before wearing them, and to examine your feet again after removing your shoes. Additionally, when the shape of your foot changes, make sure to measure your feet every time you buy a new pair of shoes. If you have uneven feet (for example, if one foot is slightly larger than the other), invest in a pair of shoes that accommodates the larger size, even if they cost more.

The third form of diabetic neuropathy is called autonomic neuropathy, and it commonly occurs alongside the other two forms. The autonomic nervous system controls various functions, including the muscles of the heart, the arterial muscles, and the glands. In your feet, the sweat glands and arteries are regulated by this system. When the sweat glands do not function properly due to autonomic neuropathy, your feet may not perspire normally, leading to dry skin. Severe dryness can cause cracks and fissures, especially around the heel. Although these cracks and fissures can be painful, sensory neuropathy may prevent you from feeling them. Unfortunately, these skin breaches can serve as entry points for bacteria, increasing the risk of infection.

Dry skin is also vulnerable to fungal infections. In long-standing diabetes, the body's defense mechanisms can become compromised. Fungal infections between the toes are particularly dangerous when they become secondarily infected with bacteria. Because the area between the toes is not well exposed to air, anaerobic bacteria—those that do not require oxygen—can grow and spread rapidly, leading to further infection of the foot and leg. (See Fig. 2-8)

Inspect between your toes daily and keep the area dry. Never apply moisturizer or ointment between your toes, as these can trap moisture and make the skin more prone to breakdown. As mentioned earlier, the autonomic nervous system regulates the functions of the arteries in the foot, controlling blood flow. In autonomic neuropathy—or when there is increased blood flow to the foot—this can become problematic. One such condition is diabetic neuropathic arthritis, also known as Charcot's diabetic foot (see Chapter 7).

Charcot foot disease is defined as the chronic, progressive, and painless destruction of the bones and joints in the foot and ankle. This condition was first described by Dr. Charcot in the last century when he observed joint changes in patients with syphilis. In diabetic patients with extensive nerve damage, the weight-bearing bones and joints of the foot can gradually undergo painless destruction, leading to a severely disfigured foot. Excess blood flow can wash away the minerals that normally give bone its hardness, rendering the bone soft and more susceptible to damage—a process that can be triggered by even minor trauma.

Although the affected foot may be hot, swollen, and red, sensory neuropathy prevents the patient from feeling any discomfort or pain. Continued weight-bearing on an affected foot can result in multiple fractures, as seen on X-rays, even though the patient remains unaware of the damage. If your foot becomes hot, swollen, and red, you should immediately offload weight from the affected foot using crutches and seek podiatric attention, as this may indicate an infection or an acute Charcot process. Further details on Charcot foot are discussed later in Chapter 7.

The tingling, burning, sharp pain, and throbbing discomfort associated with the early stages of neuropathy can be managed by keeping blood sugar levels within normal limits (see Chapter 1). In some cases, taking aspirin, ibuprofen, or another non-steroidal anti-inflammatory medication can help reduce nerve inflammation and alleviate discomfort.

Vigorous blood sugar control can help reverse the early nerve damage associated with diabetic neuropathy. Once you begin experiencing signs of diabetic foot neuropathy, it's essential to pay special attention to your feet. The shoes and socks you wear can contribute to future problems (see Chapter 10). Socks should be made of 100% cotton and should not be reinforced, while shoes should be well-cushioned and spacious enough to accommodate any bone deformities, such as hammertoes, bunions, and calluses. Shoe inserts are also essential to prevent blisters and calluses.

If you already have calluses, they should be examined periodically by a podiatrist rather than managed on your own. Regular removal of calluses by a professional can prevent the build-up of thick, dead skin that creates undue pressure on the underlying bones. Attempting to cut your calluses yourself—especially in the bathtub—is particularly dangerous. Accidental cuts with a rusty razor blade can lead to devastating foot infections. Remember, a podiatrist is the only medical specialist specifically trained to diagnose and treat foot and ankle problems, especially those related to diabetic foot disease.