What is Small Fiber Neuropathy?
Peripheral neuropathy, a common issue affecting around 15 to 20 million Americans over 40, is often the reason for visits to the neurologist’s office and is responsible for more than $10 billion in healthcare costs every year. Affected individuals usually experience problems with their small nerve fibers, including certain types of nerves – thinly-protected Aδ fibers and unprotected C nerve fibers.
When these small nerve fibers are affected, it’s called ‘small fiber neuropathy’ (SFN). This condition often leads to pain, a burning sensation, numbness, and tingling that usually follows a ‘stocking-glove’ pattern (meaning it starts in the feet and moves up the body).
What Causes Small Fiber Neuropathy?
Small Fiber Neuropathy (SFN) impacts smaller nerves called Aδ-fibers and C-fibers. This medical condition can influence the nerves controlling our senses and automatic body functions, causing changes in sensation, disruptions in automatic bodily functions, or a mixture of both issues.
The nerves related to automatic body functions control things like body temperature regulation, heart function, digestion, sweat regulation, urinary and reproductive system functions, among others. On the other hand, the nerves controlling our senses are responsible for detecting pain, changes in temperature, and itchiness.
People with SFN often experience tiredness, mental disturbances, headaches, and pain in their muscles and bones that can impact their day-to-day lives. SFN can occur alongside various other diseases but it can also happen without any obvious causes, in which case we say it’s “idiopathic”.
There are many possible reasons why someone might develop SFN, which we can group into six categories:
1. Hereditary (passed down in families): This may include diseases like Fabry’s disease, mutations in sodium channels in your body, Wilsons disease, and familial amyloidosis.
2. Infectious: Caused by infectious diseases like HIV, Lyme disease, and Hepatitis C.
3. Toxic: This might include exposure to alcohol, chemotherapy, neurotoxic drugs (drugs that harm the nervous system), or as a side effect from vaccinations.
4. Immune-Mediated: This can occur in conditions that involve the body’s immune system such as Ehlers-Danlos syndrome, fibromyalgia, monoclonal gammopathy, acute inflammatory SFN, lupus, connective tissue disease, chronic inflammatory demyelinating polyneuropathy, sarcoidosis, rheumatic diseases including disorders related to connective tissues, arthritis, and psoriatic arthropathy, Sjögren syndrome, or primary systemic amyloidosis.
5. Metabolic: Can happen due to conditions that affect the body’s metabolism such as diabetes, impaired glucose tolerance, vitamin B12 or copper deficiency, or abnormal thyroid function.
6. Idiopathic: Sometimes, SFN can occur without an identifiable cause.
Risk Factors and Frequency for Small Fiber Neuropathy
Small Fiber Neuropathy (SFN) is a condition that had a recorded rate of 12 new cases every year per 100,000 people in a study conducted in the Netherlands. The study also found that SFN currently affects 53 out of every 100,000 people. This illness seems to occur more often in men, and it is usually diagnosed in patients who are over 65 years old. However, it’s important to note these figures might be lower than the actual numbers, since awareness about SFN is still growing and there are no standardized criteria to diagnose it yet. As more people learn about SFN, the number of recognized cases is expected to rise in the years ahead.
Signs and Symptoms of Small Fiber Neuropathy
Small fiber neuropathy, or SFN, is a condition that patients often initially show symptoms of sensory changes like pain, burning, numbness, and tingling. For majority of patients, these symptoms starts in the feet and slowly moves upwards (this is known as a length-dependent pattern). However, some patients may experience symptoms in varying parts of the body like the mouth, face, trunk, scalp, and upper and lower limbs in a non-length-dependent distribution. Others may experience sensory changes in just one nerve or a few nerves, often associated with specific syndromes like burning mouth syndrome or vulvodynia. There are different causes for these patterns. For example, length-dependent SFN is often attributed to metabolic issues including diabetes or exposure to neurotoxins, while non-length-dependent SFN is typically due to paraneoplastic disorders or immune-mediated issues such as Sjögren syndrome.
Patients’ first symptoms often include neuropathic foot pain, that may feel like discomfort in feet. This might be numbness in the toes, a wooden sensation in the feet, or feeling as though walking on sand, golf balls, or pebbles. Pain akin to burning in the feet, which spreads in a stocking-glove distribution, is a common symptom. Accompanying symptoms can include pains similar to aching, stabbing, pins and needles, electric shock, or cramping in feet and calves. These symptoms tend to become more noticeable at night and some may even experience restless legs, intolerance of bed sheets, or sensations such as allodynia (overactive sense of pain) or dysesthesia (abnormal sense of touch) caused by clothes. Some patients might not experience pain but report swelling and tightness in the feet. Involvement of autonomic fibers can lead to other symptoms such as dry mouth, dry eyes, constipation, bladder incontinence, orthostatic dizziness, sexual dysfunction, red or white skin discoloration, or trouble sweating.
When evaluating a patient for SFN, comprehensive medical history should be taken into account, including family history, alcohol consumption, and usage of neurotoxic medications like colchicine, metronidazole, and chemotherapy drugs. Questions relevant to risk factors such as sexual history, intravenous drug use, and blood transfusions are needed as conditions like hepatitis C and HIV are known to be associated with SFN. A detailed neurologic physical exam can distinguish SFN from large fiber neuropathy. Examination might come up with symptoms like allodynia (pain from non-painful stimuli), hyperalgesia (increased pain sensitivity), or reduced thermal and pinprick sensation in the area affected. Other findings like weakness, reduced proprioception (sense of body’s position), and absence of deep tendon reflexes are not observed in SFN as these imply large fiber involvement. If autonomic fiber involvement is suspected, examinations for orthostatic hypotension and skin disorders resulting from vasomotor or sudomotor abnormalities are conducted. In addition, any systemic disorders associated with SFN that were mentioned earlier are evaluated in the physical examination.
Testing for Small Fiber Neuropathy
If your doctor suspects you have small fiber neuropathy (SFN), a condition affecting the small nerves in your skin, they might need to consider several tests. Some of these tests might not show clear results because they are designed for larger nerves. However, they can still be useful in assessing other conditions that might be affecting your larger nerves too.
One of the tests used is the nerve conduction study, which measures how quickly your nerves transmit signals. However, in the case of SFN, small fibers transmit signals slowly which makes it hard for the test to capture their responses, leading to normal results even when SFN is present.
In such cases, a skin biopsy is usually performed. In this procedure, small skin samples, typically from the leg or thigh, are taken and sent off to a lab for analysis. If the test shows fewer nerves in your skin than expected, it’s an indication of SFN.
The amount of sweat produced in your limbs in response to a substance known as acetylcholine can also be tested. This is known as Quantitative Sudomotor Axon Reflex Testing (QSART). This test may further assist doctors in diagnosing SFN.
Other tests that might be useful include the tilt-table test and cardiovagal testing, mainly used if you’re experiencing disturbances in your heart rate or blood pressure. Thermoregulatory sweat testing might also be performed, especially if you’re having abnormal sweating patterns.
Diagnosing SFN is challenging due to a lack of a universally accepted test. Still, a combination of certain test results, including skin biopsy and QSART, can aid doctors in arriving at an accurate diagnosis.
The cause of SFN can be tricky to identify, and in some cases, it remains unknown. However, finding the underlying cause is vital to determine a suitable treatment. Therefore, various tests are performed to check for different potential causes including blood disorders, kidney or liver disease, inflammation, thyroid problems, diabetes, vitamin deficiencies, celiac disease, and some infectious diseases like HIV and Hepatitis C.
Further investigations might be carried out if the primary tests don’t indicate a clear cause. These can include tests for sarcoidosis, vitamin deficiencies, systemic amyloidosis, paraneoplastic disease, and autoimmune autonomic ganglionopathy.
In some cases, genetic testing might also be necessary to check for specific genetic disorders that cause SFN, such as Fabry disease and familial amyloidosis.
Treatment Options for Small Fiber Neuropathy
The best way to manage Small Fiber Neuropathy (SFN), a condition that affects the nerves, touches on treating the cause of the condition. For example, where diabetes is identified as the cause, measures to control blood sugar and lower cholesterol, exercise and focus on nutrition are the corrective steps to take. Since walking and running can be uncomfortable for people with SFN because of the pain, indulging in alternative exercises like water therapy, swimming, and stationary cycling is advised.
A Vitamin B12 shortage is sometimes related to low absorption than a dietary deficit, so often, injectable supplements are suggested rather than taking it orally. Pain and issues with the autonomic nerves (which control basic body functions such as breathing and heartbeat) linked to Sjögren syndrome, a disorder that causes dry eyes and mouth, have been successfully treated with corticosteroids, special proteins called intravenous immune globulins, and other substances that suppress the immune system.
SFN related to sarcoidosis, an inflammatory disease, can be treated by using similar methods. SFN caused by celiac disease, a condition where you cannot eat gluten, may be aided by a gluten-free diet. It is essential to manage the pain associated with SFN since the pain from nerve damage can be extremely severe, leading to decreased function and depression.
Medications for SFN include a range of drugs such as medicines for seizures, depression, anesthetics, narcotics, non-narcotic pain relievers, and drugs for irregular heartbeats. However, treatments not involving medications can also offer relief, such as heat, ice, massage of painful areas, and transcutaneous electrical nerve stimulation (TENS) – a therapy that uses low voltage electric current to relieve pain.
Also, a combination treatment of gabapentin (a seizure medication) and nortriptyline (a depression medication) was found to be highly potent for neuropathic pain when used together rather than using each separately. Even though medications like cannabis alleviate pain – especially in patients with diabetic neuropathy and HIV, it is important to be cautious about the possible side effects such as sleepiness, extreme happiness, and cognitive disruption. Holistic therapies, which involve the mind and the body, such as yoga, tai chi, and meditation may help patients with neuropathy deal with pain, improve their quality of life and balance. Another option is neuromodulation, a method that alters nerve activity by delivering electrical stimulation directly to a target area.
What else can Small Fiber Neuropathy be?
When diagnosing neuropathy, it is crucial to distinguish between the two types: small fiber and large fiber neuropathy. Different symptoms help to differentiate between the two. For example, if a person is experiencing weakness, has less awareness of body position (reduced proprioception), and doesn’t react to deep tendon reflex tests, these suggest they might have large fiber neuropathy.
Additionally, there are specific tests like EMG (electromyography) and nerve conduction studies that can be useful to tell the two apart. If these tests show normal results, it could mean small fiber neuropathy. Abnormal results, on the other hand, suggest large fiber neuropathy. An EMG can also help to rule out a condition known as lumbar radiculopathy, which is sometimes mistaken for neuropathy since it can cause similar symptoms like burning and numbness in the lower extremities.
Determining the correct type of neuropathy is crucial for treatment, as the management strategies for these conditions can vary significantly.
What to expect with Small Fiber Neuropathy
Small fiber neuropathy (SFN) typically progresses slowly in most patients, with symptoms gradually increasing over time until they reach a steady state. A study found that after two years, only 13% out of 124 patients with SFN started to show signs of large-fiber issues. Large-fiber neuropathy is a related condition that can cause weakness, imbalance, foot ulcers, and Charcot joints, which represent a type of severe bone and joint damage. However, none of these symptoms were observed in the study’s SFN patients.
Interestingly, over half of the patients witnessed either improvement or no change in their condition over the two years. To better understand how SFN develops and changes over extended periods of time, more studies that last longer than two years would be helpful.
Possible Complications When Diagnosed with Small Fiber Neuropathy
The chance of complications for patients with SFN (Small Fiber Neuropathy) typically depends on the source of the disease. It’s crucial for patients to receive and follow the right treatment to prevent their symptoms from getting worse due to their health conditions. SFN can cause substantial pain, leading a person to limit their activities. This can cause other issues such as weight gain and depression, so it’s essential to manage the pain effectively so that the person can continue their physical activities.
Complications can include:
- Worsening of underlying health conditions
- Decreased physical activity due to pain
- Weight gain due to decreased activity
- Depression linked to reduced activity and persistent pain
Preventing Small Fiber Neuropathy
If you have been diagnosed with SFN, or small fiber neuropathy, it’s important to learn about how to manage the discomfort caused by this nerve disease and how to treat any conditions that might be causing it. Here are a few tips based on what might be causing your SFN:
If you have diabetes, learn about how to manage your diet, exercise, and insulin (if you need it). Plan to regularly see your primary care doctor, foot doctor (podiatrist), eye doctor (ophthalmologist), hormone specialist (endocrinologist), and heart doctor (cardiologist).
If your SFN is due to drinking too much alcohol, use resources to help you stop drinking.
If you’re lacking certain nutrients like vitamin B12, learn about how to correctly add them to your diet or take them as supplements.
If you have SFN because of an infectious disease like HIV, Lyme disease, or hepatitis C, learn about the proper antiviral treatment.
If your immune system or a genetic condition is causing your SFN, learn more about how to treat it and what other complications could happen.
Along with learning how to manage any conditions causing your SFN, it’s important to remember to keep up physical activity. This can help your condition and overall health.
