Neuropathy is a medical term describing damage or disease that affects the nerves, disrupting their ability to transmit signals throughout the body. This results in a malfunction of the communication network connecting the brain and spinal cord to the rest of the body. This disorder is common, affecting an estimated 20 million or more people in the United States, with prevalence rising significantly in the older population.
Understanding How Neuropathy Affects the Nerves
The nervous system includes the Central Nervous System (CNS)—the brain and spinal cord—and the Peripheral Nervous System (PNS). Neuropathy primarily involves the peripheral nerves, which branch out to the limbs and organs, carrying information to and from the CNS. Damage to these nerves is specifically termed peripheral neuropathy.
Peripheral nerves are classified into three functional types:
- Sensory nerves transmit information about touch, temperature, and pain.
- Motor nerves control the movement of voluntary muscles used for activities like walking and grasping.
- Autonomic nerves regulate involuntary functions such as heart rate, blood pressure, and digestion.
Damage can manifest in different anatomical patterns. Mononeuropathy involves damage to a single nerve, such as Carpal Tunnel Syndrome, where the median nerve is compressed. Polyneuropathy affects many different nerves simultaneously. This is the most common presentation, typically involving the longest nerves first, often starting symmetrically in the feet and hands.
The damage itself is categorized into two main pathological types. Axonal damage targets the axon, the projection of the nerve cell that conducts electrical impulses. Demyelinating damage affects the myelin sheath, the protective coating around the axon, causing nerve signals to slow down or stop. Many forms of neuropathy involve a mixture of both types.
Identifying the Primary Causes and Risk Factors
The causes of peripheral neuropathy are diverse, ranging from chronic metabolic diseases to acute physical trauma.
Diabetes Mellitus
Diabetes is the most frequent underlying cause, often resulting in diabetic neuropathy. Chronically high blood sugar levels damage the small blood vessels supplying the nerves, leading to nerve fiber breakdown. Approximately half of all individuals with long-term diabetes eventually develop this condition.
Trauma and Compression
Physical trauma or compression is another significant cause. A sudden injury, such as a fracture, can sever or crush nerve fibers, causing immediate localized neuropathy. Nerve compression also occurs over time in entrapment syndromes, where repetitive motions or swelling create pressure on a specific nerve.
Infections
Certain infections can trigger nerve damage by invading the tissue directly or initiating an autoimmune response. For example, the varicella-zoster virus (shingles) can damage nerves, leading to postherpetic neuralgia. Other infectious agents, including those responsible for Lyme disease and HIV, are also known causes.
Toxins and Medications
Exposure to various toxins and certain medications is a risk factor. Heavy metals, such as lead and mercury, are neurotoxic. Additionally, certain chemotherapy drugs used in cancer treatment can induce chemotherapy-induced peripheral neuropathy because they affect healthy, fast-growing nerve cells.
Autoimmune Disorders
Autoimmune disorders are a significant cause, where the immune system mistakenly attacks healthy nerve tissue. Guillain-Barré Syndrome (GBS) is a rapid-onset example where the immune system attacks the myelin sheath, causing muscle weakness. Systemic autoimmune diseases, like lupus and rheumatoid arthritis, can also cause nerve damage through chronic inflammation.
Recognizing the Different Types of Symptoms
Symptoms depend entirely on which of the three nerve types—sensory, motor, or autonomic—have been damaged.
Sensory Symptoms
Sensory nerve damage often manifests as abnormal feelings or a loss of sensation. Positive symptoms include tingling, a “pins-and-needles” sensation, burning pain, or sharp discomfort. Numbness is a negative symptom, typically beginning in the toes and feet before progressing up the legs in a “stocking-glove” pattern.
This loss of sensation increases the risk of unnoticed wounds or inability to detect temperature changes. Individuals may also experience allodynia, where typically non-painful stimuli, like the light touch of clothing, causes intense pain.
Motor Symptoms
When motor nerves are affected, primary symptoms relate to muscle function and movement. Muscle weakness is common, often leading to a loss of dexterity in the hands or difficulty lifting the feet, which can cause tripping. Chronic motor nerve damage can result in muscle atrophy and painful muscle cramps or twitching.
Autonomic Symptoms
Damage to the autonomic nerves controls internal organ functions, leading to a diverse array of symptoms. These may include digestive issues, such as feeling full quickly or constipation. Autonomic dysfunction can also cause orthostatic hypotension—a sudden drop in blood pressure when standing up, leading to dizziness. Other manifestations involve sweating changes, bladder control problems, and sexual dysfunction.
Diagnosis and Management Options
Diagnosis begins with a detailed medical history and a thorough physical examination to assess reflexes, muscle strength, and sensation. Since neuropathy is often a complication of another condition, blood tests are frequently ordered to screen for common causes like diabetes, vitamin deficiencies, and autoimmune markers.
To confirm the presence and extent of nerve damage, electrodiagnostic tests are utilized.
Electrodiagnostic Testing
Nerve Conduction Studies (NCS) measure the speed and strength of electrical signals traveling through a nerve, helping determine if the issue is primarily axonal or demyelinating. Electromyography (EMG) is often performed alongside NCS. EMG involves inserting a thin needle electrode into muscles to record electrical activity, assessing the function of the motor nerves.
Management of neuropathy is a two-pronged approach: treating the underlying cause and alleviating the symptoms.
Treating the Cause
Addressing the root cause is the most effective way to halt the progression of nerve damage. For instance, strict control of blood sugar levels is paramount for diabetic neuropathy. For cases linked to vitamin deficiency or toxin exposure, correcting the deficiency or eliminating the exposure is the appropriate treatment path.
Symptom Relief
Symptom relief focuses largely on managing neuropathic pain, which often does not respond well to standard pain relievers. Prescription medications, such as anticonvulsants and some antidepressants, interfere with pain signals transmitted by damaged nerves. Topical treatments, including patches or creams containing lidocaine, can also provide localized relief.
Physical and occupational therapy can help address muscle weakness and loss of coordination, improving balance and mobility. Lifestyle adjustments, such as maintaining a healthy diet, exercising regularly, and meticulous daily foot care, are recommended to prevent complications like ulcers and falls.