Wednesday, 2 October 2013

Small Fibre Neuropathy: A Definition

Today's post from (see link below) is a comprehensive definition of small fibre neuropathy and includes some very useful information for those who have been diagnosed with this particular type of neuropathic complaint. Small fibre neuropathy affects the autonomic system more than large fibre neuropathy. The autonomic system is the range of involuntary actions that the body carries out - things our body does without our having control over them. However, as you will see if you read this article, the end result is very often the same as that for patients with other forms of neuropathy and the symptoms are just as recognisable. The article looks also at the genetic reasons for and the possibility of inheriting small fibre neuropathy but your neuropathy is still far more likely to have been caused by outside factors (such as diabetes, HIV and up to 100 other causes). Many of the links in this article provide more useful definitions of the sort of vocabulary you're likely to meet in the course of your diagnosis and treatment.

What is small fiber neuropathy?
Reviewed: November 2012
Published: September 23, 2013

Small fiber neuropathy is a condition characterized by severe pain attacks that typically begin in the feet or hands. As a person ages, the pain attacks can affect other regions. Some people initially experience a more generalized, whole-body pain. The attacks usually consist of pain described as stabbing or burning, or abnormal skin sensations such as tingling or itchiness. In some individuals, the pain is more severe during times of rest or at night. The signs and symptoms of small fiber neuropathy usually begin in adolescence to mid-adulthood.

Individuals with small fiber neuropathy cannot feel pain that is concentrated in a very small area, such as the prick of a pin. However, they have an increased sensitivity to pain in general (hyperalgesia) and experience pain from stimulation that typically does not cause pain (hypoesthesia). People affected with this condition may also have a reduced ability to differentiate between hot and cold. However, in some individuals, the pain attacks are provoked by cold or warm triggers.

Some affected individuals have urinary or bowel problems, episodes of rapid heartbeat (palpitations), dry eyes or mouth, or abnormal sweating. They can also experience a sharp drop in blood pressure upon standing (orthostatic hypotension), which can cause dizziness, blurred vision, or fainting.

Small fiber neuropathy is considered a form of peripheral neuropathy because it affects the peripheral nervous system, which connects the brain and spinal cord to muscles and to cells that detect sensations such as touch, smell, and pain. 

How common is small fiber neuropathy?

The prevalence of small fiber neuropathy is unknown. 

What genes are related to small fiber neuropathy?

Mutations in the SCN9A or SCN10A gene can cause small fiber neuropathy. These genes provide instructions for making pieces (the alpha subunits) of sodium channels. The SCN9A gene instructs the production of the alpha subunit for the NaV1.7 sodium channel and the SCN10A gene instructs the production of the alpha subunit for the NaV1.8 sodium channel. Sodium channels transport positively charged sodium atoms (sodium ions) into cells and play a key role in a cell's ability to generate and transmit electrical signals. The NaV1.7 and NaV1.8 sodium channels are found in nerve cells called nociceptors that transmit pain signals to the spinal cord and brain.

The SCN9A gene mutations that cause small fiber neuropathy result in NaV1.7 sodium channels that do not close completely when the channel is turned off. Many SCN10A gene mutations result in NaV1.8 sodium channels that open more easily than usual. The altered channels allow sodium ions to flow abnormally into nociceptors. This increase in sodium ions enhances transmission of pain signals, causing individuals to be more sensitive to stimulation that might otherwise not cause pain. In this condition, the small fibers that extend from the nociceptors through which pain signals are transmitted (axons) degenerate over time. The cause of this degeneration is unknown, but it likely accounts for signs and symptoms such as the loss of temperature differentiation and pinprick sensation. The combination of increased pain signaling and degeneration of pain-transmitting fibers leads to a variable condition with signs and symptoms that can change over time.

SCN9A gene mutations have been found in approximately 30 percent of individuals with small fiber neuropathy; SCN10A gene mutations are responsible for about 5 percent of cases. In some instances, other health conditions cause this disorder. Diabetes mellitus and impaired glucose tolerance are the most common diseases that lead to this disorder, with 6 to 50 percent of diabetics or pre-diabetics developing small fiber neuropathy. Other causes of this condition include a metabolic disorder called Fabry disease, immune disorders such as celiac disease or Sjogren syndrome, an inflammatory condition called sarcoidosis, and human immunodeficiency virus (HIV) infection.

Read more about the SCN9A and SCN10A genes.

How do people inherit small fiber neuropathy?

Small fiber neuropathy is inherited in an autosomal dominant pattern, which means one copy of the altered SCN9A gene or SCN10A gene in each cell is sufficient to cause the disorder. In some cases, an affected person inherits the mutation from one affected parent. Other cases result from new mutations in the gene and occur in people with no history of the disorder in their family.

When the genetic cause of small fiber neuropathy is unknown or when the condition is caused by another disorder, the inheritance pattern is unclear.
Where can I find information about diagnosis or management of small fiber neuropathy?

These resources address the diagnosis or management of small fiber neuropathy and may include treatment providers.
Genetic Testing Registry: Small fiber neuropathy

You might also find information on the diagnosis or management of small fiber neuropathy in Educational resources and Patient support.

General information about the diagnosis and management of genetic conditions is available in the Handbook. Read more about genetic testing, particularly the difference between clinical tests and research tests.

To locate a healthcare provider, see How can I find a genetics professional in my area? in the Handbook. 

Where can I find additional information about small fiber neuropathy?

You may find the following resources about small fiber neuropathy helpful. These materials are written for the general public. 

MedlinePlus - Health information
Health Topic: Peripheral Nerve Disorders

Additional NIH Resources - National Institutes of Health (2 links)

Educational resources - Information pages (4 links)

Patient support - For patients and families (2 links)

You may also be interested in these resources, which are designed for healthcare professionals and researchers.

Genetic Testing Registry - Repository of genetic test information (1 link) - Linking patients to medical research
PubMed - Recent literature
OMIM - Genetic disorder catalog

What other names do people use for small fiber neuropathy?

small nerve fiber neuropathy

For more information about naming genetic conditions, see the Genetics Home Reference Condition Naming Guidelines and How are genetic conditions and genes named? in the Handbook. 

What if I still have specific questions about small fiber neuropathy?

Ask the Genetic and Rare Diseases Information Center.
Where can I find general information about genetic conditions?

The Handbook provides basic information about genetics in clear language.
What does it mean if a disorder seems to run in my family?
What are the different ways in which a genetic condition can be inherited?
If a genetic disorder runs in my family, what are the chances that my children will have the condition?
Why are some genetic conditions more common in particular ethnic groups?

These links provide additional genetics resources that may be useful.
Genetics and health
Resources for Patients and Families
Resources for Health Professionals

What glossary definitions help with understanding small fiber neuropathy?

autosomal ; autosomal dominant ; axons ; cell ; channel ; diabetes ; diabetes mellitus ; differentiation ; fainting ; gene ; glucose ; HIV ; hypotension ; immunodeficiency ; infection ; inheritance ; inheritance pattern ; ions ; mutation ; nervous system ; neuropathy ; nociceptors ; orthostatic ; palpitations ; peripheral ; peripheral nervous system ; peripheral neuropathy ; prevalence ; sarcoidosis ; sensitivity ; sodium ; sodium channel ; subunit ; syndrome ; virus

You may find definitions for these and many other terms in the Genetics Home Reference Glossary.

See also Understanding Medical Terminology.

The resources on this site should not be used as a substitute for professional medical care or advice. Users seeking information about a personal genetic disease, syndrome, or condition should consult with a qualified healthcare professional. See How can I find a genetics professional in my area? in the Handbook.

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